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Detection and H igh Resolution Tracking of Vehicles at 
H ypersonic Velocities
The D efense Intelligence Reference D ocument provides nonsubstantive but 
authoritative reference information related to intelligence topics or methodologies.
Prepared by:
Technology Warning Division (DWO-4)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:
A A P Person 8 3
(U ) CO PYR IG H T W A R N IN G : F urther dissem ination of the photographs in this publication is not authorized.
This product is one of a series of advanced technology reports produced in FY 2010 under the 
Defense Intelligence Agency, Defense W arning Office's Advanced Aerospace W eapons S ystem 
Applications (AAW S A) Program. Comments or questions pertaining to this document should be 
addressed to |AAP Person 1 lAAW S A Program Manager, Defense Intelligence Agency,
ATTN: JU1AF Ul/DWU-3, bldg bUUU, Washington D.C. 20340-5100
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Contents
Introduction........................      1
Chapter 1: Theory Governing Objects in Flight............................  3
S ubsonic Flow and Drag ............      3
S upersonic Flow...............        6
Chapter 2: Hypersonic Compressible Flow Theory...................  10
Chapter 3: Detection Technologies................................................................................11
Electromagnetic Methods............................................................................................13
RADAR (Reflected Energy)...................    13
Doppler RADAR...........................................................  14
Radio Reflection Detection....................  15
Optical Methods........................................................................................................16
S ky Cameras and Photographic Methods..................................................................16
Infrared Detection..................................................................................................17
LIDAR....................................................... .........................................................23
Acoustic and S eismic Methods....................................................................................25
Infrasound...........................  25
S eismic.................................................................................................................27
Chapter 4: Vision of Progress Over the Next 30 Years......................................................28
Detectability of New Hypersonic Aircraft......................................................................29
Recommendation # 1 - Build a Database of the W ake Characteristics for Existing Aircraft 
...........................................................................................................................29
Recommendation # 2 - Exploit the Detectability of Hypersonic Aircraft Based on the
Features That Allow Them To Fly Efficiently at High S peeds........................................30
Recommendation # 3 - Explore the Detection of Vehicles Designed To Be Undetectable30
Recommendation # 4 - Explore the Development of Novel Detectors..........................31
S ummary........................................................  31
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Figures
Figure 1. Boundary Layer Development in S ubsonic Flow about a Projectile.........................4
Figure 2. S ubsonic Flow Past a Projectile...............................  5
Figure 3. S trouhal Frequency as a Function of Reynolds Number for Flow over a 
Circular Cylinder.....      6
Figure 4. S upersonic Flow Past a W edge-S haped Body.......................................................8
Figure 5. S upersonic Flow Past a Blunt Body (S pheroid).....................................................8
Figure 6. Ratios of Mach Number, Temperature, and Pressure Across a Normal S hock......10
Figure 7. Hypersonic Vehicle Detection Techniques.................  11
Figure 8. Laser-Based Chronograph................................................................................12
Figure 9. Electromagnetic S pectrum as a Function of W avelength......................................13
Figure 10. S chematic of a RADAR S ystem.......................................................................14
Figure 11. Radio Reflection Detector...............................................................................16
Figure 12. Energy S pectrum of the S un at 5,778 K..............    19
Figure 13. Hypersonic Flow Past a S phere.......................................................................20
Figure 14. Infrared Energy S pectra for Mach 3 and Mach 3.5............................................21
Figure 15. Peak W avelength of the Infrared Radiation Emitted by Hypersonic Objects..........22
Figure 16. Peak S hock Layer Temperature for Hypersonic Objects.....................................22
Figure 17. S chematic of a Pulsed LIDAR S ystem..................    24
Figure 18. Mach Cone Generated by a S upersonic Object.................................................26
Figure 19. Comparison of Detection Technology for Hypersonic Objects.............................32
Tables
Table 1: Normal S hock Values for Mach 3 Flow......................    9
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Introduction
An object is supersonic when its speed through the atmosphere is greater than the local 
speed of sound. The Mach number is defined as the speed of the object divided by the local 
speed of sound. For Mach numbers greater than 1 (supersonic flow), shock waves develop in 
the flowfield and near the surface of the object due to the air's compressibility. Traditionally, 
the lower Mach number limit for the so-called hypersonic speed regime is about Mach 5 (1.7 
km/sec). "Low hypersonic" values range between Mach 5 to about Mach 10, while "high 
hypersonic" values range between approximately Mach 10 to Mach 30 or above. Mach 30 
(10 km/s), for example, is close to S pace S huttle reentry velocity. Few objects can travel at 
hypersonic velocities. The most common object that we see moving at these speeds are 
meteors entering the Earth's atmosphere. As meteors fall to the Earth's surface, their 
velocities may reach 30 miles per second (48 km/s),1 and their corresponding Mach number 
as they enter the upper layers of the atmosphere will exceed 150. Meteors are preceded by 
a bow shockwave as they compress the air immediately in their path. Temperatures and 
pressures increase dramatically across the shockwave to a point where the gases in air ionize 
and disassociate, leading to the emission of visible light and radio waves. These conditions 
also lead to rapid heating of the meteor surface causing them to fracture and break up as 
they enter the atmosphere. Optical and RADAR-based surveillance systems are now used to 
scan outer space to detect asteroids and other objects with orbits that may lead them to 
collide with Earth.
A second class of hypersonic objects includes reentry vehicles moving into the Earth's 
atmosphere from orbit. In the case of a reentry vehicle returning from low-Earth orbit at 
100 km altitude, the velocity of the vehicle will reach 8 km/s (about 5 miles per second). 
The Mach number of this vehicle will exceed 26.5 in the upper atmosphere. As the vehicle 
moves through the atmosphere, flow-induced drag forces will slow down the vehicle; if the 
vehicle has sufficient thermal protection, it can survive reentry and be recovered.
A third class of hypersonic vehicles includes the reentry payload used in ICBMs 
(intercontinental ballistic missiles). The payload in these missiles is launched using a rocket 
that boosts them above the atmosphere to a suborbital velocity. The payload velocity may 
exceed 7 km/s as they reenter the atmosphere, equating to M > 23. The coupled high 
velocity and high kinetic energy of these objects creates an ionized wake and shock wave 
that can be used to detect their position and to determine their velocity. This information, 
along with their ballistic trajectory, is used to locate their target and the time to impact.
A fourth class of hypersonic vehicles includes manned and unmanned rockets and aircraft. 
The North American X-15, for example, exceeded a speed of 7,274 km/hr in 1964, equating 
to about M = 6.5. This aircraft also exceeded 100 km in altitude on two occasions, qualifying 
the X-15 as a spacecraft. The S R-71 Blackbird, an air-breathing strategic reconnaissance 
aircraft, has exceeded Mach 3.2 at 80,000-ft altitude, and its actual maximum speed may 
have reached into the hypersonic flow regime. Other real and conceptual aircraft capable of 
reaching hypersonic speeds include the alleged Aurora S R-91 (Mach 4 to 6), the Boeing X- 
51, an unmanned Mach 6 scramjet, the HTV-2 from the DARPA Falcon project (designed for 
M=20 flight to low-Earth orbit), the HyperS oar (M = 12), the Russian Leninetz Ayaks, and 
the S kyion (designed for single-stage-to-orbit flights). Certain rockets are also designed to 
travel at hypersonic speeds. The Patriot missile, for example, travels at Mach 5, a high 
speed necessary for it to be able to intercept and destroy other missiles while they are in 
flight.
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There are a number of different techniques available to detect hypersonic objects and to 
determine their position, velocity, and trajectory. These detection methods can be broadly 
divided into electromagnetic, optical, and acoustic/seismic methods. Each method relies on 
the properties of the surface of the object to reflect incident radiation or upon the properties 
of the hot ionized gases caused by the bow shock and entrained into the object's wake. The 
theory of external fluid flow around objects in subsonic, supersonic, and hypersonic regimes 
will be reviewed and various detection methods will be compared in this report.
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Chapter 1: Theory Governing Objects in Flight
Many of the detection technologies for hypersonic aircraft are based on the properties of the 
air flow around the object. The fluid mechanics affecting supersonic and hypersonic aircraft 
are well described in textbooks on compressible flow by J.D. Anderson;2'3 subsonic flow, 
including the affect of boundary layers, is covered in the textbook by F. M. W hite.4
Hypersonic flow usually refers to the regime where objects are moving faster than Mach 5, 
but technically refers to the Mach range where pressure, temperature, and density ratios 
across shock waves reach constant values, and this does happen at about Mach 5. Fluid flow 
changes dramatically at Mach 1 defined at the point where the velocity of a projectile, 1/, 
equals the local speed of sound, a, in the atmosphere:
V
M=- (1)
a
Flow regimes are defined by the following:
• M < 1, subsonic flow
• M = 1, sonic flow
• M > 1, supersonic flow
• M > 5, hypersonic flow
The speed of sound varies with temperature and with the type of gases in the atmosphere. A 
simple equation for the speed of sound includes the ratio of specific heats, y, the gas 
constant, R, and the temperature, T:
a = y[y~RT (2)
For air at room temperature, y = 1-4, R = 287 J/kg-K, and T = 20° C or 293 K. The 
subsequent speed of sound is 343 m/s (1,125 ft/s or 767 mph). It is easier for a projectile 
to exceed the speed of sound at higher elevations since the temperature of the atmosphere 
is lower, reducing the speed of sound and the subsequent velocity necessary to break the 
sound barrier.
The viscosity of the fluid flowing around an object induces drag and retards its forward 
motion. S upersonic flow has some of the viscous characteristics of subsonic flow, but adds in 
the complication of shock waves. To understand the nature of how a projectile affects the 
flow of fluid around it, consider the case of a subsonic projectile.
SUBSONIC FLOW AND DRAG
S ubsonic flows, such as the flow shown in Figure 1, are heavily influenced by collisions 
between molecules of air and the surface of the projectile. The equations for this interaction 
were first developed by S ir Isaac Newton in the seventeenth century, where he determined 
that fluids have a property called "viscosity" that is the cause of drag on projectiles. The key 
parameter that affects subsonic fluid flow is the ratio of viscous forces to inertial forces, 
where inertial forces are associated with the tendency for a moving fluid to keep moving in 
the same direction. The ratio of these two forces is defined as the Reynolds Number, Re.
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1^ _ viscous forces _V D 
inertial forces v
(3)
In this expression, V is the velocity of the projectile, D is its effective diameter, and v is the 
kinematic viscosity of the fluid. As flow passes over the projectile in Figure 1, viscous forces 
are especially strong near the surface where the velocity changes from zero at the surface of 
the projectile to the free stream velocity.
Direction of Flow
Figure 1. Boundary Layer Development in Subsonic Flow About a Projectile.
The region where this transition in velocity occurs is called a "boundary layer," which grows 
in thickness as flow moves along the surface from the nose to the rear of the projectile. 
The boundary layer forms around the surface of the projectile and "separates" from the 
surface at the back end of the projectile. Pockets of fluid spill off of the back end of the 
projectile and form rotating pockets of fluid called S trouhal eddies or vortices. These eddies 
"shed" from the surface at a rate that is strongly dependent upon the Reynolds number in 
the flow. Figure 2 demonstrates that these eddies form a "wake" downstream of the 
projectile that can extend far into the fluid.
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B oundary Layers
Projectile
D irection of F low
Figure 2. Subsonic Flow Past a Projectile.
Visible examples of a "wake" are the contrails observed in the upper atmosphere when water 
vapor in the exhaust of jet aircraft freezes into ice crystals, highlighting the wake generated 
by the aircraft and its engines. In still air, the wake can be visible for many miles behind the 
aircraft.
The wake rapidly becomes turbulent as it extends downstream. The S trouhal eddies are 
detectable and have a shedding frequency that is expressed in terms of the S trouhal 
number, S t = f D/V, where f is the S trouhal shedding frequency (hz), D is the diameter (m) 
of the projectile, and V is its velocity (m/s) through the air. The S trouhal number is a 
function of Reynolds number, as shown in Figure 3. By measuring the S trouhal frequency, 
the velocity of the projectile can be determined with moderate accuracy.
At low speeds, and corresponding low Reynolds numbers, the flow is "laminar" where 
streamlines move in smooth "laminae" or layers over projectiles. At high Reynolds numbers, 
the fluid flow becomes unstable and forms a widely distributed set of small eddies defined as 
"turbulence." In turbulent flow, rapid mixing of the flow in the wake, even in the boundary 
layers, will occur.
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R eD  = V D /v
Figure 3. Strouhal Frequency as a Function of Reynolds Number for 
Flow Over a Circular Cylinder.
The high velocities present in the wake generate low pressures on the downstream side of 
the projectile that pull the projectile backwards into the wake. This is defined as "wake" or 
"form" drag and is a predominant cause of drag forces on aircraft or on any object moving 
through the atmosphere. The viscous forces that create the boundary layers on the surface 
of a projectile also contribute a small amount of drag, usually referred to as "skin friction." 
Boundary layers and wakes also affect supersonic flow, but the effect of shock waves tends 
to be the predominant mechanism affecting flow around supersonic projectiles. Total drag 
force, Fd, exerted on projectiles or aircraft is usually summarized by this equation:
FD=y2pAfCDV2 (4)
The drag force is dependent upon the density of the air, p, the frontal area of the aircraft, Af, 
the aircraft velocity, V, and the drag coefficient, Cd. The drag coefficient is a function of the 
geometry of the aircraft and the two "dimensionless" groups, Mach number, M, and Reynolds 
number, Reo, that relate inertial, viscous, and elastic forces in the flow:
CD = /(Reo, M, geometry) (5)
Drag coefficients for various types of aircraft and projectiles are obtained from theory, from 
computational fluid mechanics, or, most commonly, from experiments in wind tunnels. 
These coefficients are usually presented in tables or in graphical form.5'6' 7
SUPERSONIC FLOW
Air is predominantly made up of molecules of nitrogen (78% ) and oxygen (21% ). At room 
temperature, air behaves as an ideal gas where the density, p, the pressure, p, and the 
temperature, T, are related through a gas constant, R, which is a property of the air. The 
ideal gas law gives the relationship between these values. The gas constant for air is 
R = 287 J/kg-K.
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p-pKT (6)
It is possible to statistically model the motion of the molecules of oxygen and nitrogen in air, 
and their kinetic energy, KE, can be determined based on the mass, m, of each molecule and 
the temperature of the gas.
KE^-mV1 =-kT (7)
In this equation, k is the Boltzmann constant (1.3807 x 10'23 J/K). W e can calculate the 
velocity of air molecules based on the temperature of the air.
Or, for the average velocity of a molecule:
For air near sea level (p = 101,320 Pa, T = 293 K) the average velocity of molecules in air is 
463 m/s or 1,035 MPH. This value is just a little higher than the speed of sound in air (343 
m/s or 767 MPH) as computed earlier. These molecules only travel a short distance before 
they collide with each other. This distance is defined as the "mean free path" given by the 
symbol A.
In this expression, d is the diameter of a molecule, which is approximately 0.3 nanometers. 
For air at 20° C and 101,325 Pa, the mean free path (A) is approximately 100 nanometers or 
about 333 molecular diameters.
W hen the supersonic projectile in Figure 4 moves through the air, molecules of nitrogen and 
oxygen in the air bounce off the vehicle's surface and collide with other molecules of air a 
short distance away. At the speed of sound, these molecules are not moving fast enough to 
get out of the way and a large number of molecules pile up along a straight line that 
emanates from the nose or leading edge of the projectile as a "shock wave."
S upersonic flow in the atmosphere labeled as region 1 passes through the shock and moves 
parallel to the surface of the body. The flow "expands" through a Prandtl-Meyer expansion 
fan at the end of the airfoil and speeds back up to its original Mach number. Flow in the 
boundary layer separates from the end of the airfoil and forms a highly turbulent wake 
downstream of the airfoil. The wake is also composed of S trouhal eddies that can subsist in 
the air long after the airfoil has passed by. The flow density, pressure, and temperature 
increases dramatically across the bow shockwave and returns to the original Mach number 
downstream of the airfoil.
For a blunt-nosed object, as shown in Figure 5, the magnitude of the impact that the 
shockwave has on the flow is easier to explain. For a blunt object, the shock detaches from 
the surface of the object into the freestream in front of the object. S ince the original flow 
moving at Mi is traveling at 90° with respect to the shock near the nose of the object, the 
shock in this region is referred to as a "normal" shock. The properties across a normal shock
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are described by the Rankine-Hugoniot equations, which yield the ratio of certain
thermodynamic properties.
Figure 4. Supersonic Flow Past a Wedge-Shaped Body.
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These equations can be expressed in a form where the ratio of density, temperature, and 
pressure across the normal shock are a function of the upstream Mach number, Mi.
M2
1+Z—M^
2 1
z —1 (ID
7]
2 + (/-l)M ,2 
(r+D M ?
p,= (/TOM ,2 
p, 2 + ^-1)^
(12)
(13)
(14)
As an example, if the object shown in figure 4 is traveling at Mach 3 through air at sea level, 
the pressure in the region between the nose of the object and the shock changes as shown in 
the following table:
Table 1: Normal Shock Values for Mach 3 Flow
Freestream Ratio of Values
Values Properties Downstream
Across the of the
Normal Shock Normal Shock
M i = 3 M z/M ^ 0 .158 M 2 = 0 .475
Pi = 10 1,325 Pa p2/pi = 10 .333 p2 = 1,0 47,0 0 0  Pa
Tj = 293 K T2/Tj = 2.679 T2= 78 5.0  K
= (20 ’C) = (512’C)
Pi = 1.293 kg/m 3 P2/P1 = 3.8 57 p2 = 4.976 kg/m 3
For a normal shock, the Mach number on the downstream side of the shock is always 
subsonic, and in this example, the Mach number abruptly drops from Mi - 3 to M2 ~ 0.475. 
The temperature always increases across a shock, and for a Mach 3 flow, the temperature 
rises from room temperature at 20° C up to 512° C. It is apparent that, for supersonic 
aircraft traveling at this speed, the fuselage and wings will need to be made of materials that 
can withstand the high temperatures and the dramatic pressure increase of 10.333. S hocks 
are always accompanied by significant increases in pressure, temperature, and density in the 
flow across the shock.
Flow about an Ogive or wedge-shaped airfoil as shown in Figure 4 can be analyzed in a 
similar fashion; however, the wall deflection angle, 0, will affect the flow and the formation of 
the oblique shock wave that surrounds the airfoil.
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Chapter 2: H ypersonic Compressible Flow Theory
Hypersonic flow is typically assumed to apply to objects traveling at M > 5. Figure 6 shows 
that, for Mach numbers above 5, the ratio of Mach numbers across the shock approaches a 
constant value of 0.378, although temperature and pressure ratios continue to increase. In 
hypersonic flow, the bow shock, or Mach angle, /3Z approaches the half-angle, 6, of slender 
airfoils and the drag coefficient reaches a constant value that does not change with Mach 
number. The drag coefficient actually becomes a simple function of the half-angle of the 
airfoil, and the boundary layer is squeezed between the shockwave and the airfoil surface.
Figure 6. Ratios of Mach Number, Temperature, and Pressure Across a Normal Shock
In hypersonic flow, the Rankine-Hugoniot equations, which are based on the perfect gas law 
(p = pRT), fail to predict the real behavior of air at extremely high temperatures. Equation 
12, for example, predicts a temperature of 29,787° C in the air near the surface of a blunt 
object traveling at the reentry Mach number of 26.5, while the actual temperature only 
reaches 7,600° C. The reason for this discrepancy is due to ionization of molecules of air as 
electrons are stripped away by the high temperatures that exist across the shock. S ome 
energy is used to produce this ionization, and above temperatures of 550° C, equation 15 
does not accurately predict air temperatures due to shocks. Ionization is responsible for the 
glowing wake that follows reentry vehicles and meteors as electrons rush to recombine with 
ions releasing x-rays and visible light. The ionization also interferes with radio transmissions, 
but provides a convenient way to identify hypersonic objects due to the emitted light. A 
"rule of thumb" is that the peak shock layer temperature in degrees kelvin is 1,000 times 
larger than the aircraft speed in km/s. By this standard, a reentry vehicle at 8 km/s 
(Mach 26.5) would have a maximum shock layer temperature of 8,000 K. At temperatures 
above 2,000 K, nitrogen and oxygen gas in the form of O2 and N2 will disassociate into 
individual ions, consuming more energy from the flow.
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Chapter 3: Detection Technologies
Hypersonic objects passing through the Earth's atmosphere leave traces that can be 
observed using a number of detection methods. The bow shock introduced by such objects 
reflects and refracts radio waves, RADAR pulses, and visible light. The ionized air 
surrounding the object also creates electromagnetic interference that is visible on RADAR and 
the emitted visible light in the wake is observable in photographs. The shock and turbulent 
wake also create low frequency sound that can be sensed by ground-based sensors. These 
detection methods can be characterized as electromagnetic, optical, and acoustic and seismic 
as shown in Figure 7. Each method will be discussed along with the limitations and 
advantages for the detection of hypersonic objects passing through the atmosphere.
Electromagnetic
R A D A R
E nergy R eflection
D oppler R A D A R
R adio R eflection D etection
Optical
Sky Cam eras and Photographic M ethods
Infrared D etection
LID A R
Chronograph
Acoustic and Seismic
Infrasound
Seism ic
Figure 7. H ypersonic Vehicle Detection Techniques.
One of the fundamental tools for detecting the velocity of high-speed objects is the 
chronograph. These devices are used to measure the velocity of automobiles using 
pneumatic tubes, and simple systems using laser diodes or LEDs are used to measure the 
velocity of bullets. Figure 8 outlines a laser-based chronograph that is commercially 
available for measuring the velocity of projectiles in two-stage gas guns capable of speeds in 
a vacuum of 12 km/h, or approximately Mach 36 if the projectile passed through air. As a 
high-speed object passes through the first laser beam followed rapidly by the second, 
photodetectors sense the change in intensity of the laser beam and send these signals to a 
storage oscilloscope or to a counter. The time delay between the two signals, At, is used to 
find the velocity of the object based on the distance between the two photodetectors, L. 
Digital clocks are capable of accurately measuring time delays to within a fraction of a 
nanosecond (10’9 seconds), so chronographs are capable of very high accuracy in 
determining the velocity of objects. As noted earlier, their use at gun ranges or in high­
speed gas guns requires that the path of the hypersonic object must pass through the laser 
beams for the chronograph to be effective, which limits their usage for the detection of 
hypersonic aircraft except at instrumented test ranges.
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Hypersonic
A ircraft
Figure 8. Laser-Based Chronograph.
Currently, the best devices for the detection of hypersonic vehicles are based on the 
reflection of radio and microwave radiation through RADAR and upon the detection of 
infrared energy emitted by the ionized gases surrounding high-speed vehicles and in their 
wake. Optical and RADAR methods are also based on electromagnetic energy, yet make use 
of different parts of the electromagnetic spectrum as shown in Figure 9. Longer wavelength 
photons, or quanta of electromagnetic energy, have less energy than short wavelength 
photons. As a result, detection of short wavelength radiation tends to be more line-of-sight 
as in infrared or other optical methods. Longer wavelengths, used in RADAR, tend to bend 
around the Earth and can provide over-the-horizon detection of ICBMs, reentry vehicles, and 
other hypersonic vehicles.
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Electromagnetic Spectrum
Wavelength Type of
Radiation
1 nm x-rays
10  nm
10 0  nm U ltraviolet (U V)
1 pm Visible Light (0 .4to 0 .7 pm )
10  pm N ear Infrared
10 0  pm Therm al Infrared
( 1 m m F ar Infrared
10  m m M icrow aves
10 0  m m R adar
1 m R adio W aves
10  m
10 0  m A M  R adio
Figure 9. Electromagnetic Spectrum as a Function of Wavelength.
ELECTROMAGNETIC METH ODS
RADAR (Reflected Energy)
RADAR, or "radio detection and ranging/' has been heavily used since W orld W ar II for the 
detection of aircraft and sea vessels. First patented by Christian Hulsmeyer in 1904, this 
technique was first demonstrated by Nikola Tesla in 1917; RADAR was exploited in the 
defense of Britain through the Chain Home RADAR network initiated in the late 1930s.8 
RADAR uses an antenna or a dish to transmit pulses of microwaves or radio waves toward a 
potential target. Energy reflected from the target is collected by the antenna and the time of 
flight of the transmitted and reflected signals yields the distance to the target. Early RADAR 
systems emitted radiation in the high-frequency band (HF) from about 10 to 30 MHz. 
Modern systems can operate well above 300 GHz.
As a single pulse travels at the speed of light toward a target, the time delay between the 
generation of the pulse and the time that its echo is received is 2L/c where L is the distance 
to the target and c is the speed of light. For a target located 30 km from the source, the 
time delay is 200 microseconds, an easily measurable delay. By using microwave radiation 
emitted from a dish, target distance and bearing can both be measured.
W hile it was possible to use low-frequency radio waves (~30 MHz) for RADAR systems in the 
1940s, the wavelength of these waves (100 meters) made it difficult to resolve small targets, 
including aircraft. This made it desirable to develop systems that could operate at higher 
frequencies. UHF energy (300 to 1,000 GHz, 0.3- to 1-meter wavelength) radiation is used 
for long-range surveillance, including the detection of intercontinental ballistic missiles. Air 
traffic control uses the L band (1 to 2 GHz, 0.15- to 0.3-meter wavelength). X-band (8 to 
12 GHz, 25- to 37-millimeter wavelength) energy is now used for airport RADAR where the 
range to aircraft is short. Higher frequency radiation is used for imaging systems and for 
meteorological data acquisition.
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RADAR systems are widely used for aircraft surveillance and can detect the distance to 
objects and the position of an object (through triangulation); with Doppler RADAR, the 
velocity can be directly measured. RADAR does have limitations. W hile low-frequency 
RADAR systems are capable of following the curvature of the earth, the long wavelength of 
this energy makes it difficult to resolve small objects. Higher frequency RADAR tends to be 
more "line of sight." S ince it depends upon reflected energy, several problems are 
highlighted by the RADAR Equation:
preceived
fjran sm ittd_G ^
(4^)2 R; R; (15)
In this equation, Ptransmitted represents the transmitted power, Preceded the received power, G 
the transmitting antenna gain, A the aperture area of the receiving antenna, cr the RADAR 
cross section, F the pattern propagation factor, and Rt and Rr are the distance from the 
transmitter to the target and the target to the receiver, respectively. If the RADAR 
transmitter and receiver are in the same unit, the power returned as a reflection from a 
target decreases as R4 . This means that energy received in reflected energy decreases by 
94%  every time the distance to the target is doubled.
RADAR depends upon the reflection of electromagnetic waves off of a target's surface, 
thereby making objects with a small RADAR cross section difficult to detect (e.g., stealth 
aircraft). Atmospheric phenomenon, including inversions and turbulence, can lead to 
interference with RADAR reflections. Even with these limitations, RADAR is widely used to 
monitor high-speed aircraft, meteors, and man-made objects reentering the atmosphere 
from Earth orbit.
Velocity Vector
Doppler RADAR systems yield velocity data for a target. Pulsed RADAR systems send out a 
short burst of high-frequency radiation that is reflected from a target. The RADAR unit 
processes the delay time between the transmitted pulse and the received echo to determine 
the range or distance to the target. The energy reflected from the target is, however,
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Doppler-shifted from its original frequency due to its velocity toward or away from the 
RADAR transmitter.9 The frequency shift, Af, is given by the following equation:
2V 2W .a £   £ __ £ _ £ Jtrasn m itia f 1
J reflected J tran sm ittd y J tran sm ittd
In this equation, c is the speed of light, and V is the target speed in the direction toward or 
away from the RADAR transmitter. The frequency shift is typically small since I/ << c, but 
the shift is easily observable due to "beating" between the transmitted and reflected signals. 
The development of fast Fourier transforms (FFTs) greatly increased the rate that Doppler 
reflections could be processed to compute target velocity. By triangulating between two 
RADAR transmitters, the position of a target can be determined along with its velocity and 
direction of travel. Doppler RADAR systems are used for monitoring high-speed aircraft, 
turbulence and shear in meteorology, and even in hand-held police RADAR guns.
RADAR technology has been used for the detection of missiles, reentry vehicles, and space 
objects near the Earth since the 1960s. On 9 S eptember 1961, for example, a Naval 
Research Laboratory RADAR installation at the Chesapeake Bay Annex was used to detect a 
Mercury/Atlas flight during its launch phase.10 The RADAR system sensed the range to the 
rocket, its rate of climb, and the range of its exhaust plume. This RADAR system worked 
"over the horizon" and demonstrated that a RADAR system could be used to identify targets 
and compute their trajectory.
A wide range of RADAR systems are now available to monitor the Earth and space for high­
speed objects in the atmosphere.11 Cobra Dane and Cobra Judy, L-band phased-array 
RADAR systems located on land (Alaska) and on ships (X- and S -band), are used for the 
detection of reentry vehicles above 35 km. RADAR systems in Florida and Massachusetts are 
capable of monitoring objects in space at a distance of 5,000 km to yield data on their size 
and shape. Ballistic missile early warning systems still operate in Alaska, Great Britain, and 
in Greenland to detect and track missile launches. On Kwajalein Atoll in the Pacific Ocean, 
Altar tracks reentry vehicles at distances of up to 2,500 km with high resolution using a 100- 
kW , millimeter-wave RADAR system. TRADEX, a multitarget tracking system using L- and S - 
band RADAR was developed in 1963 to track missile signatures at a distance of up to 1,400 
km. The venerable RC-137 (converted Boeing 707) can also be deployed with onboard 
RADAR and optical systems that can track missile launches and reentry vehicles.
Radio Reflection Detection
This method can be used to detect meteors or any other object entering the Earth's 
atmosphere during the day or night. The principle of this method is based on passive 
electromagnetic energy emitted from the Earth's surface in the form of radio or television 
signals. If these signals are relatively high frequency, the signal is line of sight and cannot 
normally be received over the horizon. A radio or television located over the horizon and 
tuned into the signal frequency will only detect static or hiss. W hen a meteor or other object 
enters the Earth's atmosphere, its bow shock and turbulent wake containing ionized air 
reflects radio signals causing an over-the-horizon receiver to hear pings and whistles that 
change pitch as the meteor passes by and breaks up in the atmosphere.12 It is possible to 
receive reflections off of the surface of the hypersonic meteor in addition to the wake and the 
shock. S ound files from the Aurigid Meteor S hower caused by radio echoes are available13 
with data obtained from a directional antenna at 61 MHz and 217 MHz. The data 
demonstrates that it is possible to differentiate reflections from the bow shock and the
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ionized wake. This technique for detecting hypersonic meteors is not fully developed and 
appears to be employed only by hobbyists.
S ince a receiver can be tuned to a specific over-the-horizon transmitter, it may be possible to 
develop a network of radio receivers to provide information on high-speed objects in the 
atmosphere. Multiple stations would allow triangulation of the object to determine velocity 
and position as a function of time, i.e., tracking and path prediction. The sound files also 
contain information that could provide information on the hypersonic object. Detection of the 
shock provides information on the position of the object while the transient "whistles" 
reflected from the ionized wake may provide information on the S trouhal eddy frequency that 
could directly infer the object's velocity. This detection technology should be further 
developed.
Figure 11. Radio Reflection Detector.
OPTICAL METH ODS
Sky Cameras and Photographic Methods
A classic example of a hypersonic object passing through the Earth's atmosphere is a 
meteor. Meteorite hunters and astrophysicists have worked to develop methods to detect 
these objects as they fall to the Earth. Motion-detection video systems provide information 
on location and direction of meteors passing through the atmosphere by detecting their 
visible wake. The El Paso All S ky Camera project14 is one example and even provides live 
streaming video of meteoric events.
Photographs of objects in orbit with diameters ranging from 0.1 meter to several meters 
have been used to determine their expected trajectory as meteors as they entered and fell 
through the Earth's atmosphere.15 This technique was first used to monitor the Pribram 
meteorite simultaneously by several observatories in 1959. This technique was even used to 
locate a 1.75-kg enstatite chondrite meteorite called Neuschwanstein reported in 2003. The
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European Fireball Network16 uses cameras at several stations separated by distances of 
about 100 km and covering an area of about 1,000,000 km2. Fisheye lenses allow each 
station to observe the sky every night to monitor meteors and other nebular objects. The 
network is operated by the German Aerospace Center (DLR) and the Institute of Planetary 
Research in Prague.17 Other networks include the Meteorite Observation and Recovery 
Project (MORP) in Canada and the Prairie Network in the United S tates; all have been in 
operation since the 1960s and 70s.
Another recent meteorite recovery was made when asteroid 2008 TC3 entered the Earth's 
atmosphere on 7 October 2008 over S udan.18 The meteor's path left a wake that was visible 
through dawn. A total of 280 fragments weighing 11 pounds were collected in the S udanese 
desert. The photographic evidence of this hypersonic reentry was interesting. The 4.2- 
meter W illiam Herschel Telescope in the Canary Islands optically recorded the spectrum of 
this asteroid 2 hours before it burned up in the Earth's atmosphere. This information can be 
used to identify the chemical makeup of the surface of the object or of the vaporized 
products in its wake. A sequence of photographs taken at 4-second intervals over a period 
of about 50 seconds provided information on its velocity as it reentered the atmosphere and 
its brightness as a function of time.
Although these photographic methods have been applied to meteors, data from the European 
Fireball Network and similar camera systems can be used to monitor the flight of hypersonic 
aircraft that produce a visible wake through the ionization of air. The photographic methods 
described here are limited to nighttime use. Multiple stations recording the same event have 
been used to measure both position and velocity of objects moving at hypersonic velocities 
through the Earth's atmosphere and the computed trajectories have been used to 
successfully predict meteorite impact points on at least two occasions.
Optical systems are constantly monitoring the Earth for possible missile launches. Two KH-II 
satellites are constantly in orbit 250 to 500 km above the Earth's surface with multispectral 
cameras that resolve objects to within 2 to 3 meters. Big-Bird, at an orbit 160 to 280 km 
above the Earth, can take video or photographs with a resolution of 250 mm. The photos 
must be sent back to Earth for processing. Close-Look, operating from 130 to 300 km, has 
a resolution of 50 to 150 mm. Landsat, with an orbit of 800 km, can transmit multispectral 
video of the Earth with a resolution of 20 to 30 meters. The space shuttle, the U-2 aircraft, 
and the S R-71 have all been used to provide high-resolution images of objects from high 
altitude.
Infrared Detection
Infrared detection of reentry vehicles and ICBMs by satellites has been available since the 
early 1960s, with the United S tates leading the effort through detection of possible missile 
launches from the S oviet Union. In addition to RADAR, infrared detection is one of the best 
techniques for the detection of hypersonic vehicles.
Theory of Infrared Detection Systems.
All objects emit radiation that is a function of their temperature according to equation 17, the 
S tefan-Boltzmann Law:19
q = saAT^ (17)
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In this equation, q represents the amount of radiant energy emitted in watts, e is the 
emissivity (a dimensionless quantity that gauges the relative ability of an object's surface to 
emit energy by radiation), A is the surface area, a is the S tefan-Boltzmann constant (u 
= 5.67 x 10'8 J s^ m^ K’4), and T is the temperature of the object's surface. A simple 
example of emission of heat as radiant energy is an infrared bathroom heater where electric 
current is passed through a metallic element that reaches several thousand degrees and 
emits considerable thermal radiation.
All objects in the environment exchange radiant heat with each other and seek equilibrium 
temperatures. At high temperatures, many objects behave as "black body radiators" defined 
by their surface emissivity, e = 1. The surface temperature of an object causes 
electromagnetic radiation to be emitted with a spectrum given by Planck's Distribution Law, 
equation 18, where the energy density is emitted by the surface of the object per 
wavelength per unit volume:
, , 8 7i h c 1
“ ------IT------ <18>
- 1
The terms in this equation are as follows:
• c: speed of light, (3 x 108 m/s).
• h: Planck's constant, (6.62 x 10'34 J s).
• A: wavelength of the emitted radiation, (m).
• k: Boltzmann constant, (1.38 x 10’23 J/K).
• T: surface temperature, (K).
• u(A,T): spectral energy density, (Jm^-m’1).
The surface of the sun behaves as a black body radiator3 with a surface temperature of 
5,778 K. For the sun, the Planck distribution of energy versus wavelength looks like that of 
Figure 12. The horizontal axis is the wavelength in units of meters and shows that peak 
energy occurs at a wavelength of about 5.02 x IO*7 meters or 0.502 microns (1 micron = 10’6 
meters). This falls within the visible band (0.38 microns to 0.65 microns) and corresponds to 
the yellow color of our sun. The peak wavelength, equation 19, is given by W ien's 
Displacement Law:
4,x = 2.8977685 x 10’3 (m ■ K)/T (19)
This shows that as the temperature of an object increases, the amount of radiant energy 
released by the surface increases and the value of the peak wavelength becomes smaller.
a A black body is an idealized object that absorbs all electromagnetic radiation that falls on it. 
S ince a black body is a perfect absorber of radiant energy, by the laws of thermodynamics it must 
also be a perfect emitter of radiation.
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x
Figure 12. Energy Spectrum of the Sun at 5,778 K.
Application of Planck's Law to Hypersonic Vehicles.
As described in the theory of hypersonic fluid flow, any object moving through the 
atmosphere with an M > 1 will result in a shock wave emanating from the leading edge of 
the object. If the object has a blunt nose, such as a meteor entering the atmosphere, the 
shock will be detached from the surface. If we consider a spherical object moving through 
the atmosphere at hypersonic speed, as shown in Figure 13, the expected spectrum of 
emitted radiation can be computed. In this example, the altitude is assumed to be 10 km 
above the Earth's surface. At this altitude, the ambient static temperature is 223 K (-50° C), 
the pressure is 26.1%  of sea level atmospheric pressure (see Appendix A for the properties 
of the U.S . S tandard Atmosphere).
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Figure 13. H ypersonic Flow Past a Sphere.
If the sphere in Figure 13 were traveling through the atmosphere at Mach 3, could we 
identify the speed of the sphere by the peak wavelength of the infrared radiation that it 
emits? From compressible aerodynamics relationships, conditions behind the bow shock, in 
region 2, can be calculated. W e find that the peak static temperature would reach 597 K and 
the peak wavelength 4.85 microns. Figure 14 shows the expected infrared spectrum. For 
comparison, the spectrum for Mach 3.5 is also shown. In the figure, the wavelength has units 
of microns (i.e., 10'6 meter).. As the Mach number increases, the peak temperature also 
rises while the peak wavelength decreases (shifts to the left). The higher the Mach number, 
the greater the amount of infrared energy emitted. For Mach 3.5, the peak wavelength is 
3.92 microns. The energy difference between a Mach 3 and Mach 3.5 aircraft would be 
significant enough to detect with an infrared camera or FLIR (forward looking infrared) 
detector.
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Figure 14. Infrared Energy Spectra for Mach 3 (red line) and Mach 3.5 (blue dashed line).
The peak wavelength changes for hypersonic aircraft with speeds that range from Mach 1 to 
Mach 100 are included in Figure 15. S atellite systems currently monitor 2.5- to 5-micron IR 
emissions and could detect aircraft with speeds as low as Mach 3. W hile Figure 15 plots the 
peak wavelength, aircraft at Mach 5 or higher have lower temperatures in the turbulent wake 
that would still be detectable. The peak temperatures are shown in Figure 16. Above 550 K, 
the perfect gas law cannot be used to accurately predict the peak temperature due to 
ionization and disassociation of molecules in the air. Figure 15 and Figure 16 use the "rule of 
thumb" described earlier in the theory of hypersonic vehicle flight to determine peak 
temperature and the corresponding wavelength.
Military satellite systems monitor infrared energy emitted by objects on the surface of the 
Earth. Kidd and Caldwell20 reported on the use of IR systems for defense support in 1992 
and highlighted the problems in resolving missile launches and wakes from background 
"clutter" caused by the infrared emissions from other objects on the surface of the Earth. 
Typical satellite IR systems detect energy between 2 and 5 microns. To verify that an object 
is an actual missile, the object must be detected in the 2.6- to 3.2-micron band, which 
corresponds to the emission from water vapor in the hot exhaust of a rocket. It must 
simultaneously be identified in the 4.1- to 4.8-micron range to differentiate the missile from 
background clutter. One problem encountered in missile detection involves the "glint" that 
occurs as aircraft at high altitude passing over land at night reflects sunlight to an overhead 
satellite. Computer software to discriminate signals from background heat combined with 
detection in the two different infrared bands helps to identify actual missile launches. FLIR 
(forward looking infrared) is also now used for atmospheric surveillance to detect objects in 
the 3- to 5-micron band and in the 5- to 14-micron band.
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Peak W avelength of E m itted R adiation
Figure 16. Peak Shock Layer Temperature for H ypersonic Objects.
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Commercial and Governmental IR Systems
There is a long history in the United S tates of the usage of IR detectors to sense launches 
and to track the trajectory of ICBMs.21 ARPA initiated studies of the possible use of IR 
detection of aircraft and missiles in the 1950s. Under their Defense S upport Program (DS P), 
Joseph Knopaw studied the possible use of IR detectors in satellites to detect missiles and 
the hot exhaust plumes of ICBMs. Rand Corporation submitted a report in 1955 that outlined 
the detection of ICBM launches from satellites in Earth orbit. In 1956, the U.S . Air Force 
chose Lockheed as the lead designer with Joseph Knopaw as the project manager for 
S ubsystem G, ICBM Attack Alarm S ystem (W S -117L).
By 1958, control of the Air Force system shifted to ARPA (Advanced Research Projects 
Agency) as the MIDAS  missile defense alarm system. The satellite IR detection system could 
alert the S trategic Air Command of possible S oviet missile launches 15 minutes earlier than 
the DEW system could. The first successful satellite launch occurred in 1960 with a 300-mile 
orbit. In 1963, MIDAS 9 with a 2,250-mile polar orbit succeeded in detecting nine missile 
launches using an 8-inch concentric telescope and an Aerojet-General IR detector.
In 1964, the Air Force launched the RJS -2 satellites into geosynchronous orbit over the 
equator. The MIDAS system was renamed the Defense S upport Program in 1969 and an 
agreement between the United S tates and Australia provided communication to the 
constellation of MIDAS satellites from the Overseas Ground S tation (OGS ) in Australia and 
the Continental Ground S tation (CGS ) at the Buckley Air National Guard S tation in the United 
S tates. Four Phase 1 Integrated Missile Early W arning S atellites (IMEW S ) were launched 
between 1970 and 1973 followed by Phase 2 satellites from 1975 to 1977, the Multi-Orbit 
S atellite/Performance Improvement Modification (MOS /PIM) models from 1979 to 1984, two 
S ensor Evolutionary Development (S ED) satellites from 1984 to 1987, and DS P-I satellites 
since 1989. The DS P satellites are designed for the global monitoring of ICBMs, S LBMs, and 
tactical missiles. Their operating life is 5 to 7 years, and they weigh approximately 5,000 
lbs. W ith 6,000 IR telescopic detectors, these satellites monitor IR emissions between 2.7 
and 4.3 microns. The 23rd, and last, DS P satellite was launched in November 2007.
The DS P system is capable of monitoring more than ICBMs. In 1972, the system detected a 
large meteor passing over several western states 94 km above the Earth. This meteor, 
moving at 18 km/s, was on a trajectory over S alt Lake City that would have done significant 
damage if it had impacted the Earth. In 1991, DS P satellites detected the launch of 88 Iraqi 
S CUD missiles. The DS P system is currently controlled under the ALERT (attack and launch 
early report to theater) system under the ALERT Control Center located at S hriever Air Force 
Base in Colorado. The current operational inventory is classified.
Other commercial IR detection equipment includes the Lucid Dimensions S pherical Detection 
S ystem (S DS ) using a 3D spherical sensor array.22 This system is designed to track ballistic 
missiles, aircraft, and vehicles. Developed under an S BIR, this system can be mounted on 
ground-based vehicles, ships, or aircraft.
LIDAR
LIDAR, or "light detection and ranging," uses pulses of laser light directed toward a target. 
Reflected light is detected and, through the time of flight of the laser beam, the distance to 
the target can be computed. W hile LIDAR systems are similar in function to RADAR systems, 
the highly directional nature of the laser beam permits a very accurate determination of 
target distance. As an example, reflected panels left on the moon's surface by the Apollo 
astronauts are now used to accurately monitor the distance from the Earth to the moon using
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LIDAR. By attaching a rotating mirror to vary the direction of the laser beam, it is possible to 
traverse the laser across surfaces to develop 3-D maps of objects using "imaging LIDAR." As 
an example, during the DARPA challenge held in the Nevada desert in 2005, autonomous 
automobiles used LIDAR to build a map of all objects in front of the vehicle and used it for 
collision avoidance and navigation. LIDAR systems have also been used to obtain accurate 
maps of the surface of mars from the Mars Orbiting Global S urveyer. LIDAR systems for 
meteorological studies are often mounted in aircraft and fired from the side of the fuselage to 
obtain a two-dimensional horizontal map of the atmospheric conditions.
A measurement technique known as Laser Doppler Velocimetry (LDV) uses the Doppler shift 
in the transmitted and reflected laser light to measure the velocity of the target and the 
properties of the air along the axis of the laser beam.
LIDAR systems using pulsed YAG lasers are capable of measuring many properties of air 
along the path of the laser beam. As shown in Figure 17, if a laser pulse is fired toward a 
distant target, light is reflected from particles in the atmosphere and this light is collected 
through a beamsplitter and directed to a photodetector. By knowing the time interval 
between generation of the laser pulse and the time that each reflected signal is received, the 
distance along the beam can be accurately determined for each data set. By recording this 
optical information as a function of time, using Raman spectroscopy, particle light scattering, 
and absorption theory, multiple properties of the air and particles in the air along the path of 
the laser can be measured as a function of distance from the transmitter. Particle density, 
velocity, and chemical species can be measured using pulsed LIDAR systems. Concentration 
of airborne gas species (oxygen, nitrogen), air temperature, and air velocity can also be 
measured in this fashion.
Figure 17. Schematic of a Pulsed LIDAR System.
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LIDAR, can be used to detect and monitor hypersonic objects in several ways. By using a 
micropulse laser, a LIDAR system could be used to scan the sky for supersonic airborne 
objects. LIDAR can also detect the presence of a hypersonic vehicle by measuring the air 
velocities in the long turbulent wakes that they leave in the atmosphere. Electro Optic 
S ystems in S ydney, Australia,23 has announced a laser tracking system that can track objects 
in low-Earth orbit that are as small as 100 mm (4 inches). These objects pose a risk to 
satellites and manned space vehicles due to their high velocities (~ 8 km/s). There are an 
estimated 200,000 such objects greater than 10 mm in size currently orbiting the Earth.
In the testing of hypersonic aircraft, a dedicated LIDAR system trained on the turbulent wake 
could measure the temperature and velocity distribution within the wake. This information 
can be used to design aircraft that would minimize detectability and increase vehicle 
reliability. The U.S . Army, for example, reported on the use of a LIDAR system to monitor 
atmospheric particulates from the erosion of nose-cones on Athena-H reentry missions at the 
W hite S ands Missile Range in New Mexico in 1973.24 The LIDAR system monitored the path 
of the reentry vehicles at altitudes of 9.3 km and 14.3 km and determined the concentration 
of erosion products and ice particles along the laser beam path.
ACOUSTIC AND SEISMIC METH ODS
Infrasound
Microphones can be used as a chronograph to measure the velocity of airborne objects. A 
supersonic aircraft generates a three-dimensional bow shock referred to as a "Mach cone." If 
the aircraft's Mach cone, which is actually a pressure pulse, sweeps over two microphones 
separated by a distance, the velocity of the aircraft can be measured. In Figure 18, two 
microphones can sense the dramatic pressure increase that occurs behind a shock wave and 
determine the velocity by dividing the microphone separation distance by the delay time.
S ound waves generated by the aircraft and the shock do not reach a terrestrial microphone 
until the Mach cone reaches the detector. The Mach cone has a half angle given by 
/3 = arcsin (1/M), and this yields a method of detecting the aircraft altitude, H:
M =
D2
H2
(20)
"Infrasound" refers to subaudible (< 100 Hz) sound waves that are generated by explosions, 
sonic booms, and rocket exhaust. Infrasound can be sensed by pressure transducers that 
detect the very small changes in atmospheric pressure that are induced by these events. 
The U.S . Air Force installed a network of infrasound stations around the country in the 1960s 
to detect the detonation of nuclear devices. Each station was composed of multiple pressure 
transducers so that the direction of the sound waves could be detected at the location of 
each station.
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Figure 18. Mach Cone Generated by a Supersonic Object.
The Los Alamos National Laboratory still operates four of these stations that were installed in 
1983 at various locations in the United S tates.25 These infrasound detector stations are 
relatively inexpensive and are easy to maintain. W hile they have not yet been used to sense 
nuclear detonations, they do detect sonic booms, and they were responsible for identifying 
and tracking two large bolides, or brilliant meteors, that entered the Earth's atmosphere in 
2000 and 2001. W hile capable of sensing meteors as small as a baseball, in 2001, a large 
bolide entered the Earth's atmosphere over the Pacific Ocean and exploded near Baja 
California. The bolide was estimated to have a 3.6-meter diameter and exploded with the 
energy of 6,000 tons of TNT.
Even a single infrasound station can track a meteor. On 4 October 1996, a station in 
Colorado observed a meteor near Bakersfield, California.26
Another infrasound station network includes the International Monitoring S ystem, a network 
of 321 stations around the world designed to monitor explosions that would represent 
violations of the Comprehensive Nuclear Test Ban Treaty. Although this network may be a 
useful worldwide resource for the detection of meteors, reentry vehicles, and hypersonic 
flights, its use in the detection of nuclear explosions is dubious given the circumstances of 
November 1999. During this event, a sonic boom was monitored in the skies above northern 
Germany. The information provided by the network could not differentiate between a meteor 
and a nuclear explosion.
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Infrasound stations, or observatories, could also be used to monitor the acoustic signature of 
meteors or supersonic aircraft based on their shock waves or the disturbances caused by 
their turbulent wake. S ome meteors, for example, produce subsonic sound from 1 to 5 Hz 
for several minutes during their flight through the atmosphere. This data could be developed 
to identify the speed, altitude, and type of object moving through the atmosphere.
Seismic
W hen a sonic boom is generated by a meteor or an aircraft, the sound waves pass through 
the atmosphere at the speed of sound in air at about 343 m/s. Infrasound stations pick up 
the pressure variations caused by the shock wave many minutes after the event occurred, 
depending upon the distance between the supersonic object and the station. W hen a Mach 
cone generated by a shockwave strikes the ground as shown in Figure 18, the pressure pulse 
is propagated into the ground and travels as a high-speed N-wave at a velocity far exceeding 
the speed of sound in air.
Ground-based vibrations caused by sonic booms have been detected by seismic sensors 
originally designed to detect earthquakes as documented by Cates and S turtevant.27 Caltech 
and the U.S .G.S . operate 200 seismographic stations throughout S outhern California as part 
of the TERRAscope seismic network. The network employs detectors that can sense ground 
vibration between 1 and 20 Hz.
The network was actually used to detect the sonic boom and pressure vibration induced by 
the flight of supersonic aircraft and reentry vehicles. On 9 December 1993, the network 
successfully detected the flight of an S R-71 flying over Edwards Air Force Base at a 
maximum speed of Mach 3.2. On 30 January 1992, the network was also used to detect the 
reentry of the space shuttle Discovery, S TS -42, over the Pacific Ocean and landing at 
Edwards Air Force Base.
Another Discovery landing on the west coast was monitored by 66 seismic stations in 
W ashington and Oregon. The shuttle was monitored by this seismic network for 500 km of 
its trajectory through the atmosphere. The seismic data from these stations showed the 
hyperbolic shape of the Mach cone as it impacted the surface of the earth. The angle of the 
Mach cone showed that the shuttle was traveling at Mach 14, corresponding to an altitude of 
55 km.
One interesting investigation that involved the use of these seismic arrays involved 
mysterious sonic booms heard over S outhern California in 1991 and 1992. By using data 
from the TERRAscope array, the source of these disturbances was traced to two F-4 Phantom 
aircraft flying over Edwards Air Force Base at speeds near Mach 1.
S ince the TERRAscope array is designed to detect seismic activity, including earthquakes and 
volcanoes, the detection of meteors and supersonic aircraft generate unwanted data. 
Caltech has been adding atmospheric pressure transducers to its seismic network to detect 
and remove airborne signals from the seismic data. Conversely, by combining the infrasound 
network data operated by the Los Alamos National Laboratory with data from seismic arrays, 
data from explosions, volcanic eruptions, and other terrestrial sources can be eliminated 
from infrasound data to better detect the presence of meteors, reentry vehicles, and 
supersonic aircraft.
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Chapter 4: Vision of Progress Over the Next 30 Years
W hile there are several methods that can be used to sense high-speed supersonic objects, 
technology changes in aircraft will generate the need for newer detection and tracking 
techniques. Listed below are five important categories of objects capable of hypersonic 
speeds. Over the next 30 years, the ability to reliably and accurately detect them will be 
critical.
• S pace Debris - Technology is currently available to detect space debris using RADAR and 
cameras. Novel methods using coronographs28 during daylight have been shown to 
detect the reflected light from millimeter-size debris particles in orbit, and existing 
telescopes are capable of detecting meteors that pose a risk to Earth. S pace debris 
reentering the Earth's atmosphere can reach speeds of over 17,000 miles per hour or 
Mach 23.
• Reentry Craft - The highest recorded speed for any manmade vehicle was the Apollo 10 
reentry capsule in 1969. It reached a velocity of 24,790 mph, or Mach 36. Reentry 
vehicles are currently monitored by ground-based telescopes and RADAR and through 
satellite imagery.
• Meteors - The systems used to monitor missiles and satellites based on RADAR and 
optical devices can currently detect and track meteors entering the Earth's atmosphere. 
Further development of acoustic and radio detection techniques should be pursued over 
the next 30 years.
• Missiles - The Cold W ar generated the need for equipment to monitor hostile missile 
launches, to track their path, and to estimate the location of their targets based on 
ballistic trajectories. The United S tates S pace S urveillance Network, for example, is 
composed of a worldwide collection of RADAR, electro-optical detectors, and passive RF 
devices that can track satellites, missiles, and aircraft. They maintain an inventory of 
satellites in Earth orbit and, through the S pace S urveillance Telescope program (S S T), 
asteroids and other faint objects in space are detected and tracked. GEODS S (Ground­
Based Electro-Optical Deep S pace S urveillance) telescopes monitor space at 3 different 
locations around the globe with high enough resolution to detect a basketball 20,000 
miles from the Earth. Continued vigilance from governments around the world will likely 
increase the development of technology to detect and track high-speed objects in the 
atmosphere and in space over the next 30 years with an emphasis on RADAR and optical 
detection.
• Hypersonic Aircraft - Over the next three decades, hypersonic aircraft development will 
likely spur an increased need for new technology to detect their presence. W hile the X-15 
rocket plane exceeded Mach 6 (4,520 mph) as early as 1967, continued development of 
high-speed aircraft will continue to meet needs in the following areas:
o 5S TO (S ingle S tage to Orbit) - Long the dream of NAS A and space exploration 
enthusiasts, technology has been under development to replace the S pace S huttle 
using aircraft that can take off from a conventional runway and attain low-Earth 
orbit. The X-43A scram jet, part of NAS A's Hyper X Program and fueled by 
hydrogen fuel was successfully tested in 2004. This aircraft is capable of 7,000 
mph or Mach 9.8 flight.
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o Manned military jet fighters - The fastest military jet is the MIG-25 Foxbat at 
2,115 mph (M = 3). The fastest U.S . jet is the F-15 at 1,875 mph (Mach 2.6). 
Current U.S . efforts are concentrating on unmanned vehicles, such as the X-43A.
o S urveillance aircraft - The S R-71a "Blackbird" achieved 2,056 mph (Mach 3.3) in 
1976 using an air-breathing engine. Its rumored replacement, the S R-91 Aurora, 
would have been capable of Mach 6. In 2008, the U.S . Air Force entered into an 
agreement to design a Mach 6 unmanned "blackswift" aircraft using a scramjet.
o Passenger jets - The current record holder is the Tupolev Tu-144 "charger" at 
1,553 mph (Mach 2.15). Reaction Engines Limited has proposed the A2 hypersonic 
passenger liner using liquid hydrogen to fuel the aircraft and to cool the wings and 
fuselage. This aircraft is expected to achieve Mach 5. In 2000, Russia and the 
People's Republic of China entered into a 25-year agreement to develop the 
Leninetz AYAKS hypersonic transport plane using kerosene fuel and a variety of 
techniques to attain Mach 8 to 10. An MHD (magnetohydrodynamic) engine is 
used to further accelerate combustion products produced by the engine. Lasers 
and RF generators are employed to disrupt the bow shock, reducing drag and 
doubling the velocity of the vehicle.
o S tealth missiles - The US AF Falcon Project planned for building a Mach 6 HTV-3V 
"Blackswift" platform but was cancelled in 2008. The NAS A X-51 scramjet vehicle 
is based on the Falcon scramjet and is scheduled for testing in 2010. This vehicle 
is capable of Mach 7+.
DETECTABILITY OF NEW H YPERSONIC AIRCRAFT
W hile conventional RADAR and optical systems may be able to track existing vehicles, future 
aircraft capable of Mach 8 and above are now being designed and tested. New propulsion 
technology and drag reduction techniques also offer new ways to detect such aircraft. Based 
on a review of hypersonic vehicle detection and tracking, four recommendations are made for 
research and technology development over the next 30 years to increase the detectability of 
hypersonic objects passing through the Earth's atmosphere.
Recommendation # 1 - Build a Database of the Wake Characteristics for 
Existing Aircraft
There is considerable information about the aircraft contained within the turbulent wake that 
trails it. The wake provides an indication of the vehicle's path and its current bearing. By 
analyzing the S trouhal eddy formation, it is possible to ascertain its velocity and hydraulic 
diameter. The wake can also provide information on the chemical combustion products that 
can be obtained through LIDAR and can indicate the fuel used by the aircraft. S ome aircraft, 
such as the X-43A and the proposed Reaction Engines Limited A2, use hydrogen as the fuel, 
so the absence of carbon molecules in their turbulent wakes could aid in their identification.
Authors Mark Garnet and Aaron Altman from the University of Dayton29 proposed the 
development of a database of turbulent wake data for conventional aircraft to allow their 
identification. They point out that, although the aircraft may be designed for stealth, their 
wakes cannot be hidden. They presented data on the unique characteristics that can be used 
to identify the wakes generated by the F-15, F-16, F-18, and B-52. They also point out the
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need for computational modeling of hypersonic wakes to predict the characteristic signature 
of new, unknown aircraft.
A hypersonic vehicle database of wake characteristics for existing aircraft should be 
developed based on optical and Doppler RADAR data. Doppler RADAR provides information 
on turbulent intensities and eddy formation within the wake. Computational fluid mechanics 
models of hypersonic wakes have been done since the early 1960s, such as the work done by 
Zieberg and Bleich.30 S uch modeling work should continue capitalizing on the increasing 
computer performance that will be available during the next 30 years.
Recommendation # 2 - Exploit the Detectability of H ypersonic Aircraft 
Based on the Features That Allow Them To Fly Efficiently at H igh Speeds
S everal new propulsion systems for hypersonic aircraft will be tested over the next 30 years. 
These include pulsed detonation engines (PDE) successfully tested in 2008 by the Air Force 
Research Laboratory and Innovative S cientific S olutions, Inc. over the Mojave Desert. W hile 
these engines are capable of Mach > 6, they have one characteristic that makes them very 
detectable. The pulsed detonation of combustible fuel creates a wake with periodic bulges 
referred to as "donuts on a rope." As mentioned earlier, measurements of the turbulent 
wake behind hypersonic aircraft can be used for detection, identification, and tracking.
The AYAKS hypersonic transport plane under development in Russia uses an MHD engine, 
lasers, and RF generators to decrease aircraft drag. The lasers and RF ionize the air ahead of 
the aircraft, creating a plasma to disrupt the bow shock and significantly reduce aerodynamic 
drag. Drag reduction allows the aircraft to double its maximum speed. The RF and laser 
emissions from this aircraft can possibly be used to detect the aircraft in flight from ground­
based antenna or telescopes or from satellites. The MHD engine also generates 
electromagnetic radiation that can possibly be detected.
Every new propulsion or drag reduction system under development will have some 
characteristic that may make it more detectable from the ground or from satellite. The 
Aerospike engine proposed for the S S TO and under development by Rocketdyne and Garvey 
S pacecraft involves the use of a novel virtual Laval nozzle that is external to the engine. The 
IR emissions from an Aerospike engine may increase its detectability.
Recommendation # 3 - Explore the Detection of Vehicles Designed To Be 
Undetectable
Over the next 30 years, hypersonic cruise missiles will have the ability to travel at speeds of 
up to Mach 8 decreasing the chances of being detected or defeated by antimissiles. The 
unmanned NAS A X-51 tested in 2010 is capable of greater than Mach 7 (over 4,000 mph), 
for example, and is designed to be launched from a B-52. The DARPA/US AF Falcon Project 
has resulted in the testing of the HTV-2 at Mach 6.7 over the Pacific Ocean, designed as a 
hypersonic cruise vehicle.
Efforts to disguise aircraft through the use of novel vehicle shapes and stealth materials can 
also increase their detectability in other ways. Golovitichev and Hansson presented a paper 
in 199931 that stated the following:
"The concentrated energy drag reduction concept has proven to be effective for the 
cruise range vehicles, preventing the missile deceleration with the benefit that the 
spike "products" could envelope the missile in a film of ionized gas which would be 
impervious to RADAR pulses, thereby rendering it electronically "invisible". The infra-
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red penalty at close range would be significantly offset by the high speed of the 
flight."
This discussion about the AJAX engine concerned the generation of ion combustion products 
that would be further accelerated by an MHD (magnetohydrodynamic drive). The increased 
number of various carbon-based molecules in the wake of this aircraft could be detected 
using LIDAR. The increased temperature near the surface of this aircraft would also increase 
its detectability due to increased emission of infrared radiation. For AJAX (AYAK) vehicles, 
development of enhanced LIDAR systems and IR detectors should proceed over the next 30 
years to increase their detection and tracking.
Recommendation # 4 - Explore the Development of Novel Detectors
One characteristic of hypersonic flight (M > 5) is the ionization of air in the region 
downstream of the bow shock and the disassociation of oxygen and nitrogen at very high 
speeds. W hen electrons recombine with ions generated by the shock, they emit 
electromagnetic radiation that can be detected. This radiation extends from the visible 
range, as evidenced by the glowing trail left by meteors entering the Earth's atmosphere, but 
the emitted radiation also extends into the ultraviolet and x-ray range. Although such 
electro-opticaI sensors are likely part of the existing DARPA/US AF space surveillance system, 
telescope systems that monitor the UV range should be developed, along with x-ray 
detectors capable of monitoring the atmosphere.
Hypersonic objects passing through the atmosphere disassociate nitrogen and oxygen which 
recombine in the object's turbulent wake. During recombination, new molecules are created, 
including NO, NOz, ionized O and N, and combustion products.32 LIDAR can detect the 
presence and concentration of these compounds in vehicle wakes and improved LIDAR 
systems should be a priority.
As hypersonic objects pass through the atmosphere, the object becomes ionized with a 
significant positive charge. Technology should be developed to sense the flight of objects 
with an electrostatic charge as they pass through the atmosphere.
SUMMARY
Each detection system discussed in the previous section has both benefits and problems. A 
comparison of the various detection techniques is shown in Figure 19 based on their ability to 
detect specific properties, relative cost, and accuracy.
Infrared detection systems and multispectral cameras installed on satellites, such as the KH- 
II, provide surveillance of large portions of the Earth for possible missile launches, reentry 
vehicle trajectories, meteor wakes, and hypersonic vehicles. RADAR systems based on 
satellites, ships, and aircraft can detect the position and velocity of such hypersonic objects 
at varying range. AW ACS , for example, can detect objects at up to 370 km with a resolution 
of 0.5 meters. TRADEX can track up to six objects at a distance of 1,400 km with an 
accuracy of 3 meters and a velocity resolution of 0.01 m/s.
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Figure 19. Comparison of Detection Technology for H ypersonic Objects.
Detection Method Properties Detected Availability Useful for Detection of:
Distance Velocity Altitude Whole SkyCommerciallyExisting Expense Meteor Reentry Aircraft
To Target Or Directed Available Network DetectionVehicles
Electromagnetic
Radar X X X Both X X Modest X X X
Radio Reflection Detection Whole Sky X Low X X
Optical
Sky Cameras, Photographic Whole Sky X X Low X X x
Infrared possible Both X Modest X X X
Lidar X X Directed X Medium X X X
Chronograph X Directted Low
Acoustic and Seismic
Infrasound X X Whole Sky X X Low X X X
Seism ic X X Whole Sky X Modest X X X
LIDAR systems provide detailed information on velocity, temperature, particle velocity, and 
atmospheric composition along the path of the laser beam that can be used to interrogate 
vehicle wakes and exhaust plumes. S eismic and infrasound systems continuously operate 
and can provide information on the path followed by a high-speed aircraft or object by 
tracking its sonic boom and low-frequency acoustic signature. Finally, optical cameras 
aimed toward the Earth provide valuable information on hypersonic object tracks by 
recording their tracks. Aimed toward space, optical cameras are tracking asteroids and other 
spaceborne objects that may enter the atmosphere as a meteor posing a risk.
32
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Appendix A: MathCad Program Used To Estimate H ypersonic 
Vehicle Surface Temperature
Modeling of the Thermodynamic Properties 
Of Airflow about H ypersonic Vehicles
I Purpose Com pute the properties across the bow shock of a hypersonic vehicle m oving 
through the earth's atm osphere.
A  D efine the A ircraft Velocity
B Com pute the M ach num ber for the Vehicle
V 1= 3300 M PH
AW Vehicle Velocity,
H := 10-kmAW Vehicle Altitute
N ote that the Karm an Level corresponds to 10 0  km  altitude 
w here the atm osphere essentially ends
Tatm(H) = 222.986 K
a := ^'Rair TatmW a = 299.326­
s
M j = 4.929
C Com pute the Stagnation Properties U pstream  of the B ow Shock
T1 :*W H ) Tj - 222.9S6K
( 1-1 2^
Toi =
Pl “ Patnr ~
( 1-1 2V-1 
Pol-Plj1+~—M 1 |
Tol = 1.306 x 103K
Pl = 0261 atm
Pol = 126.94 atm
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UNCLASSiFiED/yron ornciAh use only
D . Com pute the Properties on the D ow nstream  Side of the B ow  Shock at the N ose
1. The follow ing functions describe the change in properties across a norm al shock
1 + ^.M ,2
2. Com pute the properties across a norm al shock for this problem .
2 1
. . (Y  + D I^2 
Pratio(M l):=
2+ (y - D  M !
wM ’I + TTiSM
2+ (y - 1)-M ]
(Y  + D M  j
a. Stagnation tem perature and pressure ratios.
^oratio^l) : ^
Po.ratio(^l)' Pratio(^l)‘
Y ~ 1
2
(M2(M1))2
^M 2(M 1)
T2-TrTratio(M l)
P2:=PrPratio(M l)
Pol^PorPratio^l)
M 2 = 0.416
T2= 1.263 X 103K
p2 = 7.447 x IO5 Pa
po2 = 3.624 x 10 Pa
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Tol^orW *1))
To2 = 7.396 X KIK
11.707* deg This is the Mach angle for a blunt-nosed object.
E . Com pute the Peak W avelength of E m itted Infrared E nergy
1. U sing W ien's D isplacem ent Law , com pute the peak w avelength of em itted radiation.
- 3 b :- 2.897 7685-10 -m-K
. h „ 3= 2.295 x 10 nm [Tula t
X v = 2.295* mic ro n
2.295 x 10“ 6 m
2. U sing Planck’s Law , com pute the distribution of em itted energy w ith w avelength.
u(l,T) := 8 -rc h e
X k-T 
ke
Spectral Energy Density for a Black Body 
Radiator in energy per unit volume per unit 
wavelength.
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UNCLASSIFIEDj^'+^fl-^MttAWJM^
IL D efinition of U nits
M PH =
mile 
hr
- 34h = 6.6260689610 ^ Js
- 73 J
k= 1.3806504*10 *-
K
c = 2.997 92458* 10 —
Planck’s Constant.
Boltzmann Constant.
Speed of light in a vacuum.
III. Properties of A tm ospheric A ir
A . F undam ental properties.
Y = 1.4 Patio of specific heats for air.
R„ir= 287 —W kgK Gas constant for air.
B . U .S. Standard A tm osphere
1. The raw  data is included in the m atrix "data."
2. F unctions w ill be developed from  a cubic spline interpolation of the data
data = csort(data,0)
■ ( 3
height = \IO data; m
(3 r
pressure = U0 data/ Pa
temperature = data K
a kgdensity = data-----
3m
pres s ure^* = csplin efheigh L  press ure)
37
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< 0 000 101,32 288 1.226
0.305 97.72 286 1.191
0.610 94.21 284 1.156
0.914 90.81 282 1.122
1.219 87.51 280.1 1.089
1.524 84.31 278.1 1.056
1.829 81.2 276.1 1.025
2.134 78.18 274.1 0.994
2.438 75.26 272.2 0.963
2.743 72.43 270,2 0.934
3.048 69.68 268.2 0.905
3.353 67.02 266.2 0.877
3.658 64.44 264,2 0.850
3.962 61.94 262.2 0.823
4,267 59.52 260.3 0.797
4.572 57.18 258,3 0.771
4.877 54.92 256.3 0.747
5.182 52.72 254.3 0.722
5.486 50.6 252.3 0.699
5.791 48.45 250.4 0.674
6.096 46.56 248,4 0.653
6.401 44.64 246.4 0.631
6.706 42.79 244.4 0.610
7.010 41 242.4 0.589
7.315 39.27 240.5 0.569
7.620 37.6 238,5 0.549
7.925 35.99 236.5 0.530
8.230 34.43 234.5 0.512
d ata =
8.534 32.93 232,5 0.493
8.839 31.48 230.5 0.476
9.144 30.09 228.6 0.459
9.449 28.74 226.6 0.442
9.754 27.45 224.6 0.426
10.058 26.2 222.6 0.410
10.363 25 220.6 0.395
10.668 23.84 218.7 0.380
10.973 22.73 216.7 0.365
11.278 21.66 216.5 0.349
11.582 20.65 216.5 0.332
11.887 19.68 216.5 0.317
12.192 18.75 216.5 0.302
12.497 17.87 216.5 0.288
12.802 17.04 216.5 0.274
13.106 16.24 216.5 0.261
13.41 1 15.47 216.5 0.249
13.716 14.75 2165 0.237
14.021 14.06 216.5 0.226
14.326 13.4 216.5 0.216
14.630 12.77 216.5 0.206
14.935 12.17 216.5 0.196
15.240 11.6 216.5 0.187
16.180 10 216.3 0.161
18.442 7 216.3 0.113
20.576 5 217.2 0.080
23.849 3 220,3 0.047
26.481 2 223.1 0.031
31.055 1 227.6 0015
< 100 0 0 0
38
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temperature^ = cspline( height, temperature)
dens ity^ e cspline(height,dens ity)
3. These functions provide atm ospheric pressure, tem perature, and density for the U .S. 
Standard A tm osphere from  0  to 10 0  km  in height.
Patm^ 5 interp^pressuregpheight,pressure,h) Pressure (Pa).
^atm^^ = in terp^temp erat ure^, height, temp eratu re, h) Tem perature (K).
Patm^ “ interp(density^t,height,density,h) D ensity (kg/m A 3).
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height
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UNCLASSIFIED/yFOR OmGIAh USE OMI Y
1 Dyrud, L. P., et al, "Meteor velocity determination with plasma physics," Atmospheric Chemistry and Physics, 4, pp. 817­
824, 2004.
2 Anderson, John D., Modern Compressible Flow, 3rd ed., McGraw-Hill, 2003.
3 Anderson, John D., Fundamentals of Aerodynamics, 5th ed., McGraw-Hill, 2007.
4 W hite, Frank M., Fluid Mechanics, 7th ed., McGraw-Hill, 2003.
5 W hite, Frank M., Fluid Mechanics, 7th ed., McGraw-Hill, 2003.
6 Carter, R. T., et al, "Estimating the Drag Coefficients of Meteorites for All Mach Number Regimes," 40th Lunar and 
Planetary S cience Conference, pp. 2059-2060, 2009.
7 Macrossan, M. N., "S caling parameters for hypersonic flow: correlation of sphere drag data," Center for Hypersonics, 
University of Queensland, Brisbane, Australia.
8 Brown, Louis, A RADAR History of W orld W ar II, Institute of Physics Publishing, Bristol, England, 1999 (IS BN 0-750 3­
0659-9).
9 Luck, David G., Frequency Modulated RADAR, RCA Labs, McGraw-Hill, 1949.
10 Headrick, J.M., et al, "A MADRE ICBM Detection: S ome Trajectory Parameter Determinations when Employing 
Ionospheric Refraction," Naval Research Laboratory Report, AD596673, December 1, 1961 .
11 "Technical Verification S ystems, Detection of Reentry Vehicles," Bulletin of the Atomic S cientists, Vol. 41, No 1, p. 39, 
January, 1985.
12 W est, Adnan, "Detecting Meteors using Radio," 
ng-radio/, 7/15/2010.http://newburyas.wordpress.com/2009/12/12/detecting-meteors-usi
13 Barricklow, S am, "Aurigid Meteor S hower Radio Echoes, S eptember 1, 2007," , 
7/15/2010.
http://www.k5kj.net/Aurigid_2007.htm
14 Gamble, Jim, , 2010.http://elpallskycamera.us
15 P. S pumy, J. Oberst, D. Heinlein (2003). "Photographic observations of Neuschwanstein, a second meteorite from the 
orbit of the Pribram chondrite’1. Nature 423 (6936): 151-153.
16 Oberst, J.; Molau, S .; Heinlein, D.; Gritzner, C.; S chindler, M.; S pumy, P.; Ceplecha, Z.; Rendtel, J.; Betlem, H.. "The 
’’European Fireball Network'1: Current status and future prospects’’. Meteoritics & Planetary S cience 33 (1): 49, 1998.
17 .rcktmom.com/njlworks/MeteorTrackingPpr.htmlhttp://www
18 Beatty, Kelly, "Meteorites Found from Asteroid 2008 TC3, 
.html, retrieved 7/10/2010.http://www.skyandtelescope.com/community/skyblog/newsblog/41873107
19 Bird, R. B., S teward, W ., Lightfoot, E., Transport Phenomenon, John W iley & S ons, p. 432, 1960.
20 Kidd, J., and Caldwell, H, US AF, "Defense S upport Programs to S upport a Changing W orld," AIAA S pace Programs and 
Technologies Conference, Huntsville, AL, March 24, 1992.
21 Bayne, W alter J., Air W arfare: An International Encyclopedia,Volume 1, ABC-CLIO, Inc., S anta Barbara, CA, pp. 169 - 
172, 2002.
22 "Lucid Dimensions S pherical Detection S ystems (S DS ) 3D Passive Infrared (IR)", - 
dimensions-spherical-detection-sds-3d-passive-system, downloaded 7/14/2010.
http://www.defensereview.com/lucid
23 "Australian laser system to track space junk," 
2010, 7/20/2010.http://news.yahoo.eom/s/afp/20100720/sc_afp/technologyaustraliasciencespacelaser_
41
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UNCLASSiFiED/yron ornciAh use only
24 Roberto, Rubio, "LIDAR Detection of S ubvisible Reentry Vehicle Erosive Atmospheric Material," Army Electronics 
Command, Fort Monmouth, New Jersey, DTIC Number ADA041085, March, 1977.
25 Britt, Robert Roy, "Listening for Nukes: A Meteor Detection Project," 
010524.html, May 24, 2001.http://www.space.com/scienceastronomy/solarsystem/nuke_meteor_
26 Bedard, A. J., "Infrasonic and Near Infrasonic Atmospheric S ounding and Imaging," U.S . Department of Commerce, 
.noaa.gov/psd/programs/infrasound/infrasound.html, 7/15/2010.http://www.esrl
27 Cates, Joseph E., and S turtevant, Bradford, "S eismic detection of sonic booms, " Journal of the Acoustic S ociety of 
America, vol. Ill, no 1, part 2, pp. 614 - 628, January, 200 2.
20 Koutchmy, S . and Nitshelm, C., "Optical Detection of S pace Debris using a Large Achromatic Coronograph," 
Astrophysics and S pace S cience, vol. 143, pp. 45-49, 1988.
29 Garnet, M., and Altman, A., "Identification of Any Aircraft by Its Unique Turbulent W ake S ignature," AIAA, Journal of 
Aircraft, vol. 46, no 1, pp. 263-268, 2009.
30 Zeiberg, S . and Bleich, G., "Finite-Difference Calculation of Hypersonic W akes," AIAA Journal, vol 2, no 8, pp. 1396, 
1964.
31 Golovitchev, V. I., and Hansson, J., "S ome Trends in Improving Hypersonic Vehicles' Aerodynamics and Propulsion 
(Fundamentals of AJAX Project)," XIV IS ABE Conference, S eptember 5-10, 1999, Florence Italy.
32 S tepanov, M., "S tudy of Thermochemical Nonequilibrium in Hypersonic Far W akes in the Atmosphere," Fluid Dynamics, 
vol. 37, no 1, pp. 138-146, 2002.
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