MILITARY COMMUNICATION249 MILITARY COMMUNICATION In general, military communications systems are different from ordinary systems in various specific security requirements, namely, robustness in hostile environments, shielding from adversaries, and immunity from unfriendly eavesdropping. Hostile environments to communications are radio frequency interference (RFI), spoofing, jamming, and fading, either natural or human-made. RFI is unintentional but can J. Webster (ed.), Wiley Encyclopedia of Electrical and Electronics Engineering. Copyright # 1999 John Wiley & Sons, Inc. In recent years. low probability of detect (LPD) or low probability of intercept (LPI) becomes critical for designing military systems. Besides providing better quality and resolution of the image. Eavesdropping is a technique used to surreptitiously intercept intelligence information from an enemy. UNINTENTIONAL INTERFERENCE The radio frequency interference (RFI) from another unintentional interferer can be substantial in a multiuser. referred to as the adjacent channel interference (ACI). making use of channel coding to enhance the power efficiency of the desired signal can mitigate the CCI problem. frequency hopping. Thus. Consequently. In view of four GPS satellites. Greater accuracy of less than 1 meter can be achieved by using correction information from another GPS receiver at a known location. or deception to a receiver by an external source using a ‘‘lookalike’’ signal. The ACI is caused by leakage of the signal power from adjacent channels next to the desired signal bandwidth. in poor weather conditions. However. which basically has a confined power spectrum with no side lobes. A good military communication system generally requires the addition of an antenna nulling technique that can isolate the effect of an intentional jammer or spoofer. Spoofing can cause greater damage than other intentional interference. Other schemes such as channel coding. SPOOFING Spoofing is defined as the purposeful degradation. the location of a soldier or a command post can be identified and life can be jeopardized. resulting in significant performance degradation. referred to as the co-channel interference (CCI). portions of the power spectra of adjacent channels overlap each other. Fading can severely disrupt communications. it can be difficult if not impossible to obtain any image using optical equipment. a GPS receiver can determine its three-dimensional position to an accuracy of better than 16 m. Other important areas in military communications include target recognition (classification) and navigation. with its wide expanses of featureless desert. and time hopping. and so on. the SAR system can be operated regardless of the weather condition. such as direct sequence spreading and frequency hopping (to be discussed later). laser or infrared light. or leaked from the adjacent channels. The global positioning system (GPS) is a satellite-based navigation system with global coverage. such as optical lenses. Therefore.S. decoder. position location. evasion. because armed forces personnel may make a costly or deadly mistake in response to a deceived command without knowing it. is the ideal combat environment in which to prove the value of GPS. such as direct sequence. eavesdropping requires the right demodulator. the RFI can be located within the intended receiving bandwidth. is crucial for tactical movement. This can always change the outcome of a battle or a war. In general. the mitigation technique for the ACI rejection includes the adoption of a bandwidth-efficient modulation scheme. in addition to sophisticated spread-spectrum techniques. using electronic equipment for communication. From the spectral point of view. the jammer must act to lower the SNR. resulting in leakage. The Persian Gulf region. multiser- vice communication system. A conventional communication system is not able to survive in such hostile environments. such as jamming. such as Gaussian minimum shift keying (GMSK) or filtered phase shift keying (FPSK). Unlike detecting the existence of a signal. is always a way of collecting target images. Techniques such as channel interleaving can be employed to ‘‘whiten’’ the channel in the presence of fading to reduce their efficacy. the U. forces could not have performed the maneuvers that contributed to the success of Operation Desert Storm in 1991. or back lobes of the receiving antenna. and code division multiple access (CDMA) scenarios. to raise the bit error rate (BER) of the communication system beyond acceptable levels: Eb /Ntotal = Eb /(N0 + J0 ) = [(1/Eb /N0 ) + (1/Eb /J0 )]− 1 (2) (1) . In order to be immune to eavesdropping. and hence the Eb / Ntotal. an adversary can detect the signal that is intended for the friendly party. Jamming is also intentional and can completely shut down the entire communication. Spoofing is intentional and can cause a great deal of confusion. or equalization are also important to make a system robust under a jamming or a fading environment. All spread-spectrum schemes can be utilized for counterattacking the measurements from spoofer and jammer. can be realized. Optical remote sensing methods. RFI can penetrate the receiver from main. The CCI can be initiated from intermodulations. Without a reliable navigation system like GPS. In addition to the channel coding. The jammer power at the target antenna system can be represented by J = Pj Gj /(4π R2 j )Bs /j /Lj where J ϭ jammer power at target antenna Pj ϭ RF power delivered to the jammer antenna Gj ϭ gain of the jammer antenna Rj ϭ range from jammer to target antenna Bs/j ϭ ratio between the bandwidth overlap between the energy inserted into the target spectrum and the jammer’s transmitted spectrum Lj ϭ losses in propagation between jammer and target antenna To be effective. This leakage exists because no ideal brick-wall filter. and so on. synthetic aperture radar (SAR) has been extensively utilized for remote sensing. The requirement of shielding from adversaries refers to the ability of not being detected by enemies. which passes 100% of the signal power within a certain range of frequencies and also completely cuts off the power beyond this range. In modern warfare. leakage from spatial discriminated co-channel signals. which can usually be achieved by using a radiometer. JAMMING A jammer is a device that seeks to nullify a communication system by inserting energy into the target spectrum. denial. side. Target recognition involves a great deal of data collection and processing. diversity.250 MILITARY COMMUNICATION significantly degrade performance. and rescue. However. a system needs a cryptography technique to ensure the secrecy of communications. combat. which is rich in harmonics and can cover a wide instantaneous bandwidth. Pulsed Jamming A pulsed jammer. and frequency chirp jammers. to maximize BER in the joint probability computation. 4. Spectrum of a multitone jammer. to obtain statistics of BER. Figure 3. for a given total power it will maximize jamming energy at the detector. a stand-off distance can be computed for a known J / S value where the jammer becomes ineffective: Stand-off distance = Rt [(Pj Gj )/(Pt Gt )(S/J )(Bs/j )(La /Lj )]1/2 (4) The jammer nominally attempts to disrupt communications with minimal resource use. Bj ϭ ͱBs. A pulsed jammer in time domain. and N0 is the one-sided power spectral density of the received additive white Gaussian noise (AWGN)... Carrier frequency Jammer Bj = Bs MILITARY COMMUNICATION 251 N-tone jammer Frequency Bj = Bs Frequency Figure 1. For the case of equidistant frequency tones. seeks to obtain a high instantaneous power output by reducing its duty cycle. (8) Figure 2. 3. Spectrum of a partial-band jammer. as shown in Fig. 0. Smart Jammers The category of smart jammers includes the frequency follower. this raises the noise density over that part of the band: Noise density = J /Bj = J /(α Bs ) = J0 Worst-Case Partial-Band Jamming A worst-case partial band jammer (WCPBJ) is employed against a system whose instantaneous bandwidth is less than allocated spectrum. . The power per tone for the equal amplitude case is Pj = J /N (7) (Bj < Bs ) (6) If there are differences in range between the communications transmitter and jammer from the receive antenna. the fraction of bandwidth occupied. a pulse. with the waveform and noncoincidence. where Eb is the signal energy per bit. the power production is for a fraction of time. (1 Ϫ 0). (Bj = Bs ) (5) where N is the number of tones. Broadband Noise Jamming A broadband noise jammer. the store and forward. as shown in Fig. Bs Carrier frequency Jammer Bj Desired signal Frequency Pulse with power No jamming power between pulses Time Figure 4. employs a noise source of bandwidth Bj that covers the entire allocated spectrum Bs of the communication system under attack. Spectrum of a broadband noise jammer. One must compute the joint probability of likely coincidence. . that is. resulting in frequency spacings that are equidistant. 2. Generally these are generated by a harmonic source. employs a noise source that covers some fraction Ͱ of the allocated spectrum of the communication systems under attack. as shown in Fig. 1. Some microwave tubes have the ability to produce large amounts of instantaneous but not continuous power. the total jamming bandwidth occupied is Bj = f s (N − 1 ) where f s is the frequency spacing. The WCPBJ would attempt to vary the value of 0. employs sets of sinusoids instead of noise sources to cover the jamming band. that is. A measure of jammer power versus signal power is J /S = (Pj Gj )/(Pt Gt )(Bs/j )(Rt /Rj )2 (La /Lj ) where S ϭ signal power at antenna Pt ϭ power delivered to transmit antenna Gt ϭ gain of the transmit antenna Rt ϭ range from transmit to receive antenna La ϭ losses in propagation between transmit and receive antenna (3) For a given jammer power J. The jammer can use several waveform strategies to maximize its effectiveness and reduce Eb / J0. The source waveform for small duty cycles is generally rectangular—that is. Multitone Jamming A multitone jammer. J0 is the jamming energy per bit. as shown in Fig. The noise density is Noise density = J /Bj = J0 Partial-Band Noise Jamming A partial-band noise jammer (PBNJ).. a hybrid system that includes more than one antijamming technique is implemented. Practically. Let d(t) and c(t) be denoted as the original data and PN sequence. PN sequence c(t). frequency. which is Rd (ϭ 1/ Td). Other types of multiple access include frequency division multiple access (FDMA) and time division multiple access (TDMA). As a result of this. For a fixed amount of jammer power J. antispoofing and antijamming techniques need to be implemented. the chip time Tc equals Td / Nc. frequency sweeps or chirps the intended jamming band to put energy into the victim receiving system.’’ Therefore. is the ordinary BPSK signaling. as its name implies. In a CDMA system. the value of the power spectrum density function of n(t) at the carrier frequency Ͷ0 is approximately 1/ NJ that of J(t). Since c(t) changes its polarity Nc times faster than d(t). respectively. All techniques described here can be used for antispoofing purposes as well without being particularly specified in the context. Therefore. the bandwidth of x(t). Again. the received signal at the receiver can be represented by r(t ) = x(t ) + J (t ) (14) The receiver multiplies the received signal r(t) by a duplicate of the PN signal c(t) to obtain z (t ) = c(t )(x(t ) + J (t )) = s(t ) + c(t )J (t ) (15) dk pd (t ) (9) ck pc (t ) (10) where dk and ck are either ϩ1 or Ϫ1. and spatial discriminations. Different techniques should be adopted to counterattack different types of jamming. In the DS spread PSK system. after the signal z(t) passes through the front-end filter of the BPSK demodulator. the N can be between 100 and 106 or higher. A pseudorandom (PN) binary sequence whose elements have values of ϩ1 or Ϫ1 are generated by a PN sequence generator Nc times faster than the data rate. This implies that the direct-sequence spreadspectrum approach is based on the concept of power discrimination. Conventionally. These jammers would be employed if certain aspects of the victim system are known and can be exploited by a nonstationary signal. users each have their own unique code ID. They can be based on the concepts of power. the effective noise component contributed to the decision rule is significantly reduced. this technique essentially enhances the effective SNR input to the demodulator. Hence. respectively. Frequency Hopping A frequency hopping (FH) system is driven by a frequency synthesizer that responds to a PN sequence from a PN code . the modulating signal is the multiplication of d(t) and c(t). The multiple access scheme that adopts the DS technique is called code division multiple access (CDMA).252 MILITARY COMMUNICATION The frequency follower and the store-and-forward class of jammers is nominally frequency-agile and captures or senses the victim signal and mimics the carrier frequency and perhaps the modulated waveform. The same idea of using process gain stated early in this section is the concept used to discriminate the unwanted user from the desired one. Then d (t ) = and c(t ) = ∞ k =−∞ ∞ k =−∞ d(t): Data duration Td Tc (PN chip duration) c(t): DS spread sequence: d(t)c(t): Figure 5. It is worthwhile to mention that the DS technique can be also used for the purpose of multiple access. which is approximately Rd. Direct Sequence Direct sequence (DS) is a spread-spectrum technique that is usually used with the phase-shift-keying (PSK) signaling. depending on the capability of antijamming (antispoofing) and/or the spread bandwidth of the system. c(t). the bandwidth of n(t) will be approximately NJ (ϭ Rc / BJ) times wider than that of J(t). time. These jammers are effective against stationary or slow frequency hoppers where the signal stays at frequency long enough for the jammer to copy and transmit to the victim in one frequency hop period. The DS spread binary PSK (DS/BPSK) signal can be expressed as x(t ) = where s(t ) = √ 2Sd (t ) cos ωc t (12) √ 2Sd (t )c(t ) cos ωc t = c(t )s(t ) (11) since c2(t) ϭ 1. The processing gain of this DS/BPSK system is simply PG = WDS = Nc Rd (13) In the presence of jamming signal J(t). and pd(t) and pc(t) are unit pulse functions with duration Td and Tc. where Td is the data bit duration. the effective noise component at the input to the BPSK demodulator becomes n(t) ϭ c(t)J(t). is Nc times wider than that of d(t). and d(t)c(t). The frequency chirp jammer. Waveforms of data d(t). denoted by WDS. In most cases. It has been shown (1) that the BPSK data modulation with QPSK direct sequence spreading is a robust antijamming system to combat either continuous-wave (tone) or random (partial-band) jammer. for the case that jammer’s bandwidth BJ is much smaller than Rc ϭ 1/ Tc. and modulating signal d(t)c(t). Figure 5 illustrates the waveforms of d(t). the unit of each element of a PN sequence is called a ‘‘chip. ANTISPOOFING AND ANTIJAMMING TECHNIQUES To make a system robust in hostile environments. the frequency hopper outputs a continuous sinusoidal waveform that frequency jumps from one value to another. Therefore. In order to provide a nulling capability. the FH/MFSK system becomes vulnerable to the intelligent partial-band and multitone jammers. • Slow Frequency Hop. the antenna system tries to sense the presence of a spoofer or jammer within the antenna field of view by sampling the received signal from each element antenna and determining whether there is significant energy outside the ex- WFH Td Data duration WFH: hopping bandwidth (b) Time Figure 6. The hopping rate is slower than or equal to the date rate. It is usually implemented in the FFH system to counterattack the partial-band or multitone jamming. Antenna Nulling A spatial discriminating technique for antispoofing and antijamming is antenna nulling or sidelobe cancellation (3). Based on the output sequence from a PN code generator. A TH system can force a jammer to stay on at all times in order to be effective. (b) Fast frequency hopping. as shown in Fig. For a majority of time. each of which can be pointed and controlled independently. the effective corrupting power fallen into a single MFSK band is J/PG. • Fast Frequency Hop. follow the hopping pattern. To solve this problem. To make the counterattack more effective. in turn. First. (a) Slow frequency hopping. For a fixed jammer power J. the signal is free of jamming. The most commonly used modulation schemes when the FH technique is adopted are M-ary frequency-shiftkeying (MFSK) modulations. The received signals from all element antennas are phase shifted and amplitude attenuated independently and individually. an FFH must be adopted so that the jammer is not able to follow the hopping pattern.MILITARY COMMUNICATION 253 Frequency Td (Data duration) MRd (MFSK frequency band) WFH Hopping time Th WFH: hopping bandwidth (a) Time Frequency Th (Hopping time) MRd (MFSK frequency band) Because the PG is usually very large. Instead of hopping the carrier frequencies in a much wider frequency band. other sub-bits of the same bit may be hopped to a frequency outside the jamming band and detected without errors. and concentrate its total power J to jam the full band of M MFSK carriers. Time Diversity Time diversity is a technique in which each information bit is subdivided into multiple equal spaced sub-bits before entering the modulator. Under the constraint of a fixed energy. Unfortunately. the coding and time diversity becomes desirable is this scenario. For a stationary partial-band or multitone jammer. the partial-band or multitone jammer is intelligent enough to be able to detect the transmitted FH/ MFSK signal. In addition. the FH system can be categorized into slow frequency hopping (SFH) and fast frequency hopping (FFH). 6(a). Note that the FH/MFSK system has a much wider range of frequencies than the ordinary MFSK system. the time hopper controls the time stamps for turning on and off the signal transmission in the time domain. which implies that there are one or more data bits in each hop. These frequencies are. time hopping (TH) is also driven by a PN sequence generated by a PN code generator. if one sub-bit is jammed. Hence. This implies that the FH is based on the concept of frequency discrimination. are needed. The combination of this array of element antennas is called phased-array antenna. Therefore. used as the carrier frequencies of a MFSK signal. an FH system is also considered to be robust. where PG is the processing gain of the FH/MFSK system and is defined as PG = WFH MRd (16) . Obviously. This technique is effective only to the pulsed jamming. the TH system is based on the concept of time discrimination (2). the broadband jammer needs to spread its power over the entire FH bandwidth WFH. as shown in Fig. the FH/MFSK is very effective in counterattacking broadband jamming. Time Hopping Like FH. the hop duration has to be equal to or less than the sub-bit duration. the jammer needs to reduce its transmitting power. and then combined to form a single signal. which implies that there are multiple hops within each data bit duration. 6(b). This is due to the fact that the signal is lost for only a small portion of time when the hopping frequency falls into the fixed jamming band. generator. At least two spot beam element antennas. The hopping rate is faster than the data rate. Depending on the hopping rate that the frequency changes. resulting in less interference. This antijamming technique is also based on the concept of time discrimination. the antenna system needs to be equipped with an array of element antennas and associated electronics for beam forming. such as the military satellite communications (MILSATCOM) systems. and (3) nuclear-blast-induced plasma. satellite to terminal locations. time of day. (2) ionospheric effects. particularly at high latitudes and at the 100 50 Fade duration (s) 1 L1 L φ + 0 – 20 10 5 Lmin 0 τ Time Depth of fade 20 log Li 20 dB 30 dB 40 dB 50 dB 2 1 . Equatorial scintillation is caused by electron gradients at altitudes of several hundred kilometers.’’ Experiments have shown that both fades and their duration would be proportional to the combined amplitude L as Prob[(Lmin/L ) ≤ X ] = X . The received signal is a composite of constructive and destructive interference of the primary and coherent reflections to induce the scintillation behavior. a nuller algorithm processor decides and adjusts accordingly to the phases and attenuation weights of all element antennas. and Brayer of MITRE also performed several investigations in this area (10. A conceptual antenna pattern of nulling. Recently. from the earth’s where R is the instantaneous amplitude and is the instantaneous phase of the ‘‘composite echo. smooth. In multipath fading. This causes the fading characteristics to be highly geometry-dependent. PROPAGATION CHANNEL CHARACTERISTICS The term fading channel is used when the physical medium affects radio-wave propagation such that the received signal appears to have amplitude fading and/or phase jitter. with tan γ = R sin θ 1 − R cos θ (18) pected bandwidth. Figure 9 indicates the change of ionospheric scintillation as the frequency changes (13). As a result. Duration of fading at a 4 GHz on a 30 mile path.254 MILITARY COMMUNICATION Location of jammer Angle of boresight Original antenna pattern Resulting antenna pattern surface.1/2) convolutional code with Viterbi decoding has been widely used due to its good FEC capability (4). that occur along the propagation channel. Ionospheric scintillation can be a major factor for satellite communications depending on carrier frequency. Three principal fading phenomena are (1) multipath. a null is generated at the direction to the spoofer or jammer so that the interference attack becomes completely ineffective. season. and magnetic activity. The Reed– Solomon code has been shown to be powerful for correcting burst errors (5). Fade condition is dependent on the terrain encountered.99 Figure 8. It is convenient to define the single-frequency case after Bullington (12) for the primary and echo without modulation: ν = 1 + Rei (θ + (n− 1 )·π ) = Le− i·γ (17) Null Antenna gain (dB) Figure 7.01 . Forward Error Correcting Code In general. (7.1/2) convolutional code as the inner code and Reed–Solomon code as the outer code has been known to have a significant anti-corruption capability (6). Multipath Fading Multipath fading is usually associated with terrestrial communications or low-elevation-angle satellite communications where the transmit and receive signals are subject to reflection from terrain and objects.11). . using a forward error correcting (FEC) code to improve the power efficiency can be also considered as an antispoofing or antijamming technique. After the spoofer or jammer is detected. Figure 7 illustrates a conceptual diagram of forming an antenna null. the signal is a composite of the line-of-sight wave and reflections. such as mountainous. or oceanic. Various coding schemes have been included in various systems for this purpose. Measurement data have been provided from NASA missions (8. where 0 ≤ X ≤ 1 (19) Figure 8 illustrates a relationship between fade duration and percent of fades for an example of multipath fading at 4 GHz. The electron densities in the ionosphere tend to align with the earth’s magnetic field lines. fixed or moving. In most military communications systems. Ionospheric Effects Fading or scintillation occurs in the ionosphere due to the influence of electron densities in the propagation medium. a newly invented turbo code has demonstrated its ability to provide a near-Shannon-limit coding gain (7). Concatenated code structure with (7. the antenna nulling scheme is required to be implemented on spacecraft.9). lake.01 1 10 50 90 Percent of fades 99 99. High-latitude scintillation occurs from the visible aurora region (regions D and E) and from the polar cap to the aurora (14).9 99. 82 ϫ 10Ϫ5 m) c ϭ light speed (3 ϫ 108 m/s) NL(t) ϭ large scale (slow component of electron density) f c ϭ carrier frequency f r ϭ 1/(2ȏ0) ϭ Rayleigh phase variance. If the random component is zero mean and normally distributed. Wittwer (13) has given the variance of this component g( f) as τ0 ( f c /r0 c )2 δ( f − f ) g ∗ ( f )g ( f ) = [a 2 + ( 2 π f τ 0 ) 2 ] 3 / 2 0 where for f ≤ f r for f > f r (24) Geomagnetic latitude 06 40° 20° 22 24 02 04 Sunrise g(Ϫf ) ϭ g*( f) a2 ϭ (r0 и c и NL(t)/ f c)Ϫ2 r0 ϭ classical electron radius (2. . For frequency-selective fades ( f0T Ͻ 1). (2) spatial diversity... Corroboration of such characteristics were conducted by the ‘‘Starfish’’ experiments in the 1950s and by STRESS test in which barium clouds were set up in the upper atmosphere through which radiowave propagation was studied. we have ( f .. . .. . The depth of scintillation fading is proportional to the density of crosshatching. Nuclear Blast Induced Plasma Nuclear-induced scintillation is postulated when such ordinance is detonated in the upper atmosphere to cut off communications..MILITARY COMMUNICATION 255 50 Peak-to-peak fade depth (dB) The generalized power spectrum ⌫( f. .. Frequency dependence of ionospheric scintillation fading. (4) polarization.. ∞ −∞ −2x2 where 2 1 /2 f = f 0 (1 + C1 ) poles. . where ͳ is the Dirac delta function. and (5) equalization... In addition. . . . we have ∞ −∞ ∞ −∞ (22) ( f ..000 Frequency (MHz) Figure 9. particularly via satellite. . τ ) df dτ = 1 (23) Local time Sunset Figure 10. there are random time-varying components of the electron density. (3) time diversity (interleaving).. Figure 10 shows the geographic distribution of the ionospheric scintillation (15). ... 80° 60° 20 In scintillation. also mitigate fading... coding and frequency hopping (with or without chirp combined). . (20) and (21). Geographic distribution ionospheric scintillation. the severer the fade. Rayleigh scintillations ϭ 0 phase-only scintillation FADING MITIGATION TECHNIQUES 20° 40° 60° 80° There are five major techniques that can be employed to specifically combat the effects of fading.572 (τ0 f )2 ]2 (20) 40 30 20 for f 0 и T Ͼ 1 with T being the minimum symbol time. C1 = delay parameter (≈ 0... where the hop rate is faster than the data rate. in which the darker the region. τ ) = 1. τ ) = 2. They are (1) frequency band selection and diversity. ) of the scintillation fading can be characterized by the scintillation decorrelation time 0 and the frequency-selective bandwidth f 0 (16). ( f.864τ0 δ(τ ) [1 + 8.981 10 f τ0 1 exp − 2 [(πτ0 f )2 − 2π f τ ]2 − (πτ0 f )2 1 /2 2C1 C1 exp −x4 C1 21 /2 1+ 1 ((πτ0 f )2 − 2π f τ ) 2 C1 dx (21) × 0 100 200 300 400 500 1000 2000 3000 5000 10.25 ) For both Eqs. . I2. . with a convolutional or turbo code as the inner code and a Reed–Solomon code as the outer code.. the continuous data need to be subdivided into blocks. I3. So. if I1. are the input symbols. . I3. or circularly polarized. I14. I7. X. The wave can be linearly. the effects of the fading phenomena are spatially selective as well. the interleaving pattern can be any format of permutation. Furthermore.. Thus. This is due to the nature of the wave propagation and multipath reflections. circular polarization is preferable. However. incorporated with interleaving is required. At the receiver. Providing multiple receptions by utilizing more than one ground station and then combining the received signals can build up a robust system in fading environments. Theoretically. elliptically. the convolutional Start Start I1 I7 I2 I8 I3 I9 I4 I5 I6 I1 I7 I2 I8 I3 I9 I4 I5 I6 I10 I11 I12 I10 I11 I12 I13 I14 I15 I16 I17 I18 I19 I20 I21 I22 I23 I24 I25 I26 I27 I28 I29 I30 Figure 11. X. Figure 12 illustrates the structure of a convolutional interleaver. Polarization Polarization is the orientation of the plane on which the electric field vibrates when an electromagnetic wave propagates through the medium. . where X is a dummy symbol. 11. I8. In the transmitter. I19. . . Due to the process of deinterleaving. Therefore. . I2. X. I5. • Convolutional Interleaver. I19.. Lee and Messerschitt (18). transmitting the signal on multiple carriers and employing a diversity combiner is also an effective way to combat the fading loss. X. the modulated symbols at the output of modulator are permuted before entering the channel. easily corrected by an FEC decoder. . It can be seen that the permutation cannot take place until the entire block is filled up with the input symbols. I13 I14 I15 I16 I17 I18 I19 I20 I21 I22 I23 I24 End I25 I26 I27 I28 I29 I30 End Read pattern Write pattern . As was shown in the previous section. A synchronization sequence must be transmitted to aid the adaptation process. It should be pointed out that interleaving/deinterleaving results in a delay that depends on the interleaving depth and type. It can be seen that the convolutional interleaver reads out the symbols on diagonals. the outputs become I1. . In order to ensure the randomness of the channel symbol errors after deinterleaving. I2. Equalization Equalization attempts to compensate for the time dispersion effect in the fading channel. I25. For a 5 ϫ 6 block interleaver. X. Jordan suggested that multipath effects can be limited by use of circular rather than linear polarization (17). Fading is more sensitive to spacing with vertical than with horizontal distance by about an order of magnitude. Spatial Diversity Due to link geometry.256 MILITARY COMMUNICATION Frequency Band Selection and Diversity Scattering and scintillation are two major factors that cause fading in communication systems. X. in a severe fading environment where the fade duration is long. I4. Coding Using an effective FEC code is an important antifading scheme. X. and Orfanidis (20) discussed the concept of equalization and adaptive signal processing. the interleaving depth should be linearly increased as the fade duration increases. I14. I10. Block and convolutional interleavings are the two most commonly used patterns. Widrow and Stearns (19). I20. in a fading environment. Spacing the transmit/receive apertures to decorrelate fading provides immunity. I8. a block interleaver is a regular interleaver in that the input symbols are written in rows and read in columns. the errors generally come in bursts. the outputs are I1. X. Because fading is usually on and off so that the received signal is corrupted only in a small portion of a duty cycle. I13. . a deinterleaver rescrambles channel symbols using a reverse permutation pattern so that the order of the originally modulated symbols is preserved. Reed–Solomon code is known to be good for burst error correction. I4. • Block Interleaver. in turn. . I25. A common equalizer structure is the mean square error (MSE). I13. As shown in Fig. A 5 ϫ 6 block interleaver. the burst channel symbol errors are broken into scattered random errors that will be. For the same input symbols I1. X. the concatenated code structure. I4. X. X. I15. Therefore. it generally comes with an FEC code. a block interleaver of size N suffers a delay of N symbol intervals. X. where the sum of the squares of ISI and noise power is minimized. when an interleaving technique is adopted. . I3. I7. X. X. The channel symbols are corrupted in bursts in a fading environment. In addition. properly selecting a frequency band for a satellite system to operate becomes very important. Interleaving/Deinterleaving Interleaving is essentially a permutation among the transmitted channel symbols. I9. the effectiveness of these two factors depends on the operating frequency. Therefore. The effects of the multipath channel has the effect of time smearing the signal introducing intersymbol interference (ISI). X. I2. each of which matches with the frequency band of each channel so that the radiometer can de- . which maximizes the likelihood ratio p1(z)/ p0(z) for a given Pfa. Therefore. In this section. where Rs is the channel symbol rate. the radiometer will not be able to effectively detect the presence of a signal. the observable z is compared against a threshold. in a radar detection problem we might select two hypotheses—a target is present (H1) or no target is present (H0). be defined as the probability of observable being at z given conditions of H0 and H1. the most commonly used theory is the detection theory. 2 2 and variances. we assume that the input filter bandwidth of the radiometer is equal to Rs of the s(t). while the probability of false alarm Pfa is defined as the probability that the signal is declared to be present when no signal is actually transmitted. A convolutional interleaver of depth 6. The test statistic z out of the radiometer can be expressed as z= 1 T0 t 0 + T0 t0 (s(t ) + n∗ (t ))2 dt (25) x(t) Input filter Magnitude squared Integrate and dump z Figure 13. assuming an ideal filter is used. assuming that the entire packet is to be observed within the observation period T0. where Ȑ is the duty cycle of the packet within T0.8 times the Rs for QPSK signaling (23). p0(z) and p1(z). It operates as a square-law device that outputs the average power of the input signal within a certain bandwidth and over a certain period of time T0. respectively. It is possible that the receiver misdetects a radio signal when it thinks that only the noise is received. The signal is declared to be present if z is above or equal to the threshold. When a single fixed-length packet is transmitted with a packet duration less than the observation time T0. In order to prevent signals from being intercepted. where B is the bandwidth of the input filter of the radiometer. a channelized total power radiometer is used. assuming to be ideal. and n*(t) is the band-limited noise component. The hypotheses are statements of the possible decisions that are being considered. a system may require the ACI at the edge of the input-filter bandwidth to be at least Ϫ20 dB of the center carrier power so that the system can provide a good link quality to the customer. The input x(t) to the radiometer includes the signal component s(t) of power Ps and the noise component n(t). It can be shown (22) that p0(z) and p1(z) can be approximated by Gaussian probability density functions with mean values. which is close to the 3 dB bandwidth. Hypothesis testing is one of the most important statistical tools for making such decisions (21). the Neyman-Pearson criterion (21). In this type of detection problem.I1 Time division multiplexer Buffers Time division multiplexer Figure 12. which is not desirable. To the contrary. although the signal will not be likely missed. Detection of Signals in the Presence of Noise In communication systems.I2. Figure 13 shows the block diagram of a total power radiometer. radio signals from transmitters can be detected by adversaries. If it is set too low. As a result. given by m0 = Ps + N0 B m1 = N0 B interleaver can start its output without having the entire block filled up. the receiver may present a false alarm by declaring that a radio signal is present when no signal actually exists. To decide whether it is H0 or H1. the bandwidth of the input filter can merely match the 3 dB bandwidth of the power spectrum of the signal. if the receiver is simply used to detect the presence of a radio signal. is generally adopted for setting up the threshold. receivers are designed with good capability of low probability of detect (LPD) or low probability of intercept (LPI). This refers to the technique of making a decision as to whether a radio signal is received in the presence of noise. the bandwidth of the input filter. It should be pointed out that the design of the front-end filter bandwidth of a receiver is based on the requirement of adjacent channel interference (ACI). 0 and 1 . I4. T0 is the observation period. It can be seen that the sensitivity of the detection (interception) depends on the threshold set. Block diagram of a total power radiometer. This is a practical assumption as long as T0 is much larger than the packet duration. the delay is one-half of the interleaver depth. has to be at least 1. The total power radiometer is a commonly used device that detects the existence of a radio signal in the presence of noise. Let the likelihood function. For example. As a result. the Pfa will be high. where t0 is a particular time instant for starting the observation. 2 σ0 = (2Ps + N0 B )N0 /T0 2 2 σ1 = N0 B/T0 (26) LOW PROBABILITY OF DETECT/LOW PROBABILITY OF INTERCEPT (LPD/LPI) In electronic warfare. for any specified values of Pfa and Pd there is a corresponding threshold. In order to be effective. and absent if z is below the threshold. For example.I3. m0 and m1. which is white with power spectrum N0. in that a bank of filters and integrate-and-dump circuits are implemented. On the other hand. the location of soldiers and command posts can be identified.MILITARY COMMUNICATION 257 …. On the other hand. respectively. Let us define the probability of detection Pd as the probability that the signal is declared to be present (z is above or equal to the threshold) when the signal indeed exists. In this case. jeopardizing human lives and success of operations. if the threshold is set too high. we will replace the above Ps by ȐPs. T is the receiver system temperature in kelvin. and B is the bandwidth of the input filter in hertz. and L is the loss during propagation. is used to quantify the capability of the LPI/LPD of a system. The received noise power can be represented by N = N0 B = kTB (32) where k is Boltzmann’s constant (ϭ1. in which case the free space loss is only considered in determining the received radio power. and hr is the radiometer antenna height (24). Combining Eqs. defined as η= is such that 1 Pfa = √ 2π ∞ η It is well known (24) that the received power of a radio signal at the receiver can be represented by Pr = Pt Gt Gr /L (31) where Pt is the transmitting power. Therefore. ground collection and airborne collection. which is defined as the range from the adversary within which a radio can be detected. Thus. The ground vulnerability range is associated with an adversarial radiometer. Blake showed that when a radio wave is reflected from the ground. which may vary for different systems. the range of vulnerability is meaningful only under specified values of Pfa and Pd. From Eq. (26). This implies that the range of vulnerability is dependent on the level of threshold. Gt is the transmit antenna gain. From Eq. it can be illustrated that R∝ EIRP · µ η T0 Rs e (36) where e ϭ 1/4 for ground collection and 1/2 for air collection.379 ϫ 10Ϫ23 J/K). a much finer integration time can be used for detection. where R is the distance between a radio and the radiometer. From above. Therefore. In this case. the vulnerability range becomes R = [(EIRP )(λ/4π )2 (Gr /T )/(kRs )/SNRdetection]1/2 (35) Plugging Eq. Pfa (for each channel) ϭ (channel bandwidth in MHz)/3600. Based on the Neyman-Pearson criterion. Note that adopting . this so-called intelligent radiometer is able to pinpoint the time at which each received packet starts and ends. Because different communication systems may allocate different bandwidths for the entire system. (34) and Eq. where is the wavelength of the carrier. the received signal is subject to 2 2 a loss of L ϭ R4 /(ht hr ). The range of vulnerability can be studied under two categories. defined as Ps / N0B. it was shown that the sensitivity of detect (intercept) of a radiometer depends on the threshold set. the vulnerability range for the ground collection can be expressed as 2 1 /4 R = [(EIRP)(h2 t hr )(Gr /T )/(kRs )/SNRdetection] Using this threshold and the fact that B ϭ Rs. for a FAR ϭ 1/h/MHz and T0 ϭ 1 s. (31) and (32). Let us consider a case where the threshold of the radiometer is set such that the probability of correctly detecting a radio signal. required for this detection is SNRdetection = η √ µ Rs T0 (30) When a very sensitive detection device is implemented so that the radiometer can detect the signal given a very small amount of energy. Accordingly. the dominant loss of the radio-wave power is due to the reflection from the terrain. The airborne vulnerability range is associated with radiometer placed on the aircraft. the longer the range for a radio not being detected. the receiving SNR at the radiometer can be expressed as SNR = (EIRP )(Gr /T )/(LkB ) (33) mean difference standard deviation (27) exp − x2 2 dx (28) Thus. For example. Range of Vulnerability The range of vulnerability. L is simply the free-space loss that is equal to (4ȏR / )2. it can be seen that a radio with higher channel symbol rate Rs is less vulnerable to the radiometer detection. (33). (34) Airborne Collection. For airborne collection. the total observation time can essentially match with the actual packet duration. Pd. the threshold depends on only Pfa. That is why the LPI/LPD capability is significantly improved when a CDMA system is utilized. Ground Collection. The lower the threshold. FAR is defined as the number of false alarm declarations within a unit of time and for a unit of bandwidth. As a result. the duty cycle is always 1. Pfa (for each channel) ϭ T0[FAR] [channel bandwidth]. Therefore. (36). from Eq. for the sake of fairness. the LPD/LPI capabilities for different systems should be compared based on the same length of observation period T0 and the same specification of false alarm rate (FAR). Hence. Gr is the receive antenna gain. the SNRdetection used in the intelligent radiometer will be different under the same specifications of Pd and Pfa. ht is the radio’s transmit antenna height (from ground). the normalized threshold. In this case. the received signal-to-noise ratio (SNR). exceeds 50% (with FAR ϭ 1/h/MHz).258 MILITARY COMMUNICATION tect the entire transmitted frequency band. which is on or near the ground. it can be seen that µPs η= N0 rT 0 B (29) where EIRP (ϭPtGt) is the transmitted equivalent isotropic radiated power from a radio and Gr / T is defined as the figure of merit of the receiving antenna. (30) into both Eq. (35). This is due to the fact that when an intelligent radiometer is used.34). and modification. All subscribers keep secret their corresponding decryption keys for decrypting their received messages. the shape of the diffuse return of . Cryptography is a practical method of protecting transmitted information from being intercepted. each ROI must be further enhanced to reduce background noises. omission of false alarms. Ȑ Ͻ 1) and the Ȑ in Eq. resulting in a reduction of range of vulnerability by a factor of Ͳ3/2. In order to prevent the eavesdropper from learning it. in the classic cryptography system. Furthermore. computer generated hologram filters can be used (36). the observation time is reduced by a factor of 1/ Ȑ (T0 becomes ȐT0. More recently the Dempster–Shafer theory of evidence was developed and refined to address the evidence accumulation issue in target recognition (33. It must handle every pixel in the input scene. an eavesdropper can easily intercept it. TARGET RECOGNITION OR CLASSIFICATION Identifying a target in a sense with object distortions and background clutter present is a challenging problem for military applications. the transmitter operates on the plaintext with an invertible transformation to produce a ciphertext or cryptogram. Various types of correlator (detection filter). deletion. Eq. and identification of macroclass (large-sized) target. the transmitted data are susceptible to unauthorized interception. These processes will help remove false alarms and achieve identification. Wang developed an algorithm of generating a significantly long pseudo-random number (PN) sequence using exponentiation in finite fields (26). In optical sensing systems. and locate all candidate regions of interest (ROIs). were developed (30). rank order H-M. Early rule-based inference systems such as MYCIN used certainty factors and developed a simple calculus to compute an overall certainty factor for a hypothesis (32). In general. (36) is replaced by 1. They are (1) detection. This PN sequence can be used as an encryption/decryption code that is applied to the plaintext by the same way as in the direct sequence (DS) spread spectrum system. McEliece suggested using a linear error-correcting code. This system employs a public directory in that each subscriber places a key to be used by other subscribers for encrypting their transmitted messages addressed to each recipient. (36) also explains why the radio is more vulnerable to an intelligent radiometer than a basic radiometer. Such unwanted exposure of data may jeopardize national security. it merely attempts to locate ROIs. and (5) identification. Optical morphology (31) is a technique used to enhance the optically collected images. Public Key Cryptosystems The public-key cryptosystem is the first secrecy system that does not rely on exchanges of secret keys to obtain its security from cryptanalysis (25). there are five levels of processing required to complete a target recognition. a popular approach to evidence accumulation is via Bayes nets (35). accommodate target distortions. Because it conceivably must process every pixel in every image. for the encryption algorithm (28). it must contain simple and fast algorithms to avoid long processing time. Methods of Encryption In any cryptosystem. Therefore. Two basic mechanisms. etc. Segmentation Segmentation refers to the inference about objects within each ROI and includes rejection of clutter.. Thus. can be adopted for target image collection. Image Enhancement Once ROIs have been located. a TDMA system becomes vulnerable when an intelligent radiometer is utilized. Detection Detection is the most computationally demanding stage. Diffie and Hellman used the finite field exponentiation as the operation for encipher or decipher (25). Merkle and Hellman designed a socalled trapdoor knapsack n-vector as the public encryption key (27). and infrared light are commonly used for optical sensing. Bayes nets are graphs. Classical Cryptology When a transmitter generates a plaintext or unenciphered message to be communicated over an insecure channel to a legitimate receiver. reject clutter. It does not attempt to recognize the object from the background. Lens. such as hit-miss (H-M). This implies that a TDMA system also has a good LPI/LPD property. resulting in an increase of vulnerability range R by a factor of ͙1/ Ȑ. Without appropriate safeguards. and fill in holes and sharp edges. Currently. an extensive amount of processing is required. the extracted features might be the locations of scattering centers. (2) image enhancement. (3) segmentation. while the classic radar and synthetic aperture radar (SAR) (29) are used for electronic sensing. In SAR. (4) feature extraction. addition. primarily tree-structured but not necessarily so. the most important thing is to design a means of encryption so that it is practically impossible for cryptanalysis to break it. After the image is collected. Goppa code. This system requires exchanges of the secret keys among communicators. optics and electronics. that use Bayes’s rule to lay out all of the conditional probability relationships in assessing the probability that a given hypothesis is true. The inverse transformation (or called ‘‘key’’) is either already known by or transmitted via a secure channel to the legitimate receiver. laser. the receiver can decipher the received ciphertext by applying the key and recover the original plaintext. Feature Extraction The next step for target recognition is to examine the ROI and extract features that would support the inference.MILITARY COMMUNICATION 259 TDMA in a system will reduce the duty cycle Ȑ and increase the Rs by the same factor of Ͳ. CRYPTOGRAPHY One of the more important elements that differentiates military communications from commercial communications is secrecy. and altitude). In the following section we will briefly describe the current MILSATCOM systems and Milstar architecture. User Segment GPS receiver equipment. feedforward neural networks have been used for target identification (39). time. Ascension Island. which are accurate to within 10 ns. The systems are able to support voice. establishes the user’s position. submarine. less noisy. Hawaii. Diego Garcia. and mobile. less than 1 m. or the location of shadows. data. have been developed to support communication beyond line of sight and to provide global dispersed forces and global power protection (42–44). Ascension Island. Space Segment The complete GPS space segment consists of 24 satellites. with four operational satellites in each plane. has been developed and launched (41). The system can also support polar regions and oceans. and extremely high frequency (EHF). The choice of frequency bands is critical in designing the MILSATCOM systems. submarines. The receivers detect. The satellites are positioned so that six are observable nearly 100% of the time from any point on the earth. Currently. The concept is that appropriate features be detected. and shore sta- . To achieve them. in turn. In recent years. transportable. Control Segment The worldwide GPS ground control segment includes monitor stations. and is getting smaller. Ground antennas are located at Kwajalein. Colorado. and Cape Canaveral. air. This gives the ranges to the known satellites. are available and each provides different advantages. UHF systems are inexpensive. one can classify the current systems into three categories. SHF. global positioning system (GPS). identification. Today. cars. ship. Three basic frequency bands. ships. The GPS monitor stations are located in Kwajalein. located. The typical hand-held receiver is about the size of a cellular phone. EHF frequency bands provide the highest data rates and are the most jam-resistant of the three bands because the operating frequencies are allocated in the range of 30 GHz to 300 GHz.S. which must be synchronized with a master system clock. a space-based navigation system. and operation duration and intensity. it is possible to establish the three coordinates of a user’s position (latitude. and more accurate. By measuring the distance to four GPS satellites. The concept involves measuring the times of arrival of radio signals transmitted from satellites whose positions are precisely known. inclined 55Њ. can be placed onboard aircraft. ranging and navigation are essential. the master station is at Falcon Air Force Base. they can support higher data rates and hence provide more jam resistance than UHF (because we spread the signals wider than UHF). and Colorado. and used as a reference. Current MILSATCOM Systems Based on the frequency bands allocated to the MILSATCOM systems. as well as GPS time. The satellites travel in 12 h circular orbits 11. By 1994. namely. There the data are processed to establish the satellites’ clock correction factors and current orbital elements for transmission back to the satellites via the ground antennas. decode. The problems of this method are selecting the threshold and requiring the number of classes that were known a priori. and characterized so that they can be matched against predicted features in the final stage. record their positions and status. shore. UHF with frequency ranging from 300 MHz to 3000 MHz is suitable for mobile systems. Identification The K-nearest neighbor (K-NN) is a classic algorithm for target identification or classification (37). GPS can determine a user’s position with an accuracy of better than 16 m. Diego Garcia. Designed systems are also required to provide maximum flexibility for situations such as unpredictable conflicts in location. This process is also slow since it uses a feedforward unsupervised learning method (38). The hand-held units distributed to U. trucks.S. longitude. Each is equipped with a combination of rubidium or cesium atomic clocks. and a master control station. Since the operating frequencies for SHF systems range from 3 GHz to 30 GHz. The objective of GPS is to provide accurate. The FLTSAT serves Navy surface ships. ultrahigh frequency (UHF). GLOBAL POSITIONING SYSTEM In military applications. can be obtained by using corrections sent from another GPS receiver at a known location. imagery. It can also classify multitarget and multibackground images (40). To be effective. or other vehicles.000 nautical miles above the earth. there are more than 100 different receiver models in use for a wide variety of military and civilian applications. text. trains. They occupy six orbital planes. Figure 14 shows a typical MILSATCOM system. ground antennas. the GPS had already completed its full 24-satellite constellation. which. Receivers at the monitor stations track the GPS satellites. submarines. atomic clocks are installed onboard each satellite. or it can be hand carried. unique to each application. Greater accuracy. MILITARY SATELLITE COMMUNICATION SYSTEMS Military satellite communication systems of the U. UHF systems consist of two types of satellites: • FLTSATCOM and AFSATCOM. continuous position location information in three dimensions anywhere on or near the earth in all weather conditions. super high frequency (SHF). armed forces personnel during the Persian Gulf war weighed only 28 ounces. which can work in bad weather conditions and dense foliage. UHF Systems. and video. namely UHF. and relay information to the master control station. This algorithm is fast. aircraft. The systems have been designed to have both interoperability and compatibility features so that they can support users of all types of platforms such as land.260 MILITARY COMMUNICATION the object. and process the GPS satellite signals. Transmission frequencies are selected to minimize timing errors caused by the earth’s ionosphere and to be unaffected by rain and weather. and EHF systems. Moreover. The two systems share a set of eight satellites in synchronous equatorial orbits. The Milstar system and satellites are built to be survivable throughout all levels of conflict. DSCS provides required national security and maintains thorough communications during crisis and conflict. The strategic missions include strategic intelligence relay. The services are provided for both stressed and unstressed environments.S. The program is managed by the Navy as a lead service with a plan for 10 satellites.2 GHz for the downlink. tactical warning/attack assessment data relay. which is the latest addition to and the most advanced in the MILSATCOM architecture. However. and Navy task force connectivity also using an MDR mode. These satellites also have EHF on-board signal processing packages. EHF systems can be classified into two systems: • Milstar. Unstressed environments include ATM. which are built to replace the FLTSAT.0–21. Milstar is a joint MILSATCOM program consisting of a six-satellite constellation operating at UHF (225 MHz to 400 MHz). tions. tactical intelligence dissemination using both LDR and medium data rate (MDR) modes. exploitation. The Defense Satellite Communication System (DSCS) has been developed to provide the Department of Defense (DoD). These satellites were built by TRW with a design life of 5 years and a weight of 1860 kg at launch.5 GHZ). high-speed computer to computer. airborne command posts. MILSATCOM system. and ground terminals.5 GHz to 45.MILITARY COMMUNICATION 261 Cross link Cross link Cross link EHF uplink: 43. The FLTSATs 7 and 8 have fleet EHF packages. and it provides LDR capability only. SHF Systems. UFO stands for UHF follow-on satellites.5–45.5 GHz to 45. Other salient features associated with Milstar include LDR and MDR communication services using robust signal waveform.2 GHz EHF crosslink: 60 GHz Bomber (UHF) Recon (UHF) ACMS Ground command post ABNCP (EHF) Ship Several channel antijam portable (SCAMP) Sub Secure mobile antijam reliable tactical terminal (SMART-T) ACMS Mission control segment Shore Sensor sites LEGEND ABNCP: Airborne Network Command Post Figure 14. other government agencies. This is the ultrahigh frequency follow-on/EHF with operating frequencies in the range of 43. force management. and force report back. The satellites operate in the frequency ranges of 240 MHz to 400 MHz with on-board signal processing for SHF uplink. provides service for mobile users for both strategic and tactical missions. The Milstar communication links have high threat mitigation features such as LPI. and tactical antijam (AJ). These satellites are built by Hughes Aircraft Company with a 14-year design life. This system.5 GHz for the uplink and 20. antijam. SHF (20. • UFO/E.2 GHz). and interoperable terminal base. . These satellites are built by TRW for Air Force Space Systems Division with a design life of 5 years and operating frequency ranging from 7200 MHz to 8400 MHz. wideband and high capacity during peace and precrisis. Milstar satellites can provide narrow coverage spot beams and MDR nulling antenna capabilities. flexible network configuration. and U. UFO/E provides high-speed fleet broadcast capability. dedicated voice and data.2 GHz to 21.2 GHz to 21. which is not visible from the equatorial satellites. and antiscintillation capabilities. LPD. The DSCS provides services that cannot be provided by other media.5 GHz SHF downlink: 20. allies with global communications services. nuclear scintillation. • UFO. Army mobile subscriber equipment using an MDR mode. EHF Systems. with two satellites in each of five coverage areas. The AFSAT serves Air Force strategic aircraft. and EHF (43. Stressed environments contain jamming. The satellites are hardened to resist the effects of nuclear radiation. The UFO/E does not support the crosslinks. The tactical missions are command and control using a low data rate communication (LDR) mode. The Air Force also has communications payload on several satellites in high inclination orbits to provide coverage of the north polar region. J.. voice. Near Shannon limit error-correcting code: Turbo code. The crosslink payload also allows for command and telemetry to and from all satellites from a single ground station. W. J. 33-695. Proc. J. 1975. Principles of Digital Communication and Coding. The following are some of the features associated with the LDR payload: • Data rate: 75 bps to 2400 bps. and resolution of complex satellite anomalies. MDR terminals can provide all of the features that LDR can provide. Channel Coding and Data Compression System Consideration for Efficient Communication of Planetary Imaging Data. Jet Propulsion Laboratory. Nitzberg. as well as UHF force element (also referred to as AFSATCOM dual modem upgrade II). 135. Space Segment. Support Facilities. and crosslinks. 1971. Spread Spectrum Communications. and data. • Satellite Control Subsystem. California. Navy. Rice. 1985. M. • Mission Support Element (MSE). targeting updates. R. 1974. Memo. 1064–1070. shore. • Multiplexing: TDM (up to 70 channels)/FDM (32 channels) on the uplink and single TDM on the downlink. 1993. The three segments are space. This provides UHF uplink with 2 GHz bandwidth and SHF downlink with 1 GHz bandwidth. This supports EHF uplink with 2 GHz bandwidth and SHF downlink with 1 GHz bandwidth. pp. LDR terminals provide survivable tactical and strategic user communications. Milstar satellites are placed in geosynchronous orbits that can provide coverage up to Ϯ65Њ latitude. I. This simultaneously allows LDR and MDR communication data transmissions and reception between satellites. MSAT-X Report.8 kbps to 1. Mission Control Segment. CA. These include ship. 5. and Army terminals. Multipath Measurements for the Land Mobile Satellite Radio Channel. These include secure mobile antijam reliable tactical terminal (SMART-T) and single/multiple channel antijam portable terminal (SCAMP) Block I and II. Hubbard. MA: MIT Press. A Study of Multipath Propagation in Land Mobile Satellite Systems. • Capacity: Maximum throughput of about 45 Mbps. New York: McGraw-Hill. These include EHF/UHF command post-ground and transportable. mission control. and Air Force–developed terminals. • MDR Payload. J. Thitimajshima. and mobile subscriber equipment range extension. The following are some of the features associated with the MDR payload: • Date rate: 4. 1991. There are three basic types of terminals: Air Force Milstar. W. Adaptive Signal Processing for Radar. 3. California.544 Mbps. New York: Wiley. C. The terminal designs and communications protocols are required to provide for interoperable communications among Army-. . 1979. The payload has onboard signal processing and resource control. This subsystem uses LDR terminal EHF/SHF communications to control Milstar satellites. • Multiplexing: TDM/FDM on the uplink and TDM on the downlink. This provides software and databases to control Milstar satellites. and detailed communications network planning. This element also supports launch. 9. 4. This provides distributed command and control via multiple satellite mission control subsystems (SMCS) and preplanned response to satellite. The payload has onboard signal processing. Jr. and routing provides interconnections from EHF/SHF links to UHF uplinks and downlinks.262 MILITARY COMMUNICATION Milstar Architecture The Milstar system consists of three segments and the support facilities. conflict resolution. Conf. Cambridge. The space segment includes orbiting satellites with satellite bus. mission support. • Army Terminals. • Mission Development Element. Error-Correcting Codes. R. Dixon. Berrou. A. Omura. and emergency message dissemination. MSAT-X Report. • Modulation: Filter symmetrical DPSK on the uplink and DPSK on the downlink. respectively. • Modulation: FSK on the uplink and DPSK/FSK on the downlink. Weldon. 2. R. Geneva.. Viterbi and J. Pasadena. tactical command and control. K. including imagery. namely. It provides crosslink processing of MDR data. MA: Artech House. This provides communications planning software and generates satellite and terminal database information. • LDR Payload. Satellites use EHF and SHF for the uplink and downlink. submarine. W. C. • Air Force Milstar Terminals. contingency planning. 1985. Switzerland. Vol. LDR terminals also provide force direction/report back. Terminal Segment. 8. • Crosslink Payload. Jamnejad. and system simulation supports training and software/database validation. It also provides fleet broadcast services. Commun. Spread Spectrum Systems. 1986. MD: Computer Science Press. K. The two basic support facilities are Milstar auxiliary support center and on-orbit test facility. • Mission Planning Element. mission development. BIBLIOGRAPHY 1. F. This element also supports communication resource apportionment. 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