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Satellite communication is widely used for military and civil purposes. Several regional, communications, national, international systems of increasing capacity and effectiveness of SATCOMS have been and are being implemented over the years. Military Satellite Communications (milsatcoms) have been well-formed for a long time. It is useful and preferred tools for military communication for countries like USA, UK, France, NATO and nations of the former Soviet Union. Milsatcoms systems are different than civilian ones with respect to frequency bands, survivability and interoperability. The SATCOM has given the military users the potentiality of more mobility, flexibility and strategic and tactical communication for land, sea and air operation.
The journey of UK and US milsatcom started with the experiment of using the moon to bounce radio signals off for military communication traffic (3). The UK military satellite communication began in 1969 with the launch of SKYNET 1. It was world's first geostationary defence communication satellites. Then with the advancement of technology, SKYNET 2, 3, 4 and 5 projects were taken and some of them deployed (2).
The necessity of milsatcoms is constantly growing. One reason is that the increased communication requirements in the face of enemy threats and attacks. Other reasons are -exchanging and analysing of digital information, commanding or controlling computers, sensors or network.
Figure 1: SKYNET-4 (7)
2 Military Applications
High reliability and high capacity service over a wide coverage made satellite communication very attractive for military applications. This service is instant and virtually available any part of the world without any national infrastructure. Thus, communication can be established in such area where very little probability or no infrastructure or in such environment where the local infrastructure is strictly denied or unreliable.
The wide area coverage facility may come as a disadvantage for milsatcoms. The visible signal can be detected by enemy quite easily. Then, the interception and the disruption of the satellite signal may be a threat to military forces. However, satellite communications are playing a growing important role for out-of-area operations. For example, peacekeeping operations in Bosnia by UK forces (2). Satellite communications were broadly used for this activity.
Milsatcoms also proved the suitability for very large fixed links as well as small portable or mobile links and are utilized by all three armed services. Again, many kinds of data rates, modulation and terminals and modems are used in armed forces. All can be accommodate in one satellite systems. One crucial reason behind the major demand of milsatcom is the interoperability between different networks and systems and flexible.
3.1 Propagation factors
Propagation for the earth-space path has been measured with great length and depth. Only few important points will be discussed here. Spilker's book (5) can be referred for brief overview. There is an in-depth handbook (6) has been made for NASA on propagation below 10 GHz.
Three factors can be considered for propagation factors. They are
The propagation phenomenon
Its effect on the electro magnetic wave
The consequence to the communications link.
Phenomena influencing propagation add precipitation, ionization, hydrometeors, particulate matter and reflections.
The primary effects on electro-magnetic propagation are multipath, absorption, scattering, scintillation and depolarization.
The major consequences on the received signal are fading and attenuation.
These propagation factors degrading SATCOM performance and making highly frequency dependent.
3.2 Precipitation effects
Rain as a precipitation has little effect on SATCOM links except at EHF bands. The EHF bands are taken for the milsatcoms are 30-44GHz for uplink and 20GHz for downlink. The main reason for going to the EHF band is the very small propagation effects caused by ionosphere. There is also much more tolerance to the effects of nuclear scintillation than at lower frequencies. But the rain attenuation is more severe at these frequencies unfortunately. Even the snow has very little propagation effect at EHF.
3.3 Frequency Bands
Currently UHF and SHF bands are used for milsatcoms. SKYNET 4 experimenting with the EHF bands and this band is supported for US forces via Milstar. All the bands normally use circular polarization. The brief characteristics are given below of all bands:
UHF (ultrahigh frequency) system is used for tactical ground, sea and air forces to run the application of developed technologies (4). Broad range 225-400 MHz. Simplicity and cheapness of the ground equipment and popularity with mobile equipment are the main attraction. There are many disadvantages of UHF. One of the main disadvantages is interfering with other systems and deliberate jamming.
SHF (super high frequency) is used for defence satellite communication systems (4).
Uplink 7.9-8.4 GHz, downlink 7.25-7.75 GHz. Parts of the bands are used for other communication except milsatcoms. The ground antennas have reasonable directivity. As a result, interference from other users is not a problem. The directivity also allows frequency reuse by other satellites within the geostationary arc. Transponders are transparent and wideband. They are carrying numbers of channels simultaneously. The frequency of 725 which is a translation frequency is used for interoperability.
EHF (extremely high frequency) runs the uplink frequencies range from 43.5-45.5 GHz, downlink frequencies range 20.2-21.2 GHz. This is not very old development for operational use. Future application may be used in these frequencies for milsatcoms in need of highly survivable communication. The EHF has better results than SHF in terms of antijam performance, intercepts and lower ability of exploitation and lower vulnerability to geolocation.
3.4 Interference and noise
This factor should be kept in mind at the early stage of planning any SATCOM system. The mutual interference between proposed system and other systems must be kept in an acceptable level. This factor become crucial in the case of Geostationary Earth Orbit (GEO) is proposed.
Figure 3(a): Satellite Systems with common uplink and Downlink Frequencies
Figure 3(b): Satellite Systems Where the Uplink Frequency of One Coincides with the Downlink of Other
Interference between satellite systems: Probable interference paths between satellite systems are shown in figure 1(a). There can be two cases. First, the uplink and downlink frequencies are common (shown in figure 1(a) ). Second, it is shown in figure 1(b), where the uplink frequency band of one system coincides with the downlink frequency of the other system.
Figure 3 (c): Interference between a satellite system and a terrestrial system.
Interference between satellite systems and Terrestrial systems: the interference between satellite paths and terrestrial paths are shown in the figure 1(c).
4 Satellite Orbit
Most of the military communication satellites are in geostationary orbits. Russian's have used highly-inclined elliptical orbits (Molnya) to cover polar region for many years. Three satellites have used for coverage and each one 8 hours per day. The main advantage of molnya orbit is coverage. Drawbacks are: the need for three satellites, increased orbit decay, increased satellite fuel and greater environmental radiation levels.
LEO and MEO can be used for milsatcoms and the investigations are in progress. The satellites at this orbits can be used for military mobile communication.
5 Traffic, Terminals and Multiple access
The traffics for military communication are strategic and tactical. Strategic traffic at SHF frequency is dealt as civil traffic. There are fixed large terminals as well as small mobile terminals. Tactical terminals are handled with great importance. The number of small terminal may outnumber the number of big terminals and all are separately dependent with single-channel-per-carrier (SCPC). Their capacity is limited. The traffic is combined to a few data and speech channels. Data are sent at 50/75 baud. Prior to transmission several channels may be multiplexed into higher data rates. Slow scan television, computers and sensing devices exchange other forms of data.
The data of speech in military systems is in digital form. The forms of traffic here. 2.4 kbit/s vocoded speech and 16 kbit/s continuously-variable slope-delta modulation (CVSD) which is a simple form of adaptive delta modulation, akin to one-bit differential pulse-code modulation (PCM).
Many kinds of carrier-modulation schemes are used in milsatcoms. It is generally important that signals are constant envelope within a transponder. The use of PSK of FSK rather than ASK are recommended. Binary PSK (BPSK) and four-phase PSK (QPSK) are generally used with the change of marginally improve upon BW and envelope characteristics. Phase coherence is usually lost between hops in a frequency-hopped spread-spectrum system (FHSS) which makes FSK the preferred modulation scheme for this type of spread spectrum.
Error-control coding is used here widely. It is a feature of modern designs. It helps to maintain high integrity for data communication and speech links and sometimes provide link power benefits.
Coding is particularly important for frequency-hopped spread spectrum where a combination of bit interleaving. Coding and symbol diversity maintain the data inergrity in the presence of jammed hops. Convoluational and Reed Solomon codes are employed for advanced spread-spectrum modems.
Various terminals are used for milsatcoms. They range from fixed station to smaller portable or mobile terminals. There is a main base or anchor stations for each milsatcom system. That works as an interface between satellite traffic and fixed networks; switching and routing centre and often be collocated with TT&C facilities and the sytem control and operational staff.
Local loopback facilities provided by Anchor stations to allow communication between small satellite communication terminals. It is because of the power constraints. Anchor systems also provide patching and switching connections to a variety of terrestrial networks.
Besides large stations, a number of small fixed terminals sometimes employed to handle traffic. Most of them are from an anchor stations. Some of them also communicate directly. Land tactical terminals are variety in sizes. Again a smaller terminal is the UK man-portable PSC 505 Manpack terminal or land mobile terminals. They offer many convenience and flexibility. Ships are the main users of milsatcoms and they used both SHF and UHF. The features of EHF are their current interest as it has the feature of antijam and LPE (low-probability of exploitation) performance. SCOT can be a good example for milisatcoms ship terminal.
5.5 Multiple access techniques:
A transponder for milsatcoms may require handling a large number of links, different form of traffic and protocols simultaneously from a large number of terminals. The requirements may change very rapidly and system controller need to respond to this.
Time-division multiple access (TDMA) can be used for milsatcoms. Frequency Division Multiple Access (FDMA) is generally used in milsatcoms. A frequency slot and power allocation is given to each link, and these are placed at appropriate intervals over the transponder BW.
users may be independent in terms of power, traffic and modulation scheme. For example analogue, PSK, FSK etc.
no overall time synchronization is required
implementation is relatively simple and cheap
significant intermodulation products owing to mixing of frequencies in the transponder; these can interfere with other links, especially where strong and weak (i.e. high and low data rate) links are mixed; intermodulation power also wastes satellite EIRP;
Signal suppression of weak carriers by strong carriers may be important, especially with mix of link types.
Code division multiple access (CDMA) is another method for milsatcoms. It is a kind of spread-spectrum (SS) communication and is also known as spread-spectrum multiple accesse (SSMA). Most CDMA access is direct sequence (DS) form of SS but frequency - hoping (FH) version are being used increasingly.
6 Threats to Satellite Communications
Military and civil systems are different in operations. It is very important to have a clear perception of the threat to the satellite communication system. Protections against the threats are very costly and have a direct influence of the design of the satellite. Physical damage like Ground station destruction is more or less understandable. On the other hand, an enemy can employ more economical facilities like signal jammer.
There are many threats of milsatcoms. The important one's are described here.
Jamming is an attack by an enemy to block communication by saturating a system with radiated power. It may both directional-uplink and downlink.
Figure 7: Uplink Jamming Concept
Uplink Jamming: This type of attack is a serious threat and attractive to an enemy because many links can be affected simultaneously. If a large ground station can radiate high amount of power, the jammer can radiate any higher amount of power at similar scale. The development of very high power gyrotrons for use in nuclear fusion research is of particular concern. Using gyrotrons at higher frequency may allow intensely high jammer EIRPs. In the above figure the uplink jamming cocept is shown. The signal will be affected directly resulting in reduced signal-to-(jammer) noise ratio, as the transponder is power limiting, downlink power will be captured. As a result, the wanted downlink EIRP is reduced. The transponder throughput is extremely reduced and traffic rates will be hard to maintain. (1)
Downlink jamming: It is from an aircraft or some source. Airborne platforms can be very effective in downlink jamming because of mobility and facility to cover large area. This type of jamming is not as much serious as uplink jamming. One reason is that it can be easily eliminated by attacking (shot down). (1)
6.3 Nuclear Threat
Nuclear weapons draw special threats. There are radiation, EMP and ionosphere enhancement consideration apart form any effect from direct blast. Nuclear effect may be categorized as follows:-
effects on the propagation path.
effects on the space segment.
effects on the ground segment.
Threats to the propagation path: High altitude nuclear detonations are the most serious threat to the propagation path. It can cause serious degredation over very large areas of the earth's surface because of the extended effects in the space and time. EMP (electromagnetic pulse) is the result of the same detonation. The nuclear detonation effects on the propagation path can be divided into two parts. "Non-scintillation effects" and "scintillation effects".
Non scintillation effect is the consequence from enhanced total electron content of the propagation path and can disrupt communications via the links interdiceted by the fireball. The scintillation effects last much longer and constitute the major threat to the propagation path.
The Threat to the Space Segment:
Spread spectrum techniques
Low probability of exploitation
7 Threat Implications and Countermeasures
Spread spectrum techniques
Spread Spectrum techniques
8 Future Trends in milsatcom
Politcal and military context
Use of LEOs/MEOs-military use of commercial satellite communications
Enhanced onboard processing
EHF and optical frequencies
System management and networking