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Communication with deployed forces has always been problematical, particularly when, in recent years, those forces have been mechanised and fast moving. Radios using VHF or UHF are limited to line of sight, and so HF frequencies have been used for longer distances, but these are subject to the vagaries of the ionosphere. Satellite communications are far more reliable. In fact, over seventy per cent of American overseas military communications are conducted via satellite. Most communications satellites use geo-stationary orbits, which is at about 36,000 km distance where the satellite's speed matches the earth's rotation, in an equatorial orbit. Coverage in this orbit is limited to below 70 degrees north and south. In the more extreme latitudes, a different orbit has to be used, one that was pioneered by the Soviets in their Molniya system. This had a highly elliptical orbit, one that at apogee was 40,000 kin from the earth, but at perigee, only 450 km. Thus the satellite would dash around the perigee, but slow substantially as it approached apogee, and thus could be used. A third type of orbit is used for covert communications, which has an altitude of around 1,500 kin, and as both the receiver and sender are unlikely to be both visible to the satellite, it can make use of a recording devise to playback messages. Satellites can use either UHF (300 MHz - 3 GHz), SHF (3 - 30 GHz) or EHF (30 - 300 GHz).
Navigation satellites fulfil an important requirement in that a relatively small number can give full Earth coverage, compared to a ground-based system. Navigation satellites first came about due to a need by the US Navy for a navigation system that would help provide accurate data for their ballistic missile armed submarines to accomplish their mission - to hit enemy cities with nuclear warheads - and to have a truly global system that would not limit their areas of operation. Thus the US Navy developed what became known as the Transit system. It operated on a principle of transmitting a radio signal, which would be received with a shift in frequency depending on whether the satellite was approaching or going away from the recipient (a Doppler effect). The receiver then calculates their position using the measured Doppler shifts combined with the precise satellite positions that are transmitted by the satellites every two minutes. It can take as little as six minutes to get a fix, but the best accuracy is usually achieved after about ten minutes and takes into account the second frequency the satellites transmit on as well, with accuracy down to 25 metres being possible.
The Transit and Tsikada systems were among the first generation of navigation satellite. The second generation, which is now in service, includes the Navstar Global Positioning System (GPS) by the USA and Glonass, by the Soviet Union / Russian Federation. GPS works by the principle of ranging by time difference. Each carries an atomic clock, so is able to transmit an accurate time signal and precise orbital parameters. Both the receiver and satellite generate codes at the same time, but because the satellite is transmitting from orbit, its signal will be late by certain amount of time. By multiplying this by the speed of light the receiver obtains his range from the satellite. If he gets a three-dimensional navigational fix, that is, a latitude, longitude and altitude. This assumes that the receiver's clock is as accurate as the atomic clock on the satellite. Having an atomic clock in every receiver mounted on every tank, plane and ship in a country's arsenal would be ludicrously expensive, but the problem is removed if we use a cheap quartz clock and take a signal from a fourth satellite to remove time uncertainties. The receiver can finally calculate their three-dimensional velocity using the Doppler shifts on the four signals used for navigation. Accuracy is maintained by, what is in effect a control loop, with monitor stations receiving two navigational signals from each satellite and passing them back to a Master Control Station, located at the Consolidated Space Operations Centre, Falcon Air Force Base, Colorado Springs. The station takes data from each satellite and updates its estimates of each satellite's orbital parameters and clock errors, and then passes it back (via ground stations) to the satellites to update their messages. The Soviet / Russian Glonass system is very similar to Navstar GPS in many ways.
Reconnaissance and surveillance both refer to monitoring activities performed by certain types of satellites, but whereas surveillance can be described as a regularly performed activity, reconnaissance is usually looking for more specific intelligence, sometimes conducted as a more urgent activity. Satellites use an array of sensors including infra-red, ultraviolet radio, radar and optical wavelengths. The development of these systems has tended to concentrate around increasing the resolution, not only in optical wavelengths, but other parts of the electromagnetic spectrum as well. The resolution of a system is defined by distance, and refers to the distance within which the system cannot distinguish objects. So if a system has a resolution of one metre, two objects that are closer than one metre apart will probably be seen as one object. Resolution also serves as a description of the smallest increase in size of an object that will be detectable by a satellite. So if a two-metre object increased in size by five per cent, the minimum resolution that would detect that would be 10 centimetres. Obviously the better the resolution, the better the intelligence that can be gathered.
History is fill of examples of how weather affected the course of a campaign or battle. Military commanders can use the knowledge of predicted weather to their advantage in maximising the impact of their resources and minimising the effect of the enemy's. Weather (or meteorological) satellites can acquire information on conditions far from the home base provide information on the temperature on the ocean's surface, the position of storm fronts, icebergs, fog and pack ice. Weather satellites use a variety of sensors looking at both the visible and infra-red part of the spectrum. With normal photographs, cloud cover can be assessed as to what type of weather it holds, and the direction and speed of the cloud movement. Infra-red photographs are used to determine the range of temperatures within a region and if used over an area of water, the relative surface temperatures can be analysed, and the position of sea fronts noted. These satellites tend to be in either low polar orbits inclined at 98 degrees to make them sun-synchronous, or geostationary orbit, at 36,000 km above the equator.
Finally, there are early warning and attack assessment satellites. As an ICBM can take only thirty minutes to reach its target, and an SLBM take as little as ten, it is vital to identify, assess and track a nuclear attack as quickly as possible. Both the USA and Russia use satellites with infra-red sensors to detect the massive amount of heat generated by a missile on take-off and in flight, as well as missile tests and satellite launches. By plotting an infra-red source over several scans, it is possible to determine whether the source is stationary (a forest fire for example) or moving. An actual missile launch can be confirmed within a minute of initial detection, which mostly occurs as it breaks through the cloud cover.