INTRODUCTION
The military use of space can be divided into two main classes:
1 Services and data in support of terrestrial military activity
2 The use of force (warfighting) in the outer space domain
Although we are now seeing some precursors of class 2, it is by far class 1 activities that make up most of the military use of space.
Class one activities include many of the civilian uses of space albeit with some differences in application. The diagram below gives an overview of the military use of space.
COMMUNICATION
Communication satellites account for between one quarter and one third of all dedicated military satellites. Most of these are in geosynchronous orbits. They range from small bandwidth systems in the UHF spectrum to extremely high bandwidth relay satellites operating in the military Ka band frequency allocation.
The three main dedicated military frequency bands are:
FLTSATCOM satellites are narrow band communication satellites using the UHF band. These support both USAF and US Navy mobile communications.
One of the long term milcomsats is the US DSCS system operating at X-band.
DSCS has now been supplemented and replaced by the much higher bandwidth Wideband Global SATOM system at Ka band.
Space-X has also designed and launched a military version of the Starlink LEO constellation. This, as with its commercial counterpart provides low latency communications which are vital in many two way internet situations.
Most military communications are encrypted. In the US Department of Defense there are two separate communication networks, the NIPRnet and the SIPRnet. The second one is the secure (S) network for the handling of classified information up to 'secret' level. The NIPRnet provides users access to the internet, whereas the SIPRnet is a totally separate network.
RECONNAISSANCE
Reconnaissance satellite were the first military satellites to be launched into space and now account for about one fifth of all military satellites. This classification is often reserved for optical and infrared imaging satellites but has recently been extended to Synthetic Aperture Radar satellites (SARsat). It usually excludes missile warning and signals intelligence satellites which are each classified separately. However, the distinction is becoming blurred, especially when one satellite carries multiple sensors.
Unlike many other military satellites, reconnaissance satellites generally have a very low orbit (Soviet RORsats) or at least a highly elliptical orbits with a very low perigee to achieve maximum resolution. They also carry a large fuel tank to enable movement to new areas of interest.
The theoretical highest resolution achievable with an optical system of aperture a is:
If we assume that the satellite is at height h of 200 km and directly overhead the ground being imaged then the theoretical resolution gives a minimum pixel size of:
The above table shows theoretical resolution. The actual resolution depends upon the quality of the optical system and also the resolution of the camera used. In early days before digital cameras, photographic film was used and when the film was exhausted the film canister and container was ejected from the satellite and after surviving reentry it deployed a parachute and was snatched from the lower atmosphere by an aircraft.
After the development of high resolution CCD digital imagers the data was relayed digitally in real time via a relay satellite to a ground station. The imagery was of course encrypted.
The final determinant of resolution is the turbulence in the lower atmosphere just above the site imaged. This typically limits the ground resolution to a minimum of ten centimetres. Thus it is not possible to read vehicle number plates from orbit.
The largest optical reconnaissance satellite in orbit has a mirror of 2.4 metres in diameter and achieves a resolution of around 0.2 metres. Advances in commercial Earth Observation Satellites can now achieve a similar resolution.
When the US was the only nation with this EOS capability the US government imposed a limit of 1 metre resolution on commercial imagery. However, when other nations made sub-metre resolution imagery available, the US lifted this restriction on US commercial firms.
The Soviet Radar Ocean Reconnaissance program to observe US naval movements would orbit around 200km altitude and below. At this height a satellite with solar panels would deorbit within one day due to atmospheric drag. The Soviets thus streamlined these satellites as much as possible and used nuclear power to run the systems, thus extending the RORsat life to around one week.
POSITION NAVIGATION & TIMING
The PNT function is performed by a series of satellites in middle Earth orbit. These GNSS (Global Navigation Satellite Systems) satellite constellations are:
The principle of operation is time of flight. By measuring the time or phase delay between the signals from at least 3 satellites a position is computer.
The satellite signals are spread spectrum using a pseudo random code to modulate to carrier frequency. There are two code sequences used for each satellite, a coarse acquisition code and a precision code. The C/A code is made available to all users, whereas the encrypted precision P(Y) code is only available to the US military and other approved users. There are five allocated frequencies for the GPS system, L1 to L5, all in L-band (1 to 2 GHz). The L1 frequency at 1575.42 MHz is the one used by all GPS users. The L5 frequency of 1176.45 contains the precision code.
On each satellite there is also a nuclear bomb detonation (NUDET)detector, which broadcasts a continuous encrypted signal on the L3 frequency of 1379.913 MHz. This device was included with the GPS satellites, because their precision time-keeping allows the exact place of detonation to be readily calculated by a ground station receiving several satellites.
METEOROLOGICAL
Global meteorological observation from space is a joint function shared by both civilian and military satellites. The US military maintains two DMSP (Defense Meteorological Satellite Program) satellites in low Earth orbit. The function of these satellites is to collect cloud, atmospheric, space weather and surface data. They orbit at an altitude of around 830 km in sun-synchronous polar orbits with an orbital period of 101 minutes.
DMSP satellites have the ability to store data for later transmission (when over the appropriate downlink site) and also to transmit data directly to ground terminals anywhere in the world. The downlink frequency is around 2252 MHz and all the data is encrypted.
Imagery from civilian metsats is also used by the military, particular when operations are conducted away from home or for countries that do not have dedicated military metsats. This includes the USA who have let their military meteorological programs fall behind in recent years.
The importance of accurate weather information was emphasized in 1980 when Operation Eagle Claw using six US transport planes and eight helicopters proved a disastrous attempt to rescue American Hostages from Iran. The operation was essentially thwarted due to desert dust storms that were not predicted by USAF meteorologists.
MISSILE WARNING
Advance warning of ICBM strikes on the USA has been one of the core uses of US military space assets. The first satellites used for this purpose were the MIDAS (Missile Defense Alarm System) satellites. The first launch of a MIDAS satellite occurred in 1960. These were replaced by Defense Support Program (DSP) satellites starting in 1970.
The satellites are in geosynchronous orbits, and are equipped with infrared sensors operating through a wide-angle Schmidt camera. The entire satellite spins so that the linear sensor array in the focal plane scans over the Earth six times every minute. The linear array of around 2048 lead sulphide detectors has peak spectral response at a wavelength of 2.7 microns in the near infrared spectrum. This was to detect the emissions from the hot exhaust of a ballistic missile (1000 - 2000 K) during the first part of its flight (diagram below).
The DSP IR sensors have peak spectral response at 2.7 microns which is right in the middle of the near IR atmospheric transmission window.
The satellites were kept rotating at 6 RPM to give a revisit rate every 10 seconds. They proved very effective at detecting SCUD missile launches by Saddam Hussein in the IRAQ war.
As another side benefit they were also found good for detecting small asteroids or large meteoroids impacting the Earth's atmosphere. The diagram below shows the location of such atmospheric impacts from 1994 - 2013.
DSP have been supplemented and replaced with the SBIRS (Space Based Infrared Satellite System).
SIGNALS INTELLIGENCE
Russia, China, the USA and others maintain satellites whose purpose is to gather intelligence on radio emissions around the world. Signals intelligence (SIGINT) is usually broken down into communications intelligence (COMINT) and electronic intelligence (ELINT). The former is concerned with transmissions originating from humans whereas the latter is concerned with data transmissions.
SIGINT satellites are used to monitor signals originating from ground stations, whereas SIGINT ground stations are concerned with signals originating in or relayed by satellites.
One of the most advanced SIGINT satellite series is the ORION satellites with masses of 5 tons and a collection dish of 100 metres in diameter (unfurled after launch into geosynchronous orbit). The frequencies monitored probably span from 50 MHz to 30 GHz. This wide a frequency range requires multiple collecting surfaces and antenna feeds. These satellites send processed signals to ground stations, one of which is the Pine Gap station near Alice springs in the Australia Northern Territory.
The following redacted image is believed to have originated from the National Reconnaissance Office who is responsible for launching these satellites.
SPACE WEAPONS
There are now satellites in space that are used to spy on the nature and the activities of other satellites. Some military satellites in both LEO and GEO orbits are designed to move close to other satellites and detect the signals being received and transmitted by those satellite as well as closely observe the satellite architecture. Any deliberate approach closer than 10km would generally be regarded as hostile act. The Chinese have publically released images of a US 'spy' satellite approaching one of their satellite to within this distance (image below)
Russia, China, the USA and India have now all tested satellite weapons in orbit. So far, in so doing, it has only been there own satellites that they have destroyed, but it doing so large amounts of space debris has been produced. Two types of destructive weapons have been tested.
The first type is a direct ascent ASAT missile launched from a ground site. The second type is a co-orbital weapon that moves close the target satellite and the either physically moves the satellite from orbit, or explodes whereon the debris destroys the satellites.
Ground lasers have been reported blinding satellites with optical sensors, so far only temporarily, but with higher power there is the possibility of permanent damage to the optical sensor.
Australian Space Academy