Signals Intelligence or SIGINT is an activity whereby radio signals are intercepted (received) and analysed for the information they contain which might be of use to the receiver (interceptor). Although this activity has been carried out since the development of the radio, space based signal sensors are of particular value because of the proximity to which a sensor may approach a potential target.

SIGINT is generally broken down into two subdivisions, COMINT or Communications Intelligence and ELINT or Electronic Intelligence. This latter activity is concerned with the reception and analysis of non-communication type radio emissions such as radar, telemetry and navigation beacons.

COMINT

The world today is a vastly different place than yesterday in considerable part due to our ability to communicate instantly from anywhere in the world to anywhere else. Such communications use vast amounts of the radio spectrum and may be solely over the terrestrial surface, or may involve satellite relay stations. Optic fibre is also carrying an ever larger volume of communication traffic both within countries and also between continents. However, even as optic fibre use increases, the local use of wireless technology is increasing. Communications may be analog or digital, voice or message or data. They may be between two people but increasingly they may also be between two computers. This can blur the distinction between COMINT and ELINT.

Space based communications intelligence assets may be placed in low Earth orbit (LEO -below 2000 km) or in geosynchronous orbit (GEO). Each has its advantages and disadvantages.

LEO COMINT satellites are required to monitor low power transmissions that have short range and/or antenna beam patterns that confine most of the signal to the horizontal plane. LEO satellites may approach as close as 300 km above a transmitter of interest and within about 1000 km in the horizontal beam of a signal. In contrast to this, LEOsats will only have a particular area of the Earth in sight for less than 10 to 20 minutes, and also must record and store any signals of interest for later download when over a control station. The short time available for download (10 to 20 minutes say 4 times a day) severely limits the spectral bandwidth that can be monitored by a LEO COMINT asset. Such satellites must thus be frequently retasked to monitor different small segments of the total radio spectrum.

A satellite in geosynchronous orbit can see approximately one third of the Earth's surface and it remains stationary or nearly so with respect to a ground station. This allows such a satellite to monitor a large part of the Earth's surface and to continuously downlink to a fixed station in 'friendly' territory. Because of this, almost the entire radio spectrum may be monitored and downlinked in real-time. However, the downlink frequency must be extremely high to cope with such a wide bandwidth. This necessitates the use of frequencies that are subject to significant attenuation by water vapour in the Earth's atmosphere, and the placement of ground stations in low rainfall areas is often desirable. The disadvantage of geosychronous orbit is that the sensor is always at a great distance (a minimum of 36,000 km) from the desired signal. This implies a need for very large receive antennae, if low power signals are to be received.

It is believed that US space based COMINT assets have deployed the largest space structures ever made, as antennae, some around 40m in diameter.

The radio spectrum used for communications extends from around 500 kHz to 20 GHz (some experimental and military systems may exceed this upper limit by an order of magnitude). In the early years of radio communication, most message traffic (analog/voice and digital/data) was transmitted in the high frequency (HF) band (3 to 30 MHz), making use of the reflective property of the Earth's ionosphere to achieve global distances. However, the limited reliability of this mode drove a move to higher frequencies when communication relay satellites became available. As a consequence, it has been reported that some intelligence agencies have ceased significant monitoring of signals below 20 MHz (this also makes the design of wideband antennae easier - because of size considerations). However, HF technology has not stood still, and both commercial companies and amateur organisations have established HF packet radio networks that carry considerable traffic around the world. Recent amateur 'intercepts' have hinted that some terrorist organisations are making use of these networks to convey their encrypted and sometimes even open messages.

Many communications are now encrypted and so COMINT analysis involves a large component of cryptanalysis.

ELINT

Electronic intelligence was used widely in the second World War to identify enemy radars. Wide band receivers would monitor the emissions from a radar and the characteristics of the signal would be used to identify the type of radar and thus its purpose. Such characteristics would include the radar pulse repetition frequency (PRF), the pulse width (PW) and even the pulse profile. Experienced operators could sometimes even identify not just the type of radar, but an individual radar, and thus the ship that carried it before a visual sighting of the vessel was made.

Because radio signals diminish as the inverse square of distance and radar returns vary as the inverse fourth power of the distance, ELINT analysis of a radar signal can usually be safely made before the radar is capable of detecting the ELINT vessel.

ELINT now involves not just the analysis of radar signals (although this still remains a large part of ELINT), but also the reception and analysis of telemetry signals (that might be transmitted during a missile test firing) to provide information on opposition systems. As with COMINT, most military and even a large component of civilian telemetry (even scientific data from satellites) is encrypted, and thus ELINT also relies very heavily on the cryptanalysis community.

Space based ELINT offers the same advantages that it does to COMINT - unrestricted access to the space directly above the desired signals. This is particularly important to radar signals which almost universally use frequencies that propagate only along line of sight.