Radio-frequency collection

Radio-frequency (RF) collection is the passive reception from orbit of radio signals that people and their machines transmit, as distinct from radiation reflected or emitted by the Earth's surface. The satellite carries a receiver, either one tuned to a specific broadcast such as ship or aircraft transponders, or a wideband software-defined radio that records a broad range of emitters.[^soldi-2021-maritime-surveillance][^eoportal-hawkeye-360] The technique has two arms that can share hardware: decoding what an emitter reports about itself, and estimating where an emitter is.

Message decoding is best established for ships and aircraft. The Automatic Identification System (AIS) is a ship transponder broadcast on very high frequency (VHF) radio that carries a Maritime Mobile Service Identity (MMSI) identifier, position, speed, course and voyage fields, sent every 2 to 10 seconds depending on ship type.[^soldi-2021-maritime-surveillance] AIS was designed for short range, and satellite receivers have operated since 2008; a receiver at 600 km altitude has a horizon range of about 1440 nautical miles.[^soldi-2021-maritime-surveillance] The Radio Regulations list the AIS 1 and AIS 2 channels at 161.975 and 162.025 MHz.[^itu-r-m2372-2015] For aircraft, Automatic Dependent Surveillance-Broadcast (ADS-B) Out transmits GPS-derived position, velocity, altitude and heading, on 1090 MHz for aircraft above 18,000 ft or flying outside the United States.[^fcc-wrc15-implementation-2026] The 2015 World Radiocommunication Conference allocated 1087.7 to 1092.3 MHz to space station reception of ADS-B emissions, extending reception beyond terrestrial line of sight, including over oceans and remote regions.[^fcc-wrc15-implementation-2026]

Emitter geolocation addresses transmitters that do not report their position. The same signal captured at spatially separated receivers yields time difference of arrival (TDOA) and frequency difference of arrival (FDOA) measurements, from which a ground emitter position can be estimated.[^ho-chan-1997-tdoa-fdoa] HawkEye 360 flies clusters of three satellites carrying a software-defined radio that tunes from 70 MHz to 6 GHz, and publishes results as a 95 percent confidence ellipse.[^eoportal-hawkeye-360] ESA Earth Online describes Unseenlabs satellites as each detecting, characterising and geolocating RF emitters on their own, and relays the operator's statement that location can reach 1 kilometre accuracy.[^esa-earth-online-unseenlabs]

Demonstrated examples include an ESA-hosted AIS receiver on the International Space Station Columbus module, which received nearly 30 million messages from more than 60,000 transmitters in four months from June 2010,[^esa-iss-ais-2010] and the Proba-V ADS-B receiver, which had recorded more than 25 million aircraft positions from over 15,000 aircraft by May 2015.[^esa-proba-v-ads-b-2015] Aireon hosts ADS-B receivers on the Iridium NEXT constellation and states that its service has run since March 2019, with latency under 1.5 seconds at the 99th percentile.[^aireon-icao-suricg5-2020]Applications include vessel tracking beyond coastal range, aircraft tracking over oceans and remote regions, and fishing effort mapping: one study processed 22 billion AIS messages and tracked more than 70,000 industrial fishing vessels from 2012 to 2016.[^kroodsma-2018-science] Combining synthetic aperture radar (SAR) with AIS identifies ships that do not broadcast; ESA uses this approach with Sentinel-1,[^esa-sentinel1-maritime-traffic] and a longline fishing study found non-broadcasting vessels accounted for about 35 percent of activity north of Madagascar and 10 percent near French Polynesia and the Line Islands.[^kroodsma-2022-longline-sar]

Limits are set by the emitters. AIS monitoring covers only cooperative ships; AIS has no authentication or encryption and relies on GPS, so it can be spoofed or switched off.[^soldi-2021-maritime-surveillance][^kpler-ais-fundamentals] A satellite field of view spans many AIS organised areas, so messages overlap and detection is poor in dense traffic.[^soldi-2021-maritime-surveillance] Satellite AIS systems flying as secondary payloads on sun-synchronous orbits leave gaps of up to 9 hours near the Equator.[^soldi-2021-maritime-surveillance] For ADS-B, ESA notes that signals are beamed sideways rather than in all directions and that detectability varies with the age and make of the equipment.[^esa-proba-v-ads-b-2015]