SIGINT satellites pick up electromagnetic signals: ELINT (radars) and COMINT (communications). There are two basic forms:
| Form | How it works | Examples (open source) |
|---|---|---|
| Large, in GEO | A very large antenna that continuously listens to a whole region | American SIGINT satellites with antennas reportedly tens of metres across |
| Clusters in LEO | Two or three satellites fly in formation and locate a transmitter from differences in time and frequency (TDOA/FDOA) | American NOSS (naval targets), Russian Lotos-S (Liana), Chinese Yaogan triplets |
A ship or air defence site that switches on its radar can thus be found from space — the principle is in the EW Academy (§ 24.7). Emission control (EMCON) therefore also applies against satellites.