What is the difference between TDOA and FDOA geolocation, and which does a satellite constellation use?

Time-Difference of Arrival (TDOA) computes position by measuring the nanosecond-level delay between the same signal arriving at two or more satellites. Frequency-Difference of Arrival (FDOA) exploits the Doppler shift between satellites moving at different velocities relative to the emitter. LEO RF constellations almost always fuse both techniques — TDOA-only is sufficient for stationary emitters, but FDOA is essential for moving targets such as ships or aircraft. Combining them reduces CEP from kilometres to hundreds of metres.

How many satellites does a sovereign nation actually need to build a useful geolocation capability?

A minimum viable constellation for regional (not global) coverage with 60-minute revisit is approximately 6–9 satellites in a carefully chosen LEO plane, flying in clusters of three. For continuous sub-30-minute global revisit, 18–24 satellites is the practical threshold, based on HawkEye 360's operational experience and Spire's RF monitoring service. A phased programme — launching a 3-satellite demonstrator, then scaling — is the standard sovereign procurement path and keeps upfront cost manageable.

Can this capability detect GPS jamming as well as other emitters?

Yes, but detection of GPS jamming from space is a secondary use-case here; it is covered in depth in the GPS Jamming Detection application (§7.4.2). Emitter geolocation satellites can certainly detect and fix the position of ground-based GPS jammers — the L1/L2 band emissions are well within standard RF payload sensitivity ranges. However, the signal-processing pipelines and alerting thresholds are different enough that the two functions are typically treated as separate payload modes.

What happens if a targeted nation objects to being observed — is spaceborne RFGEO legal?

Under the Outer Space Treaty (1967, Article II) and long-established customary international law, observation from space is legal: there is no sovereign airspace in orbit. RFGEO is a passive receive operation — no signal is transmitted toward the target — so it falls outside ITU interference rules. It does exist in a grey zone under national intelligence law for some jurisdictions, but no binding international treaty prohibits passive RF collection from orbit. Nations operate these systems openly (e.g. the US National Reconnaissance Office, French DGA's CERES constellation).

How quickly can geolocation data reach a decision-maker after a signal is detected?

End-to-end latency depends on downlink architecture. With direct-to-ground downlink and cloud processing, current commercial systems (HawkEye 360, Kleos) deliver geolocation products in 30–90 minutes after satellite pass. With onboard processing and inter-satellite links, latency can be reduced to under 10 minutes. A sovereign military constellation with dedicated ground stations and classified processing pipelines can achieve sub-5-minute alert-to-fix cycles, which is the threshold for tactical relevance.

What payload does a sovereign emitter geolocation satellite actually carry?

The core payload is a wideband RF receiver covering the frequencies of interest (typically 100 MHz–18 GHz, sometimes extended to millimetre-wave), a precision timing reference (often a space-qualified OCXO or CSAC), and a digital signal processor running TDOA/FDOA algorithms. Secondary payloads often include an AIS or ADS-B receiver to correlate RF emitters with vessel or aircraft identity. The whole payload can fit within a 12U–16U cubesat form factor for narrowband missions, or a 50–150 kg microsatellite for wideband collection.

Why not just buy HawkEye 360 or Spire data rather than building a sovereign constellation?

Commercial data is fine for peacetime maritime domain awareness and spectrum enforcement — the use-cases those firms publicly serve. The problems are access, priority, classification, and longevity. A commercial provider answers to its board and to US government tasking priority; in a crisis, your account may be deprioritised or data may be withheld under national security directives. Sovereign ownership means you control tasking, processing, classification level, and retention policy — and you retain the capability regardless of vendor corporate events or geopolitical shifts.

How does a sovereign programme handle the emitter library needed to identify and attribute signals?

Signal identification requires a database of known emitter characteristics — frequency, modulation, pulse parameters, and platform associations. Sovereign programmes typically build this from three sources: ingested allied intelligence (via NATO STANAG 4592-compatible exchange), national signals intelligence archives, and the constellation's own accumulated observations. This library is itself a classified national asset that grows in value over years of operation, representing a strategic intelligence advantage that cannot be purchased from any commercial vendor.

7.4.1Live

Emitter Geolocation

Analyst in uniform studying multiple screens displaying frequency waveforms and map overlays in a dimly lit operations room — Server infrastructure like this processes raw satellite RF signals to pinpoint emitter locations, enabling precise interference detection worldwide.

Satellite RF Emitter Geolocation Signal Chain

1
RF Intercept Satellite
Captures emitter signal from low orbit
2
TDOA Downlink
Time-difference data relayed to ground stations
3
Geolocation Processing
Multilateration fixes emitter coordinates precisely
4
Intelligence Consumer
Operator receives geolocated target on map

Multiple satellites measure signal arrival-time differences, enabling ground processors to pinpoint a radio emitter's precise geographic location.

Page in the build queue

This application has its place in the atlas. The full page — short and long descriptions, what-matters bullets, sovereignty rationale, and the eight-field architecture block — is being written next.

Maturity reading: Operational systems exist with paying customers. Phase 1 built §4.1 (Maritime Intelligence) end-to-end as the depth-and-format reference. Section 4 is the next section to graduate from scaffold to full content; the rest follow on the roadmap.

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7.4.2 · Next

Spectrum Survey

Bring this into your sovereign programme.

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