EPIRB and PLB signal detection from orbit
The Cospas-Sarsat system turns 406 MHz distress transmissions into geolocated alerts via LEO Doppler processing and MEO near-instantaneous relay. Understanding its geometry, latency and gaps is essential for any government operating search-and-rescue coordination.
Sensors
- Cospas-Sarsat LEOSAR (Low-Earth Orbit Search and Rescue): Instruments aboard NOAA POES and Russian Meteor satellites in roughly 850 km polar orbits receive 406 MHz transmissions and measure Doppler shift across the pass. A single satellite pass of 10–15 minutes yields one Doppler curve; two solutions exist per pass (ambiguity resolved by subsequent passes or GPS data). Maximum detection latency without GPS: up to 90 minutes at mid-latitudes, longer near the equator where polar-orbit geometry is worst.
- Cospas-Sarsat MEOSAR (Medium-Earth Orbit Search and Rescue): Hosted payloads on GPS Block IIF/III, GLONASS, Galileo and BeiDou satellites at roughly 19,000–24,000 km altitude. Multiple satellites see the same transmission simultaneously, enabling near-instantaneous Doppler-difference and time-difference-of-arrival (TDOA) position fixes within minutes. Galileo's SAR/Galileo payload is the most capable European contribution; MEOSAR is now the primary detection layer for most ocean basins.
- Cospas-Sarsat Return Link Service (RLS): Galileo and BeiDou satellites carry a downlink channel that sends a short acknowledgement message back to the activated beacon, confirming receipt by the rescue system. This is a one-way confirmation only; it does not enable two-way communication. Deployment of RLS-capable beacons is ongoing as of the mid-2020s.
- GPS-encoded 406 MHz beacons: Modern EPIRBs and PLBs embed a GPS fix in the 406 MHz message itself. When present, position accuracy is typically better than 100 m. Without an embedded fix, LEOSAR Doppler-derived positions carry uncertainties of 2–5 km under good geometry, degrading significantly near the equator.
- Spire Global RF payload (LEMUR-2 constellation): Spire's LEO constellation carries software-defined radio payloads that can be tasked to monitor specific frequency bands including the 406 MHz distress band. Revisit is a function of constellation size and tasking priority. This is a commercial supplement, not a certified safety-of-life system; it cannot substitute for Cospas-Sarsat in regulatory SAR frameworks.
What Doppler shift actually tells you
When a 406 MHz beacon transmits and a LEO satellite passes overhead at roughly 7.5 km/s, the received frequency rises as the satellite approaches and falls as it recedes. The shape of that Doppler curve, measured over a pass of 10 to 15 minutes, encodes the beacon's position relative to the satellite's ground track. Two mirror-image solutions emerge from a single pass, one on each side of the track. A second pass, or an embedded GPS fix in the beacon message, resolves the ambiguity.
The physics is elegant but the geometry has a well-known weakness. Near the equator, polar-orbiting satellites pass at a shallow angle, compressing the Doppler curve and inflating position error. Mid-latitude users in the North Atlantic or Southern Ocean get better geometry and shorter wait times simply because polar orbits are denser there. This is not a flaw in the system design; it is an inescapable consequence of orbital mechanics. Anyone planning SAR coverage for equatorial island states should account for it explicitly.
Why MEOSAR changed the latency calculation
LEOSAR's 90-minute worst-case detection window was acceptable when it was designed. It is not acceptable now that MEO constellations offer near-continuous global visibility. A 406 MHz transmission is simultaneously visible to multiple GPS, GLONASS, Galileo and BeiDou satellites. Their separation in space allows time-difference-of-arrival and frequency-difference-of-arrival calculations that converge on a position fix within minutes of the first burst, without waiting for a satellite to pass overhead.
The practical result is that MEOSAR has collapsed the detection latency for most ocean basins to under five minutes in normal conditions. The caveat is satellite health and ground segment availability. Not every MEO satellite carries a SAR payload, and the ground infrastructure that processes MEOSAR signals, the Local User Terminals and Mission Control Centres, must be operational and connected. The system is more capable than LEOSAR but it is not immune to ground-segment outages.
False alarms consume real resources
Cospas-Sarsat statistics published by the organisation consistently show that the large majority of 406 MHz activations are accidental or false. Beacons triggered during maintenance, battery replacement, or rough handling generate alerts that task coast guard aircraft and vessels. The 15-character hexadecimal beacon identifier encoded in every 406 MHz message is the primary tool for filtering these: a registered beacon with a known owner can be telephoned before a helicopter departs.
Unregistered beacons, or beacons with outdated registration data, are the persistent problem. Some administrations have moved to mandatory registration with renewal requirements. From an analytics standpoint, a history of false activations from a specific vessel or identifier is detectable and useful for prioritising response. That kind of pattern analysis sits entirely within the public Cospas-Sarsat data stream.
Return Link Service: confirmation without conversation
RLS is the most significant recent addition to the system. When a Galileo or BeiDou satellite detects a valid 406 MHz signal and the ground segment processes it, a short coded message is transmitted back to the beacon on a separate downlink frequency. The beacon's display or indicator light confirms that the distress alert has been received.
This matters psychologically and operationally. A survivor who knows the alert has been received is less likely to attempt risky self-rescue. But RLS is strictly one-directional. It carries no position correction, no estimated rescue time, and no two-way messaging. Beacons must be RLS-capable to receive the acknowledgement, and the installed base of such beacons is still growing. Governments procuring or mandating beacon equipment should specify RLS compatibility explicitly in their standards.
Coverage gaps that planners routinely underestimate
Three gaps deserve attention. First, terrain masking: a beacon activated in a steep valley or inside a metal vessel structure may not have a clear line of sight to any satellite, LEOSAR or MEOSAR. Second, the 406 MHz band is shared and subject to interference; strong co-channel signals in congested port areas can degrade detection probability, though the system's narrow-band design and message structure provide some protection. Third, the ground segment is not globally uniform. Local User Terminals are concentrated in North America, Europe, Russia and Australia. Ocean regions distant from these terminals rely on store-and-forward from LEO satellites or MEOSAR relay, which introduces its own dependencies.
Polar regions above roughly 75 degrees latitude present a specific case. MEOSAR geometry is actually favourable at high latitudes because MEO satellites are visible at lower elevation angles for longer periods. LEOSAR coverage is also dense near the poles. The gap that remains is ground-segment connectivity in the Arctic, where LUT density is lower and communications infrastructure is sparse.
What an analytics layer adds to a certified SAR system
Cospas-Sarsat is a safety-of-life system operated by national administrations. No commercial analytics provider replaces it or sits in its signal chain. What analytics can do is work alongside it: correlating beacon activation events with AIS vessel tracks, weather model outputs and historical incident databases to support rescue coordination centres in prioritising search areas and estimating drift.
Satellize builds analytics on open and commercial satellite data for government clients. The methodology for correlating 406 MHz event data with oceanographic and vessel-tracking layers is straightforward and well-documented in the published SAR literature. Governments establishing or upgrading national SAR coordination infrastructure should treat the Cospas-Sarsat data stream as one input among several, not as a self-contained picture. The beacon tells you someone is in distress. Everything else tells you where to look first.
Typical figures
| Distress frequency | 406.028 MHz (406 MHz band, internationally protected) |
| LEOSAR detection latency | Up to 90 minutes worst-case (equatorial); typically 30–45 minutes at mid-latitudes without embedded GPS |
| MEOSAR detection latency | Under 5 minutes in most ocean basins under normal ground-segment conditions |
| LEOSAR position accuracy (no GPS in beacon) | 2–5 km at favourable geometry; degrades significantly near equator |
| Position accuracy with embedded GPS | Better than 100 m (dependent on beacon GPS fix quality) |
| Beacon identifier length | 15 hexadecimal characters (encodes country code, beacon type, serial or vessel MMSI) |
| MEOSAR host constellations | GPS Block IIF/III, GLONASS, Galileo, BeiDou (coverage varies by payload deployment status) |
| Return Link Service availability | Galileo and BeiDou RLS operational; requires RLS-capable beacon to receive acknowledgement |
| Beacon transmission power | 5 W nominal (regulated minimum); burst duration approximately 0.5 s, repeated every 50 s |
| False alarm proportion | Historically high; Cospas-Sarsat annual reports indicate the majority of activations are accidental (exact figures vary by year and region) |
Analytics Satellize can run
| Beacon event correlation with AIS vessel track | Temporal and spatial join of Cospas-Sarsat alert data against AIS position history; identifies most probable distressed vessel when beacon MMSI matches or vessel is in proximity | Incident report with vessel identity, last known position, and AIS track replay; delivered within minutes of alert ingestion |
| Ocean drift projection for search area estimation | Leeway and current modelling using CMEMS or HYCOM oceanographic fields combined with beacon last-known position; standard IAMSAR drift methodology | GIS polygon of probable search area at 6, 12 and 24 hours post-activation, updated as new position fixes arrive |
| False alarm probability scoring | Beacon identifier history cross-referenced against registration database and prior activation records; vessel port-call data used to flag activations in harbour | Alert triage score (high/medium/low confidence of genuine distress) delivered to rescue coordination centre dashboard |
| National beacon registration gap analysis | Statistical analysis of activation events against national registration database to identify unregistered or expired-registration beacons by flag state or region | Quarterly report for maritime administration with registration compliance rate and priority follow-up list |
| SAR response time audit | Reconstruction of detection-to-rescue timelines from Cospas-Sarsat event logs and incident records; comparison against IAMSAR benchmark intervals | Annual performance report for national SAR authority, with gap analysis by ocean region and season |
| Ground-segment coverage modelling | Geometric visibility analysis of LUT network against LEOSAR and MEOSAR orbital parameters; identifies ocean areas with elevated latency risk | Coverage map with latency contours; used to inform LUT siting decisions or supplementary commercial RF monitoring tasking |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.