Aviation ELT distress signal detection from LEO
406 MHz Emergency Locator Transmitters alert the Cospas-Sarsat constellation when aircraft crash or declare distress. LEO geometry, false-alarm rates and terrain masking all shape how quickly that alert becomes a rescue.
Sensors
- Cospas-Sarsat MEOSAR payloads (GPS, GLONASS, Galileo, BeiDou SAR instruments): Search-and-rescue instruments hosted on GNSS satellites at medium Earth orbit (~19,000-24,000 km). Near-global coverage at any instant; MEOSAR can detect and localise a 406 MHz signal within minutes of first transmission, with TDOA/FDOA position accuracy typically better than 5 km when multiple satellites are in view. No waiting for a satellite pass.
- Cospas-Sarsat LEOSAR (NOAA-15, NOAA-18, NOAA-19, MetOp-A/B/C): Sun-synchronous polar orbiters at roughly 850 km altitude carrying SARSAT/COSPAS transponders. LEOSAR provides Doppler-based localisation (accuracy 2-5 km under good geometry) but coverage is intermittent: at mid-latitudes a satellite passes over every 60-90 minutes on average, creating a detection latency that can exceed 90 minutes in the worst case.
- Spire LEMUR-2: Commercial LEO constellation of over 100 small satellites in polar and inclined orbits at ~500-600 km. LEMUR-2 carries GPS-RO and AIS payloads as primary instruments; the constellation's orbital density reduces average revisit to under 30 minutes globally, and the platform architecture is compatible with hosted RF-monitoring payloads including 406 MHz band reception, though Spire's primary Cospas-Sarsat role is augmentation rather than replacement of the certified system.
- Cospas-Sarsat GEOSAR (GOES, Meteosat, INSAT, Elektro-L): Geostationary relay satellites that provide near-instantaneous alert forwarding for 406 MHz signals visible from their footprint. No Doppler localisation is possible from GEO alone; GEOSAR alerts must be paired with encoded GPS position from the beacon itself or with MEOSAR/LEOSAR fixes. Polar regions above roughly 75-80° latitude fall outside GEOSAR coverage.
What a 406 MHz ELT actually transmits
A modern 406 MHz ELT broadcasts a digitally encoded burst every 50 seconds, lasting approximately 0.5 seconds. The encoded message carries a unique 15-hex-digit beacon identification number registered to a specific aircraft, operator and country. This is what separates the 406 MHz standard from older 121.5 MHz beacons, which Cospas-Sarsat stopped processing by satellite in 2009: the digital ID allows ground stations to cross-reference a registration database before dispatching rescue assets, and to filter known false alarms.
Many modern ELTs also include an internal GNSS receiver. When the beacon encodes its own GPS or GNSS position into the transmission, MEOSAR ground stations can resolve location to within 100 metres, bypassing the need for satellite-geometry-based localisation entirely. When no internal GNSS fix is available, the system falls back to TDOA/FDOA calculations across the satellite constellation, with accuracy degrading to the 2-5 km range depending on geometry and signal quality.
LEOSAR latency: the gap that costs lives
LEOSAR satellites localise a beacon by measuring the Doppler shift of the 406 MHz signal as the satellite passes over. This is elegant physics, but it requires the satellite to actually be in view. At mid-latitudes, the average wait for a LEOSAR pass is 60-90 minutes; at high latitudes, polar orbital geometry means passes are more frequent, sometimes under 30 minutes. In the equatorial band, coverage gaps can reach two hours.
MEOSAR closed most of this gap. With SAR payloads distributed across the GPS, GLONASS, Galileo and BeiDou constellations, there are typically several MEO satellites visible from any point on Earth at any time. The Cospas-Sarsat programme reports that MEOSAR can deliver an alert with position within three minutes of signal acquisition in favourable conditions. The key word is 'acquisition': terrain masking in mountainous regions can prevent the signal reaching any satellite at all if the aircraft has come down in a deep valley with the antenna obstructed.
False alarms consume the system
Cospas-Sarsat's own published statistics show that the overwhelming majority of 406 MHz activations are inadvertent. Hard landings, maintenance without a beacon guard, baggage handlers, and aircraft sales where ownership transfer is not reflected in the registration database all generate alerts that rescue coordination centres must investigate. The false-alarm rate for 406 MHz ELTs has been reported at well above 95% of all activations in some national systems.
This matters analytically. An organisation monitoring ELT alert traffic to assess search-and-rescue system performance or coverage gaps must distinguish between registration-database mismatches, known-test signals, and genuine emergencies. The encoded beacon ID is the primary discriminant. Signals from unregistered or improperly registered beacons are a persistent problem and place a disproportionate burden on rescue coordination centres in countries with less rigorous registration enforcement.
Terrain masking and the polar edge case
A 406 MHz ELT transmits omnidirectionally, but the signal still needs line-of-sight to a satellite. In steep terrain, the effective sky view from a crash site can be severely limited. A aircraft down in a narrow Alpine valley or a fjord may transmit for hours before a satellite geometry exists that allows the signal to propagate clear of the surrounding ridgelines. This is not a theoretical concern: accident investigation reports have documented cases where beacon signals were received only intermittently or with significant delay due to terrain obstruction.
Polar operations face a different constraint. GEOSAR satellites at geostationary altitude cannot see above roughly 75-80° latitude. Flights along transpolar routes, increasingly common as airlines exploit great-circle efficiency over the Arctic, depend entirely on MEOSAR and LEOSAR for distress detection. MEOSAR coverage at high latitudes is actually geometrically favourable because MEO satellites subtend large elevation angles from polar regions, but the absence of GEOSAR redundancy means the system has no instantaneous relay fallback if MEOSAR processing is delayed.
What space-based ELT monitoring tells an operator
For a government operating a rescue coordination centre, or an airline with polar or remote-area operations, the analytically useful questions are not simply 'did the beacon fire?' They are: where are the coverage latency hotspots on our routes, what is our realistic detection-to-alert time for a given flight corridor, and how does terrain along that corridor interact with the satellite geometry at the times our aircraft are overhead?
These questions can be answered by combining published Cospas-Sarsat constellation ephemeris data with terrain elevation models and route schedules. The output is a time-resolved coverage probability map: for each segment of a route, at each hour of the day, what is the expected latency before a distress signal would be acquired by at least one MEO or LEO satellite with sufficient elevation above the local terrain mask? Satellize builds this class of analysis for sovereign clients who need to understand the real performance envelope of the system they are depending on, not the headline specification.
The Cospas-Sarsat programme is a multilateral intergovernmental system; no private operator replaces it. The analytical value is in characterising its gaps honestly and in designing complementary procedures, such as position reporting intervals or HF voice check-in requirements, that reduce the window of uncertainty when a beacon has not yet been acquired.
Honest limits of the current architecture
MEOSAR is a genuine improvement over LEOSAR-only detection, but it is not instantaneous everywhere. Signal acquisition still requires line-of-sight. Encoded GNSS position from the beacon itself is the fastest path to a precise fix, but older ELT models without internal GNSS remain in service on thousands of aircraft worldwide, particularly in general aviation fleets in lower-income countries. For those beacons, the system falls back to Doppler localisation with its associated geometry-dependent accuracy.
The 406 MHz band is also not immune to interference. Intermodulation products from other RF sources and, in rare cases, deliberate or accidental jamming in the 400-406 MHz region can degrade reception. The Cospas-Sarsat system includes signal quality metrics to flag suspect detections, but a degraded signal may still generate a position estimate with unquantified error that is not flagged as uncertain in the alert passed to the rescue coordination centre.
Typical figures
| ELT transmission frequency | 406.028 MHz (primary); 121.5 MHz homing signal (secondary, not processed by satellite) |
| MEOSAR alert latency (favourable conditions) | Under 3 minutes from signal acquisition to rescue coordination centre alert (Cospas-Sarsat published figures) |
| LEOSAR alert latency (mid-latitudes) | 60-90 minutes average; up to 2 hours in worst-case equatorial geometry |
| TDOA/FDOA position accuracy (no internal GNSS) | Typically 2-5 km; degrades with poor satellite geometry or terrain masking |
| Position accuracy with encoded internal GNSS | Better than 100 m (dependent on beacon GNSS fix quality at time of activation) |
| GEOSAR polar coverage limit | Approximately 75-80° latitude; no GEOSAR coverage beyond this |
| Beacon identification | 15-hex-digit unique ID encoding country, operator and aircraft registration |
| Transmission duty cycle | 0.5-second burst every 50 seconds |
| False-alarm rate (published national statistics) | Exceeds 95% of activations in several national systems; primary cause is inadvertent activation |
Analytics Satellize can run
| Route coverage latency map | Satellite ephemeris propagation combined with terrain elevation masking (SRTM or Copernicus DEM) and Cospas-Sarsat constellation geometry modelling | Time-resolved GIS layer showing expected detection latency by route segment and hour of day, exported as GeoTIFF or vector polygon set |
| Terrain obstruction risk index | Sky-view factor calculation from digital elevation model at candidate crash-site locations along a flight path, intersected with satellite elevation angle envelopes | Per-waypoint risk score report identifying segments where terrain masking could delay acquisition beyond a client-defined threshold |
| False-alarm pattern analysis | Statistical analysis of beacon ID registration database cross-reference rates and activation timing patterns using publicly available Cospas-Sarsat incident data and national RCC reports | Summary report quantifying false-alarm burden by aircraft type, operator category and geographic region, with recommendations for registration audit priorities |
| Polar route gap assessment | MEOSAR-only coverage modelling above 75° latitude using published MEO constellation almanacs, with GEOSAR exclusion zone applied | Annotated route profile showing GEOSAR-dark segments and MEOSAR acquisition probability windows, formatted as a PDF briefing for flight operations teams |
| SAR system performance baseline | Aggregation of historical Cospas-Sarsat annual report data on detection rates, alert times and rescue outcomes, normalised by region and aircraft category | Benchmarking dashboard comparing a client's operational area against global and regional published performance figures |
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.