Definition
A surveillance technique that determines an aircraft's position by measuring the time difference of arrival of its Mode S transponder replies at multiple ground receiver stations, calculating position from those arrival time differences rather than from the aircraft's own reported position.
Key Points
- Determines aircraft position by measuring the precise time difference of arrival of the same transponder reply at several ground receiver stations, calculating position from those differences rather than from the aircraft's own reported position
- Relies on the aircraft's Mode S transponder replies as its underlying signal source, meaning MLAT works with equipage aircraft already carry for Mode S surveillance without requiring any additional broadcast capability
- Provides an independently-derived position rather than depending on the aircraft's own GNSS receiver and self-reported position, giving MLAT a different failure mode and integrity profile than GNSS-dependent surveillance like ADS-B
- Commonly deployed specifically as a complementary or backup surveillance layer alongside ADS-B, since MLAT's independence from aircraft-reported position provides a cross-check or fallback if ADS-B data integrity is ever in question
- Requires multiple, precisely time-synchronized ground receiver stations covering the surveillance area, making ground infrastructure density and synchronization quality direct factors in MLAT position accuracy
Concept
When an aircraft's Mode S transponder replies, that same reply reaches several ground stations at very slightly different times depending on each station's distance from the aircraft — MLAT measures those precise arrival time differences across the receiving stations and calculates the aircraft's position from the geometry those differences imply, entirely independent of anything the aircraft itself reports about its own position.
Explainer
The independence MLAT provides from aircraft-self-reported position is precisely its complementary value alongside ADS-B: ADS-B surveillance depends on the aircraft's own GNSS receiver and onboard system correctly determining and broadcasting position, meaning a GNSS error or spoofing condition affecting the aircraft's own receiver would propagate directly into degraded ADS-B position data, while MLAT's ground-based time-difference calculation isn't subject to that same specific failure mode at all. This is exactly why MLAT is frequently deployed specifically as a surveillance integrity cross-check alongside ADS-B rather than as a wholesale replacement — comparing MLAT-derived position against ADS-B-reported position gives air traffic control a way to detect exactly the kind of GNSS-related ADS-B data integrity issue that neither surveillance method alone would necessarily reveal. For CE aviation verticals, MLAT deployment is most relevant in busy terminal areas and other locations where surveillance integrity and redundancy justify the ground infrastructure investment multiple synchronized receiver stations require, complementing rather than replacing ADS-B's broader-coverage, lower-infrastructure-cost surveillance model. This surveillance integrity role places MLAT squarely within the CNS/ATM framework's surveillance component, providing air traffic management with an independent cross-check specifically where CNS/ATM's broader surveillance picture depends on more than a single position source to remain trustworthy.
Cite this entry
Multilateration. ConnectedEarth Knowledge Base. https://connectedearth.space/knowledge-base/multilateration/ (updated 13 September 2026).