SSR, Mode S and ADS-B dependencies

What information does a transponder add?

A cooperative reply can carry identity and pressure-altitude information.

Interrogation and reply

Secondary surveillance uses an interrogator and an airborne transponder. The aircraft receives an interrogation and transmits a reply. This active reply is a different mechanism from the weak reflected echo of primary radar. Conventional Mode A conveys a code; Mode C adds pressure-altitude information. Mode S supports selective addressing and additional data.

The ground surveillance system combines timing, antenna direction and reply information. A code or label is not by itself proof that identification and every associated parameter are correct. Crew settings and data sources remain part of the chain.

Altitude reference

Conventional transmitted pressure altitude uses the standard-pressure reference, independently of the pilot’s local QNH setting. A wrong encoder/source can therefore misreport altitude even when the cockpit altimeter appears reasonable. Compare and report discrepancies using the applicable procedure; do not assume that changing QNH repairs the reported source.

Trace an altitude discrepancy

The pilot sets local QNH correctly and sees 5,000 ft on the altimeter, while the surveillance report contains an unexpected pressure altitude. First recognise that those values use different reference conditions and need not be numerically identical. A raw standard-pressure report cannot be compared directly with a local-QNH altitude without the appropriate context and processing.

If a genuine discrepancy remains after using the proper references, investigate the transmitted source and follow the reporting procedure. Turning the QNH knob merely to make a displayed pair match could corrupt the pilot’s altitude reference while leaving the encoder error unchanged. Mode S addressing and additional data improve the information available but do not eliminate the need for correct source data, crew settings, identification and surveillance-system correlation.

Why does ADS-B position depend on aircraft navigation?

Broadcast surveillance reports position; primary radar measures an echo.

Dependent surveillance

ADS-B Out broadcasts aircraft-derived information, including position from an approved navigation source. It does not wait for a conventional radar interrogation to provide each broadcast. The word dependent matters: erroneous source position can affect the report, subject to system monitoring and quality indicators.

Primary radar position and GNSS-derived ADS-B position therefore do not have the same dependency chain. A GNSS disruption may affect navigation and ADS-B-related functions together. Two cockpit or ground products can agree because they share the same underlying source.

Losses and correlation

The laboratory independently switches the transponder, GNSS source and primary echo visibility. A primary echo may survive a transponder failure; an SSR reply can remain available while GNSS position is unavailable, depending on equipment. Real system behaviour is installation-dependent, so the diagram explicitly describes its assumed chain instead of promising a universal failure pattern.

Compare a common-source failure

Assume the installation broadcasts GNSS-derived ADS-B position, transmits pressure altitude through its transponder and remains detectable by a primary radar. If GNSS position becomes unavailable, those three chains need not fail together. Primary echo detection uses reflected energy, and the pressure-altitude reply has a different source. The ADS-B position chain depends on the navigation source and its valid-output logic.

If the GNSS position instead becomes misleading, several dependent products may present the same plausible displacement. A controller display and a cockpit map could then appear to corroborate one another while repeating a shared source. The laboratory’s switches reveal the assumed dependencies. A real aircraft’s fault messages and remaining transmissions depend on its design, so use installed documentation and operational procedures rather than generalising every switch outcome.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.