Why the transponder doesn't reply?
An interrogation may fail to obtain a reply from the wanted aircraft transponder for a number of reasons.
- Busy replying to another interrogator
The aircraft may already be in the process of making a reply to another interrogator or recovering after it. The recovery time or „dead” time may be up to 125 µs for some older transponders but modern transponders are much less (typically 45 µs).
Also Mode S transponders may be replying with a Mode S reply which is much longer than a SSR Mode A/C reply. In addition, Mode S transponders make autonomous replies known as „squitter”, for acquisition of their address by ACAS.
- Transponder Dead Time
There is a period of dead time occurring after receipt of a valid interrogation sequence (up to 125 µs), during which no replies will be issued to arriving interrogations while the valid interrogation is being processed. The aircrafts transponder may also be suppressed on board the aircraft when the aircraft makes other transmissions – e.g. by navigational Distance Measuring Equipment (DME) and also ACAS interrogations.
In a very highly populated interrogator environment, it is possible that the transponder has reduced its sensitivity in response to over-interrogation. Mode A/C transponders should suppress replies in a range of circumstances related to the maximum interrogation and reply rates. They must be able to send at least 1,200 up to a maximum of 2,000 (programmable maximum) Mode A or C replies in any one second interval. If the transponder is subject to over-interrogation by multiple ground stations, it should not issue replies.
- Transponder in Suppression
A Mode A/C transponder may suppress its reply by interrogations received via the sidelobes of the antenna. During suppression, the transponder will not accept interrogations. Any interrogation therefore arriving at that transponder will be ignored and no reply issued. The duration of suppression is defined in SARPs as being 35 µs (±10 µs.)
- Transponder Antenna Shadowing
SSR operates on a line of sight basis and if a nearer target or object is obscuring a far target, it will not reply. This principle is known as antenna shadowing. When the aircraft turns, the SSR antenna may be shadowed by other parts of the airframe. This is less so nowadays for civil aircraft with both top and bottom antennas.
- Transponder Malfunctioning
Apart from the „no reply” reasons listed above, which can all occur from a perfectly working / operational system, it is also possible to have a transponder failure or malfunction of one of the two active components of the system which is mostly the airborne part, the transponder. It is noted that most aircraft today carry a backup transponder in case of failure.
It is clear that problems with classical A/C SSR will be compounded as the volume of air traffic increases each year and the need for replacement systems has become apparent. The current surveillance infrastructure is reaching saturation point in some parts of the world where high-density air traffic is common. Europe airspace is one example.