Secondary Radar– is used to determine the range of the a/c and bearing with respect to the station.
It requires a Transponder Equipment to be carried on board the A/c.
Operation– A continuous beam of pulse energy is transmitted from the Transmitter in all directions into the atmosphere. This beam is received by a Transponder in the a/c, which amplifies these signals and transmits them back to the ground Transmitter on a different frequency. When the Speed of the Beams (which travels at a constant rate) is known and is reference to the Time taken by the beams to be received at either unit, the Range is calculated.
The Aircraft Receives on 1030 Mhz and Transmits on 1090 Mhz.
Range- approximately 50-60 N.M.
The Range is almost double since the beam requires only the strength to reach the target receiver, which has its own Transponder to transmit these signals back.
Unlike the Primary Radar beam which requires the Strength to go and comeback after hitting the target.
Uses– It is used by the Area Control Centre to assist the a/c on approach, provide Radar Vectoring (Radar Vectoring = A/c is made to follow certain maneuvers, before being brought in line with the long finals to expedite traffic and maintain a safe separation between Air Traffic), keep a watch on the a/c and prevent collision, provide En route coverage.
Mode A – Aircraft Identification is available to the Ground Radar unit on their Radar Screen.
Mode C – Aircraft Identification as well as Altitude Reporting in steps of 100 feet, is available to the Ground Radar unit on their Radar Screen.
Mode S – Aircraft Identification, Data Transfer capability as well as Altitude Reporting in steps of 25 feet is available to the Ground Radar unit on their Radar Screen.
Identification– on pressing Identification button located in the Transponder the a/c illuminates brightly on the radar Screen of the Ground Radar Unit for the purpose of identification.
Squawk 2332– means the code given is issued to the a/c for the specified portion of the flight, you are expected to confirm the code and select the same on your Transponder.
On tuning the a/c Transponder with this code, the Ground Radar Unit can have A/c bearing as well as Altitude of the a/c.
The Altitude Reported on SSR unit is with reference to QFE (Pressure Altitude setting of 1013.2)
Fruiting – is caused when two SSR station causes signal to overlap each other, and causes Radio aids to hunt for the reliable signal.
Grass – is the disturbance caused by the electrical discharge interfering with the Radar beams, they travel to the Radar Screen via “Y” plates passing through the beam and looks like tiny grass spread over the screen.
The Grass can be controlled by the Gain Control.
Synchronous Garbling – is caused when an a/c comes in a straight line behind another a/c to be invisible from the SSR operator screen. This occurs as a result of inability of the Radio wave to search the farther a/c in the straight line behind another a/c. The resultant echo shows only one a/c on the Radar Screen, when there are two a/c operating in the vicinity. It usually does not last for a long time since two a/c have their relative speed that constantly changes their position with reference to each other.
7500- Hijack
7600- Communication Failure
7700- Distress
0000- Transponder Failure
2000- No Squawk Code Issued