10 SSR Flashcards
SSR equipment
A ground radar that can trigger a data exchange with transponder.
SSR comprises a ground based interrogator (phased array antenna) and an airborne transponder (Responds on a omnidirectional antenna).
1030MHz from the Ground Station and 1090MHz response from the aircraft.
How does interrogation occur? - Side lobe suppression.
Purposes of pulses P1, P2 and P3.
GS transmits 3 interrogation pulses
P1 and P3 are interrogation pulses(pulse paiurs) Spacing determines the infomation request.
P2 is a side lobe suspression pulse, to improve the clarity from the omnidirectional arial on the aircraft.
It is used to check that the aircraft it in the main beam of the trasnmitter
Fruiting?
Each ground station incorporates it own interrogater identifier code (IC) or a station ID (SI) to prevent fruiting.
squitter?
Pulses on regular timed rate (1/sec) even when not being interrogated for TCAS purpose and preformatted infomation line in NATS airspace.
Mode S interrogation methods?
Aircraft addressing
All-Call
Selective call (Roll Call)
Mode S principle?
Mode S survalience implicitly uses the principle of selective addressing. Messages are addressed to individual aircraft, or groups of aircraft using each aircrafts unique ICAO aircraft address.
How does the AC transponder know its being interrogated by a Mode S interrogator?
Mode S does not transmit the P3 pulse, but has an additional P4 pulse. Transponder without mode S will ignore P4 impulses. P4 pulses can be either long or short in duration.
The Mode A/C/S all-call interrogation elicits a Mode A or Mode C reply (depending on the P1-P3 pulse spacing) from a Mode A/C transponder because it does not recognize the P4 pulse. A Mode S transponder recognizes the long P4 pulse and responds with a Mode S reply. (ICAO Annex 10 Aeronautical Telecommunications, Chapter 3)
This means that a mode S transponder transmits or receives extra pulses after the mode a and mode c transponder pulses.
How is the response adminitered?
- Pulses P1 and P2 are framing pulses 20.3us apart
- Between the framing pulses 12 more pulses that determine the ident.
2000
7000
7007
IFR
VFR
Open skies.
SSR modes of operation are determined by the time spacing between pulses, and are:
Mode A – identification 4 number identification 0-7.
Mode C – altitude.
Mode S – addressing and flight data. ADS-B, TCAS
Mode A?
Mode A replies are transmitted as one of 4096 ATC assigned four-digit identification codes.
Mode C?
Mode C replies are transmitted as pressure altitudes, in 100 ft increments. Relative to 1013mb
Mode S?
- Mode S must be compatible with Mode A and C systems.
- Mode S transponder receives interrogations from interrogators and aircraft Traffic Collision Avoidance Systems (TCAS).
- Mode S has enhanced vertical tracking, using 25 ft increments.
SPI?
Special position identification (SPI) signals can be transmitted by the pilot, on pressing the IDENT button, in response to ATC requests.
Mode S address?
Each aircraft is allocated a unique ICAO Mode S address, which is hard-coded into the airframe, and entered into the transponder during installation into the aircraft.
The 24-bit ICAO address is included in each Mode S interrogation, to address specific aircraft.