Secondary Surveillance Radar SSR) – Ibex Aviation Academy

Secondary Surveillance Radar

SSR provides positive identification of aircraft in the line of sight on the UHF band frequencies with active co-operation of target
Secondary Surveillance Radar detects aircrafts at higher range using lower power on the principle of interrogation and response

Interrogator of SSR is a ground-based equipment transmitting on 1030 MHz in horizontal plane in a specified direction
SSR Interrogator has a flat directional antenna mounted on top of primary radar antenna to synchronise with primary radars
The Interrogation Message is contained between two framing pulses called P1 and P3
Interrogator can transmit in Mode A for a duration 8 micro seconds or Mode C with duration of 21 micro seconds
Interrogator receives the aircraft’s response on 1090 MHz in the horizontal plane

SSR transponder replies to the ground interrogator using an omni-directional pulses on 1090 MHz after a delay of 50 micro seconds
The reply consists of a unique 4-digit code set on the aircraft transponder and connected to flight plan of every aircraft
The reply code is received on ground providing display of aircraft call-sign, track and height on radar scope

Ground based Interrogator transmits unwanted side-lobes that has to be identified and not replied by aircraft
Interrogator transmits an additional framing pulse P2 in all directions from a separate ariel for side-lobe suppression
P2 pulse is transmitted 2 microseconds after the first framing pulse P1 with a strength lesser than P1 but exceeds the side lobe strength
If P2 is stronger than P1, it is considered as a side-lobe and the aircraft would not respond to this side-lobe interrogation

Aircraft transponder replies to the interrogation by a message enclosed between two framing pulses called F1 and F2 spaced 20.3 micro seconds apart
The information is contained in three pulses for each digit of SSR code making a total of 12 information pulses
Code relayed by information pulses by three digits for each number of the code
For example, 7 is relayed as 1+2+4, 3 is relayed as 1+1+1 and 6 is relayed as 2+2+2
The maximum value of code is 7777 which results in a combination of 4096 codes

When ATC requests Squawk Ident, Special Position Identification or Ident spring-loaded button is pressed
Ident button activates an Ident pulse transmission 4.35 micro seconds after F2 for 20 seconds
ATC can identify the aircraft by a ring around the return on the radar screen

Mode A transponders replies provide Identification, Bearing and Range with pulses train of 8 micro second spacing
Mode C interrogation pulses are at 20.5 micro second spacing for transmitting height referenced to 1013 HPa using an encoder altimeter
Mode C is activated through an ALT button providing an accuracy of 50 feet with an overall accuracy on the radar scope is 100 ft

Special Codes are reserved in SSR for Emergency, Communication Failure, Hijack and Code not Allotted st departure
7700 indicates Emergency, 7600 indicates Communication Failure, 7500 indicates Hijack, 2000 indicates code not allotted and 7000 is the Default code

SSR requires lesser power and the range can extend to 200 NM, ensuring positive identification of aircraft
SSR is clutter free and not dependent on returning pulses due to aircraft aspect or echo area
In addition to raw position calculated Speed, Track and Altitude are displayed to the Radar Controller
SSR mode A or C provides for only 4096 Codes which is grossly insufficient due to increase in civil aviation

Garbling is caused by overlapping replies from two aircrafts when aircraft transponders are within 1.7 NM apart
Fruiting is the result of interference at one interrogator (ground) by replies from transponder (aircraft) to another interrogator

Mode S overcomes disadvantages of normal SSR by selective addressing using 16,700,000 codes incorporated during manufacture
Data link in Mods S SSR helps in transmitting written message when supported by ground data network
Uplink and downlink over the horizon are possible facilitating ground to air and air to ground data exchange
Mode S SSR continuously transmit every second without interrogation reducing radio communication while under radar control
SSR Mode S also operates on 1030 and 1090 MHz frequencies providing height to an accuracy of 25 Ft
Mode S SSR interrogates on All Call, Broadcast, Selective and Inter Mode
All Call mode is used to interrogate all mode S transponders (aircraft)
Broadcast mode is used to transmit information to all transponders
Selective mode is used to get response only from selected transponders
Inter-mode is used to interact with mode A and C transponders
If Mode A or C ground station interrogates mode S transponder, the aircraft will reply in Mode A or C using 4096 codes

SSR Mode S enables Unique Aircraft Identification by Unique Address incorporated during manufacture
Selective Calling facility in addition to All-call facility provides Integrity of Identification with reduced Garbling and Fruiting in SSR Mode S
Mode S provides Simplified Aircraft Identification and improved Air Picture and Tracking in Horizontal and Vertical direction
More aircrafts can be accommodated by large number of addresses improving Short Term Conflict Alert
Mode S reduces Radio Transmission and provides an altitude Accuracy of 25 Ft with a display accuracy of 100 Ft