10 SSR

Question 1

SSR equipment

Answer 1

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.

Question 2

How does interrogation occur? - Side lobe suppression.

Purposes of pulses P1, P2 and P3.

Answer 2

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

Question 3

Fruiting?

Answer 3

Each ground station incorporates it own interrogater identifier code (IC) or a station ID (SI) to prevent fruiting.

Question 4

squitter?

Answer 4

Pulses on regular timed rate (1/sec) even when not being interrogated for TCAS purpose and preformatted infomation line in NATS airspace.

Question 5

Mode S interrogation methods?

Answer 5

Aircraft addressing
All-Call
Selective call (Roll Call)

Question 6

Mode S principle?

Answer 6

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.

Question 7

How does the AC transponder know its being interrogated by a Mode S interrogator?

Answer 7

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.

Question 8

How is the response adminitered?

Answer 8

• Pulses P1 and P2 are framing pulses 20.3us apart
• Between the framing pulses 12 more pulses that determine the ident.

Question 9

2000
7000
7007

Answer 9

IFR
VFR
Open skies.

Question 10

SSR modes of operation are determined by the time spacing between pulses, and are:

Answer 10

Mode A – identification 4 number identification 0-7.
Mode C – altitude.
Mode S – addressing and flight data. ADS-B, TCAS

Question 11

Mode A?

Answer 11

Mode A replies are transmitted as one of 4096 ATC assigned four-digit identification codes.

Question 12

Mode C?

Answer 12

Mode C replies are transmitted as pressure altitudes, in 100 ft increments. Relative to 1013mb

Question 13

Mode S?

Answer 13

• 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.

Question 14

SPI?

Answer 14

Special position identification (SPI) signals can be transmitted by the pilot, on pressing the IDENT button, in response to ATC requests.

Question 15

Mode S address?

Answer 15

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.

Question 16

Mode S interrogation contains either:

Answer 16

• Specific aircraft address.
• All-call.
• Selective surveillance calling (Roll-call).

Question 17

The transponder has several selectable modes including:

Answer 17

OFF
SBY (standby)
ON (Mode A)
ALT (Modes A, C, and S)
TEST

Question 18

The reply lamp?

Answer 18

The reply lamp indicates the transponder is being interrogated.

Question 19

Range and direction infoamtion from SSR?

Answer 19

Azimuth infomation from the Tx bearing

Question 20

SSR has the following advantages over primary radar:

Answer 20

• It requires much less transmitting power to provide coverage up to 200 to 250 NM.
• It is not dependent on an aircraft’s echoing area or aspect.
• It gives clutter free responses as it does not rely on returning reflected pulses.
• It positively identifies an aircraft’s primary response by displaying its code and call sign alongside.
• It indicates an aircraft’s track history, speed, altitude, and destination.
• It can indicate on a controller’s screen that an aircraft has an emergency, has lost radio communications, or is being hi-jacked.
• It is a required component for ACAS/TCAS.

Question 21

Code for transponder mailfunction?

Answer 21

0000

Question 22

FRUIT

Answer 22

Since all aircraft reply on the same frequency of 1090 MHz, a ground station will also receive aircraft replies originating from responses to other ground stations. These unwanted replies are known as FRUIT (False Replies Unsynchronized with Interrogator Transmissions or alternatively False Replies Unsynchronized In Time). Several successive fruit replies could combine and appear to indicate an aircraft which does not exists. As air transport expands and more aircraft occupy the airspace, the amount of fruit generated will also increase.