Secondary Radar Principles

Question 1

Define Secondary Surveillance Radar

Answer 1

Ground interrogators and airborne transponders determine the range and azimuth of aircraft. Height and identity will be displayed also, if equipment allows.

Question 2

What frequency does the ground interrogator transmit on?

Answer 2

1030MHz

Question 3

What frequency does an airborne transponder reply on?

Answer 3

1090MHz

Question 4

Which wavelength is used for SSR?

Answer 4

30cm

Question 5

Describe the interrogator process

Answer 5

The interrogator transmits Mode A pulses

For vertical position info, the interrogator transmits Mode C pulses

Question 6

What is the difference between Mode A and C pulses?

Answer 6

The timing between the pulses

Mode A - 8.5uS
Mode C - 21uS

Question 7

Describe the Reply process for Mode A

Answer 7

The airborne transponder recognises Mode A interrogation.

12 bits of binary are transmitted in reply that correspond to the allocated Squawk

Question 8

How many discrete squawk codes are available?

Answer 8

4096

Question 9

What are the three binary bit numbers used?

Answer 9

4, 2 and 1

Question 10

Why are squawk codes issued to aircraft?

Answer 10

To ease the task of identification and the monitoring of an aircrafts progress.

Question 11

What are the ways squawk codes can be used?

Answer 11

Discrete - Single codes issued to individual aircraft
Non-discrete - Single codes issued to numerous aircraft
Special Codes applied by aircraft on certain occasions

Question 12

What is CCAMS?

Answer 12

Central Code Assignment and Management System

Question 13

What is the purpose of CCAMS?

Answer 13

To organise squawks, by use of an algorithm, so that an aircraft can have only one squawk from takeoff to touchdown. This will reduce RTF and cockpit workload.

Question 14

Describe Conspicuity Codes

Answer 14

Non-Discrete codes assigned to individual positions to identify aircraft being controlled by a particular unit.

Question 15

What are the emergency codes?

Answer 15

7500 - Taken Alive
7600 - Radio Tricks
7700 - Going to Heaven

Question 16

Describe the Reply Process for Mode C

Answer 16

Mode C interrogation is recognised, then 11 bits of data which correlate to the vertical position information are replied.

Vertical position is based on the 1013.2hPa only, from which computer processors determine and display the accurate FL or A based on inputted local QNH and Transition Level.

Question 17

Describe Garbling

Answer 17

When responses to Mode A interrogation overlap due to aircraft proximity to each other. Usually when aircraft pass over each other.

Ground equipment can be fitted with a de-garbler.

Question 18

Describe Fruit

Answer 18

False Replies Un-synchronised In Time

When an interrogator receives a reply from a transponder triggered by a third party rather than itself.

Removed by a de-fruiter.

Question 19

What are SSRs vulnerable to?

Answer 19

Antenna Shadowing, mitigated by placing more than one antenna