SSR Summary

SSR

Requires Transponder in aircraft and Interrogator at ground

station.

Advantages over primary radar.

Aerial on top of primary radar.

Displays call sign, pressure altitude or FL, ground speed, destination.

Frequencies

Ground station transmits narrow beam at 1030 MHz and receives at

1090 MHz.

Aircraft receives at 1030 MHz and transmits omni-directionally at

1090 MHz. (in the UHF band).

Modes/Replies

Mode A

For identity (8 µs interrogation pulse spacing) 12 reply

pulses give 4096 combinations (20.3 µs spacing between

framing pulses). Extra pulse (SPI) for squawk Ident (for

20 s).

Mode C

For automatic pressure-altitude (21 µs interrogation

spacing). Transmitted and displayed every 100 ft (± 50 ft

from given level). Switch off if difference > 300 ft (200 ft

UK).

Special codes

7700 – emergency

7600 – radio failure

14

7500 – unlawful interference

Disadvantages of SSR

Garbling - overlapping replies if aircraft < 1.7 NM apart

Secondary

Fruiting - interference caused by replies to other interrogation.

Limited codes (4096).

Surveillance

Mode S Features

Selective addressing. Nearly 17 million codes from 24-bit address.

Data link air-to-ground, ground-to-air, air-to-air.

Height read-out in increments of 25 ft.

Radar

Interrogation modes

All Call - mode S.

Broadcast (no reply).

(SSR)

Selective calling (unique aircraft address).

Intermode - A/C/S All call.

Benefits of mode S

Unambiguous aircraft identification.

Improved surveillance (eliminates garble, resolves overinterrogation

and reflections).

Improved situation awareness for radar controller.

No code shortage.

Reduced R/T.

Improved short term conflict alert.

a.

7700

7600

7500

b.

7700

7600

7500

c.

7600

7500

7700

d.

7500

7600

7700

a.

primary

pulse

SHF

b.

primary

pulse

UHF

c.

secondary

FM

SHF

d.

secondary

pulse

UHF

3.

If the SSR transponder IDENT button is pressed:

a.

it causes a momentary distinctive display to appear on the controller’s screen

b.

an identification pulse is automatically and continuously transmitted for 20

seconds, 4.35 µs before the last framing pulse

c.

an identification pulse is automatically and continuously transmitted for 10

seconds, 4.35 µs after the last framing pulse

14

d.

an identification pulse is automatically and continuously transmitted for 20

seconds, 4.35 µs after the last framing pulse

Questions

4.

When using SSR the ground controller will ask the pilot to cancel mode C if there is a

discrepancy of more than ............... between the altitude detected by the radar from

the reply pulses and the altitude reported by the pilot read from an altitude with the

subscale set to ...............

a.

100 ft

Regional QNH

b.

300 ft

1013 HPa

c.

50 ft

1013 HPa

d.

300 ft

Regional QNH

1

2

3

4

5

a

d

d

b

c