DGCA CPL / ATPL Study Notes • Radio Navigation • Ch 14

✈ Chapter 14: Secondary Surveillance Radar
SSR — Mode A, Mode C & Mode S

📋 Contents

1. Introduction & Principle 2. Ground Equipment 3. Airborne Transponder 4. Interrogation Modes 5. Transponder Codes 6. SSR Problems — Garbling & Fruiting 7. Mode S 8. SSR Summary 9. Practice Questions (5 Q)

1. Introduction & Principle

Secondary Surveillance Radar (SSR) is a secondary radar system: instead of relying on echoes, the ground station interrogates airborne transponders which transmit their own coded replies. This gives much more information than primary radar.

📡 SSR Advantages Over Primary Radar

Identity (callsign equivalent code)
Altitude (Mode C, no need for R/T)
Strong, reliable signal (transponder active transmitter, not a passive reflector)
Works regardless of aircraft size or material (no 4th power law limitation)
Not affected by weather, ground clutter or sea return
Mode S: Data link — downlinked aircraft data; uplinked ATC messages

Fig 14.1 & 14.2: SSR antenna (rotates with primary head) and overall principle of operation

2. Ground Equipment

📡 SSR Ground Station

Parameter	Detail
Transmit frequency	1030 MHz (UHF)
Receive frequency	1090 MHz (UHF)
Antenna type	Mounted on top of primary radar antenna; co-rotates
Antenna beam	Narrow in azimuth (1–3°)
SLS (Side Lobe Suppression)	Omnidirectional pulse P2 transmitted simultaneously with P1; if transponder sees P2 > P1, it suppresses reply → prevents side-lobe interrogations
Processor	Decodes replies, displays on PPI alongside primary return

3. Airborne Transponder

Fig 14.3: Transponder control panel — STBY, ON, ALT, IDENT positions, mode selector, code display

📡 Transponder Operation

Parameter	Detail
Receive frequency	1030 MHz
Transmit frequency	1090 MHz
Reply delay	50 µs (transponder receives and delays before replying, to allow distance measurement)
Antenna	Omni-directional (top and bottom fuselage)
STANDBY	System active, will NOT respond to interrogations (used on ground when not required)
ON	Mode A (identity) only
ALT	Mode C (altitude encoding) + Mode A
IDENT / SPI	Special Position Identification pulse — flashes radar blip for 20 seconds

4. Interrogation Modes

Fig 14.4: Pulse spacings for Mode A (8 µs) and Mode C (21 µs) | Fig 14.5: Reply pulse train (F1...SPI...F2)

📡 Mode Comparison

Mode	P1–P3 Spacing	Reply Contains	Info Provided
A (Identity)	8 µs	4096 identity codes	Aircraft identity (4-digit squawk)
B	17 µs	Identity codes	Aircraft identity (alternate)
C (Altitude)	21 µs	Altitude in 100 ft increments	Pressure altitude (re: 1013.25 hPa)
D	25 µs	Future use	—

⚡ Key Mode Details

Mode A provides 4096 identity codes (0000–7777 octal, with 1200, 7000, 7700 reserved/restricted)
Mode C altitude: encoded in 100 ft increments using Gillham code; referenced to 1013.25 hPa regardless of QNH set
Mode C displayed separately on PPI — ATC can read altitude without R/T
Transponder replies with framing pulses F1, F2 spaced 20.3 µs, with information pulses between

⚠ Required Actions

When ATC say “SQUAWK IDENT”: press IDENT button → SPI for 20 seconds
HIJACK: squawk 7500
Loss of R/T: squawk 7600
Emergency: squawk 7700
Military conspicuity: squawk 7000
VFR conspicuity (no ATC unit): squawk 7000

5. Transponder Codes

Fig 14.6: Reply codes — 4096 combinations of 4 digits 0-7 in Mode A

📡 Code Summary

Code	Meaning
7700	Emergency
7600	Radio failure
7500	Unlawful interference (hijack)
7000	Military conspicuity / VFR conspicuity (UK)
2000	Entering controlled airspace without ATC clearance
Assigned	Four-digit code assigned by ATC

6. SSR Problems

Fig 14.7: Garbling (two aircraft within 1.7 NM cause overlapping replies) and Fruiting (random interrogations cause interference)

⚠ Garbling

Two aircraft within 1.7 NM of each other appear to be in the same direction from the ground station
Their replies overlap → garbled, unreadable replies
Solution: Mode S (selective addressing)

⚠ Fruiting (Phantoms)

Multiple ground interrogators on same frequency
Transponder receives interrogation from station B but reply is picked up by station A
Station A sees a phantom target at incorrect position
Solution: ISLS (Improved Side Lobe Suppression) — synchronised interrogation timing between stations

7. Mode S

✓ Mode S Key Features

Feature	Detail
Addressing	Each aircraft has a unique 24-bit address (16.7 million combinations)
Selective interrogation	Ground station interrogates ONLY the target aircraft → eliminates garbling
Data link	Bidirectional: aircraft can downlink data; ATC can uplink instructions
Altitude resolution	25 ft (vs 100 ft for Mode C)
ACAS / TCAS	Uses Mode S for aircraft-to-aircraft interrogation to provide collision avoidance
ELS	Elementary Surveillance — squawk, flight ID, altitude, emergency
EHS	Enhanced Surveillance — adds selected altitude, airspeed, heading (for upper airspace in Europe)
ADS-B	Automatic Dependent Surveillance Broadcast — squitter every 0.5 s; position via GPS

8. SSR Summary

Parameter	Detail
Type	Secondary radar; interrogation–reply
Ground Tx	1030 MHz (UHF)
Transponder Tx	1090 MHz (UHF)
Transponder delay	50 µs
Mode A spacing	8 µs → 4096 codes → identity
Mode C spacing	21 µs → pressure altitude re 1013.25 hPa → 100 ft increments
SPI	IDENT pressed → 20 s enhanced blip
Garbling	<1.7 NM separation → overlapping replies
Fruiting	Phantom returns from multiple interrogators
Mode S address	24 bits → 16.7 million unique addresses
Mode S altitude	25 ft resolution

9. Practice Questions

Q1. SSR operates on frequencies:

(a) 1030 MHz ground Tx, 1090 MHz airborne Tx

(b) 1090 MHz ground Tx, 1030 MHz airborne Tx

(c) 9375 MHz both

(d) 960 MHz both

Answer: (a)
Ground transmits at 1030 MHz, airborne transponder replies at 1090 MHz.

Q2. Mode C provides ATC with:

(a) aircraft callsign

(b) airspeed

(c) position

(d) pressure altitude

Answer: (d)
Mode C (P1–P3 spacing = 21 µs) provides pressure altitude referenced to 1013.25 hPa in 100 ft increments.

Q3. Advantage of Mode S over Mode A/C:

(a) no transponder required

(b) not dependent on pressure altimeter

(c) eliminates garbling by selective addressing

(d) uses VHF frequencies

Answer: (d)
Mode S eliminates garbling through selective addressing — each aircraft has a unique 24-bit ICAO address; ground station interrogates only one aircraft at a time.

Q4. A transponder code of 7700 indicates:

(a) radio failure

(b) emergency

(c) unlawful interference

(d) conspicuity

Answer: (b)
7700 = Emergency. 7600 = radio failure. 7500 = hijack (unlawful interference).

Q5. The unique identifier used in Mode S is:

(a) a 4-digit octal squawk

(b) the Mode C altitude code

(c) a 24-bit ICAO aircraft address

(d) a 12-bit reply pulse train

Answer: (c)
Mode S uses a 24-bit ICAO address, providing up to 16.7 million unique aircraft identifiers, enabling selective interrogation and data link capability.

DGCA CPL/ATPL Radio Navigation Study Notes
Chapter 14 — SSR (Secondary Surveillance Radar)
Capt Pankaj Pahil | www.ghostaviator.com
For personal study use only.

✈ Chapter 14: Secondary Surveillance RadarSSR — Mode A, Mode C & Mode S