14
RADIO NAV — CHAPTER 14 · NOTES
SSR
by Ghost Aviator
LECTURE PARTS
Ghost Aviator
www.ghostaviator.com
Ghost Aviator
www.ghostaviator.com
Ghost Aviator
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.
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.
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.
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).
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.
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)
Ghost Aviator | www.ghostaviator.com
For personal study use only.
Chapter 14 — SSR (Secondary Surveillance Radar)
Ghost Aviator | www.ghostaviator.com
For personal study use only.
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