Egerton University — Department of Electrical & Electronic Engineering

Study Guide: Secondary Radar

Course EEEN 567 – Satellite Engineering • 4th Year BSc. Electrical & Electronic Engineering

Topic 7 • Cooperative Transponder Systems in Aviation, Satellites & Spacecraft

Learning Objectives

By the end of this study guide, the learner should be able to:

  1. Distinguish primary radar (echo reflection) from secondary radar (interrogation → transponder reply) and state the advantages of the cooperative approach.
  2. Describe the ground interrogator and airborne transponder subsystems of an ATC Secondary Surveillance Radar (SSR).
  3. Explain ATCRBS interrogation pulse pairs (P1–P3 spacing), the Mode A / Mode C / Mode S reply formats and the 4096 code structure.
  4. Derive the secondary radar range equation and show that the returned signal falls as 1/R2 instead of the primary-radar 1/R4.
  5. Apply secondary radar in aviation: ATC surveillance, TCAS, multilateration and ADS-B context.
  6. Apply secondary-radar (transponder) techniques to satellites and spacecraft: TDRSS, DSN coherent ranging transponders, radar transponders/calibration targets and Cospas-Sarsat distress beacons.
  7. Use the interactive animations and simulations to verify SSR range, code and link-budget calculations.

1. Principle: Primary vs. Secondary Radar

A primary radar depends on the reflection of its own transmitted energy from a target — the aircraft contributes nothing. A secondary radar is cooperative: the ground station (the interrogator) transmits a coded interrogation, and a receiver-transmitter device on the target (the transponder, from "transmitter-responder") replies on a different frequency with a coded, higher-power signal.

Components of secondary radar
Fig. 1 — Components of secondary radar: the interrogator/reply link (left) works alongside the primary echo radar (right), sharing a synchroniser and PPI display.

1.1 Why Secondary Radar Wins

1.2 Interrogator & Transponder Hardware

SSR interrogator and transponder block diagram
Fig. 2 — SSR block diagram: interrogator (synchroniser, encoder, transmitter, receiver, decoder, display) and transponder (receiver, decoder, encoder, transmitter).
ATC ground station SSR and PSR antennas
Fig. 3 — ATC ground station: SSR antenna mounted above the primary surveillance radar (PSR) antenna, with a slotted waveguide for interrogation/reply and a large vertical aperture to control side lobes.
Side-lobe suppression (SLS): the SSR antenna's control (SLS) pattern transmits P2 so that an aircraft receiving P2 stronger than P1 knows it is being interrogated through a side lobe and stays silent — preventing false azimuth replies.

2. ATC Secondary Surveillance Radar (SSR)

The air-traffic-control SSR system is the classic secondary radar. The ground interrogator transmits on 1030 MHz; aircraft transponders reply on 1090 MHz. An interrogation consists of two pulses, P1 and P3; their spacing encodes the mode:

P1–P3 spacingModeReply requested
8 µsMode AIdentity (4-octal-digit squawk code, 4096 codes)
21 µsMode CPressure altitude (100 ft increments, Gillham/Gray-code)
Mode S formatMode S24-bit aircraft address, datalink (56/112-bit payloads)
Mode A and Mode C interrogation pulse pairs
Fig. 4 — ATCRBS interrogations: P1–P3 spacing of 8 µs (Mode A) or 21 µs (Mode C). The P2 SLS pulse (2 µs after P1) is omitted here.
Mode A and Mode C interrogation timing
Fig. 5 — Mode A vs Mode C interrogation timing (8 µs vs 21 µs P1–P3 spacing).

2.1 The Reply Pulse Train

A Mode A/C reply is a framed pulse train: framing pulses F1 and F2 (20.3 µs apart) with up to 13 information pulses between them (C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4). Each present pulse = logic 1; groups of three pulses encode one octal digit (A, B, C, D). Total reply length 20.75 µs ≈ 3.35 NM of slant range — which sets the SSR dead-time after each interrogation.

Mode A C reply pulse positions
Fig. 6 — Reply pulse positions: F1/F2 framing pulses and the information pulse slots with their timings.
Mode A C reply framing
Fig. 7 — Reply framing: F1…F2 span 20.75 µs; each octal digit uses a 1-2-4 pulse-weight group (e.g. A1 A2 A4 for digit A).

2.2 Mode S and Modern Surveillance

Mode S (Select) gives each aircraft a unique 24-bit address (16.7 million codes), eliminates the all-call garble of ATCRBS, and provides a 1 Mbit/s data link. It underpins TCAS collision avoidance (aircraft interrogate each other), multilateration (time-difference positioning from 1090 MHz replies at several receivers) and complements ADS-B, where aircraft broadcast position automatically.

3. The Secondary Radar Range Equation

Step 1 — Power density at the transponder: Wₕ = PₘGₘ/(4πR²)
Step 2 — Interrogation power captured: Pₘₕ = Wₕ·Aₕₕ where Aₕₕ is the transponder antenna effective aperture.
Step 3 — Reply re-radiation: the transponder transmits with its own power Pₚ and antenna gain Gₚ (its ERP is fixed, independent of R).
Step 4 — Reply received at the interrogator:
Pₘₖ = PₚGₚGₖλₖ² / [(4π)²R²L] One-way (Friis) equation for the reply path: the reply power at the ground receiver falls as 1/R².

Combining both legs, the interrogator must simply receive enough reply power above its noise floor. Because each leg is one-way, the overall dependence on target range is 1/R² in the cooperative link budget — vastly more favourable than the 1/R⁴ of primary radar.

Rₘₗₓ = [ PₚGₚGₖλₖ² / ((4π)²kT₀BF(S/N)₁L) ]¹⁑₂
Design insight: since the transponder ERP is fixed and known, range in SSR is limited by the minimum trigger level (MTL) of the interrogation at the aircraft and by the receiver noise floor of the ground reply — not by the target's RCS. This is why small aircraft are detected at the same range as large ones.

4. Secondary Radar in Satellites and Spacecraft

The interrogator/transponder philosophy extends naturally to space: the "transponder" is the spacecraft's coherent radio, and the "interrogator" is a ground station or another spacecraft. Spacecraft applications include:

4.1 Tracking & Data Relay Satellite System (TDRSS)

NASA's TDRS satellites act as orbiting relay transponders: a user spacecraft (Shuttle, ISS, HST, launch vehicles) transmits to a geostationary TDRS, which relays the signal to the White Sands ground station. This gives near-continuous coverage of LEO spacecraft that could otherwise be seen by a ground station for only a few minutes per orbit. Ranging is done by two-way coherent transponder measurements through the relay.

TDRS constellation
Fig. 8 — The TDRSS constellation: geostationary relay satellites over the Indian, Pacific and Atlantic regions.
Shuttle TDRS communications and tracking
Fig. 9 — Shuttle–TDRS tracking and communications: the orbiter's Ku-band radar/comm system relays via TDRS to the ground station (NASA SP-504).

4.2 Deep-Space Coherent Ranging Transponders

Deep-space probes (e.g., via NASA's Deep Space Network) carry coherent transponders: they receive an uplink carrier, multiply its frequency by a rational ratio (e.g., 880/749 for S-band uplink to X-band downlink), and retransmit. Ground stations measure the round-trip delay (range) and Doppler shift (range-rate) of the coherent downlink — the spaceborne equivalent of SSR ranging, achieving metre-class range and mm/s velocity accuracy over billions of km.

4.3 Radar Transponders and Calibration Targets

Active radar transponders (corner-reflector beacons) are placed on satellites, buoys and ground sites: when illuminated by a radar (SAR, altimeter, tracking radar) they return a strong coded echo, serving as calibration targets and fiducial markers for image geolocation and radar alignment — the space analogue of the SSR reply.

4.4 Distress Beacons — Cospas-Sarsat

Emergency locator transmitters (ELTs), EPIRBs and PLBs are essentially space-interrogated transponders in reverse: they transmit a distress signal on 406 MHz that LEO and GEO satellites relay to ground stations, which compute the beacon's position by Doppler (LEO) and/or the encoded GPS position in the message — a secondary-radar service that has saved tens of thousands of lives.

System"Interrogator""Transponder"Service provided
ATC SSR (ATCRBS/Mode S)Ground interrogator (1030 MHz)Aircraft transponder (1090 MHz)Identity, altitude, datalink
TCASOwn aircraftOther aircraft (Mode S)Collision avoidance
TDRSSUser spacecraftGEO relay satelliteTracking, data relay, ranging
DSN coherent rangingGround station uplinkDeep-space probe transponderRange, range-rate, telemetry
Radar calibration transpondersSAR / tracking radarActive beacon on satellite/buoyCalibration, geolocation
Cospas-Sarsat(Beacon-initiated)ELT/EPIRB/PLB → satellitesDistress alerting & location

5. Animations

Animation 5.1 — SSR Interrogation–Reply Cycle and Slant Range

Watch the P1–P3 interrogation travel to the aircraft, the transponder reply after a fixed turnaround delay, and the slant-range computation from the total elapsed time.

Reply carries squawk 2401 / FL 310

Animation 5.2 — Mode A/C Reply Pulse Train

Set a 4-digit octal squawk code (or a Mode C altitude) and watch the transponder assemble the framed reply between F1 and F2.

6. Interactive Simulations

Simulation 6.1 — SSR Slant-Range Calculator

From the measured delay between interrogation transmission and reply reception, compute slant range — remembering the transponder turnaround delay.

Press Calculate.

Simulation 6.2 — Secondary vs. Primary Radar Link Budget

Compare the reply power received from a cooperative transponder (1/R²) with the echo from a passive target of equal RCS in primary radar (1/R⁴). Both use the same ground antenna and frequency.

Press Compare.

Received Power vs. Range

7. Self-Check Questions

Q1. State the fundamental difference between primary and secondary radar.
Answer: Primary radar relies on passive reflection of its own pulses from the target (1/R⁴). Secondary radar is cooperative: an interrogator sends a coded interrogation and an active transponder on the target replies on a different frequency with higher, known power (1/R²).
Q2. An SSR interrogation shows P1–P3 spacing of 21 µs. What information will the reply contain?
Answer: 21 µs = Mode C — the reply encodes the aircraft's pressure altitude in 100 ft increments (Gillham/Gray code). (8 µs would be Mode A identity.)
Q3. Why does secondary radar roughly double the coverage of primary radar for the same powers?
Answer: The interrogation and reply are two independent one-way links. The transponder transmits a fixed ERP regardless of range, so received reply power falls as 1/R² rather than 1/R⁴ — doubling range requires 4× the link margin instead of 16×.
Q4. An interrogation–reply total delay of 405.3 µs is measured, with a 3 µs transponder turnaround. Find the slant range.
Answer: Two-way propagation delay = 405.3 − 3 = 402.3 µs. R = c·t/2 = 3×10⁹ × 402.3×10⁻⁶ / 2 = 60.35 km.
Q5. What is the purpose of the P2 side-lobe-suppression pulse?
Answer: P2 is radiated in the interrogator's control (Omni/SLS) pattern. If an aircraft receives P2 at or above the strength of P1, it knows it lies in an interrogator side lobe and must NOT reply — suppressing false azimuth targets.
Q6. Explain how Mode S reduces garble compared with ATCRBS.
Answer: ATCRBS replies are anonymous 4096-code pulse trains — all aircraft in the beam reply together, overlapping (garble). Mode S assigns each aircraft a unique 24-bit address and uses selective (point-to-point) interrogations and scheduled replies, so only the addressed aircraft answers.
Q7. Give two satellite/spacecraft applications of the transponder principle and name each system's "interrogator" and "transponder".
Answer: (1) TDRSS: user spacecraft = interrogator, GEO TDRS = relay transponder (tracking + data relay). (2) DSN coherent ranging: ground station = interrogator, deep-space probe transponder = cooperative beacon giving range/range-rate. (Radar calibration transponders and Cospas-Sarsat beacons are also valid.)
Q8. A deep-space transponder receives a 2.1 GHz uplink and retransmits coherently at 8.4 GHz. Why must the frequencies be related by an exact rational ratio?
Answer: Coherence: the downlink phase must bear a fixed relationship to the uplink so that ground Doppler and delay measurements on the downlink reflect the true spacecraft range/range-rate (here ratio 4, typical DSN ratios like 880/749). An independent oscillator would add unknown phase noise and destroy measurement accuracy.

8. References and Further Reading

  1. M. I. Skolnik, Introduction to Radar Systems, 3rd ed., McGraw-Hill, 2001 (Ch. 8: Radar Beacons / Secondary Radar).
  2. ICAO Annex 10, Aeronautical Telecommunications, Vol. IV (Surveillance Radar and Collision Avoidance Systems).
  3. U.S. Navy Aviation Training, Components of the Secondary Radar, tpub.com 14342A, Fig. 5-21.
  4. NASA, SP-504: Communications and Tracking (Shuttle–TDRS), klabs.org.
  5. Wikipedia, “Tracking and Data Relay Satellite” and “Secondary Surveillance Radar”.
  6. AeroElectric Connection, “Mode A and Mode C Codes,” aeroelectric.com.
Study tip: This topic ties together the radar range equation (1/R⁴) and the Friis one-way link (1/R²) from earlier guides. Next, attempt the post-test quiz on secondary radar, then explore MTI and pulse-Doppler topics.