Secondary Radar Applications — ECE 5th Year

Learning Objectives

By the end of this topic, the student should be able to:

Explain the fundamental difference between primary and secondary radar systems.
Describe the operating principle of an airborne transponder and the ground interrogator.
Analyse the operation of Distance Measuring Equipment (DME) including pulse-pair timing, slant range computation and X/Y channel pairing.
Explain SSR Modes A, C and S, including squitter, selective interrogation and data-link capability.
Describe Identification Friend or Foe (IFF) and its military extensions (Modes 1, 2, 3/A, 4, 5).
Explain how secondary-radar techniques are used in satellite ground control: tracking, telemetry transponders, ranging and command (TTC).
Compute slant range from interrogation–reply time delay and apply the R = c·Δt / 2 relation.
Discuss system limitations: multipath, garbling, fruit, line-of-sight coverage and frequency congestion.

1. Theory: Primary vs Secondary Radar

1.1 The cooperative transponder principle

A primary radar detects targets by listening for echoes of its own transmitted pulse — the target is passive. A secondary radar relies on a cooperative target that carries a transponder: a receiver–transmitter that listens for a coded interrogation on one frequency and, if the code is recognised, replies on a different frequency with a coded reply. Because the reply is generated by an active transmitter on the target, secondary radar provides much greater range, unambiguous identity, altitude and data-link information from a small low-power transponder.

Primary radar

One frequency (monostatic echo).
Range from echo delay; no identity, no altitude.
Requires large target echo cross-section.
Clutter from weather, terrain, birds.

Secondary radar

Two frequencies: interrogation ↓, reply ↑.
Reply carries identity, altitude, data.
kW-class ground interrogator ↔ few-hundred-watt transponder gives ~200 NM coverage.
Targets without transponders are invisible → "cooperative surveillance".

1.2 Generic block diagram

Fig. 1 — Generic secondary-radar link: coded interrogation on 1030 MHz, coded reply on 1090 MHz (ATC bands; DME uses 962–1213 MHz, see §DME).

1.3 The range equation of secondary radar

R = ( c · Δt ) / 2
R = slant range [m or NM]; c = 3×10⁸ m/s (≈ 161875 NM/s ≈ 1 NM in 6.18 µs round trip); Δt = interrogation-to-reply delay measured at the interrogator [s]. The factor ½ accounts for the two-way path.

Why only "½ c·t"? The interrogation travels from ground → aircraft, and the reply travels back aircraft → ground along (essentially) the same path, so the measured delay corresponds to twice the one-way slant range.

1.4 Reply coding — pulse position

Replies are encoded as pulse pairs / pulse trains using pulse-position modulation. The reply frame is bracketed by two framing pulses F1 and F2, spaced 20.3 µs apart; between them, information pulses (e.g. A4 A2 A1 B4 B2 B1 C4 C2 C1 D4 D2 D1 for Mode A) are present or absent. Altitude (Mode C) uses a Gillham/Gray-code interpretation of the same slots. This fixed structure makes decoding robust against noise and allows the interrogator to measure range precisely.

Fig. 2 — Pulse-position reply frame (F1 … F2 framing pulses 20.3 µs apart; information slots between). Live decoder in the SSR section below.

Application 1 — Secondary Surveillance Radar in Air Traffic Control

SSR is the most widespread secondary-radar application. A rotating ground antenna interrogates all transponders in beam (Mode A: "who are you / what is your squawk?"), requests pressure altitude (Mode C), or addresses one aircraft individually with a 24-bit address (Mode S) to avoid garble and enable data link (ACARS/ADS-B services). Typical ATC bands: 1030 MHz interrogation / 1090 MHz reply, ~200 NM instrumented range, coverage up to FL600+.

1.1 Modes at a glance

Mode	Reply content	Use
A	4-octal identity ("squawk") code	ATC identification of flight (e.g., 1200 VFR)
C	Pressure altitude (100-ft increments)	Altitude reporting to ATC
S	24-bit address + data-link (Comm-B, EHS)	Selective interrogation, collision avoidance (TCAS), ADS-B out
1/2/3A, 4, 5 (military)	Security-coded replies	IFF (see Application 3)

1.2 Interactive Mode A / C reply decoder

Simulation 1 — Squawk / Altitude Encoder & Reply Frame

Choose a 4-digit octal squawk (Mode A) or set altitude (Mode C). The 12 µs-spacing pulse frame is drawn live; framing pulses F1/F2 are always transmitted.

Mode

Squawk code (octal, 0000–7777)

Decoded value

—

Active information pulses

—

Frame length

20.3 µs

1.3 Typical problems in SSR

Garbling — overlapping replies from two aircraft inside the same beam cell.
Fruit — replies elicited by other interrogators, appearing as asynchronous noise (mitigated by side-lobe suppression, ISLS/RSLS).
Multipath — reflections from buildings/terrain distort reply pulses.
Squitter overload — on 1090 MHz, capacity is shared with Mode S and ADS-B.

Application 2 — Distance Measuring Equipment (DME)

DME is the workhorse airborne ranging system of civil aviation (ICAO Annex 10). An airborne interrogator transmits pairs of pulses on a channel in the 962–1213 MHz band; the ground transponder (co-located with VOR or ILS, ~1 kW) replies on a paired channel 63 MHz away after a fixed delay. The aircraft measures the round-trip time and displays slant range in nautical miles. ~252 channels (X and Y pulse-spacing variants × 1 MHz spacing). Coverage ≈ 200 NM (high-power installations to 390 NM), accuracy ≈ ±0.2 NM or 1 % of distance.

Fig. 3 — DME geometry. Slant range R from pulse-pair timing; ground range found by correcting for altitude h.

2.1 Timing and channel plan

Pulse pairs: X-mode spacing 12 µs, Y-mode 36 µs between the two pulses of a pair (distinguishes channels and reduces interference).
Fixed transponder delay: 50 µs ± jitter, so the interrogator's clock starts from a known reference.
Reply efficiency: ground beacon limits replies (~1000/s, "squitter" fill-in) to protect receivers from overload.
Memory search/track: the interrogator searches the reply pulse train (≈ 150/s from one aircraft, randomised PRF), locks on, then tracks with a gate; display updates ~once per second.

R(NM) = [ ( t_measured − 50 µs ) / 2 ] · c , with c = 1 NM / 6.18 µs (round trip)
Example: measured delay 500 µs → R = (450 µs)/(12.36 µs/NM) ≈ 36.4 NM

Worked example (exam standard): A DME interrogator measures a total interrogation-to-reply time of 872 µs. Transponder fixed delay = 50 µs. Find slant range.
Δt_prop = 872 − 50 = 822 µs round trip → one-way = 411 µs. R = 3×10⁸ × 411×10⁻⁶ = 123.3 km ≈ 66.6 NM.

2.2 DME / VOR and DME / ILS pairing

DME is normally co-located with a VOR (forming a VOR/DME station giving ρ–θ fixing) or with an ILS localiser (DME/ILS giving distance-to-touchdown on approaches, e.g., "DME 4.2 NM"). Channel pairing tables (ICAO) map VOR frequencies to DME channels automatically.

Application 3 — Identification Friend or Foe (IFF)

IFF is the military ancestor and cousin of SSR: an interrogator (ground, shipborne or airborne) challenges an aircraft transponder; friendly transponders reply with a cryptographically protected code, unknown targets remain silent → "no reply = not positively identified". Civil SSR and military IFF share the 1030/1090 MHz ATCRS framework.

Mode	Reply content	Typical use
1	64-bit mission code (non-secure)	General situational awareness
2	Unique platform identity	Individual aircraft ID
3/A	Octal code — identical to civil Mode A	Joint civil/military ATC
4	Encrypted challenge–response	Positive friend identification (crypto)
5	Encrypted, GPS-position enhanced, Mode S-like data link	Modern NATO standard

Exam note: Mode S and IFF Mode 5 both use selective addressing; the key distinction is the cryptographic authentication and the ability to include position/velocity (Mode 5 level 2).

Fig. 4 — IFF principle: coded challenge, authenticated reply; silence ⇒ suspicion.

Application 4 — Secondary Radar in Satellite Ground Control

Satellite ground stations use exactly the same interrogation–transponder philosophy for Tracking, Telemetry and Command (TT&C):

Tracking / ranging: the ground station transmits a ranging tone or pseudo-random code; the satellite's coherent transponder turns it around (coherent two-way Doppler and range). Range rate gives orbit determination inputs.
Telemetry transponder: satellite health data (power, temperatures, tank pressures) downlinked on a dedicated downlink — a "reply" carrying housekeeping data, exactly like a Mode C reply carries altitude.
Command uplink: the ground interrogates with commands (analogous to a selective Mode S interrogation); the satellite executes and confirms.
Applications: TDRSS — NASA's Tracking and Data Relay Satellite System: user spacecraft interrogate relay satellites; the relay transponder re-transmits range/Doppler so ground control tracks low-Earth-orbit vehicles continuously. Deep-space networks (DSN) use ranging tones with very long integration times over 10⁷–10⁹ km.

Fig. 5 — TDRSS architecture: the relay satellite's transponder turns around ranging and data, extending secondary-radar tracking beyond the user's horizon.

Worked example: A GEO satellite transponder has a known turnaround delay of 1.2 ms. Measured ground two-way delay = 253.4 ms. Propagation delay = 252.2 ms → one-way 126.1 ms → range = 3×10⁸ × 126.1×10⁻³ = 37 830 km ≈ GEO altitude ✔ (compare 35 786 km + Earth-station geometry).

Application 5 — Other Secondary-Radar Applications

Application	Principle	Notes
Multilateration (MLAT / WAM)	Transponder replies (1090 MHz) time-stamped at ≥4 receivers; TDOA hyperbolics → position	Airport surface movement, wide-area MLAT replacing SSR in some states
ADS-B	Aircraft broadcasts position (GNSS-derived) on 1090ES or 978 UAT	Not radar, but rides the same 1090 MHz transponder infrastructure; complementary to SSR
TCAS (ACAS)	Airborne interrogator + Mode S/1090 replies → collision avoidance advisories	True airborne secondary radar; uses whisper-shout power control
Airport surface guidance (A-SMGCS)	MLAT + SSR Mode S on ground vehicles & aircraft	Safety on aprons and runways
Space debris / radar transponders	Corner-reflector transponders or coherent transponders on satellites/debris improve tracking	Used in ESA/NASA cataloguing campaigns
Search & rescue transponders (SART)	9 GHz X-band radar transponder on lifeboats replies to ship radars	GMDSS; classic maritime secondary radar

Interactive Simulations

Simulation 2 — DME Slant-Range Computer

Set aircraft distance and altitude. Watch the pulse pairs travel and read the measured round-trip delay, slant range and corrected ground range.

Distance along track 60 NM

Altitude 30000 ft

Speed (closing) 0 kt

Round-trip delay (incl. 50 µs)

—

Slant range

—

Ground range

—

Rate of change

—

Simulation 3 — Secondary Radar Range Equation Lab

Generic interrogator–transponder system: vary measured delay and transponder fixed delay. Verify R = c·Δt/2.

Measured delay 618 µs

Transponder fixed delay 50 µs

Propagation delay (2-way)

—

Range

—

Range

—

Simulation 4 — Why Secondary Radar Reaches Farther (one-way vs two-way link budget)

Compare the received power of a primary-radar echo (R⁻⁴ loss) with a transponder reply (R⁻² loss) at the same range, using P_r = P_tG_tG_rλ²/(4π)²dⁿ.

Range 100 NM

Primary echo path loss ∝ R⁴

—

Transponder reply path loss ∝ R²

—

Relative advantage

—

Summary

Secondary radar replaces echo detection with a cooperative transponder reply — giving identity, altitude, data, and a one-way (R⁻²) link budget advantage over primary radar.
Core applications: SSR (ATC) with Modes A/C/S; DME with 962–1213 MHz channels and 50 µs fixed delay; IFF with crypto-protected challenge–response; satellite ground control (TT&C, TDRSS) using coherent turnaround transponders; plus MLAT, TCAS, ADS-B and SART.
The universal relation is R = c·Δt/2 (subtract any fixed transponder delay first).
Limitations: line of sight, garble/fruit, 1090 MHz congestion, and dependence on target cooperation.

Review Questions

Q1. State two fundamental advantages of secondary radar over primary radar.

(i) The reply is actively transmitted → much greater range from a small transponder; (ii) the coded reply carries identity/altitude/data; (iii) less clutter (no terrain/weather echo dependence).

Q2. A DME transponder's fixed delay is 50 µs. The interrogator measures 1.545 ms. Compute the slant range in NM.

Δt_prop = 1545 − 50 = 1495 µs → R = 1495/12.36 ≈ 121 NM.

Q3. Why does Mode S reduce garbling compared with Mode A?

Mode S interrogates one 24-bit-addressed aircraft at a time (selective interrogation), so replies from other aircraft are not elicited — eliminating all-reply overlap (garble).

Q4. Explain how a satellite coherent transponder supports orbit determination.

The transponder phase-coherently turns around the uplink ranging tone: two-way Doppler gives range-rate, and tone/code delay gives range; a sequence of range/range-rate measurements over a pass fits the orbital elements.

Q5. Distinguish IFF Mode 3/A from Mode 5.

Mode 3/A is the non-secure octal code shared with civil SSR Mode A; Mode 5 is cryptographically authenticated, selective-addressed, and can append position/velocity (Mode-S-like data link).