5th Year Undergraduate — BSc. Electrical & Communication Engineering
Masinde Muliro University of Science and Technology (MMUST)
Course Overview
Purpose of this Study Guide
This guide introduces fifth-year Electrical & Communication Engineering students at MMUST to the
applications of secondary radar systems. Having studied the fundamentals of radar principles,
primary radar, and radar equation analysis in earlier years, this unit focuses on where and how
secondary radar is deployed in modern aviation, space, and defence systems.
Learning Objectives
By the end of this unit, the student should be able to:
Explain the operating principle of secondary surveillance radar (SSR) and contrast it with primary radar.
Describe the operation, frequency plan, and accuracy of Distance Measuring Equipment (DME).
Explain the role of SSR transponders in Air Traffic Control (ATC), including Modes A, C, and S.
Describe Traffic Collision Avoidance System (TCAS) operation and its dependence on transponder replies.
Analyse the use of secondary radar in satellite ground control (TT&C ranging and tracking).
Discuss military Identification Friend or Foe (IFF) systems and their civil SSR heritage.
Evaluate emerging applications: multilateration (MLAT/WAM), ADS-B, drone detection, and space situational awareness.
Prerequisites
Radar fundamentals (primary vs. secondary radar, range equation, Doppler principle)
Modulation and coding techniques (pulse coding, PPM, FSK)
Radio wave propagation and antenna systems
Communication systems I & II
Quick Recap: What is Secondary Radar?
Unlike a primary radar, which detects targets by listening for echoes of its own transmitted
pulse, a secondary radar works cooperatively: an interrogator on the ground (or in another
aircraft/satellite) transmits a coded interrogation, and a transponder aboard the target actively replies
with coded information.
One-way power budget only (TX → target → RX as echo)
Range ∝ echo delay; needs huge power for small targets
Cannot know target identity or altitude directly
Vulnerable to weather clutter
Secondary Radar
Two-way, active link: interrogator + transponder
Reply is a coded pulse train carrying data
Provides identity (Mode A squawk), altitude (Mode C), data (Mode S)
Lower TX power needed; no return echo required
Key point: Because the transponder replies with a coded message
rather than a passive echo, secondary radar turns a radar into a data link — and this property
underlies every application studied in this unit.
Application 1: Distance Measuring Equipment (DME)
DME is the classic precision secondary-radar-based navigation aid. It gives an aircraft its
slant range (line-of-sight distance, not ground distance) to a ground beacon.
1.1 Principle of Operation
The aircraft's DME interrogator transmits pairs of pulses (interrogation) on one of 126 channels in the
1025–1150 MHz band.
The ground DME transponder (co-located typically with a VOR station) receives the interrogation and,
after a fixed reply delay of 50 μs, transmits reply pulse pairs on a channel 63 MHz away.
The airborne interrogator measures the round-trip time t and computes:
Slant range R = (c · t) / 2, where c = 3×108 m/s
The system counts replies; when ~100 valid replies/s are received, the DME enters tracking mode and displays distance.
1.2 Frequency Plan & Channeling
Parameter
Value
Interrogation band (aircraft → ground)
1025 – 1150 MHz
Reply band (ground → aircraft)
962 – 1213 MHz
Channel spacing
1 MHz
Number of channels
126 (X and Y modes)
Pulse pair spacing
12 μs (X mode), 30/36 μs (Y mode)
Fixed transponder reply delay
50 μs
Typical accuracy
±0.2 NM (±370 m)
Usable range
Up to ~200 NM depending on installation
1.3 Why DME is a Secondary Radar Application
DME exhibits the defining secondary-radar features: an active transponder on the target/ground side,
an interrogator–reply handshake, and pulse-coded channels. The distance is extracted from
the coded reply's timing, not from an echo.
1.4 Applications of DME
VOR/DME and VORTAC navigation: bearing (VOR) + range (DME) gives a 2-D position fix.
DME/DME area navigation (RNAV): position computed from ranges to two or more DME stations — a backup to GNSS.
Approach procedures: DME arcs and step-down fixes on non-precision approaches.
Ground speed & time-to-station estimation from range rate.
Worked example: A DME interrogator measures a round-trip delay of
617 μs. Find the slant range.
R = c·t/2 = (3×108 × 617×10−6)/2 = 92,550 m ≈ 92.6 km ≈ 50 NM.
Exam focus: Be able to derive range from measured delay, explain the 50 μs
reply delay purpose (allows time for transponder processing and separates reply from interrogation), and describe
DME/DME RNAV as a GNSS backup.
Application 2: Air Traffic Control Transponder (SSR)
The most widespread secondary radar application worldwide is the ATC transponder, interrogated by
ground-based Secondary Surveillance Radar (SSR) and, increasingly, by multilateration and
satellite receivers.
2.1 Basic Operation
Ground SSR transmits an interrogation on 1030 MHz.
Aircraft transponder replies on 1090 MHz with a coded pulse train (frame pulses F1, F2 plus data pulses).
The ground station decodes identity/altitude and combines it with the antenna pointing angle to locate the aircraft.
24-bit unique ICAO address; selective interrogation; data link (ACARS-like)
Modern ATC, TCAS, ADS-B infrastructure
Special Mode A codes to remember: 7500 hijack, 7600 radio failure,
7700 emergency.
2.3 Role in Modern ATM
Radar screen labels show callsign/identity + altitude (Mode A/C) or full data block (Mode S).
Feeds automated air traffic management systems (e.g., flight data processing, minimum safe altitude warnings).
Enables sector handover by automatic code assignment.
Engineering note: Because replies on 1090 MHz are line-of-sight and
transponder power is limited (~70 W), SSR coverage extends slightly beyond the radio horizon of the interrogator.
This is why Mode S extended squitter (ADS-B) can be received by satellites — see the satellite section.
Application 3: Traffic Collision Avoidance System (TCAS)
TCAS is an airborne secondary radar application: each TCAS-equipped aircraft interrogates nearby
transponders and processes their replies to compute collision threats — no ground infrastructure required.
3.1 Operating Steps
TCAS interrogates nearby aircraft (Mode S / Mode C) on 1030 MHz.
Target transponders reply on 1090 MHz; TCAS measures range (reply delay) and altitude (Mode C reply).
Multiple interrogations over several seconds give range rate; directional antennas give bearing (on some implementations).
Threat logic (tau = time to closest approach) classifies intruders: Other → Proximate → Traffic Advisory (TA) → Resolution Advisory (RA).
For RAs, TCAS coordinates a complementary manoeuvre via Mode S data link between the two aircraft.
3.2 TCAS Versions
Version
Capability
TCAS I
Traffic Advisories only (no RAs) — used in general aviation
TCAS II
TAs + RAs (vertical manoeuvres); coordinated via Mode S
Key insight: TCAS proves secondary radar is not only a ground-to-air tool —
it works air-to-air. The same 1030/1090 MHz ecosystem supports SSR, TCAS, MLAT, and ADS-B.
Application 4: Mode S, ADS-B and Multilateration
4.1 Mode S and Extended Squitter (ADS-B)
Mode S transponders periodically broadcast their position (from GNSS), velocity, identity, and intent on
1090 MHz without being interrogated — this is ADS-B Out (1090ES). Ground stations and other
aircraft (ADS-B In) receive these broadcasts, enabling surveillance without a rotating radar.
The time difference of arrival (TDOA) at ≥4 synchronized stations gives a 3-D position fix.
Deployed at airports (surface surveillance) and in mountainous regions where radar coverage is poor.
Why this matters to a communications engineer: MLAT is essentially a
hyperbolic positioning system — the same mathematics as GPS, but using SSR reply timing. Precision time
synchronization (GPS-disciplined clocks, ns accuracy) is the critical engineering challenge.
Application 5: Satellite Ground Control (TT&C)
Satellite ground stations continuously monitor and control spacecraft through the Tracking,
Telemetry & Command (TT&C) function — and the "tracking" half of TT&C is fundamentally a
secondary radar/ranging process.
5.1 How Secondary Radar Ranging Supports Ground Control
5.1.1 Transponder-Based Ranging
The ground station transmits a ranging signal (tone, pseudorandom code, or wideband signal) to the satellite.
The satellite's coherent transponder translates and retransmits it back (bent-pipe or regenerative).
Measuring the round-trip delay yields the range:
R = (c · ΔtRT) / 2, and range-rate from Doppler shift of the returned carrier
Accurate to metres (code ranging) or even centimetre-level with tones/carrier phase. Together with angle
tracking (monopulse on the uplink/downlink), this provides the full orbit state vector.
5.1.2 Coherent Transponders and Two-Way Doppler
The satellite transponder locks to the uplink carrier and retransmits a phase-coherent downlink.
Two-way Doppler shift measurement gives line-of-sight velocity — the primary input to orbit determination.
Used for: orbit injection verification, station-keeping, anomaly recovery (finding a "lost" satellite), and re-entry prediction.
5.1.3 Deep Space and Specific Implementations
DSN (NASA Deep Space Network): ranging tones and PN codes through spacecraft transponders; precision orbit determination across the solar system.
ESTRACK (ESA): similar coherent transponder ranging.
GNSS satellites: ground antennas uplink ranging/telemetry and receive satellite-borne transponder replies for orbit control (e.g., GPS ground control segment at Schriever AFB).
Low Earth Orbit (LEO) constellations: ground gateways with ranging transponders maintain large fleets of satellites.
5.2 Why Secondary Radar Rather Than Primary?
Requirement for Ground Control
Secondary Radar Solution
Measure distance to a small, distant, passive-looking object
Active transponder replies with power independent of target size
Establish communication link (commands, telemetry)
Same transponder carries command uplink and telemetry downlink
Very long ranges (GEO: 36,000 km; deep space: 109 km)
Echoes would be impossibly weak; transponder reply is detectable
Verification of satellite identity and health
Coded replies authenticate and carry status data
Worked example (GEO ranging): Round-trip time to a geostationary satellite
(slant range ≈ 36,000 km) is t = 2R/c = 2×3.6×107/3×108 = 0.24 s — the delay you
notice in live satellite interviews!
5.3 Space Situational Awareness (Emerging)
Ground-based radars with transponder cooperation (and passive listening to 1090 MHz ADS-B from aircraft in
satellited coverage) increasingly contribute to tracking low Earth orbit objects — a growing secondary-radar application
for satellite safety and conjunction avoidance.
Exam focus: Explain the two-way ranging equation for spacecraft tracking, the role
of coherent transponders in Doppler-based orbit determination, and why TT&C cannot practically rely on echo-based
(primary) radar at deep-space distances.
Application 6: Military Identification Friend or Foe (IFF)
IFF is the original secondary radar application (World War II) and the ancestor of civil SSR.
A military interrogator challenges an unknown aircraft; its transponder replies with a secure, encrypted code,
identifying it as friendly.
Mode 5: modern, GPS-position-enhanced, encrypted replies with higher security ( NATO standard, with "lethal interrogation" features for weapon systems).
Ground, shipborne, and airborne interrogators; replies on 1090 MHz heritage frequencies, preserving civil–military compatibility.
6.2 Applications
Air defence: prevent friendly fire; engage only confirmed hostile tracks.
Naval IFF: shipboard interrogators screen air contacts near a fleet.
Battlefield airspace deconfliction with civil ATC Mode S interoperability.
Interoperability principle: Civil SSR (Modes A/C/S) and military IFF share the
1030/1090 MHz framework so that military aircraft remain visible to civil ATC while carrying secure identification —
a classic systems-engineering compatibility challenge.
Application 7: Other Secondary Radar Applications
7.1 Airfield Surface Movement Guidance
Transponder-based surveillance (Mode S multilateration) tracks aircraft and vehicles on airport aprons and
runways in low visibility — e.g., A-SMGCS systems.
7.2 Search & Rescue (SARSAT / COSPAS)
Emergency locator transmitters (ELTs/PLBs) aboard aircraft and vessels reply to satellite interrogation
(406 MHz beacons with 121.5 MHz homing) — a distress transponder application of the interrogation–reply principle.
7.3 ATC Radar Beacon System (ATCRBS) for Drones
Remote ID and drone tracking systems use transponder/ADS-B-like replies so ATC can detect UAVs in controlled
airspace — an active regulatory area (e.g., RID standards).
Cooperative transponders on upper stages/debris objects assist ground radar networks in maintaining catalogues
for conjunction warnings.
7.6 Military Data Links (Link-16)
Tactical data links use transponder-style time-slot networks — conceptually interrogation/reply expanded into
a full network (TDMA), carrying position and tactical data.
Comparative Summary of Applications
Application
Platform
Frequencies
Information Extracted
Primary User
DME
Air ↔ Ground
~1000–1200 MHz
Slant range
Civil aviation
SSR / ATCRBS
Ground ↔ Air
1030 / 1090 MHz
Identity, altitude
ATC
TCAS
Air ↔ Air
1030 / 1090 MHz
Range, altitude, threat
Aircraft crews
ADS-B / MLAT
Air → Ground / Air
1090 MHz (ES), UAT 978 MHz (US)
Position, velocity, intent
ATM modernization
Satellite TT&C ranging
Ground ↔ Space
S/L/C-band, deep-space bands
Range, Doppler (orbit), telemetry, commands
Satellite operators
IFF (Mode 4/5)
Air/Ground/Sea ↔ Air
1030 / 1090 MHz heritage
Encrypted identity
Military
SARSAT distress
Beacon ↔ Satellite
406 MHz (+121.5 MHz)
Distress ID, location
Search & rescue
Summary of Key Points
Secondary radar replaces echo detection with an interrogation–reply data link, enabling identification, altitude, data, and precise range.
DME measures slant range using the round-trip delay of pulse-pair interrogations; reply delay is fixed at 50 μs; R = c·t/2.
SSR/ATC transponders (Modes A/C/S) provide identity and altitude to air traffic control on 1030/1090 MHz.
TCAS extends secondary radar air-to-air for collision avoidance with traffic and resolution advisories.
Mode S, ADS-B and multilateration modernize surveillance with selective addressing, spontaneous broadcasts, and TDOA positioning.
Satellite ground control relies on coherent transponder ranging and two-way Doppler for orbit determination — primary (echo) radar is impractical at space distances.
IFF provides secure military identification interoperable with civil SSR.
Emerging uses include drone remote ID, surface movement guidance, and space situational awareness.
Self-Test Questions
Q1. State two fundamental differences between primary and secondary radar.
Answer: Secondary radar uses an active transponder reply (not an echo) and the reply
carries coded data (identity/altitude), whereas primary radar detects passive echoes carrying no data and is limited by
the R4 two-way loss.
Q2. A DME transponder has a fixed reply delay of 50 μs. An aircraft measures a total round-trip
time of 950 μs. Calculate the slant range in km and in nautical miles (1 NM = 1.852 km).
Answer: Propagation time t = 950 − 50 = 900 μs.
R = c·t/2 = (3×108 × 900×10−6)/2 = 135,000 m = 135 km ≈ 72.9 NM.
Q3. What is the purpose of Mode C, and what does a transponder squawk of 7700 indicate?
Answer: Mode C reports pressure altitude in 100 ft increments; 7700 indicates a general emergency.
Q4. Explain how TCAS determines the range of an intruding aircraft.
Answer: TCAS interrogates the intruder's transponder and measures the delay of the 1090 MHz
reply; range = c·Δt/2, refined over multiple interrogations to give range rate.
Q5. Why can two-way ranging through a satellite transponder achieve much greater distances than
primary radar on the same target?
Answer: The transponder amplifies and retransmits a strong, coded reply, so received signal
is subject only to one-way free-space loss on each leg with known transmit power — avoiding the tiny echo return of
primary radar and providing data demodulation capability.
Q6. Describe how multilateration (MLAT) positions an aircraft, and state the minimum number of
receiving stations required for a 3-D fix.
Answer: Stations measure TDOA of 1090 MHz replies; each TDOA defines a hyperboloid; the
intersection of hyperboloids gives position. A 3-D fix requires at least 4 synchronized stations (3 for 2-D).
Q7. Give two functions of the TT&C ground segment in satellite ground control that rely on
secondary-radar-style transponder operation.
Answer: (1) Two-way ranging for orbit determination; (2) coherent two-way Doppler for
range-rate measurement; also command uplink/telemetry downlink through the same transponder.
Q8. What is the key security improvement of IFF Mode 5 over Mode 4?
Answer: Mode 5 uses modern encryption, GPS-position-enhanced replies and higher
resistance to spoofing, plus selective (interrogation-specific) replies.
Q9. Compute the round-trip signal time for a LEO satellite at 800 km altitude (neglect elevation angle).
Answer: t = 2R/c = 2×8×105/3×108 = 5.33×10−3 s ≈ 5.3 ms.
Q10. Explain why SSR replies on 1090 MHz can be received by satellites for ADS-B surveillance,
and name the resulting concept.
Answer: 1090 MHz extended squitters are line-of-sight, high-power bursts transmitted
continuously; LEO satellites with large field of view can receive them over oceans — the concept is
space-based ADS-B (e.g., Aireon).
References and Further Reading
Skolnik, M. I., Introduction to Radar Systems, 3rd ed., McGraw-Hill — chapters on secondary radar and beacons.
Fortescue, P., Swinerd, G. & Stark, J. (eds.), Spacecraft Systems Engineering, 4th ed., Wiley — TT&C chapter.
ITU-R Recommendations on aeronautical radionavigation and satellite earth stations (relevant M-series and S-series).
Assessment: A written examination (70%) covering principles and numerical range
problems, plus an assignment (30%) requiring a short report comparing two secondary radar applications of the
student's choice (e.g., DME vs. satellite TT&C ranging) in terms of link budget, timing accuracy, and data content.