Imbedded SSR Mode-S Logic Control Unit

University of StellenboschDepartment of Electrical & Electronic

Engineering

K. Gastrow

4 December 2009

Introduction

• SSR or IFF is an electronic, radio based means of target identification.• Sensor (interrogator) transmits RF coded signal (interrogation).• Target replies if it is in main beam of interrogtor and equipped with a working transponder.• Reply message contains information essential for target identification and positioning.

Civil SSR Interrogation Modes

• Mode 3/A– Identification (4096 codes) – Aircraft identification set by pilot

as per Air Traffic Control request• Mode C

– Pressure altitude reporting (Flight Level)• Mode S (Mark XII)

– Multiple information formats to selective (negotiated) interrogations

Military SSR Interrogation Modes

Employs all civil SSR modes (A, C and S) as well as:

• Mode 1– Identification (32 codes) – Role, mission, aircraft type

• Mode 2– Identification (4096 codes) – Individual aircraft airframe

• Mode 4– Three-pulse reply to crypto coded challenge

• Mode 5– Cryptographically secured version of Mode-S and ADS-B

Project Aim

• Design SSR interrogator in VHDL• Implement on FPGA• Interrogate Mode-A, Mode-C and Mode-S enabled

transponders• Receive, decode, interpret and save reply messages• To test receipt of replies, transponder is also implemented in

project.• Mode-S surveillance is limited to Elementary Surveillance (56

bit) – No Long Message (112 bit) or Extended Length Message (multiple 112 bit strung together) capability.

Interrogator

Waveform Measurements

Mode-A interrogation signal Mode-C reply with SPI pulse

Mode-S interrogation signal Mode-S reply

Mode Interlace Program (MIP)• MIP governs the timing and behaviour of interrogations and is

set up by the Mode Generator.• MIP is determined by beam dwell time of main SSR antenna. • The MIP implemented in the project assigns ⅓ of the dwell

time to Mode-S All-Call, Mode-A and Mode-C interrogations. The remaining ⅔ of the dwell time is reserved for selective Mode-S interrogations.

Mode Interlace Program (MIP)• All-Call period houses interrogations as shown below.

• Timing between these interrogations allows a reply from a target at maximum range (450 km) to be received before the next interrogation is transmitted.

Mode-S All-Call Interrogations

All-Call interrogations:• Transponders are acquired by means of All-Call

interrogations.• All-Call interrogation answered by all aircraft.• All-Call data field contains:

– Probability of reply– Sensor Site Address

• Selective interrogations activate transponder all-call lockout.• Transponders locked out do not reply to All-Call

interrogations originating from this address – less interference and RF saturation.

Selective Mode-S Interrogations• Only processed during Roll-Call period of MIP.• Selective interrogations are transmitted to an aircraft within

an 18° bearing window around last known position.

Selective Mode-S Interrogations

• During every antenna revolution, altitude and identity interrogations are transmitted once during each roll-call period while bearing condition is satisfied until a valid reply is received.

• Aircraft is dropped from memory if no reply to a selective interrogation has been received for three full revolutions.

Interrogator Receiver and Logic

• Incoming messages can be of any type (mode).• Identify mode of reply – Mode-A, Mode-C or Mode-S. Reject

invalid signals.– Mode-S : Preamble.– Mode-A, Mode-C : Framing pulses.

• Activate relevant decoder to extract data by sampling.• Once data has been sampled, activate storage management.

Interrogator Receiver and Logic

• Storage management interprets data and saves accordingly.• Mode-A and Mode-C reply data is not saved. Directly sent to

output.• Range of target calculated by timing between transmission of

last interrogation pulse and receipt of first reply pulse. Transponder delay taken into account.

Transponder details saved to RAM

• 108-bit data field, 512 RAM slots (400 used).• Transponder details removed from RAM after 3 revolutions without

reply.• Downlink Waiting, UMData not implemented – no ELM capability.

Downlink Waiting Last Reply Last Request Interrogations Done UMData

Address Bearing Range Altitude Identity Revolutions Capability Emergency Hijack Radio Failure Flight Status

0 23 24 36 37 47 48 59 60 71 72 73 74 76 77 78 79 80 82

83 87 88 92 93 97 98 101 102 107

Transponder Components• Mode Identifier

– Indentifies interrogation Mode• Data extractor (Mode-S only)

– If Mode-S interrogation received, sample and interpret data. – Calculates interrogation and reply parity.– Set up reply data accordingly.

• Reply Generators– Only activated if address matches.– Generate replies according to specifications.– Altitude, Identity and Address hard-coded.

Hardware

• Interrogator and Transponder simulated on FPGA.

FPGA Implementation

Interrogator and Transponder externally linked to complete communication path.

Test Setup

Above setup simulated on FPGA:• 2 transponders at different bearings• LEDs employed to indicate transponder acquisition and selective interrogation.

Test Results

• Transponders successfully acquired and selectively interrogated.

• Reply data is correctly received and decoded.• Transponder information successfully saved to RAM.• Transponder information successfully removed from RAM

after three revolutions without reply.• Time-related RAM glitch.

Conclusion & Improvements• Waveforms conform to specifications• Besides RAM fault, the system functions as intended.

• Improvements:– Implement ELM.– Monopulse direction finding will improve bearing accuracy.– Sliding Window/Monopulse Plot Extractor (Mode-A and Mode-

C)– Extracting heading and velocity information of targets will allow

more accurate prediction of target’s future position and reduce the number of selective Mode-S interrogations transmitted.

Future IFF Development: ADS-B (AKA Next-Gen)

• Mode-S enables ADS-B :– Mode-S ELM employed for data communication.

• GPS information is broadcast to other aircraft and interrogator sensors.

• Improves pilots’ visual awareness of other aircraft– Nearby aircraft position, altitude and direction displayed on

cockpit instrumentation.– Allows free flight – flight path no longer limited to predefined

air corridors.– Allows more air traffic – less separation due to improved

positional accuracy.

Typical ADS-B cockpit display

Questions and Discussion