Imbedded SSR Mode-S Logic Control Unit University of Stellenbosch Department 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.

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