SSR Ground Antenna and Interrogation Modes
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Questions and Answers

What is the primary purpose of interrogation pulses transmitted by ground antennas?

  • To measure aircraft speed
  • To ask specific questions to the airborne system (correct)
  • To perform altitude calculations
  • To transmit weather information
  • What is the function of the second beam provided in the ground antenna?

  • To eliminate all side lobe interrogations
  • To suppress unwanted interrogations from side lobes (correct)
  • To increase the pulse spacing
  • To enhance altitude reporting
  • In which mode does the aircraft respond with a 12-pulse reply indicating its identity number?

  • Mode A (correct)
  • Mode 3/A
  • Mode S
  • Mode B
  • Which interrogation mode is associated with altitude information?

    <p>Mode C</p> Signup and view all the answers

    What timing is associated with the transmission of pulse P2 in relation to pulse P1?

    <p>2 µs after P1</p> Signup and view all the answers

    For interrogation mode B, how does its response compare with mode A?

    <p>It gives a similar response but was specific to Australia</p> Signup and view all the answers

    What problem can arise from the reception of interrogations from side lobes?

    <p>False aircraft indications at erroneous bearings</p> Signup and view all the answers

    Which interrogation mode is labeled as 'Undefined'?

    <p>Mode D</p> Signup and view all the answers

    What is the primary reason for transmitting data twice in the system described?

    <p>To ensure data accuracy through error correction</p> Signup and view all the answers

    What role does parity play in the communication process?

    <p>It checks for discrepancies in data transmission</p> Signup and view all the answers

    What occurs if the parity check fails after an inversion of bits?

    <p>A re-interrogation is required</p> Signup and view all the answers

    How does the ground station track the aircraft's position?

    <p>Using the predicted position data from previous replies</p> Signup and view all the answers

    What does the Mode S All-Call require from an aircraft already known to the interrogator?

    <p>To not reply again to avoid unnecessary interference</p> Signup and view all the answers

    What is the maximum duration of interference that the parity system can correct?

    <p>24 µs</p> Signup and view all the answers

    When does a Mode S transponder abort its reply procedure during the All-Call operation?

    <p>When detecting pulse P4</p> Signup and view all the answers

    What type of measurements are available in the pulses of the reply?

    <p>Monopulse angle measurements and signal strength measurements</p> Signup and view all the answers

    What is one potential outcome of garbling in radar operations?

    <p>The detection of a false aircraft</p> Signup and view all the answers

    Which of the following describes FRUIT in radar operations?

    <p>Receipts of replies from transponders interrogated by other radars</p> Signup and view all the answers

    What issue does SSR experience due to antenna shadowing?

    <p>Poor signal reception due to aircraft structure</p> Signup and view all the answers

    How does an MSSR improve upon classic SSRs in terms of interrogation?

    <p>It employs a different beam pattern for additional accuracy</p> Signup and view all the answers

    What is typically necessary when primary data reliability is compromised in aircraft tracking?

    <p>More frequent pilot reports</p> Signup and view all the answers

    What phenomenon occurs when multiple replies are received at the same time in radar systems?

    <p>Garbling</p> Signup and view all the answers

    What is one way to mitigate the effects of antenna shadowing in SSR?

    <p>Install multiple antennas in different positions</p> Signup and view all the answers

    Which of the following statements about MSSR is true?

    <p>It can determine range direction from a single pulse</p> Signup and view all the answers

    Study Notes

    SSR ground antenna

    • SSR ground antennas are highly directional but have side lobes.
    • Side lobes can lead to false aircraft detections at incorrect bearings.
    • A second, omni-directional beam is used to overcome this issue.
    • The second beam has a gain exceeding that of the side lobes but less than the main beam.
    • A third pulse, P2, is transmitted from this beam 2 µs after P1.
    • Aircraft in side lobes, detecting P2 stronger than P1, do not reply.

    SSR interrogation modes

    • Interrogation pulses vary based on pulse width between P1 and P3.
    • The different pulse spacing results in various interrogation modes.
    • Mode 1: 3us spacing, Identity (military)
    • Mode 2: 5us spacing, Identity (military)
    • Mode 3/A: 8us spacing, Identity (military/civil)
    • Mode B: 17us spacing, Identity
    • Mode C: 21us spacing, Altitude
    • Mode D: 25us spacing, Undefined
    • Mode S: 3.5us spacing, Multipurpose

    SSR Transponder Responses

    • Mode-A Interrogation (identity) elicits a 12-pulse reply.
    • Mode-C Interrogation (altitude) elicits an 11-pulse reply.
    • Responses contain parity and address information for error detection.
    • Responses are transmitted twice (once in inverted form) for robustness against garbling.

    Tracking & Interference

    • The ground station uses predicted aircraft position to determine range and bearing for interrogation.
    • Errors in the reply parity check indicate a mismatch with the expected aircraft or data corruption due to interference.
    • Mode S is designed to minimize garbling through parity checks, capable of correcting errors up to 24 µs.

    SSR Replies

    • Replies provide individual monopulse angle measurements and signal strength.
    • This information helps identify corrupted bits and allows for correction by inverting bits if necessary.
    • Failed transponders can still be tracked through primary radar data, however, level information will be less reliable.
    • Garbling occurs when two aircraft replies are received simultaneously due to same slant range and bearing.

    FRUIT (False Replies Unsynchronized In Time or False Replies Unsynchronized to Interrogator Transmissions)

    • FRUIT occurs when a radar receives a reply from a transponder interrogated by a different radar.
    • This results in false target indications because the range information cannot be accurately calculated.
    • FRUIT can lead to garbling if a valid transponder reply is received at the same time.
    • SSRs are vulnerable to antenna shadowing from the aircraft fuselage (e.g., due to bank angle).
    • This is mitigated by using multiple antennas.

    Mono Pulse Secondary Surveillance Radar (MSSR)

    • MSSR uses a different beam pattern for more accurate azimuth determination.
    • This reduces the number of interrogations needed for accurate positioning.
    • It employs additional encoding of the radio signal to provide accurate directional information.
    • The name refers to its ability to extract range and direction from a single pulse.

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    CNS Concepts PDF

    Description

    Explore the critical concepts of SSR ground antennas, their directional capabilities, and the importance of side lobes in aircraft detection. This quiz covers various SSR interrogation modes and transponder responses, providing a comprehensive understanding of aircraft identification processes.

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