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Questions and Answers
What is the primary purpose of interrogation pulses transmitted by ground antennas?
What is the function of the second beam provided in the ground antenna?
In which mode does the aircraft respond with a 12-pulse reply indicating its identity number?
Which interrogation mode is associated with altitude information?
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What timing is associated with the transmission of pulse P2 in relation to pulse P1?
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For interrogation mode B, how does its response compare with mode A?
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What problem can arise from the reception of interrogations from side lobes?
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Which interrogation mode is labeled as 'Undefined'?
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What is the primary reason for transmitting data twice in the system described?
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What role does parity play in the communication process?
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What occurs if the parity check fails after an inversion of bits?
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How does the ground station track the aircraft's position?
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What does the Mode S All-Call require from an aircraft already known to the interrogator?
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What is the maximum duration of interference that the parity system can correct?
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When does a Mode S transponder abort its reply procedure during the All-Call operation?
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What type of measurements are available in the pulses of the reply?
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What is one potential outcome of garbling in radar operations?
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Which of the following describes FRUIT in radar operations?
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What issue does SSR experience due to antenna shadowing?
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How does an MSSR improve upon classic SSRs in terms of interrogation?
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What is typically necessary when primary data reliability is compromised in aircraft tracking?
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What phenomenon occurs when multiple replies are received at the same time in radar systems?
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What is one way to mitigate the effects of antenna shadowing in SSR?
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Which of the following statements about MSSR is true?
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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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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.
