Q1Radar and TV Engineering
Question
Q.1. (a) Explain microwave landing system using neat and clean diagram. [8]
(b) Write the difference between Pulse Radar and CW Radar with respect to - (i) Maximum Range (ii) Resolution (iii) Accuracy and (iv) Power Level. [8]
Answer
The Microwave Landing System is a precision aircraft landing guidance system, developed as a more capable successor to the older Instrument Landing System (ILS), operating in the microwave frequency band (approximately 5.0 to 5.25 GHz) and providing an aircraft with precise azimuth (lateral direction), elevation (vertical glide path angle), and range guidance information during approach and landing, enabling safe landing under low-visibility conditions.
Principle of Operation
The MLS ground equipment, installed at the airport runway, consists of an azimuth station (typically located beyond the stop end of the runway) and an elevation station (typically located to one side of the runway near the touchdown zone), each transmitting a narrow microwave beam that is electronically scanned back and forth in a specific angular sector at a precisely controlled, known angular scanning rate; this scanning technique is known as Time Reference Scanning Beam (TRSB). As the scanning beam sweeps across the aircraft's position, the aircraft's onboard MLS receiver detects two distinct pulses during each scan cycle: a 'to' pulse, received as the beam sweeps in one direction across the aircraft, and a 'fro' pulse, received as the beam sweeps back in the opposite direction; the precise time interval between the reception of these to and fro pulses is directly and uniquely related, through the known, fixed angular scanning rate, to the aircraft's actual angular position (azimuth or elevation angle, depending on which station's signal is being measured) relative to the runway centerline or nominal glide path. By continuously measuring this to-fro time interval for both the azimuth and elevation stations, the aircraft's onboard receiver and flight guidance computer can precisely determine, and continuously update, the aircraft's actual angular position relative to the ideal approach path, generating steering guidance commands to correct any deviation and guide the aircraft smoothly along the correct approach path to the runway. A separate precision Distance Measuring Equipment (DME/P) co-located with the MLS ground stations additionally provides accurate slant-range distance information to the aircraft, completing the full three-dimensional position guidance (azimuth, elevation, and range) needed for a precision instrument approach and landing.
Pulse Radar versus CW Radar Comparison
| Aspect | Pulse Radar | CW (Continuous Wave) Radar |
|---|---|---|
| Maximum Range | Can achieve very long maximum range, since high peak transmitted power can be concentrated into short pulses (giving high average detection range) without exceeding average power handling limits of the transmitter, and range is directly measured from pulse round-trip time, imposing no fundamental range limitation from the measurement technique itself. | Maximum range for simple, unmodulated CW radar is not directly measurable at all (since CW radar with no timing reference cannot measure round-trip time), and even frequency-modulated CW (FMCW) variants that do measure range are generally limited to shorter maximum ranges than a comparable pulse radar, since CW radar power is spread continuously in time rather than concentrated into high-peak-power pulses. |
| Resolution | Good range resolution, achieved through the use of short transmitted pulses (or pulse compression techniques), since range resolution is directly related to the pulse duration (a shorter pulse gives finer range resolution, allowing closely spaced targets at different ranges to be distinguished). | Poor inherent range resolution for simple CW radar (since there is no pulse timing to distinguish different ranges at all), though FMCW variants can achieve reasonable range resolution related to the total frequency sweep bandwidth used. |
| Accuracy | Range accuracy is generally very good, governed by the precision with which pulse transmission and reception timing can be measured, though velocity (Doppler) accuracy in a simple pulse radar without additional processing can be more limited due to the pulsed (sampled) nature of the Doppler measurement. | Velocity (Doppler) measurement accuracy is excellent for CW radar, since the continuously transmitted, unmodulated carrier provides an ideal, continuous reference against which even very small, precise Doppler frequency shifts can be measured without any sampling-related ambiguity, though range accuracy (for FMCW variants) is generally less precise than for a well-designed pulse radar. |
| Power Level | Requires very high peak transmitter power (since all the radar's average power capability is concentrated into brief pulse intervals) but comparatively low average power, since the transmitter is only active for a small fraction of the total time (a low duty cycle). | Operates continuously at a comparatively modest, constant power level (matching the transmitter's average power capability directly, since there is no pulsed, low-duty-cycle transmission), avoiding the need for the very high peak power handling capability that a pulse radar's transmitter and associated components must be designed to withstand. |
It is further worth noting that both pulse and CW radar techniques are in practice often combined within a single radar system design to obtain complementary benefits from each: a pulse-Doppler radar, for instance, transmits a pulsed waveform (retaining the range-measurement capability of pulse radar) while coherently processing the pulse-to-pulse phase information (retaining much of the excellent Doppler velocity measurement capability characteristic of CW radar), illustrating that the pulse-versus-CW distinction addressed in this comparison represents two ends of a broader spectrum of radar waveform design choices rather than two entirely mutually exclusive radar categories.
It is also worth noting that the microwave landing system's time-reference scanning beam approach, described above, achieves its precision angular measurement using a technique conceptually related to but distinct from radar itself, since MLS relies on measuring the timing of a scanning transmitted beam as observed at the aircraft, rather than measuring the round-trip delay of a reflected radar echo; nonetheless, MLS is conventionally studied and categorized alongside radar navigational aids because it shares the same underlying microwave propagation physics, similar antenna and beam-forming technology, and the same overall purpose of providing precise aircraft position and guidance information using radio frequency signals rather than purely visual references, particularly valuable during low-visibility approach and landing conditions.
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