Signal generator, power sensor test DME
By Rick Nelson, Editor in Chief -- Test & Measurement World, 5/1/2009 2:00:00 AM
DME (distance measuring equipment) provides a way for a civilian aircraft to measure its distance from a land-based transponder. DME is often paired with VOR (VHF omnidirectional radio) to determine azimuth. TACAN (tactical air navigation) is a military variant that determines azimuthal direction as well as distance.
 With DME, a plane transmits shaped double pulses to a ground station, which replies after a set delay. A receiver on the plane uses the pulse round-trip time to determine distance. Courtesy of Rohde & Schwarz. |
With DME, a plane transmits shaped double pulses to a ground station, which replies after a set delay. A receiver on the plane uses the pulse round-trip time to determine distance.
The ICAO (International Civil Aviation Organization) specifies DME parameters that must be checked at regular intervals, and DME manufacturers and station operators may specify additional parameters. Such parameters are usually tested using BITE (built-in test equipment), but the BITE itself may fail and deliver false measurement results, leading to system malfunction.
In the application note “Test of DME/TACAN Transponders,” Rohde & Schwarz presents an external-measurement approach using an R&S SMA100A signal generator connected to an R&S NRP-Z81 wideband power sensor. To make DME measurements, the signal generator feeds DME interrogation pulses via a coupler to a DME ground-station receiver. The power sensor, via an additional coupler, detects the transponder's reply pulses. Signal-generator software analyzes the detected pulses, and the generator display indicates measurement results.
 Each channel has two different codings (X and Y) that have different pulse spacing. This graph shows the time characteristic of the DME signal envelope for each channel. Courtesy of Rohde & Schwarz. |
The setup can measure reply delay, reply efficiency, ground-station pulse-repetition rate, transmit-pulse power, pulse shape, pulse spacing, receiver sensitivity, and receiver bandwidth; it can also test decoder and monitor-alarm functions. Adding a second signal generator permits measurement of receiver recovery time and receiver sensitivity with load.
The 34-page application note provides details on making each of these measurements and offers more information on DME/TACAN operation.
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