these errors one must consider systematic errors of alignment. Radars are typically maintained with an alignment accurate to +-0.044 degrees in azimuth.
Azimuth errors are clearly the dominant error, and can be translated into positional errors as follows:
Taking into account the random noise errors only:
At 50 Nm the 0.16 degree error results in a position error of 0.14Nm
At 200 Nm this error has risen to 0.56Nm and to 0.70Nm at 250 Nm
Systematic errors of +/- 0.2Nm at
250Nm from the radar also need to
be considered.
In many monopulse SSR systems, azimuth errors are a function of the received signal strength. Strong signals allow monopulse azimuth determination to work very well whereas at very low signal strength, signal noise causes a significant deterioration. For low signal strength SSR detection (limit of coverage), the positional error could exceed the above values.
No “real time” measurement of accuracy over the total coverage area is maintained. Real time monitoring of a single pseudo aircraft (site monitor) gives a degree of comfort that the measurement accuracy is within normal bounds. It is assumed that a commissioned radar continues to deliver accurate positional data within the total coverage area independent of environmental constraints. The reality is that radar is affected by a
large range of phenomena which corrupt the positional data to some extent. These corruptions are typically the result of multipath reflection of radar signals and the "bending" of the beam around obstacles. Transient positional errors are experienced as aircraft fly through regions subject to the phenomena. These errors can result in moderately large position errors up to 0.5 Nm.
A SSR radar does not measure the aircraft’s Altitude. Rather the radar interrogates the aircraft’s transponder with a request for Flight Level. In response the Transponder replies with the Flight Level as measured by a Barometric Pressure Encoder. For a Mode C reply, the unit of encoding is 100 feet, for a Mode S reply the unit is 25 feet.
Identity Altitude
SSR radar does not measure the aircraft’s Identity. Rather the radar interrogates the aircraft’s transponder with a request for Identity (Mode-A code). In response, the Transponder replies with the Mode-A code entered on the transponder control panel by the pilot. The Controller or the radar display system translates the Mode-A code to a Flight Identity. There is a limited number of Mode-A codes (4096 – some reserved). This may lead to ambiguity in aircraft identification. Special Purpose Ident (SPI) can be activated by the pilot upon controller request to aid in resolving such ambiguity.
Velocity Vector.
SSR radar determines Velocity Vector as a derivative of successive position measurements. Velocity Vector determined in this manner is of limited accuracy. Whenever the aircraft manoeuvres, the vector calculated by the radar lags In the ADS-B system, the ADS-B transponder spontaneously transmits the exact same information as for a Mode S Transponder. Thus, for Altitude information, there is no distinction between Radar and ADS-B. 中國航空網 www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：Operational Use of ADS-B In Non Radar Airspace Generic Desig(6)
