Quantifying Errors due to Frequency Changes and Target Location Uncertainty for Radar Refractivity Retrievals

Radar refractivity retrievals can capture near-surface humidity changes, but noisy phase changes of the ground clutter returns limit the accuracy for both klystronand magnetron-based systems. Observations with a C-band (5.6 cm)magnetronweather radar indicate that the correction for phase changes introduced by local oscillator frequency changes leads to refractivity errors no larger than 0.25 N units: equivalent to a relative humidity change of only 0.25% at 208C. Requested stable local oscillator (STALO) frequency changes were accurate to 0.002ppm based on laboratory measurements. More serious are the random phase change errors introduced when targets are not at the range-gate center and there are changes in the transmitter frequency (DfTx) or the refractivity (DN). Observations at C band with a 2-ms pulse show an additional 668 of phase change noise for a DfTx of 190 kHz (34 ppm); this allows the effect due to DN to be predicted. Even at S band with klystron transmitters, significant phase change noise should occur when a large DN develops relative to the reference period [e.g.,;558when DN5 60 for the Next GenerationWeather Radar (NEXRAD) radars]. At shorter wavelengths (e.g., C and X band) and with magnetron transmitters in particular, refractivity retrievals relative to an earlier reference period are even more difficult, and operational retrievals may be restricted to changes over shorter (e.g., hourly) periods of time. Target location errors can be reduced by using a shorter pulse or identified by a new technique making alternate measurements at two closely spaced frequencies, which could even be achieved with a dual–pulse repetition frequency (PRF) operation of a magnetron transmitter.