Waveforms for improving near range performance of meteorological radars using pulse compression

Pulse Compression capability has been incorporated into the Vaisala RVP8 and RVP900 meteorological radar signal processors for more than ten years and is now used in over 60 operational radars. Pulse widths up to 80 microseconds in the RVP8 and 160 microseconds in the RVP900 are possible while retaining sub microsecond resolution, down to 5 meter gates spacing, if desired. The processing gain associated with such compression results in sensitivity improvement on the order of 20 dB as compared with the corresponding uncompressed short pulse of the same range resolution. Range time sidelobe levels are suppressed by more than 50 dB compared to the main lobe over the full range of Doppler velocities expected to be measured by the meteorological radar. Non Linear Frequency Modulation (NLFM), a window weighted down conversion filter, and careful tapering of the pulse envelope is used to achieve this performance. A graphical user interface optimizer tool is used on the processor to allow the user to set the pulse characteristics and see the resulting waveform and ambiguity diagram in real-time.

A drawback to the pulse compression was that reception was not possible until full duration of the transmitted pulse was completed, thus limiting the start range of data processing. This limitation has been overcome by enhancing the capability of the RVP900 processor to support a Time Frequency Modulated (TFM) Pulse making the start range of data processing equivalent to a radar not using pulse compression, but achieving the range resolution and sensitivity of a radar using pulse compression.

The Time Frequency Modulated waveform is formed by the RVP900 digitally and converted to analog at approximately the intermediate frequency of the radar. The TFM waveform consists of two consecutive pulses, the second immediately following the first in time. The first pulse is a long NLFM pulse and the second is a traditional short pulse without frequency modulation. To allow the processor to simultaneously process the returns of the long and short pulses, the pulses are offset from each other in frequency by a few megahertz. The digital IF (Intermediate Frequency) samples are passed into two parallel downconversion filters where one filter is the corresponding filter for the long NLFM pulse (that performs pulse compression) and the second filter is the corresponding matched filter to the short non-frequency modulated pulse. Greater than 60 dB isolation is achieved between the two downconversion filters. The two parallel I/Q data streams that are then merged together in the downstream processing at the autocorrelation stage. The merging is done over a range of the I/Q samples where the results are valid for both the long and short pulses to avoid transition artifacts. Data results using the technique described will be shown.