A waveform system extending the unambiguous velocity for weather radars employing solid state transmitters

A fundamental constraint of a pulsed Doppler radar is the so-called Doppler dilemma, where the maximum unambiguous range is directly proportional to the pulse repetition frequency (PRF) while the maximum unambiguous velocity is inversely proportional. This results in the inability to explicitly measure the spectrum of atmospheric velocities naturally possible to the range extent required by most fixed operational radar sites. Several methodologies have been employed to address this, such as range and/or velocity unfolding. These traditional approaches, however, introduce trade-offs which set practical limits on the extent to which mean radial velocities can be measured unambiguously while retaining the necessary data quality.

The use of solid state power amplifiers (SSPA) in weather radar systems facilitate new approaches to addressing the Doppler dilemma. SSPAs have approximately 100 times greater duty cycle than the traditional higher powered magnetron or klystron transmitter. This allows rapidly repeated transmission of relatively long pulses and the creation of complex waveforms previously restricted by hardware. The explicit control of amplitudes, frequencies, and phases of the SSPA waveform allow for such techniques as non-linear frequency modulation (NLFM) to be realized. Additionally, the cost structure of SSPAs allows for the cost-effective use of alternating and simultaneous polarization transmission eliminating the need for high-speed waveguide switching.

A new method to measure radial velocity is presented for SSPA-based radar systems. In this method, a complex waveform is created using multiple forms of modulation. The waveforms received by the radar system from the complex pulse emission will be processed in a manner to derive a radial velocity estimate. The methodology proposed would allow a maximum unambiguous radial velocity to exceed 100 m/s for C-band wavelengths without the need for any additional unfolding algorithm. This method de-couples the maximum unambiguous velocity from the pulse repetition interval (PRI) between complex pulses. Therefore, the PRI between complex pulses may be set to achieve the required maximum unambiguous range, reducing the occurrence of multi-trip echo, without impacting the maximum unambiguous velocity.

Results from an initial simulation will be presented which will demonstrate efficacy of the method. Bias and standard deviation values will be shown at various signal-to-noise ratios.