Planning acceptance tests of the next generation Swiss weather radar network

The Swiss Rad4Alp project has been started to renew the current third generation weather radar network with a state-of-the-art solution. The main reason for this renewal is the end-of-life of the existing radar system. For the acceptance test innovative procedures are foreseen such as measurements of the far-field antenna radiation patterns with radome.

MeteoSwiss has long experience in operational usage of radar technology in a mountainous region: the 1st generation MeteoSwiss weather radars started operation in 1961 (two sites), the 2nd one in 1977 (same two sites, but equipped with digital radars). The 3rd generation network introduced Doppler technology in 1993, which has resulted to be a remarkable breakthrough, especially as far as nowcasting and clutter rejection are concerned. Another breakthrough of the 3rd generation was the improved coverage southern of the Alps thanks to the introduction of the third site located on Monte Lema. In the planned 4th generation, the coverage will be further improved by introducing at least one, probably two, additional radars at high altitude. Furthermore, a second receiver channel will be introduced to measure both vertical and horizontal polarization: the higher “costs” (especially in maintenance, signal quality and robustness) will hopefully be counterbalanced by the higher informative content of the dual-pol measurements.

One important element in obtaining valuable polarimetric measurements is a good antenna with low side-lobes and matched radiation patterns for horizontally and vertically polarized waves in the main lobe. Unfortunately, complete measurements of antenna radiation patterns with the radome are hardly available. To our knowledge, only measurements of antenna patterns with a partially assembled radome over a 20 degree interval are mentioned in literature (e.g. Doviak and Zrnic, 1998: “Polarimetric upgrades to improve rainfall measurements”, NSSL report, 110 pp.)

Performing measurements of antenna patterns with the radome in the real environment (i.e., at the site) would certainly be an important contribution to the quality of the Rad4Alp project. However, these kinds of measurements are problematic: passive scatterers, like huge spheres or corner reflectors are inadequate, especially in heavy-cluttered mountainous terrain. With the help and support of armasuisse (the Swiss federal competence center for the procurement of technologically complex systems and equipment), we have identified two alternatives: the first one is based on one-way propagation and Time Stamp Amplitude instrumentation (TSAMP); the second one is based on two-way propagation and Radar Target Simulator instrumentation (RTS). The TSAMP is able to detect in an absolute time frame radar-transmitted pulses; each pulse is oversampled and the amplitude of its envelope retrieved; this allows estimating the transmitted peak power and retrieving the far-field antenna radiation pattern with the radome. The duration of the pulses is also useful to determine the energy. Presently, the TSAMP is successfully being fed by a Radar Simulator which is able to faithfully mimic the Swiss Weather Radar (frequency, pulse width, pulse repetition interval, scan strategy). During summer 2009, we plan to perform real measurements using the operational 3rd generation Swiss radar network. Furthermore, we are also evaluating the feasibility of two-way radar measurements using the RTS, which is able to detect radar-transmitted pulses and send them back coherently (with a time-delay and/or Doppler-shift) to the “victim” radar.