# Weather Radar in Complex Orography
## 4 Solutions and Results
### Calibration, Monitoring and Control  
Automatic calibration, comprehensive monitoring and remote control are also important in flat countries, especially if the radar is used for air traffic control, automatic warnings and other demanding applications. However, they are of even greater importance in a mountainous region where there are remote unmanned sites, long access routes and periods when no access is possible. The aim of these procedures is to automatically detect instabilities, ageing components and the risks of failures at an early stage, and to have as many data as possible available to diagnose the origin of a problem. The final goal is to maximise stability and minimise the down time. Absolute calibration strategies generally combine offline and online procedures. A number of measurements are performed offline at the factory, during commissioning, and are repeated on a regular basis during preventive maintenance operations. These include measurements of the losses along the transmission and reception path, of the characteristics of the transmitted signal (output power at the magnetron, frequency and pulse shape), and of the system dynamics (in particular the minimum discernible signal, alignment of the high-sensitivity and low-sensitivity channels). If changes in the transmitting or receiving path occur, the signal losses need to be remeasured. Ideally, the antenna gain and radiation pattern should also be measured under conditions after commissioning, with and without the radome, but this is difficult, especially in mountainous sites, and is thus rarely done. If no hardware changes occur, the losses in the receiving path should remain stable. In reality, certain components, such as receiver protection devices (e.g., TR-limiters), can degrade. Moreover, certain components are sensitive to temperature, and their performances thus vary over time. It is therefore good practice to monitor the receiving path online by means of an automatic scheme. In Switzerland, for instance, reception losses are sampled using the signal of a noise source injected every 2.5 min in the H and V reception channels, both before and after the TR-limiter [137]. Atmospheric background noise, which is sensitive to degradations in the receiving path, is also monitored. Absolute calibration is therefore achieved by automatically combining the offline and online measurements and adapting the radar calibration constant of the H and V channel accordingly. This procedure is repeated every 5 min and in this way compensates for temperature dependencies of the receiving path. The absolute and differential accuracy of the two polarisation channels can also be monitored by means of Sun measurements, obtained online in operational mode, whenever the antenna points in the direction of the Sun [142; 143; 144] and during dedicated manually triggered Sun scans performed on demand [110; 111]. The Sun's signals detected by the radar receiver are also used to monitor the pointing of the antenna. Another option to monitor the stability of radar measurements is to analyse the signals scattered back by a tower nearby, see, for instance, [145], or by mountains [12; 13]. During the renewal of the radar network in Switzerland [1], an external receiver and an external transponder were deployed on a mountain in the vicinity of the radar being tested, which allowed, for instance, the antenna gain, parts of the antenna radiation pattern (Figure 16), the shape of the pulse, the precision of the scan strategy and depolarisation to be examined. At the moment of writing (2021), the external receiver is still in use for monitoring purposes, 10 years after the renewal of the Swiss radar network. A number of additional techniques are available to monitor the stability and accuracy of dual-polarisation moments, see [106; 146; 147] and references therein.