Processing BEAM Data into Results

The measured location of the sun in the BEAM product is calculated by first thresholding the SNR data above a certain signal-to-noise level which is configured in the suncal.conf file.

A 2D second order polynomial is fitted to the resulting data. Note that this fit fails if you set the threshold too low (everything passes), or set it too high (nothing passes). Take care with selecting the threshold and the radars operating noise level. This is best done after checking some produced BEAM.

The peak power of the sun measurement, and target beam width are determined from the polynomial fit and finding the peak and width. Note that the azimuthal target beam width appears larger than expected by 1/cosine of the elevation angle because an azimuth degree corresponds to less than a degree of a great circle as you get nearer to the zenith. Because of this, we recommend that you keep the azimuth sector width at least 8°.

The RAW value is what is measured from the BEAM product. The other value is corrected for this effect. The peak power is also converted to dBm and stored in the results. All other areas are corrected for this effect.

From now on, only data with a signal strength greater than 3 dB below the peak value are used. Again a 2D second order polynomial is fitted to the Zh SNR data. The sun's position is determined by solving that polynomial for 0. The position is then compared to the sun's calculated position at the start of the scan and antenna pointing offsets are given in both azimuth and elevation.

For dual-pol data, the second LDR BEAM product is also processed. We compute the average LDR in the 3 dB peak region, and store it in the results. This number should be near 0. This number, when added to the current LDR offset is the new estimate for what the LDR offset should be. A polynomial fit is also calculated for Zv by adding the LDR to the Zh values. The sun's position in V polarization is also printed so you can check the alignment.

Note that you can skip the scanning phase, and pick up processing at the BEAM product phase by calling Suncal with the –process: command line arguments. For dual-pol data supply 2 –process: arguments, 1 for the SNR BEAM, and 1 for the LDR BEAM.