Noise source calibration

As an external, pod-mounted system, which is deployed in a wide range of altitudes from near surface to approximately 40,000 feet, HCR experiences large temperature variations. To maintain good system calibration, it is essential to monitor the radar system performance versus temperature. In order to ensure operational accuracy, a number of noise source calibration (NSC) events were performed during SOCRATES ferry flights, research flights, and on the ground. During each NSC event, a known noise signal, which is invariant to temperature changes, is injected into the radar and then used to characterize the receiver gain changes by comparing the received power (DBMVC) to a temperature-corrected noise power.

As the low noise amplifiers (LNAs) are usually the components that dictate the receiver’s performance, they are outfitted with heater circuits to maintain their temperatures between 37 and 40 C in the bench test environment. During deployment, as the heaters cycle on and off, the received power level (DBMVC) directly correlates to the temperature fluctuations, leading to a sinusoidal pattern which cycles around 27 C when installed in the system. In some extreme cases, the heaters of the LNAs could not keep up with the heat loss to the environment, leading to a significant decline of the received power. Using the correlation between the LNA temperature and the received power level during the NSC events we calculated the correlation equation which we used (together with the calibration data obtained in the lab) to correct the reflectivity field (dBZ)  for LNA temperature changes in the whole data set.

As mentioned above, the received power not only depends on the LNA temperatures but also on the pod temperature. After correcting for the LNA temperature changes we were able to establish a correlation between the pod temperature and the power output during the NSC events which was then again used to correct the whole data set. (Note that we define a mean of the Noise Source, EIK, Polarization Switch, and RF Detector temperatures as the “pod temperature”.) It is important to keep in mind that for the HCR dataset, only the reflectivity related fields were corrected using the above mentioned methods. The received power (DBMVC) field is left uncorrected for engineering purposes.