Characterizing Radiosonde GPS with a Simulation for Pressure Calculation Verification

In recent years, radiosonde vendors have been utilizing GPS calculations to obtain accurate derived pressure readings in place of an in-situ pressure cell on the radiosondes. This presents a new challenge in testing, evaluating, and characterizing radiosonde pressure accuracy. As part of its core mission the Sterling Field Support Center (SFSC) has acquired a GPS simulator to perform an evaluation of radiosonde pressure calculations from GPS. Utilizing this GPS simulator, an in-house technique has been developed to determine the characteristics of radiosonde GPS during flight in a pressure controlled altitude chamber while simulating GPS coordinates. Several challenges were overcome in order to appropriately test the pressure accuracy of these radiosondes. Some of these challenges include; GPS/Radiosonde timing agreement, data resolution differences, and cracking vendor specific signal processing algorithms. Through creative use of MATLAB and Python these challenges were overcome and a near-automated process was developed for testing. Once the GPS simulator test processes were developed and finalized, testing of the radiosondes could be completed. Modern sounding systems utilize differential GPS therefore two GPS outputs are needed for proper testing. The first output involved placing a radiosonde inside of an altitude chamber and emitting realistic GPS RF from an antenna above the radiosonde. The second output was piped directly into a signal processing system to represent a surface GPS location. Once the sounding system was turned on the radiosonde system would be ‘tricked’ into thinking a live flight was being performed. Analysis of the GPS simulator output and the radiosonde system output can reveal accuracy and precision characteristics about the radiosonde sounding system thus fulfilling one of SFSC’s missions.