Detecting Satellite Transmissions

In order to detect transmissions from Seawinds, the CGS must be listening. Because storage space is limited, the CGS predicts when the satellite passes overhead and points the antenna in the appropriate direction. The CGS records twenty seconds per beam for an outer beam only pass, and ten seconds each beam for a four beam pass. Either way 40 seconds of data is stored for each pass.

The data is stored by converting the received electromagnetic pulse into a voltage, sampling the voltage by an analog to digital, or A/D converter, and storing the samples. The 40 seconds of data is then processed by segmenting it into four hundred 0.1 second files which are stored locally. The files for each pass contain approximately 450 MB of data.

During each ten second capture, most of the data is only noise since the antenna is not pointing in the direction of the CGS. However, each time the antenna rotates past the CGS, pulses can be seen above the noise floor. This usually occurs three times during the capture due to an instrument antenna rotation rate of 3.3 seconds per rotation. To find the instances when the pulses can be found above the noise, the variance of each 0.1 second file is stored. Files with large variances indicate where pulses can be seen above the noise, files with low variance mean that pulses can not be seen. Only the files with high variance are selected for further analysis.