Development of a Versatile Groundstation Utilizing Software Defined Radio

Abstract

As software defined systems establish modern technology, the Space Systems Academic Group (SSAG) of the Naval Postgraduate School (NPS) is compelled to replace legacy hardware systems with inexpensive and flexible software defined solutions. One key aspect in ensuring stable communication with the multiple small satellites and CubeSats launched by the SSAG, is to develop and maintain a versatile satellite groundstation utilizing inexpensive off-the-shelf Software Defined Radio (SDR). This thesis presents the demodulation and decoding of a 1200 baud AFSK signal from a BigRed- Bee, as well as a 9600 baud GFSK signal from a PropCube CubeSat. The demodulating signal processing steps are designed in GNU Radio Companion. The thesis also deals with parts of the AX.25 data link layer protocol decoding. Bit-per-bit comparison of the outputs in performed tests show that the SDR can replace the currently used software. A bit error rate (BER) calculation proves the excellent performance of the GFSK demodulation scheme. The performed BER test results in a BER < 1 × 10⁻⁴ for a SNR of E(b)/N(0) = 6dB, whereas the estimated BER for a coherent MSK signal is BER(MSK) ≈ 2×10⁻⁴ for the same SNR. Additionally, this thesis presents one option of a real-time implementation of the PropCube Receiver. This thesis also provides test results of performed function tests, subsystem tests and integrated system tests, accompanied by in depth explanations on the settings and preferences of each used GNU Radio block.