Space and Ground Segments Link Performance Verification for Small Satellite TT&C Transponders

Verification of low earth orbit (LEO) satellite communication links is required for evaluation and acceptance purposes. Telemetry and telecommand transponder’s bit error rate (BER), which is the main communication link parameter to be evaluated, is required to be verified by measurement rather than by analysis. This paper introduces a novel algorithm for measuring the BER of both space and ground segments. BER could be determined only if a received pattern is compared with a locally generated one. This feature exists for those transponders that utilize direct sequence spread spectrum (DS-SS) technique for purposes of range and range rate measurements, spreading the spectrum, or for security. In this work the space and ground segments’ BER verification by measurement is achieved by exploiting the inherent locally generated pseudo random sequences (PRS). The application of this algorithm for measuring the BER of the space segment necessitates sending the information as a telemetry parameter just before the end of the communication session. This algorithm requires measuring BER over a large sample of the received chips due to the randomness of the errors. The high chip rate of the employed PRS (around 0.5MHz) and the period of the communication session (7-10 minutes in average) satisfy this condition. This algorithm is applied for measuring the BER for a digitally implemented coherent MSK DS-SS modem and the results for measuring the BER against Eb/No are presented

FPGA-based Coherent MSK Spread Spectrum Modem for Small Satellites TT&C Transponders

In low earth orbit (LEO) satellite communication links it is required to employ modulation/demodulation techniques to achieve transmission with minimum power and efficient usage of spectrum (according to international telecommunication union (ITU) and consultative committee for space data systems (CCSDS) regulations and recommendations) with minimum bit error rate (BER) at the receiver. Minimum shift keying (MSK) meets all these requirements. It is required also to measure the varying distance between the satellite and the ground station (range) and the velocity of the satellite (range rate) to facilitate the tracking of the satellite by the ground station antenna tracking system and therefore, a direct sequence spread spectrum (DS-SS) technique is used. Thus, coherent MSK DS-SS modem is chosen in this paper for LEO communication links. This paper investigates design, implementation and testing of a FPGA-based coherent MSK DS-SS modem suitable for small satellites TT&C transponders. The modem includes a MSK modulator, an automatic gain control (AGC), and MSK DS-SS demodulator/synchronizer (where a proposed novel phase ambiguity solver algorithm is presented). Demodulator performance is evaluated by adding band-limited (nearly white within the signal bandwidth) Gaussian noise to the MSK DS-SS modulated signal (resulting in Eb/No near 0dB) and measuring the BER and the phase variances of the synchronized carrier and the clock with its operating Eb/No for the extracted chips, which show good performance