Anotace:
One of the main earth stations that INPE uses to track and control its satellites is located in the city of Cuiaba (15.33°S, 56.46°W, dip latitude, 6.1°S), between the magnetic equator and the peak of the equatorial anomaly. Based on the GISM model, it is determined that the ionospheric scintillation index (S4) for the telemetry link in the S band (2208 MHz), between Cuiaba station and the SCD2 satellite, depending on the date and time, can reach values greater than 0.8. This is the first study conducted on ionospheric S-band scintillation in this region of the earth. In this article, the channel model for the link and the telemetry receiver architecture are presented in order to subsequently evaluate some effects of ionospheric scintillation on the functioning of the communication system. The modulation used is OQPSK and a fully-digital demodulator recovers the carrier phase using a Costas loop and synchronizes the symbols using a Gardner synchronizer. The design of OQPSK demodulator is detailed and the impact of ionospheric scintillation on general demodulator performance and on the functioning of its modules is discussed. The system bit error rate, the error variances of the carrier phase and symbol delay in different conditions of severity of ionospheric scintillation were figured out through computer simulation. From the presented results, it is evident that, for the adopted receiver architecture, which was designed for a space channel without scintillation, there is a substancial degradation on performance of the system even for S4=0.5 and, for the scenario where S4=0.8, the link becames practically inoperative.