1、 Rep. ITU-R SA.2065 1 REPORT ITU-R SA.2065 Protection of the space VLBI telemetry link CONTENTS Page 1 Introduction 2 2 Space VLBI system 2 2.1 Space VLBI telemetry signal, noise and interference 2 2.1.1 Signal . 3 2.1.2 Noise and interference . 3 3 Space VLBI telemetry detection. 3 3.1 Telemetry re
2、ceiver 3 3.1.1 Matched filter. 4 3.2 Telemetry bit error rate (BER) . 5 4 Space VLBI cross-correlation 5 4.1 Cross-correlation of digitized observation record 6 4.1.1 Mean value of cross-correlation product 7 4.1.2 Evaluation ofcP1+8 4.2 Cross-correlation SNR (XSNR) . 9 5 Derivation of space VLBI te
3、lemetry interference criteria 10 5.1 Degradations in XSNR due to noise and interference in the telemetry link. 10 5.2 Tolerable degradation due to interference 11 5.3 Calculation of interference criteria 12 References 14 2 Rep. ITU-R SA.2065 1 Introduction Very long baseline interferometry (VLBI) is
4、 used to achieve very high resolution of observed radio sources. VLBI has many scientific and engineering uses, from observing extragalactic radio sources to navigating and tracking of spacecraft Thompson et al., 2001. 2 Space VLBI system Space VLBI pairs a space-borne observatory with a ground-base
5、d observatory to form an interferometer as shown in Fig. 1. FIGURE 1 Space VLBI system A telemetry link returns the signal record from the spaceborne observatory-to-Earth. This signal record is degraded by thermal and other internal noise sources in the space-to-Earth telemetry link and by external
6、interference from other radio stations into the receiving earth station. An appropriate figure-of-merit for the overall space VLBI telemetry link is the degradation in the cross-correlation SNR (Thompson et al., 2001 and Recommendation ITU-R SA.1344 Preferred frequency bands and bandwiths for the tr
7、ansmission of space VLBI data). This Report characterizes the degradation to the cross-correlation SNR by the interference present on the telemetry link. 2.1 Space VLBI telemetry signal, noise and interference In the telemetry link analysis, we assume that signal s(t) and noise n(t) plus an interfer
8、ing signal I(t) are present, the received signal being s(t)+n(t)+I(t). Rep. ITU-R SA.2065 3 2.1.1 Signal The signal is a carrier with power P and frequency fc. It is data modulated using differentially encoded quadriphase-shift keying (DQPSK) and rectangular data pulses. This data modulation suppres
9、ses the carrier. The signal is of the form: )2sin()()2cos()()(21tfkTtqkdPtfkTtqkdPtsckck+=(1) where T is the quaternary symbol period, and otherwise0,0,1)(Tttq 23.7 dB, and its effect on the carrier tracking loop will be even less. References THOMPSON. A.R., MORAN J.M. and SWENSON G.W. 2001 Interferometry and synthesis in radio astronomy, 2nd Ed., J. Wiley & Sons.
