n 2 n 1 n errors x 1st cycle frame 3 lost following

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Unformatted text preview: es 1 2 3 4 5 6 7 . . . . . . n-2 n-1 n Errors X 1st cycle: Frame 3 lost. Following frames rejected Frames 1 2 3 4 5 6 7 . . . . . . n-2 n-1 n Errors X 2nd cycle: Middle frame lost, following rejected Frames 1 2 3 4 5 6 7 . . . . . . n-2 n-1 n Errors X 3 cycle: Frame n-1 lost, so last two not delivered rd W hen frame loss errors come in bursts, then the hammer pattern is vulnerable to the loss of all the repeats of a frame, so in this case the cyclic pattern could give better results. The hammer pattern is not a universal solution but it provides another tool for solving the problems of getting a complete sequence of frames on board. TCretrans7 2008Sep09 I.B. + M.A. DE LANDE LONG SOFTWARE + CONSULTANCY Options for repeated retransmissions for telecommand Page 3 1.3.1 Scenarios with sporadic frame loss The typical scenario for the application of the proposed systematic retransmissions is a deep space mission with long light delay and with link conditions where errors are sporadic, so that sometimes one frame, or perhaps two frames, are lost from a sequence of frames. When the hammer pattern of transmission is used in this scenario, it can increase the probability that the full sequence is received during a limited transmission session. 1.3.2 Scenarios with high probability of frame loss There are deep-space scenarios where the probability of frame loss is exceptionally high: • Spacecraft on the far side of Earth’s orbit. In this scenario, the RF link to the spacecraft can suffer severe solar interference, leading to extreme frame loss rates. The telemetry link is also likely to be affected by the solar interference. • Spacecraft unable to maintain correct attitude. In this scenario, the low-gain antenna at the receiving end of the telecommand link is intermittently pointing in the wrong direction. If the spacecraft is spinning, there is a periodic element to the high frame loss rate. When the spacecraft enters safe mode, the telecommand data rate is very low and telemetry transmissions have very low power and a lo...
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