korneyy>> А чего мы цитату на половине обрываем? Дальше в следующем предложении:
korneyy>> Для приема сигнала на Земле используется 34-метровые антенны сети дальней космической связи NASA, но в некоторых случаях задействуются самые большие 70-метровые антенны, и тогда скорость удается поднять до 600 и даже 1400 бит/с.
ccsr> И это дает право вам утверждать что такая скорость позволяла американцам транслировать прямой телевизионный сигнал с Луны?
Unified S Band System FM and Video Signals
The normal operating mode of an Apollo S-band downlink transmitter was PM. This mode provided for coherent Doppler tracking, uplink commands, downlink telemetry and two-way voice, but not television. Video signals, even that from the slow scan camera used during the Apollo 11 EVA, are much wider in bandwidth than the other Apollo downlink signals. The PM link margin simply could not provide an acceptable picture, even when the largest available dishes were used. A means was also needed to transmit wideband engineering and scientific data, such as that recorded on a tape recorder and played back at high speed.
The answer to both needs was wideband frequency modulation (WBFM). FM with a large modulation index exhibits a strong capture or threshold effect. The output signal-to-noise ratio (SNR) can be significantly greater than the RF channel SNR provided that the RF SNR remains above a threshold, typically around 8-10 dB. In other words, for a slight increase in signal strength there is a large decrease in background noise. At some point, the background noise is essentially zero, and the video and subcarriers are loud and clear. This enhancement comes at a price: below the FM threshold, the output SNR is worse than the RF channel SNR. Reception is "all or nothing"; a receiving antenna too small to capture the video cannot capture the subcarriers either.
Potential difficulty receiving WBFM downlinks from Apollo spacecraft were enough of an issue for NASA that engineers created an Erectable S-Band Antenna. The Erectable S-Band Antenna was first flown on Apollo 11 and was intended to provide a stronger television signal for the first lunar moon walk. Because time during the brief Apollo 11 EVA was so precious, the expected 19-minute deployment of the antenna would have a major impact of productivity. Consequently, an assessment was made of the first few minutes of the black and white TV signal coming through the Lunar Module's steerable antenna. The signal was deemed adequate, so the Erectable S-Band wasn't used. It was used on both Apollo 12 and 14. Erectable S-Band Antenna deployment was intended to be easy enough that one person could do it in about 15 minutes. However, training convinced the Apollo 12 crew that they might have to work together when aiming the antenna at Earth. On both the Apollo 12 and 14 deployments, the astronauts worked as a team doing rough alignments and then, with one of them watching the sighting glass and making small adjustment to elevation using the flexible-cable crank, the other astronaut held the antenna to keep it relatively steady and prevent a collapse.
The Command Service Module carried separate FM and PM transmitters that could operate simultaneously, so voice and telemetry continued to be transmitted by PM while the video came down by FM. The Lunar Module only carried a single transmitter that could operate in either FM or PM, but not both. FM cannot be used for Doppler tracking, so the unar Module always transmitted PM during flight, reserving FM for when video was required during moonwalks and surface roving.
Это сообщение редактировалось 15.02.2011 в 15:14