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678: 774ワット発電中さん [sage] 2024/11/09(土) 11:09:34.22 ID:vcwO23TC

Chapter 3
Voyager Telecommunications
https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--141029.pdf

3.2.1.3 Command Demodulation.
Voyager receives and demodulates the command signal from the uplink carrier. (DSN command modulation is described in the DSN Telecommunications Link Design Handbook [7], module 205, 34-m and 70-m Command.) The signal consists of 16-bps, Manchester encoded commands, biphase modulated onto a squarewave subcarrier frequency of 512 hertz (Hz).

3.2.2.4 Telemetry Modulation.
The telemetry comes to the telemetry modulation unit (TMU) separately as a “low-rate” channel and a “high-rate” channel. Low rate is 40 bps only and its routing through the TMU is such that it can only be downlinked as uncoded bits. High rate, one of a set of rates between 10 bps and 115.2 kilobits per second (kbps), is downlinked as coded symbols. The TMU encodes the high-rate data stream with a convolutional code having constraint length of 7 and a symbol rate equal to twice the bit rate (k = 7, r = 1/2).
Either modulator can biphase-modulate the telemetry symbols onto either a 22.5-kilohertz (kHz) or a 360-kHz subcarrier. A subcarrier is a symmetrical square wave signal derived from a TMU crystal oscillator that has a nominal frequency of 2.88 MHz. The 360-kHz subcarrier is required for bit rates greater than 7.2 kbps. The TMU has one modulator for the S-band downlink and another for X-band downlink. The modulated subcarrier goes to the S- or X-band exciter.9

3.6.2.1 Image Data Compression (IDC). After the Jupiter and Saturn encounters, JPL completed IDC software for Voyager. The project loaded the software into the backup flight data subsystem (FDS) computer that was reconfigured to handle just that task [12, 13].20 Uncompressed Voyager images contain 800 lines, 800 dots (pixels) per line, and 8 bits per pixel (to express one of 256 gray levels). However, much of the data content in a typical planetary or satellite image is dark space or low-contrast cloud features. By counting only the differences between adjacent pixel gray levels, rather than the full 8-bit values, image data compression reduced the number of bits for the typical image by 60 percent without unduly compromising the information. This reduced the time needed to transmit each complete image from Uranus and Neptune to Earth by the same 60%.

3.6.2.2 Error-Correcting Coding. Like other deep space links, the Voyager telemetry link is subject to noise in the communications channel changing the values of bits transmitted over the channel—in other words, causing bit errors. Error-correcting coding reduces the rate of errors in the received information
that is output. Such coding increases the redundancy of the signal by increasing the number of bits transmitted relative to the information bit rate.21 The Golay encoding algorithm used at Jupiter and Saturn required the transmission of one overhead bit for every information bit transmitted (100 percent overhead). Voyager carried an experimental Reed-Solomon data encoder, expressly for the greater communication range of the Uranus and Neptune phase of the mission. The new Reed-Solomon encoding scheme reduced the overhead to about one bit in five (20-percent overhead) and reduced the bit-error rate in the output information from 5 × 10–3 to 10–6.

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