Citizen's Band [radio]?

[quote]
On 2001-10-28 23:00, saj wrote:
Peter B -

Yes, you are correct. These days however, the transmissions are largely encrypted and in digital format with unique, complex modulation characteristics.


Got a question about encryption vs. encoding... Were the transmissions encrypted to prevent them from being intercepted and understood, or were they encoded for ease of communication? If they were encrypted, one could certianly suspect NASA of wanting to cover up something, but not if they were encoded. There is a differance...

My understanding is that the signals were not encrypted, merely encoded onto the carrier by straightforward modulation.

The shuttle's voice communications are digital and carried on a channel inside the telemetry stream. You can't listen in on the shuttle, but you could listen in on Apollo.

Today's telemetry wouldn't be possible without the phase-locked loop (PLL) which was invented for Apollo's digital communications. Everything from the space shuttle to Dish Network uses a PLL.

The PLL was designed at JPL for the purpose of narrow bandwidth signal detection and doppler extraction for space missions. It was the basis of the receivers for the Minitrack tracking network that was in place prior even to the first Sputnik launch.

http://www.ieee.org/organizations/hi...s/rechtin.html

Wow, that old?

I thought PLLs had been around since the advent of digital communications. To receive a digital signal, it must be within a certian timing tolerance with the distant end, otherwise it is nothing but noise. PLL's are the best way to slave less accurate oscillators with very precise timing standards like cesium and ribidium clocks...

If the signal is phase modulated then a PLL must be used to receive it, but not all digital communications must be phase modulated. They all are today, but I suspect that was not always true. Nevertheless I'm perfectly happy with the notion that PLLs were not invented for Apollo, but were invented considerably earlier.

PLL's fix multiple problems for space telemetry systems, the first and worst is that of tracking the doppler shift of the main carrier due to the relative motion of the S/C to the ground station. By comparing the received reference to a ground reference, the range-rate can be extracted.

Telemetry demodulation is an additional benefit. Multiplying the received signal by the reconstructed carrier allows you to extract any modulation scheme, AM, PM, or FM, analog or digital. An early example of PLL signal demodulation is the color demodulator of a TV set. The color carrier is used to phase lock a local crystal oscillator, and the chroma information is phase modulated on the color subcarrier.

The early S/C systems used analog TM, but they still required PLL receivers.

Removed extra linefeed

<font size=-1>[ This Message was edited by: Karl on 2002-04-29 20:33 ]</font>

Wow, you read my mind. I was just going to comment on the NTSC video signal standard using an analog phase modulation.

Here's an interesting historical tidbit, NASA standard telemetry has the bytes aligned with MSB first, vs. industry standard LSB first. Why? because the first digital data systems were essentially just a successive approximation A/D converter hooked directly to the encoder. A SA A/D converts the most significant bit first, successively deciding which bit is a "1" or a "0". Before integrated circuits, storage registers had to be built with discrete transistors, so storing the bits to reverse the order for transmission would take a lot of hardware, so instead, each bit was jammed into the downlink as it was converted.