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This answer to Do the Voyager probes transmit analogue or digital data? states that the Voyagers sent only digital data. This is no doubt mostly because analog data was digitized and store digitally which requires an analog to digital converter.

The earliest satellite data was sent in analog form; signal levels were encoded by a voltage-to-frequency converter and the audio frequency tones were then sequenced and transmitted.

Question: What was the first use of analog to digital conversion in a orbiting or deep-space spacecraft? What about sub-orbital spacecraft?

note: For the purposes of this question digital conversion needs to be to more than one bit. Booleans or thresholds (on/off) don't count.


Other questions about electronic items in spaces (firsts or lasts):

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  • $\begingroup$ Does the conversion have to occur in the spacecraft, or can the spacecraft send analog, which gets digitized on the ground? $\endgroup$ – SF. Feb 4 at 16:40
  • $\begingroup$ @SF. I'm primarily interested in the conversion happening in space. $\endgroup$ – uhoh Feb 4 at 22:04
  • $\begingroup$ It is difficult to build an analog to digital converter without using thresholds. $\endgroup$ – Uwe Sep 21 at 10:39
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The first analog to digital converter was flown on board Injun 1 on June 29, 1961. It was also the first spacecraft completely designed and built by a university--in this case, the University of Iowa.

From "The Origins of Space Radio and Plasma Wave Research at the University of Iowa" by Donald A. Gurnett:

At the suggestion of a newly hired Assistant Professor, Brian J.O'Brien, I was urged to start thinking about a spacecraft-borne “encoder” that would transfer the "0s" and "1s" in the binary counter to a corresponding serial sequence of "0s" and "1s" that could be transmitted to the ground. I did this by finding a very simple method of converting the binary counter (often called an accumulator) into a shift register using a voltage control line that had only one resistor and one diode per stage in the counter. This system is now called a “shifting accumulator,” and was later the subject of a patent application by the Office of Naval Research. By using one shifting accumulator for each particle detector, I devised an encoding system that could produce a binary data stream from an arbitrary number of detectors. Analog voltages, such as battery voltages, could also be encoded by the simple addition of an analog-to-digital converter.

Gurnett goes on to describe the encoding system used on Injun 1:

Using frequency-shift-keying between two modulation frequencies, 3.072 and 4.096 kHz, which represented the “0s” and “1s,” this encoder provided digital data at a bit rate of 256 bits/s from thirteen particle detectors and an auroral photometer [O’Brien et al., 1962]. Injun 1 was one of the first spacecraft to use an entirely digital data system and was the first in a series of five low-altitude polar-orbiting spacecraft built at Iowa, later known as the “Injun” series of satellites.

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    $\begingroup$ I'm having trouble finding the first suborbital instance. Since it occurred after the first orbital instance, it was less noteworthy, I guess. $\endgroup$ – called2voyage Feb 4 at 16:03
  • $\begingroup$ "Analog voltages, such as battery voltages, could also be encoded by the simple addition of an analog-to-digital converter." But was there an ADC with 3 or 4 bits in use? $\endgroup$ – Uwe Feb 4 at 22:11
  • $\begingroup$ Thanks, this is much earlier than I expected, the "pace of the space race" was amazing! However the name given to this group of Explorer satellites is cringe-worthy assuming it's this. Origins of NASA Names, NASA SP-4402 doesn't say anything about the origin of the term. $\endgroup$ – uhoh Feb 4 at 22:37
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The very first analog to digital conversion was used in the first satellite ever: Sputnik 1. A barometric switch was used to signal if the gas pressure inside dropped below 130 kPa. A mechanical analog to digital converter with only one bit. I never saw a definition that an ADC should have at least 2, 3 or 4 bits.

There were two thermal switches for internal temperature. If the temperature exceeded 50 °C (122 °F) or fell below 0 °C (32 °F) the duration of the pulses send was changed. A mechanical analog to digital converter with three different states requiring at least two bits.

See Wikipedia.

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