# Why is the operating temperature for the Voyagers' receiver noise calculation about 1550K?

In line 10 of Table 5.2 of DESCANSO IV - Voyager Telecommunications it shows a value of the uplink receiver noise spectral density of -166.7 dBm/Hz, which is 196.7 dBW/Hz which is 2.1E-20 Watts/Hz = $$k_B T$$. With $$k_B$$ = 1.381E-23 J/K that's a temperature of about 1500 K (which seems really hot!) and that agrees with the stated operating temperature of 1545K which again, seems really hot!

In Table 5.3 for the downlink receiver at DSN, the noise temperature is 21K and that's consistent with it being cooled to about 13K. But I can't understand this 1500K figure for Voyager, it seems unphysical unless the technology is so old that the source of the noise is the 1970's era front-end transistor itself.

Were they really that noisy back then?

• Was this based on a worse case environment close to Jupiter, perhaps? Jan 5, 2019 at 10:55
• @SteveLinton Same line 10 in Table 5.3 shows a detailed breakdown for contributions, including Ground, Galactic, and Atmospheric. You may be right but if so, I'd be surprised they defined the Operating Temperature to be ~1500K rather than break it down as received noise. My hunch is that this is partly related to why home satellite dishes used to be so big; old transistors were noisy.
– uhoh
Jan 5, 2019 at 11:06
• You may well be right, but I was thinking of the effect of particle strikes on the receiver/amplifier, rather than incoming radio noise. Don't know if that would be classified the same way, Jan 5, 2019 at 12:25
• @SteveLinton thats interesting thinking!
– uhoh
Jan 5, 2019 at 12:34
• @uhoh It could be that the signal goes straight from the antenna into the mixer? Jan 14, 2022 at 4:45

Voyager's antenna is pointed at the sun. The transmitter on earth must be powerful enough to stand out against the sun, at least within the (very limited) receiver bandwith. All other sources of noise are negligible and thus not listed in the link budget.

The noise power spectral density seen by voyager depends on the power spectral density of the sun in the S-band, antenna gain, and distance from the sun. Noise power spectral density is often divided by Boltzmann's constant to give more convenient numbers. This noise temperature is not a real temperature and you shouldn't read to much into it. 1545K is just an alternate representation of -166.7dBm/Hz, which again is an alternate representation of 0.0000000000000000000213W/Hz.

On the downlink, the antennas on earth are pointed away from the sun. Cosmic background radiation contributes to the overall noise figure but in this direction it's only 2.7K. There is no single dominant source of noise, hence the breakdown.

• I'm not sure this clearly answers the question Why is the operating temperature for the Voyagers' receiver noise calculation about 1550K?
– uhoh
Mar 5, 2020 at 12:22
• So for example might you be suggesting that it is an upper limit for the receiver's front end or the entire system i.e. if Voyager's antenna is pointed in the direction of an extended blackbody source with a characteristic temperature of 1550 K it will still be able to receive narrow band signals from DSN without saturating or other signal degradations? If that's what it is, can you make that a little more clear? If not, can you otherwise clarify? Thanks!
– uhoh
Mar 5, 2020 at 12:43
• @uhoh essentially, yes, the latter one. It's not the noise temperature of the receiver on its own; it's the expected noise temperature of the system including the antenna pointed at Earth (which also sees the Sun). But as a temperature it doesn't have any physical meaning other than "the temperature of a resistor that would produce -167dBm/Hz of Johnson noise". It doesn't need to bear any relation to the temperature of anything in space. Jul 17, 2020 at 18:38
• I think the noise temperature has pretty much defined physical meaning. It is the temperature of a black body that will produce the same amount of noise if the antenna is surrounded by it. These 1500K make sense if one considers the Sun with its 6000K covering 1/4 ^ 4 of the antenna beam width. Nov 20, 2020 at 22:02
• @fraxinus The Sun's power at those frequencies and at that distance is negligible compared to the receiver noise. space.stackexchange.com/a/51305/14419 Jan 15, 2022 at 22:30

Having listened to CuriousMarc's excellent Apollo video and read the DESCANSO Chapter 3 document, there is no evidence that Voyager has any amplification prior to the mixer. At that time, at those frequencies, amplification would likely have involved another travelling wave tube.

Quoting from "The feasibility of a direct relay of Apollo spacecraft via a communication satellite" by P. E. Schmid: "[The noise figure,] F is on the order of 10 db at the RF input of present spacecraft receivers. Most of the noise generated in a conventional superheterodyne receiver lacking any signal preamplification is due to the mixer stage. At a frequency of 2 GHz, present spacecraft mixers consist of semiconductor diodes either in a single-ended or balanced configuration. For reasons of reliability as well as stability, most microwave mixers employ silicon diodes. By careful design, a slight noise reduction might be anticipated in this area. For example, the stated typical noise figure for the SAGE Laboratories 1.7 GHz to 2.4 GHz balanced mixer (Sage Model 225233) is 7.0 db."

7.0 dB would take a noise temperature of 300K up by a factor of five to 1500K.

• I think this is the correct answer. The link budget for the spacecraft receiver separates out "hot body" noise from operating temperature (noise). The hot body noise is the contribution from things like the Sun and the zodaical light, and that's listed as zero (and BTW the zodaical light is optically thin). The downlink budget shows a much lower receiver temperature (13K) because the DSN uses cooled front ends. I think they probably should include the solar noise, though, because I calculated that Earth is always within roughly the HPBW of the X-band antenna. Jan 17, 2022 at 22:13

That temp ( ~ 1550 K ) looks suspiciously like the maximum Zodiacal dust grain temp before sublimation Thus the dominant thermal blackbody background Voyager is likely to ever see :