"Received power" on NASA's DSN web page

Question

Out of curiosity, the "received power" shown for a spacecraft on NASA's "DSN now" web page 1 - is that at the receiver, or at the antenna?

I'm looking at one of the antennas now receiving data from Voyager 1 and it states the signal strength is -160 dBm. If that's at the antenna it's a crazy low signal level... if it's at the receiver, after the antenna's gain, it's even more crazy!

Screenshot at 2023-05-25, 19.03.42 UTC showing Voyager 1 being received by 70 meter DSS 63 Madrid at -160 dBm:

uhoh · Accepted Answer · 2023-05-26 21:49:06Z

5

I can't find a definitively authoritative source right now (no doubt it's in (at least) one of the DESCANSO series) but that's almost certainly going to be the calculated received power in dBm at the input to the front-end amplifier based on the amplified and processed signal level and known gains.

Numbers like that are not unheard of for the deepest of space probes. See for example DrSheldon's answer to Understanding the information contained in the Deep Space Network XML data?

Interestingly, How to calculate data rate of Voyager 1? and my answer there suggest a power from Voyager 1 into a 70 m dish of about -180 dBW or -150 dBm. That was five years ago, I don't know what part of the 10 dB difference is real what part is random and what part is systematic, e.g.

RTG power dropping
distance increasing
Earth oscillating by +/-1 AU annually
pointing errors of Voyager's high gain antenna due to systems shutting down or being degraded or propellant conservation measures

However anyone reading Dr. Sheldon's answer linked above could dig back in time and see just how Voyager's signal strength has been dropping. In fact that sounds like a good question to ask - "Have there been systematic changes in received signal strength from the Voyagers, and which components are seasonal, random, and smooth-monotonic?"

update: Ah! It's -150 dBm for Voyager 2

edited May 26, 2023 at 21:49

answered May 26, 2023 at 7:06

uhoh

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1

$\begingroup$ One good illustration of the systematic changes is page 30 of descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_ed.pdf , showing the variation by time of day for a particular day of the year from 1995 to 2020. The previous dozen pages describe how those numbers were calculated. Also, as I wrote in space.stackexchange.com/questions/14317/… , the number that really matters is power spectral density (W/Hz, not raw W), and Voyager's bandwidth is really small. $\endgroup$
– Ryan C
Commented May 26, 2023 at 18:12
1

$\begingroup$ @RyanC Indeed "the number that really matters is power spectral density" but the number we can really calculate for the OP based on eyes.nasa.gov/dsn/dsn.html is power. They'll instruct Voyager to use data rates and modulation schemes du jour based on conditions which include available transmit power, distance, and ground station configuration (all fairly stable these days) somewhat related and fun: How is stacking oranges in 24 dimensions related to receiving and decoding signals from the Voyagers? and $\endgroup$
– uhoh
Commented May 26, 2023 at 21:28
$\begingroup$ Did the Voyager spacecraft use a Golay, a Reed-Solomon and/or a Hamming code for data transmission encoding for error correction? (Need clarification) and What Voyager spacecraft hardware performed transmitted data coding in such a complicated way? Did ground decoding use “a big monster filling a rack”? and my favorite How to calculate data rate of Voyager 1? $\endgroup$
– uhoh
Commented May 26, 2023 at 21:31
1

$\begingroup$ you can't push Shannon to the limit. It is a non-constructive existence proof: it calculates the best any encoding scheme could ever possibly manage, but does not say how to achieve that bound. People keep inventing new schemes in search of something closer to the limit, but we have a long way to go before we figure out how to saturate it. $\endgroup$
– Ryan C
Commented May 28, 2023 at 12:12
1

$\begingroup$ and bandwidth is a tricky thing to define. theoretically, any finite-time signal has infinite bandwidth, in the sense that the Fourier transform is non-zero for unbounded frequency. in practice, you have to use something finite, but how exactly you define the cutoff is up to you. popular definitions include the region within 3 dB or 10 dB of the peak, but there is definitely measurable power well outside that if you have decent SNR. $\endgroup$
– Ryan C
Commented May 28, 2023 at 12:42

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