Friday, November 30, 2012

setiQuest Voyager 1 redux

The NASA Voyager 1 probe has been spotted again at the Allen Telescope Array (ATA). This is fairly amazing since JPL is the only other group that can see Voyager and they use a much bigger telescope that's part of the Deep Space Network (DSN). The SETI Institute's Jane Jordon and Jon Richards were kind enough to send me this new Voyager data so that I could look at it with the baudline signal analyzer. Thank you Jane and Jon.

This Voyager signal was recorded at 8419.62 MHz and was packetized into an .archive-compamp file by SonATA. The following command line streams the 4-bit quadrature Voyager samples into baudline:

cat | extract_compamp_data 9 | baudline -session compamp -stdin -format s4 -channels 2 -quadrature -samplerate 711.1111 -record

This Unix command line of multiple pipes extracts the 9th channel from the .archive-compamp file and feeds that into baudline's standard input with the appropriate configuration settingsNote the .L. in the compamp file name.  I don't know why SonATA does this but the X & Y linear polarization .compamp files are labeled as being L & R circular polarization.

X polarization
Below is the baudline spectrogram of the X linear polarization. Note the weak diagonal signal on the left side that starts on the top at around -160 Hz. This drifting line is caused by the Earth's rotation and the Doppler Effect. I call it Doppler DriftClick on the image below for a larger and better view of this weak signal.

Y polarization
Below is the baudline spectrogram of the Y linear polarization file. Note that the signal is bit stronger than it was in the X polarization above. Compare this with the signal strength from the July 2010 Voyager data analysis.

X & Y polarizations
The two spectrograms above were  combined to create this dual channel spectrogram (see the Channel Mapping window). The X polarization is green and the Y polarization is purple. Notice how this dual channel technique makes the weak drifting Voyager signal easier to see. It also makes it easy to visually compare both polarizations.

If you click to look closely at the larger version of this image you'll see several very weak crisscrossing lines and curved whistler-like shapes that are much weaker than the main drifting signal. These very weak features could be real or they could be just random shapes in the noise floor.  Need more data to know for sure. If they are real then they're likely artifacts caused by distortion somewhere in the signal path.

Auto Drift
Baudline's Auto Drift feature was enabled and each of the X & Y polarizations were pasted into the Average window. The Y polarization (purple) is about half a dB stronger which confirms our spectrogram strength observation mentioned above. The drifting signals are about 3 - 4 sigma above the noise floor. The auto drift rate measurement window reports that the signal is drifting by -0.5770 Hz/second.

Next Auto Drift is enabled with baudline's spectrogram display. Think of this as a 4-dimensional version of the above Average display where the additional dimensions are time and vector space. Each pixel has 20865 possible vector paths and that works out to about 10 billion possible vector paths seen in the spectrogram image below. Good thing that Auto Drift is a quick O(n log n) algorithm so that image rendering is rather fast.  For reference the Drift Integrator's settings were: beam width = 326 slices (16 seconds), optimum overlap = 400%, Auto Drift quality = 8.

Notice how the weak drifting signal stands out and is easier to see. Also note how the drifting signal fades in strength between the X (green) & Y (purple) polarizations. This image reminds me a lot of ice crystals on a window pane. It isn't that far out there since the Auto Drift algorithm that generates the spectrogram vectors is somewhat similar to how ice crystals grow.

The Histogram window shows 5 vertical lines (bins) that make a Gaussian-like shape (see AWGN). This sparse histogram is not surprising since the input signal has 4-bit quantization. Note that only slightly more than two bits are being utilized.

Baudline's Waveform window shows a time series view of what the 4-bit quadrature signal looks like. This sample distribution matches what is seen in the Histogram window above.  Do you see the signal? It's the magic of DSP that allows such a weak drifting signal to be encoded in so few bits.

No modulation characteristics are visible in either the baudline spectrogram or Average window views. The Voyager signal looks like a non-information baring pilot tone. This could be due to the weak nature of the signal or because Voyager was not transmitting at the time the signal was recorded.

As mentioned above, this setiQuest Voyager 1 redux capture is much weaker than the July 2010 Voyager 1 signal I previously blogged about. A good question is why is it weaker?  Here is a list of possible explanations:
  • 28 months have passed and Voyager is now 8.3 AU farther away.
  • Voyager has entered the Heliosheath and the compressed turbulent solar wind is increasing signal attenuation.
  • Voyager's Plutonium-238 power plant has a half-life of 88 years and is producing less power for signal transmission.
  • Voyager's dish antenna could be slightly off axis and is not pointed directly at Earth.
  • The ATA dishes could be slightly off axis and are not pointed directly at Voyager.
  • Reduced collection area due to some ATA dishes being off-line.
  • Some cryo feed units require maintenance and are resulting in a higher ATA system temperature.
  • An error in the calculation of the ATA beamformer coefficients.
  • Increased local RFI mixed with Walshing artifacts are effectively reducing ATA system gain. 
  • Rainy cloudy weather in Hat Creek, California.
  • The compamp 4-bit sample quantization is pushing up the noise floor. The first Voyager data collection used 16-bit samples.
  • Aliens have found the Voyager space probe and are fiddling with its innards.
Some of these explanations are unlikely or not significant enough to account for the reduction in signal strength. What do you think? Please discuss in the comments section.

Data licensed through SETI.
Software licensed through SigBlips.


baudline said...

The day after this Voyager 1 signal was recorded Jon Richards tweeted that it snowed at the ATA. This could be a cause of signal attenuation.

robackrman said...

This was enjoyable to read. I assume instead of "reduction in signal strength" you really mean reduction in signal-to-noise ratio? Do you know what were the elevations for the two Voyager observations? With respect to that, the background sky temperature (noise) will be higher at low elevation. Moisture - alluding to Jon's suggestion - will affect the higher frequency observations, such as with the ~8GHz Voyager carrier. The inconsistent general performance (elements included and feed temperatures) of the ATA instrument must always be considered, but that meta information is not usually shared.

baudline said...

Hello Rob,

That is a good question. Does moisture in the air attenuate a radio signal, scatter it, or raise the noise floor? My guess is the first two.

Jon didn't suggest snow was the culprit. I pieced that together from a random twitter post. He did mention in a setiQuest forum post that the ATA could of been pointed near the Sun. My guess is that this would raise the noise floor.

robackrman said...

A table of Az,El coordinates -- generated using JPL
HORIZONS -- comparing the Sun and Voyager 1 tracks
from the perspective of the ATA suggests that they
were widely separated.

Sun Voyager 1
------ UTC ------ -- Az -- -- El - -- Az -- -- El -
2012-Nov-07 20:00 *m 182.9546 32.5574 127.4295 50.1510
2012-Nov-07 20:10 *m 185.7899 32.4111 130.5363 51.6258
2012-Nov-07 20:20 *m 188.6102 32.1720 133.8251 53.0318
2012-Nov-07 20:30 *m 191.4088 31.8411 137.3080 54.3601
2012-Nov-07 20:40 *m 194.1791 31.4201 140.9949 55.6011
2012-Nov-07 20:50 *m 196.9153 30.9110 144.8932 56.7446
2012-Nov-07 21:00 *m 199.6122 30.3161 149.0056 57.7795
2012-Nov-07 21:10 * 202.2651 29.6380 153.3290 58.6946
2012-Nov-07 21:20 * 204.8702 28.8795 157.8533 59.4789
2012-Nov-07 21:30 * 207.4244 28.0438 162.5596 60.1216
2012-Nov-07 21:40 * 209.9252 27.1341 167.4203 60.6131
2012-Nov-07 21:50 * 212.3710 26.1536 172.3988 60.9456
2012-Nov-07 22:00 * 214.7606 25.1057 177.4515 61.1135

jimlux said...

atmospheric moisture causes both attenuation and (because it is warm) raises the sky temperature.

I would imagine that the EIRP of the spacecraft is reasonably constant, even if the power supply is weaker. Typically, there is a voltage regulator on the bus, and in any case, the HV power supply for the saturated tube amplifier would tend to make for a constant output power. There might be some aging effect but it would be very small (<tenths of a dB)

baudline said...

Excellent point Jimlux. I didn't realize that atmospheric moisture could also raise the noise floor. Note for the reader; signal attenuation and raising the floor both reduce the Signal to Noise Ratio (SNR). In your opinion, by how many dB could the SNR be reduced by atmospheric moisture?

Unknown said...

Isn't the Voyager data on a subcarrier? If this is the raw RF spectrum I would only expect to see the carrier; the data is off to the sides somewhere.

baudline said...

Hello Phil. We've talked about where the missing data is on the recent setiQuest Voyager thread but the answer is that we really don't know. There are a couple possibilities. We could be looking at the wrong frequency or at the wrong time (Voyager doesn't transmit continuously) or the data signal is too weak to see. If it's the wrong frequency then what is the correct frequency? The original Voyager SonATA dataset was 546 kHz wide and we couldn't see it there either. It is a big mystery.