9 ms·

I was in the audience for these presentations last week, and I sat down with Neinavaie and Khalife for a couple hours to discuss their work. I’m also engaged i

I was in the audience for these presentations last week, and I sat down with Neinavaie and Khalife for a couple hours to discuss their work. I’m also engaged in very similar research myself, which I presented at the same conference. So... not directly affiliated with the original authors, but very familiar with the work. Can I answer any questions people might have?

tanvach 5y ago
I’m assuming they needed to have very accurate orbit information for each satellite. Was this provided to them by SpaceX, or was it derived from the signal?
- Rebelgecko 5y ago
  IIRC the abstract says they just used semi-publicly available ephemerides that the Space Force publishes
  - piannucci 5y ago
    Yeah, I think they said they used the TLEs. There are better ephemerides posted by SpaceX on space-track.org under contract w/ the govt, who doesn’t really want to be responsible for tracking all those objects.
mateo1 5y ago
My guess from the article language is some kind of doppler shift analysis compared with ephemeris data and ground reference stations? Is that correct? Is it better than 100m?
- brokenmachine 5y ago
  From the article: >By measuring this effect for six Starlink satellites flying overhead on different trajectories, the scientists could pinpoint their location on the ground to within 7 to 8 meters
Rebelgecko 5y ago
How long does the offline processing take? How well would this work for a moving vehicle (assuming there's no other sensors e.g. IMU)? What are they actually measuring from the satellites... signal phase? Doppler?
- piannucci 5y ago
  1. Dunno, but they’re working in MATLAB. 2. I would not expect that to work very well without an IMU, but you can do some pretty remarkably good odometry using wheel rotations, vision, and/or automotive radar if you’re dead set against using an IMU. 3. I think they were measuring instantaneous Doppler and integrating to get phase. They only looked in a 1 MHz passband, and they didn’t have a very directional antenna, so they must have been acquiring something pretty narrow-band compared to the full downlink packets.
  - zsmi 5y ago
    > 1. Dunno, but they’re working in MATLAB. Processing time would also be highly dependent on what the processing was done with and once the algorithm was established it could be ported to something less generic than Matlab. For example, an FPGA or ASIC for potentially quite short/low-power processing times in not a lot of volume.
DeathArrow 5y ago
What kind of hardware is needed? Do you envision these techniques result in a opensource software usable by general public?
- piannucci 5y ago
  The particular signal they used can be captured with a $40 low-noise block w/ feedhorn (I’m using the Visiblewave Hybrid LNB), a 12-16 VDC biasing circuit, and a garden-variety SDR. I speculate, based on my own studies, that the signal they caught is the Starlink “Idle” tone. I don’t see that signal when the downlink is busy in my own experiments, but I can’t be certain yet. Different latitude, different equipment, different assumptions. The broadband downlink signals I’m interested in require a bit more effort— I’m using a 75cm offset parabolic dish on an azimuth/elevation turret with some nice high-end servos w/ built-in 12-bit encoders. Up until yesterday, we could only capture 60 MHz of real-time bandwidth; but now we have the device we need to capture closer to the entire 2 GHz span of the 10.7-12.7 GHz downlink in one fell swoop. With all that, I still only get about 11 dB of SNR. I’ve seen hints that I might be able to get closer to 20 dB, which I could trade off for a smaller dish.
  - jdiez17 5y ago
    11 dB is pretty impressive, especially if you are just looking at the Doppler shift in the carrier. Which actually begs the question, can you decode anything (e.g. satellite ID) from the downlink/idle signal? If not, how do you figure out which satellite a given signal came from? My first guess would be that you can fit an orbit to the Doppler shift, find the closest TLE set from space-track.org, and assume that the signal came from that satellite. Then propagate the orbit to the observation epoch and that should give you a fairly accurate guess of the satellite's actual location. Or do you do something completely different?
    - piannucci 5y ago
      What you describe with finding the closest TLE is pretty much what they did. We use a highly directional antenna (~1-2 degree beam) and some tracking software, so it’s not ambiguous. I’ve been able to demodulate and reconstruct about 100 subcarriers’ worth of signal, but until recently I couldn’t record the entire bandwidth. Now we’ve got our hands on one of those awesome RFSoCs with 2 Gsps ADC, so it might be possible to decode more. But unknown error correction, bit interleaving, so it’s a tall order. We’d have to get lucky.
irjustin 5y ago
How accurate do you believe this can get? My understanding is part of GPS is knowing the precise location of the satellite[0]. Is this knowable within any useful precision? I naively imagine this is the largest degree of uncertainty in the whole system? [0] https://en.wikipedia.org/wiki/GPS_signals#Navigation_message https://en.wikipedia.org/wiki/GPS_signals#Navigation_message
- piannucci 5y ago
  Accuracy will depend on whether/to what degree SpaceX commits to providing a positioning, navigation, and timing (PNT) service. The technique used in the article is quite powerful and general, but it does have certain drawbacks too. One advantage is that it is relatively insensitive to errors in the ephemeris. The path we are exploring relies more heavily on the ephemeris, as you speculate; but these can be really quite excellent, even in the sub-meter range. Actually the bigger source of uncertainty for absolute positioning using non-Doppler techniques will probably be clock errors. For Doppler, integration time/time-to-first-fix is a sticking point, unless you have 8+ satellite signals, IIRC. We think accuracy in the low single digit meters is not too much of a stretch (note that I am making a claim about a rather different technique than the one in the article... nothing worth doing is ever totally straightforward!).
  - irjustin 5y ago
    Amazing, thanks so much!
  - dr_orpheus 5y ago
    I understand that SpaceX ephemerides they publish are more accurate than the typical two-line element set. But the TLEs using the SGP4 propagator typically have errors on the order of 10s to 100s of meters [0] (and more, the further you are from the ephemeris time). Is the estimation really enough to get to down to the meter or sub-meter accuracy? Is that why it really requires 8+ satellites to get a good fix, i.e. it takes that many measurements to reduce your variance and GDOP? [0] https://conference.sdo.esoc.esa.int/proceedings/sdc6/paper/41/SDC6-paper41.pdf https://conference.sdo.esoc.esa.int/proceedings/sdc6/paper/4...
spuz 5y ago
The article states they used a small antenna to detect the satellite signals but how small is small? After all, the SpaceX "Dishy" is fairly large, heavy and power hungry. Would this technique really be possible with an antenna small enough to strap around the neck of an animal? What about something that could fit in my phone?
- ta988 5y ago
  You don't need to emit (that uses a lot of power) so that's already one factor.
- piannucci 5y ago
  Their antenna was a CotS LNBF. Think just the bit on the end of the arm of a DirecTV dish, not the dish itself. Typical aperture maybe 10cm, full unit about hand-sized.
- vel0city 5y ago
  "Dishy" size is because its a two-dimensional phased-array antenna. It has a lot of small antennas on it to steer the energy released from the antenna directionally through interference patterns. If you're only going to receive and the signals are powerful enough you don't need that big of an antenna to receive signals at these frequencies.
- stephen_g 5y ago
  You can do satellite comms on something that small (maybe an antenna a few centimetres square) if you're only interested in moving a few bits per second or maybe kilobits per second. In the transmit direction you're generally limited by how much power you can pump out (the smaller the antenna, the less antenna gain you have and the more power you need to produce from your transmit chain). In the receive direction, you have a figure called G/T which has to do with the ratio of the antenna gain to the noise temperature of the receiver chain (which includes thermal noise picked up to the universe, plus noise introduced at each gain stage etc.). The simpler the modulation, the less power you need generally, but you can move less data in a given amount of frequency spectrum, so that's the tradeoff.
mrfusion 5y ago
Can this be combined with original gps to get super accuracy? What is the practical value of this? Stronger signals than old gps?
- nayuki 5y ago
  > These orbiting internet nodes fly closer to Earth’s surface than the satellites in the government-run global navigation systems, such as the U.S. Department of Defense’s GPS, the European Union’s Galileo, China’s BeiDou, or Russia’s Global Navigation Satellite System. Because militaries operate many of the systems, the signals can be targets for jamming or scrambling—spurring researchers to look for alternatives. > Because the satellites of internet megaconstellations fly so much closer to Earth than those used in traditional GPS, their signals can be thousands of times stronger. This could make them easier to pick up in dense jungle environments where GPS sometimes struggles to determine position, Langley suggests. The brighter signals might also allow for better data collection of animals outfitted with tracking collars, he adds.
deleted 5y ago
[deleted]

tanvach 5y ago

I’m assuming they needed to have very accurate orbit information for each satellite. Was this provided to them by SpaceX, or was it derived from the signal?

Rebelgecko 5y ago
IIRC the abstract says they just used semi-publicly available ephemerides that the Space Force publishes
- piannucci 5y ago
  Yeah, I think they said they used the TLEs. There are better ephemerides posted by SpaceX on space-track.org under contract w/ the govt, who doesn’t really want to be responsible for tracking all those objects.

mateo1 5y ago

My guess from the article language is some kind of doppler shift analysis compared with ephemeris data and ground reference stations? Is that correct? Is it better than 100m?

brokenmachine 5y ago
From the article: >By measuring this effect for six Starlink satellites flying overhead on different trajectories, the scientists could pinpoint their location on the ground to within 7 to 8 meters

Rebelgecko 5y ago

How long does the offline processing take? How well would this work for a moving vehicle (assuming there's no other sensors e.g. IMU)? What are they actually measuring from the satellites... signal phase? Doppler?

piannucci 5y ago
1. Dunno, but they’re working in MATLAB. 2. I would not expect that to work very well without an IMU, but you can do some pretty remarkably good odometry using wheel rotations, vision, and/or automotive radar if you’re dead set against using an IMU. 3. I think they were measuring instantaneous Doppler and integrating to get phase. They only looked in a 1 MHz passband, and they didn’t have a very directional antenna, so they must have been acquiring something pretty narrow-band compared to the full downlink packets.
- zsmi 5y ago
  > 1. Dunno, but they’re working in MATLAB. Processing time would also be highly dependent on what the processing was done with and once the algorithm was established it could be ported to something less generic than Matlab. For example, an FPGA or ASIC for potentially quite short/low-power processing times in not a lot of volume.

DeathArrow 5y ago

What kind of hardware is needed? Do you envision these techniques result in a opensource software usable by general public?

piannucci 5y ago
The particular signal they used can be captured with a $40 low-noise block w/ feedhorn (I’m using the Visiblewave Hybrid LNB), a 12-16 VDC biasing circuit, and a garden-variety SDR. I speculate, based on my own studies, that the signal they caught is the Starlink “Idle” tone. I don’t see that signal when the downlink is busy in my own experiments, but I can’t be certain yet. Different latitude, different equipment, different assumptions. The broadband downlink signals I’m interested in require a bit more effort— I’m using a 75cm offset parabolic dish on an azimuth/elevation turret with some nice high-end servos w/ built-in 12-bit encoders. Up until yesterday, we could only capture 60 MHz of real-time bandwidth; but now we have the device we need to capture closer to the entire 2 GHz span of the 10.7-12.7 GHz downlink in one fell swoop. With all that, I still only get about 11 dB of SNR. I’ve seen hints that I might be able to get closer to 20 dB, which I could trade off for a smaller dish.
- jdiez17 5y ago
  11 dB is pretty impressive, especially if you are just looking at the Doppler shift in the carrier. Which actually begs the question, can you decode anything (e.g. satellite ID) from the downlink/idle signal? If not, how do you figure out which satellite a given signal came from? My first guess would be that you can fit an orbit to the Doppler shift, find the closest TLE set from space-track.org, and assume that the signal came from that satellite. Then propagate the orbit to the observation epoch and that should give you a fairly accurate guess of the satellite's actual location. Or do you do something completely different?
  - piannucci 5y ago
    What you describe with finding the closest TLE is pretty much what they did. We use a highly directional antenna (~1-2 degree beam) and some tracking software, so it’s not ambiguous. I’ve been able to demodulate and reconstruct about 100 subcarriers’ worth of signal, but until recently I couldn’t record the entire bandwidth. Now we’ve got our hands on one of those awesome RFSoCs with 2 Gsps ADC, so it might be possible to decode more. But unknown error correction, bit interleaving, so it’s a tall order. We’d have to get lucky.

irjustin 5y ago

How accurate do you believe this can get? My understanding is part of GPS is knowing the precise location of the satellite[0]. Is this knowable within any useful precision? I naively imagine this is the largest degree of uncertainty in the whole system? [0] https://en.wikipedia.org/wiki/GPS_signals#Navigation_message https://en.wikipedia.org/wiki/GPS_signals#Navigation_message

piannucci 5y ago
Accuracy will depend on whether/to what degree SpaceX commits to providing a positioning, navigation, and timing (PNT) service. The technique used in the article is quite powerful and general, but it does have certain drawbacks too. One advantage is that it is relatively insensitive to errors in the ephemeris. The path we are exploring relies more heavily on the ephemeris, as you speculate; but these can be really quite excellent, even in the sub-meter range. Actually the bigger source of uncertainty for absolute positioning using non-Doppler techniques will probably be clock errors. For Doppler, integration time/time-to-first-fix is a sticking point, unless you have 8+ satellite signals, IIRC. We think accuracy in the low single digit meters is not too much of a stretch (note that I am making a claim about a rather different technique than the one in the article... nothing worth doing is ever totally straightforward!).
- irjustin 5y ago
  Amazing, thanks so much!
- dr_orpheus 5y ago
  I understand that SpaceX ephemerides they publish are more accurate than the typical two-line element set. But the TLEs using the SGP4 propagator typically have errors on the order of 10s to 100s of meters [0] (and more, the further you are from the ephemeris time). Is the estimation really enough to get to down to the meter or sub-meter accuracy? Is that why it really requires 8+ satellites to get a good fix, i.e. it takes that many measurements to reduce your variance and GDOP? [0] https://conference.sdo.esoc.esa.int/proceedings/sdc6/paper/41/SDC6-paper41.pdf https://conference.sdo.esoc.esa.int/proceedings/sdc6/paper/4...

spuz 5y ago

The article states they used a small antenna to detect the satellite signals but how small is small? After all, the SpaceX "Dishy" is fairly large, heavy and power hungry. Would this technique really be possible with an antenna small enough to strap around the neck of an animal? What about something that could fit in my phone?

ta988 5y ago
You don't need to emit (that uses a lot of power) so that's already one factor.
piannucci 5y ago
Their antenna was a CotS LNBF. Think just the bit on the end of the arm of a DirecTV dish, not the dish itself. Typical aperture maybe 10cm, full unit about hand-sized.
vel0city 5y ago
"Dishy" size is because its a two-dimensional phased-array antenna. It has a lot of small antennas on it to steer the energy released from the antenna directionally through interference patterns. If you're only going to receive and the signals are powerful enough you don't need that big of an antenna to receive signals at these frequencies.
stephen_g 5y ago
You can do satellite comms on something that small (maybe an antenna a few centimetres square) if you're only interested in moving a few bits per second or maybe kilobits per second. In the transmit direction you're generally limited by how much power you can pump out (the smaller the antenna, the less antenna gain you have and the more power you need to produce from your transmit chain). In the receive direction, you have a figure called G/T which has to do with the ratio of the antenna gain to the noise temperature of the receiver chain (which includes thermal noise picked up to the universe, plus noise introduced at each gain stage etc.). The simpler the modulation, the less power you need generally, but you can move less data in a given amount of frequency spectrum, so that's the tradeoff.

mrfusion 5y ago

Can this be combined with original gps to get super accuracy? What is the practical value of this? Stronger signals than old gps?

nayuki 5y ago
> These orbiting internet nodes fly closer to Earth’s surface than the satellites in the government-run global navigation systems, such as the U.S. Department of Defense’s GPS, the European Union’s Galileo, China’s BeiDou, or Russia’s Global Navigation Satellite System. Because militaries operate many of the systems, the signals can be targets for jamming or scrambling—spurring researchers to look for alternatives. > Because the satellites of internet megaconstellations fly so much closer to Earth than those used in traditional GPS, their signals can be thousands of times stronger. This could make them easier to pick up in dense jungle environments where GPS sometimes struggles to determine position, Langley suggests. The brighter signals might also allow for better data collection of animals outfitted with tracking collars, he adds.

deleted 5y ago

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