Scientists create their own GPS by spying on internet satellites

https://www.science.org/content/article/scientists-create-their-own-gps-spying-internet-satellites

by akeck • 5 years ago

144 63 5 years ago

piannucci8• 5 years ago

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• 5 years 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?

Rebelgecko2• 5 years ago

IIRC the abstract says they just used semi-publicly available ephemerides that the Space Force publishes

mateo1• 5 years 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• 5 years 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

Rebelgecko2• 5 years 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?

piannucci8• 5 years 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.

DeathArrow2• 5 years ago

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

piannucci8• 5 years 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.

irjustin2• 5 years 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

piannucci8• 5 years 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!).

spuz• 5 years 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• 5 years ago

You don't need to emit (that uses a lot of power) so that's already one factor.

piannucci8• 5 years 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.

vel0city2• 5 years 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• 5 years 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• 5 years 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• 5 years 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.

trhway• 5 years ago

SpaceX wasn’t involved in the research and can’t do much to try to block such techniques, though it’s unclear whether it would try.

well, government can always mandate to add artificial randomly floating frequency shift to break that Doppler based calculation to supposedly block the Taliban's ICBMs from precise navigation.

DeathArrow2• 5 years ago

True, but not only US based companies are launching sattelites.

kortilla• 5 years ago

Spacex is the only one with enough volume right now for this technique to have good accuracy.

bell-cot2• 5 years ago

I understand that Russia's "GPS" system ( https://en.wikipedia.org/wiki/GLONASS ) has been fully operational for a while now. The Taliban (or North Koreans, or others non-friends of the U.S.) don't need to count on U.S.-run geolocation services for their Important Stuff.

6nf• 5 years ago

I assume regulations might prevent this but technically I guess Starlink could add a little bit of extra data to their signals and make a pretty decent GPS alternative?

piannucci8• 5 years ago

Can’t speak to the regulations, but essentially the answer is “yes”. Our publications can be found under the moniker “fused LEO GNSS”. Those of Zak Kassas and his students can be found on their website.

bagels• 5 years ago

Let's assume gps disappeared. Will starlink still operate?

nine_k• 5 years ago

It should. It does not depend on GPS, only on ground stations, to correct its orbits.

jdiez172• 5 years ago

I don't have any concrete knowledge to say Starlink satellites rely on GPS, but based on what I know about satellites with thrusters and autonomous formation flying capabilities, they almost certainly have one or more GPS receivers onboard.

My understanding is that the SpaceX ground stations uplink the orbital parameters of other satellites and/or debris and it is mostly up to the satellites to avoid a collision. You need very precise onboard location knowledge for that. Also, I bet they are using GPS time to synchronize the fleet.

Of course you can still synchronize and maneuver the satellites manually from ground stations if necessary. And they probably do that when a satellite is misbehaving. But overall the Starlink operations concept is probably tailored to mostly automated operations.

So IMO they would definitely have big issues if GPS went away suddenly.

bell-cot2• 5 years ago

The Pentagon is (plausibly) Starlink's by-far most valuable potential customer. It seems pretty safe to assume that Starlink was designed to keep working after anti-satellite weapons, jammers, hackers, etc. knocked out GPS.

a_zaydak2• 5 years ago

Good question. Also consider that most of the threats to GPS are ground based jamming. It is fairly easy and cheap to make a jammer out of hobbyist parts and knock out a large area of GPS coverage. However, just because someone jams GPS for an entire city doesn't mean that they are able to jam the LEO satellites flying over that city.

bloak• 5 years ago

Is this the "differential" technique in which you have a number of ground stations observing each satellite, so you can compute the positions of the satellites, and the device that wants to know its own position both observes the satellites and communicates with the ground stations so that it knows where the satellites are? (Obviously the communication with the ground station doesn't have to be low-latency or line-of-sight. If you had line-of-sight communication with the ground stations you could just use them instead of the satellites!)

Or does this work without the "device" having to communicate with ground stations?

mensetmanusman• 5 years ago

This is really interesting, if SpaceX allows organizations to subscribe to higher levels of GPS-like accuracy, I wonder if it would shut down other country’s attempts to launch their own GPS competitors.

On the other hand, I can imagine the Air Force not wanting this to happen, as high precision location data can be used for missile and ICBM delivery. However, if it is an inherent property of the low orbit network, there is no point in blocking it from consumers.

sesutton• 5 years ago

Iridium already offers a positioning service with its satellites.

https://www.reuters.com/article/idUSKCN0YE1HZ

numlock86• 5 years ago

Is GPS a general synonym for GNSS these days? I was always under the impression that GPS is just one specific GNSS among others like Galileo, GLONASS or Beidou. This article is describing a new concept for a GNSS system that has no relation to GPS? Or what am I missing?

detaro• 5 years ago

Yes, at least for laypeople. if it tells your position it is "GPS", and nobody knows what GNSS is.

dstick• 5 years ago

It's the escalator of GPS!

kzrdude• 5 years ago

And on the phone, it might as well be cell networks or other sources that tell you the position - not GPS - yet people call it GPS. The phone calls it location service.

chrisseaton2• 5 years ago

GPS stands for 'Global Positioning System', but if you instead think of it as 'global positioning system', then all these satellite systems are GPS, because yes they are literally all a 'global positioning system', they're just not the 'Global Positioning System'.

That's what happens when you give something specific a very generic name. People shouldn't do that.

garaetjjte2• 5 years ago

That's what happens when you give something specific a very generic name. People shouldn't do that.

Yeah, but wasn't it named Navstar-GPS?

chaps• 5 years ago

This might be a US thing, but probably even wider. "GPS" seems to have taken on the same effect as "kleenex" here. What makes it frustrating is that you can still do research in academic papers, white papers, etc on "GPS" alone!

vel0city2• 5 years ago

Re-inventing Transit.

https://en.wikipedia.org/wiki/Transit_%28satellite%29

If you know the orbits of a few satellites, and you know how those sound from your current spot, you can roughly figure out your location. This isn't a new concept in the slightest, scientists were doing it in the 1950s.

Ultimately though, to get extremely accurate positioning data you need to be very synchronized (like, atomic clocks?). You'll also need updated data of where these satellites actually are as things flying tend to drift. This is especially true of things in LEO.

Scientists create their own GPS by spying on internet satellites

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