Scientists create their own GPS by spying on internet satellites

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!).

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

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.

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

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.

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.

"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.

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.

> 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.

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

>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?

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.

It's the escalator of GPS!

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

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.