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I think there is a bit of an issue here that isn't addressed in the article. They reference using the SpaceX published ephemerides which are created from the GP

I think there is a bit of an issue here that isn't addressed in the article. They reference using the SpaceX published ephemerides which are created from the GPS receivers onboard the SpaceX satellites: "SpaceX satellites regularly downlink accurate orbital information from onboard GPS. We use this orbital information, combined with planned maneuvers, to accurately predict future ephemerides, which are uploaded to Space-Track.org three times per day" [0] So using the positioning information of the SpaceX satellites is already dependent on GPS. Saying that it could be used as a backup to GPS is a bit non-sensical to me. Sure you could go back to using ranging measurements for each of the satellites to get TLEs from NORAD or LeoLabs or one of the other commercial space tracking companies, but it would likely be less accurate and not updated as frequently as the SpaceX satellites don't have a precision clock onboard for timing and propagation like the GPS satellites do. [0] https://www.spacex.com/updates/index.html https://www.spacex.com/updates/index.html

jenny91 4y ago
They're different failure modes. It's unlikely the GPS constellation will go dark anytime soon. It's much more likely you're in an area without good view of the (few) GPS satellites required for a fix; or that there is GPS jamming, etc.
- dr_orpheus 4y ago
  Ah, that is true I didn't consider it from that perspective. It may also serve as a decent anti-spoofing measure if you compare the two results for position that you are getting directly from GPS vs from the Starlink satellites.
helsinkiandrew 4y ago
Presumably GPS can be jammed locally on the ground and confuse ground terminals but not at the range required to block the SpaceX satellites reception in low earth orbit.
- ortusdux 4y ago
  Are the phased array antennas utilized by the starlink satellites and user terminals more resilient to jamming?
  - adgjlsfhk1 4y ago
    no, but they're a different frequency so if someone was only jamming GPS, this would get through.
    - Tuna-Fish 4y ago
      They are massively more resistant to jamming. The antennas are very good at rejecting signals that are coming from the wrong direction, that is, not from above.
      - WJW 4y ago
        This is the correct answer, phased array antennas have much more directivity so they are much harder to jam with a ground-based jammer. Although I suppose in some cases you might want to put the jammer on a drone anyway, to cover a larger area. In that scenario you would probably not see a significant difference for phased array antennas vs "normal" antennas. Caveat btw: for both starlink and GPS, the satellite you are talking to will not always be "up". For GPS in particular, it is possible that some of the satellites are only barely above the horizon. So an antenna that only looks "up" is generally not what you want anyway, which makes ground-based jammer more effective again.
      - mike_d 4y ago
        Phased arrays are good at rejecting incidental interference, which is why they were used to address radar jamming. Of course that also meant that billions were poured into developing technology to counter that resilient property. Anyone capable of blocking GPS on a non-trivial scale would be easily capable of blocking Starlink as well.
      - brandmeyer 4y ago
deleted 4y ago
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gpm 4y ago
It's not clear to me if GPS is necessary here, or just a convenience? Could you reasonably update the location/timing information on starlink satellites every time they passed over (for example) north america from fixed ground stations? Or do you need to update the satellites clocks more frequently than that or something?
- tonyarkles 4y ago
  Honestly, and this isn’t something I’d ever thought of until just now, it’s probably reasonable to assume that most Starlink ground terminals (the ones that users install at their homes and businesses) sit in a fixed location. It would be complex to do, but I suspect you could probably get quite good orbital positioning in reverse by using all of the fixed ground terminals as a giant reference set. If the terminals themselves have GPS receivers built-in, the whole system could pretty readily bootstrap itself (each GT sits and continuously averages its own GPS position and clock offset, then transmits that up to the overhead SVs as they pass by). Even without GNSS receivers in the GTs, though, you could probably still rely on the Starlink-owned GTs to know their own positions very accurately and augment the whole thing using the “static user GT” assumption. GTs that did move would just be treated as noise in the whole thing. It’s been a while since I’ve done this kind of math, but it sounds like a really fun problem!

jenny91 4y ago

They're different failure modes. It's unlikely the GPS constellation will go dark anytime soon. It's much more likely you're in an area without good view of the (few) GPS satellites required for a fix; or that there is GPS jamming, etc.

dr_orpheus 4y ago
Ah, that is true I didn't consider it from that perspective. It may also serve as a decent anti-spoofing measure if you compare the two results for position that you are getting directly from GPS vs from the Starlink satellites.

helsinkiandrew 4y ago

Presumably GPS can be jammed locally on the ground and confuse ground terminals but not at the range required to block the SpaceX satellites reception in low earth orbit.

ortusdux 4y ago
Are the phased array antennas utilized by the starlink satellites and user terminals more resilient to jamming?
- adgjlsfhk1 4y ago
  no, but they're a different frequency so if someone was only jamming GPS, this would get through.
  - Tuna-Fish 4y ago
    They are massively more resistant to jamming. The antennas are very good at rejecting signals that are coming from the wrong direction, that is, not from above.
    - WJW 4y ago
      This is the correct answer, phased array antennas have much more directivity so they are much harder to jam with a ground-based jammer. Although I suppose in some cases you might want to put the jammer on a drone anyway, to cover a larger area. In that scenario you would probably not see a significant difference for phased array antennas vs "normal" antennas. Caveat btw: for both starlink and GPS, the satellite you are talking to will not always be "up". For GPS in particular, it is possible that some of the satellites are only barely above the horizon. So an antenna that only looks "up" is generally not what you want anyway, which makes ground-based jammer more effective again.
    - mike_d 4y ago
      Phased arrays are good at rejecting incidental interference, which is why they were used to address radar jamming. Of course that also meant that billions were poured into developing technology to counter that resilient property. Anyone capable of blocking GPS on a non-trivial scale would be easily capable of blocking Starlink as well.
    - brandmeyer 4y ago

deleted 4y ago

[deleted]

gpm 4y ago

It's not clear to me if GPS is necessary here, or just a convenience? Could you reasonably update the location/timing information on starlink satellites every time they passed over (for example) north america from fixed ground stations? Or do you need to update the satellites clocks more frequently than that or something?

tonyarkles 4y ago
Honestly, and this isn’t something I’d ever thought of until just now, it’s probably reasonable to assume that most Starlink ground terminals (the ones that users install at their homes and businesses) sit in a fixed location. It would be complex to do, but I suspect you could probably get quite good orbital positioning in reverse by using all of the fixed ground terminals as a giant reference set. If the terminals themselves have GPS receivers built-in, the whole system could pretty readily bootstrap itself (each GT sits and continuously averages its own GPS position and clock offset, then transmits that up to the overhead SVs as they pass by). Even without GNSS receivers in the GTs, though, you could probably still rely on the Starlink-owned GTs to know their own positions very accurately and augment the whole thing using the “static user GT” assumption. GTs that did move would just be treated as noise in the whole thing. It’s been a while since I’ve done this kind of math, but it sounds like a really fun problem!