Comment by dr_orpheus | Hacker News Reader

Comment by dr_orpheus | Hacker News Reader

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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 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

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.

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.

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.

At least one party in the conflict is generating enough interference on the ground to obstruct both GPS and GLONASS navigation in orbits passing over the region. Its loud enough to detect as soon as the region is visible (thousands of miles away). Source: we are operating a constellation of sophisticated GNSS receivers in LEO.

Hawkeye360 (a different operator of sophisticated RF instruments in LEO) detected GPS interference and geolocated it to Russian forces shortly before the invasion, but at this point it could easily be both parties.

Satellites don't need extremely frequent GPS fixes though, or do they? I think they can afford waiting half a revolution to correct their predicted location.

Are the phased array antennas utilized by the starlink satellites and user terminals more resilient to jamming?

GPS jamming occurs local to the receiver, not the transmitter. So, you'd have to worry about the dish on your roof / vehicle, not the ones on the satellite.

I'm that sense, a phased array can help by filtering out signals that are not in line of sight with your satellite, but only one satellite fix does not provide a great estimate of the receivers position. Esp when signal strength is used to estimate bearing.

If you have a phased array, you can point it at a series of sats in rapid succession, or even all at once with more electronics.

That is a good point about pinging many sats. However all at once doesn't make sense to me in the context of a jamming scenario

well, anyone having to deal with you (and I) do. I wouldn't put it past a APT to be able to do better and take action in space if their hand were forced.

Interesting! Is it theoretically possible to launch a "GPS jamming" satellite constellation? And if so, could you do it with far less satellites than the quantity you are attempting to jam?

It's much cheaper to just shoot them down.

... with friggin' lasers.

The gps antennas on the starlink satellites will be pointed “up” i.e. away from the earth’s surface. Jamming from the ground to a receiver in space pointed away from earth would be… difficult. Especially when the satellites orbit about every 90 minutes, so you’d really have to do a global very very loud gps attack… it’d be simpler trying to hit several gps satellites with missiles than to accomplish that.

no, but they're a different frequency so if someone was only jamming GPS, this would get through.

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.

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.

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.

It depends strongly on how the phased array is being steered. If the components prior to phasing and summation are saturated by the interference then phased array beamforming will not reject the interference.

Phased array antennas are mostly for getting programmable beam forming, and don’t have much to do with jamming, which works by destructively interfering with incoming waves. Starlink operating on a different frequency means that GPS jammers won’t be able to jam them out of the box, but presumably if they’re jammers built in the last 20-30 years they’d have onboard SDRs which can dynamically hop frequencies. It’s a neat trick to get GPS from Starlink, but won’t stop nation state attackers from jamming access to GPS.

There are different kinds of jamming. It's my understanding that interference jamming is fairly uncommon nowadays. If it's not perfect, it mainly gets you a reduction in SNR and GPS is already way down below the noise floor, so receivers are inherently designed to mitigate the effects even if they aren't explicitly designed for anti-jamming. Even when they work, spectral filters exist. Broadband and spoofing are what Russia typically use. The defenses against those benefit from lots of processing power and large, beamforming arrays.

It's an arms race though.

Jamming isn't destructive interference, which would require knowledge of the exact signal being sent as well as the exact location of the transmitter and receiver. Jamming is overwhelming the receiver with a stronger signal in the same frequency band so the weaker one cannot be received. Think someone screaming over someone else whispering.

Phased array beamforming absolutely helps make jamming more difficult. Jamming is all about reducing the signal to noise ratio in the channel until it is unusable. Directional antennas (of which phased arrays are electronically steerable versions) have more gain in the direction of the desired signal and less towards unwanted signals located in another direction.

Aren't the Russian drones using GPS? Why wouldn't Ukraine jam GPS?

The drones have inertial guidance and gps is used for corrections only. That means when the drone is guided to blow up the railway junction, maybe you throw it off a bit so it hits the apartment next door. Not necessarily a good plan. There was also the case of a drone being damaged by ground fire, and veering off course to hit an office building next to an energy junction that it was being aimed at. And the problem of all those people shooting small arms into the sky causes damage when the bullets fall. For these reasons, Ukraine has asked people to stop shooting at the drones -- the time to shoot down drones is when they are flying over fields towards the city, not when they are already in the populated areas.

You have the same problem with putting AA missiles in populated cities -- sometimes they miss and slam into an apartment/office building -- that has happened 3 times so far at least as publicly reported (most likely it happened more often). You are supposed put the AA missiles in defensive rings around your assets, not in your downtown area. Ukraine's problem is they don't have enough to deploy as effective rings -- e.g. they don't have a real AA system, and so they put the missiles directly in populated areas, in which case it's better to turn them off when the cheap drones fly by and save them for enemy jets. Those missiles are not cheap.

Really the drones are exploiting the fact that AA missiles are so expensive and there is a gap between the guy with a rifle standing on the hood of his police car and a $2 million Amraam. Even manpads cost far more than the drone. The production costs of one of these is about $1000 (although they are sold for 10-20x that retail) - using parts sourced from Alibaba. A cheap moped motor, a mirocontroller, it's really simple stuff. Like V1 [corrected] rocket tech. Israel is working on the "iron beam" laser defense, but right now, there doesn't exist a working defense that doesn't involve firing million dollar missiles at $1000 drones. It's something all the major powers should be investing in, and I'm worried the U.S. defense complex just isn't able to do "cheap" any more. Could they even design a system that could take out such a drone cost effectively?

You meant V1 rockets? The V2 missiles were a bit more complex.

Yes, V1. It's closer to a flying bomb. I'll correct.

The random Chinese consumer drones, sure. You could also just block wifi to knock out most of them too.

Current block military drones being imported from Iran are not vulnerable to GPS manipulation because they have backup inertial navigation systems.

After Iran steered an American drone using false GPS signals, the US also implemented inertial and celestial navigation systems. https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...

This is false. All US military navigation systems, including the referenced drone, are primarily inertial and always have been. These systems can optionally accept fine-tuning GPS corrections, but only if those corrections are within the tight (classified) error margins of the inertial navigation system. It is a misconception that the US has ever used GPS for primary guidance in weapon systems; even cheap throwaway weapons like JDAM are primarily inertial. The kind of influence possible via manipulation of GPS is measured in meters. State-of-the-art INS technology is now good enough that the US military is considering phasing out GPS corrections for some newer systems.

GPS was designed for the purpose of accurately measuring the Earth during peacetime to build a precise model the world that could be used for inertial navigation systems in wartime. It was never intended to be used as a critical navigation system since it could be trivially destroyed by the Soviet Union when it was designed. Civilian systems tend to not concern themselves with this vulnerability and therefore happily use it for navigation.

In that article, they claim that the American drone was already using inertial navigation.

> American aeronautical engineers dispute this, pointing out that as is the case with the MQ-1 Predator, the MQ-9 Reaper, and the Tomahawk, "GPS is not the primary navigation sensor for the RQ-170... The vehicle gets its flight path orders from an inertial navigation system".

Your last line seems wrong. American aircraft have had inertial navigation since forever. GPS is a more recent addition. GPS is easily jammed and that's always been known to be the case. Nothing would be solely dependent on GPS.

I'd assume the Russian drones would default to GLONASS

why wouldn't that be just as easy to jam?

It's basically the same frequencies too

It's harder to jam GPS for "just one side" so doing so would harm both side's operations.

Couldn't the US re-enable Selective Availability?

Theoretically yes, but with massive disruption in the civilian space.

DGPS [0] is a workaround for SA. If you can have a ground station with a known location somewhere in the same area - even 100 miles away should be OK - you can broadcast the correction.

DGPS was already "good enough" in 2000 when SA was turned off, so I'd expect it could achieve very close to the same precision as regular GPS by now.

[0] https://en.m.wikipedia.org/wiki/Differential_GPS

Although I wouldn't be surprised if one side is more likely to have dual-band GNSS receivers...

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?

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!

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