Whats really sad for aviation is there was already an extensive network of ground based navaids. As GPS has proliferated, they've slowly been decommissioned as they age out to save on cost. I'm specifically referring to VORs which provide azimuth and range to station.
GPS is fantastic and as a primary source everyone loves the benefits it brings. However, in a century of aviation, everyone has kinda sorted out that you can't have too many backups to critical items... unless you let the accountants and bureaucrats run the show.
VOR is extremely short range and requires an extraordinary number of transmitters to provide any volume of coverage. Additionally it cannot provide coverage over oceanic areas. We USED TO have a nearly global long-range land based navigation system called LORAN, which only required one master station and a few remote stations to provide continent-scale coverage. But it was decommissioned in a cost savings move after it was determined that it could be replaced by GPS. It worked perfectly fine until it was decommissioned in 2010, not that long ago.
In the '90s I used to fly light aircraft back and forth between Seattle and Alaska. You were frequently out of range of any VOR transmitters, but that LORAN always worked.
NDBs? That didn't work so great for Ron Brown.
On VOR: It's also a PITA to use, and for inexperienced people, or someone who rarely uses it, they can confuse flying towards and away from the station.
If you are a general aviation goober like me, flying a 1960's propeller airplane, sure, that is true.
However, in a modern commercial aircraft, all position sources including VOR-DMEs, GPS, and IRS are cross-compared by the flight management system and turned into a latitude, longitude, and circular error of probability. There is no TO/FROM flag to worry about in a 777, and no reverse sensing on the back side of a localizer approach.
Yeah that makes sense. My experience is mostly in 70s to early 90's prop planes, I have had the opportunity to use a glass cockpit a few times and it was much easier. I guess i did the typical thing and contextualized this on my experiences and ignored that 99.9% of flights are not GA haha.
Time was, if you didn't understand how to use VORs, you couldn't really fly. We used them as primary en route nav, as well as for approaches, for many decades. As a CFI I think most of the confusion around VORs these days comes down to poor or inadequate training.
> As a CFI I think most of the confusion around VORs these days comes down to poor or inadequate training.
I would say it is partially true. The reason I have trouble with VORs is that the equipment inside the plane is very awkward to use. The fact that the plane is in the center of the VOR display is super confusing as it shows everything in reverse: if the line is on the left it means the plane is on the right. And having to learn Morse Code to validate you tuned in the right frequency. And so on.
I get it that when they invented that we did not have technology, but now we do and nothing is stopping us from using a digital display that can translate VORs/DMEs/NDBs fixes into GPS like routes. And knowing where every VOR is can make the map accurate too. That would make it very easy for a lot of people to use and not have to learn the arcane ways of how people used to fly 50 years ago. I don't need to know that I'm flying TO or FROM the station.
That's fair. I replied to someone else with a similar remark, but essentially doing most of my flying in 70s to 90s prop planes, I imagine the tools I'm most used to are just dated. Your point around training is likely very accurate, I had some great teachers but I also had a few instances of being given inaccurate information and having to teach the teacher so to speak.
I don't fly and I wouldn't call my self someone that plays a lot of flight simulator. But I do enjoy navigation methods and I did learn the basics and practices of VOR navigation. I got an Android app that simulates flying courses, holding patterns etc, but only the navigation part. I would say I learned some intuition to it.
A lot of those planes still exist and have not gotten panel upgrades, so IMO you still should learn how to navigate purely via only visual references and radio navigation. I believe you are still required to know how to use them to pass your private pilot check ride, too, but I may be wrong.
Nothing is completely foolproof, but LORAN is much, much more resistant to jamming than GPS. GPS satellites orbit over 20,000 km from Earth and transmit at very limited power (less than 50 W). That makes it easy to overpower their faint signals. On top of that, GPS operates at a frequency over 1 GHz which means you can broadcast a jamming signal with a small antenna. LORAN is the opposite. It uses low frequency, extremely high power signals transmitted from stations relatively closer to the receiver. Those design choices make it less precise but vastly more robust.
> Additionally it cannot provide coverage over oceanic areas.
It's really hard to jam oceanic areas though, you need a vessel capable of sustaining human life autonomously at least for days plus dealing with the high seas.
Buoys could theoretically be used for that purpose, yes, but good luck getting them stationary enough. Maybe some of the secret services could pull off such a trick but at least no ordinary civilian.
By stopping jamming for a few seconds every few hours , and navigating using dead reckoning in between? With just a compass, a log, and a watch I can sail for days and end up within two kilometers of my destination.
Eh, I doubt it, GPS signal is so weak, once the EM field in the air has so much power, I think the literal hardware ADC of any receiver would be overwhelmed/clipped with a high power jammer nearby. I mean it's possible that the front end will be literally fried, not just clipping the analog front end. GPS is usually -130dBM, it's absurdly weak.
Nothing, and if it were subtle enough it could be effective at making it drift around. Lots of ways to get a rough fix though, and you don’t need very good absolute accuracy offshore, using a compass and log to DR your way around. Even better if you have a wind vane and anemometer. The majority of your drift will be wind driven.
A bunch of us old salty sailing nerds were actually discussing Decca Navigator System¹ the other day that was still very present in the 90s on the ships we were sailing. Now all replaced and removed.
All VORs provide a radial that you’re on. Not all provide a distance to the station. VOR/DME or VORTACs provide both. Plain VORs do not.
As more and more are decommissioned, the percentage that support distance-measuring goes up, as the ones being decommissioned are more likely to be the less-useful ones, which are less likely to have supported DME in the first place. So most remaining ones support both, but it’s not assured without checking.
Are there modern positioning systems that can automatically join all those data sources into one position? Traditionally navaids are treated as completely separate sources and are shown on different kind of instruments. But they could also be used as a replacement source for GNSS data on a modern map screen.
Depending on their kind they might not return a specific position, but in combination with other sources they often do via triangulation.
PS: INS (Inertial Navigation) also still exits. It often works for quite a long time really precisely. Also precision can be calculated. So it can start as a dot on a map which becomes a bigger circle as the precision decreases. It could be automatically recalibrated whenever high confidence positioning is available.
Yes, modern flight management computers in commercial airliners do exactly this. I can't speak to smaller aircraft.
Essentially a position is calculated from VOR/DME, GPS, and INS. When there is disagreement it is flagged and procedures are in place to fall back.
The issue is VOR/DME is slowly being decommissioned eliminating one of those sanity checks. It hasn't been degraded to the point of creating massive gaps in coverage over land but it will slowly happen if the rate of decommissioning continues.
Ultimately, INS is sufficient for enroute navigation but if you're degraded to just that state, the impacts to traffic separation, particularly in congested airspace, are significant. Required Navigation Performance (RNP) levels dictate all of this.
There's GPS jamming by the military, mostly in war zones. This seldom effects airliners because they steer clear of active war zones.
Then there's GPS jamming by delivery drivers who don't want their boss watching over their shoulder all the time. When the article mentions jammers as small as a cellphone and costing less than $100, those are sold to drivers.
Delivery drivers don't jam LORAN or VOR, though - so it's not jammed by the second type.
I’d bet a lot of money that the NTSB will determine that the pilots’ failure to maintain clearance from terrain during a visual approach will be the primary cause with contributing factors of dark, moonless night and GPS jamming.
That one’s going to be on the flight crew, not the GPS jamming test.
I'm not going to argue that as it's always easy to blame dead people, but there's two different types of jamming.
One is to actually jam the signal IOW preventing reception. The other is to overpower the satellites' signals and replace them with your own. That defeats the doppler calculations of GPS needed for triangulation, and instead the receiver believes it's where the jammer wants it to be.
If the latter includes altitude information, then it's a huge problem.
If you overpower a signal, is there a difference? I'm not saying you're right or wrong here. I'm just saying there's a long history of coverups in the government and military.
If you can say, "oh we were only jamming" when in fact you were spoofing, well that's certainly a super easy excuse to hide behind.
In either case, asking for and being cleared for a visual approach (as the crew did and was) puts the onus on them to maintain continuous visual contact with the airport and to maintain their own terrain and obstacle clearance, which it’s safe to say they didn’t do.
>If you overpower a signal, is there a difference?
Yes, obviously. Losing signal on your GPS unit, and having your GPS unit tell you that you are in a different position than you actually are, are two very different things.
Also, since other aircraft reported GPS issues, that is evidence that would speak to jamming or spoofing. All three other aircraft reported a loss of GPS, not inaccurate GPS. Coverups are real, government conspiracies are real. But the completely evidenceless “I’m just saying…” or “isn’t it weird…” or “I’m just asking questions” is tiresome.
IMO tracking is just way too useful. You know when things are about to arrive (so you dont miss a parcel because you went to the toilet or you can even intercept your driver once he's in the area, removing most of the last-mile problem), routes can be changed on the fly (e.g. for on-demand public transport pickup, trunking or delay mitigation) or, well, you can track your vehicles, allowing you to find them quickly in case of theft, optimize routes or have decent evidence in case of disputes.
The real problem is not really drivers being tracked, but how the tracking data is or might be used. Timestamped delivery notifications might be just as bad in this regard.
As often, the solution is probably more about accessible and pro-human law. No idea how that should work, maybe certain sensitive data services having to be provided by a third party or even the government itself? There are probably much smarter ideas or variations of this...
So as a business owner, I'm not allowed to know where my $100,000 asset is? My insurance carrier requires me to track the vehicle as a condition of coverage, they also require a dashcam be operating at all times.
If that's all it was ever used for, then nobody would give a shit. Instead those examples you gave just end up being an _excuse_, so that you as a business owner can fire or deny a promotion to someone you don't like because you can now prove they stopped at the 7/11 for 5 minutes to pee.
If this country had good unions and real worker protection laws, like you can't punish drivers for taking a break, then we wouldn't feel as much a need for broader sledgehammer laws banning GPS tracking. (that, lets be honest, will never happen anyways)
> Instead those examples you gave just end up being an _excuse_
How is an insurance company provision an excuse? Do you propose self-insurance? If so, would that be advisable from a fiduciary responsibility standpoint for most small companies?
Who ultimately bears the increased insurance premiums and deleterious effects emerging from legislatively banning GPS on company assets and dashcams? (Hint: not the business)
Antenna tech has moved way beyond what was possible in the days GPS was invented. Planar arrays and MIMO are ubiquitous and cheap.
These antennas have a very good idea of where the signal is coming from, due to delay measurements and can detect and filter ambient noise. They are super hard to jam. The reason why drones have become such a PITA is that these inexpensive antennas are very hard to defeat even with military jammers.
If this becomes a geniune need in the civilian sector, then I'm sure the tech is up to the challenge.
The tech is already there, but the reason those antenna are not widely used is that they used to fall under ITAR, so no US companies want to touch those for civilian uses. Of course Turkey and China is not US so they don't give a damn about ITAR compliance, and so you can buy those anti jam antennas out of alibaba.
Brad Parkinson, the father of GPS and PNTAB, they've been trying to lobby the US government to remove ITAR from that technology so the civilian can benefit from anti jamming, and finally they managed to do it sometime last year. Now it falls under EAR
There's also a bunch of Western stuff you can buy off the shelves. MIMO tech is standard in every Wifi 5+ router, Cars have pretty sophisticated electronically steered radars nowadays, boats even more so. 5G towers also have phased array antennas. This tech is everywhere in consumer or industrial stuff.
Not sure what makes an antenna/sensor ITAR restricted, I suspect if you try optimizing it to military use-cases, you'll get hit with ITAR, but I suspect Brad Parkinson is right - security through obscurity just doesn't cut it any more.
I remember in the 90s, computer security was either missing or an afterthought (you could bypass the Win98 password prompt by pressing escape in the release version). Thankfully the industry got together and figured it out.
Now everyone else needs to follow suit. We already have a bunch of examples of assuming security does not matter (hackable cars, meters that can be switched with a Flipper zero).
> Not sure what makes an antenna/sensor ITAR restricted,
Intentionally-designed anti-jam features (active-nulling and electronically-steerable receivers made for this purpose) marketed towards GNSS applications are EAR items (and are incorporated into ITAR by reference.)
> These antennas have a very good idea of where the signal is coming from, due to delay measurements
"These antennas" is doing an impossibly large amount of work. What you are describing here is a delay-calibrated coherent receiver, not an antenna, and it's absolutely not "inexpensive" to pull this off.
With passive antennas, most of the attenuation of adversarial signals is being done by the relatively-high-directionality of planar arrays: off-boresight signals have far greater attenuation with these (20dB? - tricky because you still do want good satellite data close to the horizon), and most garden variety jammers are terrestrial and off-axis.
Still, these signals are always below the noise floor and mostly unsigned, it's somewhat easy for an airborne (or spacefaring) jammer to fully drown out, replay/time-shift, or spoof the signal.
Eh a month or two before the October 2023 Israel conflict - there were many reports of GPS jamming in the middle east impacting air transport aircraft. To the point where it basically caused various failure modes that shouldn't have happened with avionics (think the things you'd expect in a fuzzing scenario). It was pretty eye opening example of just how poor modern avionics are (and their integrations) when they get unpredictable inputs & the dependant downrange systems that rely upon that data.
GPS jamming is fairly detectable right? I wonder if we could create some crowd souced map of jammers. I think this kind of already exists as fairly course data but if a bunch of people installed fixed GPS sensors at home and wired them up to note every time their location lock is lost or moves. It could create a hyper detailed map of where the jammers are and track them moving in realtime.
> if a bunch of people installed fixed GPS sensors at home
Similar things already exist - although it's used for increasing GPS precision and detecting faulty satellites, moreso than detecting jamming, so the networks might not be dense enough to detect small areas of jamming.
For one state's example, see [1] - choose 'RINEX archives' if you want historical readings.
They're easy to find, but not necessarily with stuff from off the shelf. Professionals with the right equipment though have no trouble locating them. The FCC is happy to hand out 5 and 6 digit fines for operating a GPS jammer and has fined retailers tens of millions for selling them.
If you wanted to exactly pinpoint it with a single device sure, but I'm thinking of a map where nodes just report that they are experiencing jamming, and then with the whole network you can see a radius of the jam, see it moving around, and assume it's somewhere in the center. All with cheap off the shelf hardware.
The police could probably combine this data with license plate scanners and see the single truck that always exists in the jammed location.
I know we all like to jack ourselves raw to the big powerful government handing out ruinous fines for things we don't like but realistically none of that is gonna stick to some McDonalds manager who's got a side gig importing Alibaba stuff or some delivery driver who bought it. You can't get blood from a stone. You're just guaranteeing the stone always stays a stone.
Yeah the FCC could probably pull an ATF and shoot a bunch of people over fairly petty crimes but that costs political goodwill that I'm not sure the FCC has to burn.
Yesterday's actions include 7 $25k fines apparently all for companies illegally importing banned drones (a procedural fine just for ignoring the investigation attempts). You can peruse that list and find plenty of things, lots of pirate radio recently. They absolutely do go after apparent small fry and your "mcdonalds manager" absolutely has assets that can be seized.
>Yeah the FCC could probably pull an ATF and shoot a bunch of people over fairly petty crimes but that costs political goodwill that I'm not sure the FCC has to burn.
This is exactly the kind of circle jerk commentary I was complaining about. Tone doesn't make right.
The notices of liability the FCC posts to their website are not final orders. They're the federal equivalent of when your zoning sends a notice saying "your patio is 6in too close to the property line, according to the aerial photos it's been there since , you owe use ". It's essentially civil enforcement speak for "hey asshole, lawyer up and take this seriously". So then like a naive idiot you reply back "this patio predates my ownership of the property" and the money grubbing commissioner fires back "don't care, pay up". This is what the forfeiture order the FCC post on their website are. That said, the FCC is less slimy and less interested in fine money than your local commissioner probably is and I personally hold them in high regard.
Most of these end in the subject of the enforcement simply changing their behavior and paying a trivial fine (the lawyers are the real winners).
To quote those numbers as if they're final is no less dishonest than to quote lawsuit claims as if they're final judgements.
>They absolutely do go after apparent small fry and your "McDonalds manager" absolutely has assets that can be seized
Once again, you can't get blood from a stone. You might get a little but it's not worth the squeeze It is not politically sustainable to make a habit of ruining small time businesses and individuals (we're not talking about the Sackler family here) over nonviolent violations in technical laws where there is no clear chain of causation from the violation to some sort of harm to life or limb. The Mcdonalds Manager will settle for $5k, throwing away his storage unit of Alibaba stuff and not doing that anymore.
The reason I brought up the ATF is because they have a track record of forcing compliance with arcane arbitrary laws and they do it through flamboyant violence. This comes at political cost.
>The FCC doesn't have guns, they can't arrest you
Right, they'd get the FBI be the muscle like all the other Feds that don't have entry teams do.
The feds effectively all have guns. It's just a question of how many desks the papers must cross before they turn into bullets.
The FCC takes the public commons very seriously. Given how easy it is to make radio frequencies unusable, they need to hammer hard and fast to discourage people from trying. They don’t wait for a report to be filed, they can proactively search for violators. They may only be able to fine you but they can get law-enforcement involved. Especially if your city or state has laws against what you’re doing. You usually see this when you start messing with airplanes.
If I was a concerned boss, I would likely use GSM cell info to track my drivers. While not good enough to do turn-to-turn navigation it certainly is good enough to tell where people are.
Btw, don't smartphones have this built in? I remember some old and cheap Android phones without GPS that came with this.
Being much closer to the receiver and tied into terrestrial power grids, ground-based beacons can transmit at higher powers than GNSS signals and be received at much higher powers.
Received GNSS power is on the order of -130dBm (10e-16 W) - you can jam that with a 1W terrestrial source for hundreds of meters around. Way more if you're willing to scale the power from car USB power levels to bigger grid-connected supplies.
LORAN, from my quick googling, looks to be about 6-7 orders of magnitude higher received power.
Sorry but this sounds like nostalgia. loran-c was prone to error.
Yes, we can stop that with modern tech, but the problem with loran-c is you had location ambiguities. Decca was worse, unless you had a good initial fix, you were all sorts of fucked. There are of course modern alternatives, but they are not block proof, or turned on.
There is a very good reason why GPS was opened up, because the alternatives were not anywhere near as good.
Now, what are the alternatives? well hyperbolic navigation is still a thing, and with modern electronics you can get a pretty good fix without any of the nasty ambiguaty that loran-c had. If you are flying over land, then visual odometry is now at a point where you can locate pretty accurately autonomously (assuming cloud cover is your firend)
Small low power ceasium clocks are $4k now, and only really need a few hundred millitwatts to run, which opens up a whole load of other ways to navigate.
then there are IMUs, they are dirt cheap and way more accurate than 20 years ago. A calibrated array should be able to sustain reasonable accuracy after 30 minutes now
Probably be easier and more reliable to use star navigation with modern cameras, computers and atomic clocks. Especially for commercial aircraft flying above the weather.
With a quick calculation 50 ms time offset would mean about 20 meter difference. You can get that accuracy with NTP over residential internet and a better TCXO could hold that for a day. You could sync clocks and download the almanac at each airport. The almanac could be digitally signed and NTP has an option for authentication, too. That's about 10 USD of hardware (plus what you need for image capture and processing).
I'm pretty sure with dedicated hardware and better solutions you could get much better accuracy.
I would be curious on and interested in guidance from a GNSS expert on what the risk profile is when you're using jam resistant frequencies with a multi GNSS constellation chipset (GPS, Galileo, BeiDou), WAAS (GNSS corrections received via satellite), and when close enough to airports that support it, LAAS (GNSS corrections broadcast from airports). Follow up questions would be what countermeasures are available to bolster GNSS reliability during critical phases of flight (takeoff and landing), as well as how cellular tower networks could be used as distributed sensor fabrics for constantly monitoring for and triangulate GNSS jamming using SDR (or perhaps start with a handful of GNSS reference monitoring stations at major airports for this monitoring).
Because GPS and other GNSS have only a very weak signal strength there are no jam resistant frequencies, you can jam them all very efficiently, or you can jam them very efficiently in selective way (jam BeiDou, but not GPS).
GPS was jammed even as early as Iraq war.
"Coalition troops also got a glimpse of GPS’s greatest weakness during the Gulf War. Iraqi forces installed jammers, for example, on top of landmarks such as Saddam Hussein’s palaces to prevent them from being hit, Mastalir says. This helped the military realize early on that it would have to further develop its laser-guided munitions and other weapons that acquire targets when GPS is unavailable"
A lot of work is being done to make GPS more jam resistant, and - just as important - more spoof resistant. The latest batch of satellites has a whole raft of countermeasures to help a receiver to identify whether the received signal is genuine or whether it was spoofed. Receivers too can do a lot to detect spoofing by just observing the signal (for instance: unrealistic power levels on reception, impossible response to vehicle movement and others besides), but quite a few of those are unrealistic or too expensive for regular (non military) applications. Jam resistant frequencies do not exist as you already mentioned, from a physics perspective. But GP might mean this to refer to the new M-Code signal, and these are sent out on different frequency offsets from the main carrier, so effectively different frequencies.
L5 is considered to be more jam resistant, but it is still in the process of being rolled out and most receivers do not have that capability. It will take the better part of a decade to be rolled out completely but the space side should be done by 2027 so if you need it you can get it but you'll need a completely new receiver.
For obvious reasons hard information about all this is tricky to come by, you can read what is available from non-classified sources though and you can glean quite a bit from there.
> L5-band signals are 30x harder to jam and interfere with compared to L1, and they offer superior performance in difficult-to-navigate areas such as urban canyons and tree-covered regions.
If Russia’s proof of concept moves to active use, those satellites (Kosmos 2546 and any others) will need to be removed from service with anti satellite technology (either munition or X-37B equivilent). Clearly Russia is attempting to shield an overt military capability behind the satellite’s early warning launch detection primary mission to dissuade attempts to disable it. Remove the orbital capability, deny them access to replace it.
Ongoing Ukrainian strikes against Rassvet satellite production and launch sites contribute to further impairment of Russia's ability to get any new or replacement satellites to orbit.
I think Starshield (Starlink for the military) is going to use lasers as the means of communication (and location). Those are highly directionaly, and likely not possible to jam. Though I'm not sure how resistant are they to atmospheric interference, and the sats need to know where your receivers are, which I can't imagine the military is too happy about.
What about terrain following? I would guess decent topographical maps of the world aren't that hard to come by today, to get a good idea of where you are.
Although that approach depends on you being able to see the ground neither of which is guaranteed over a body of water or in poor visibility.
My guess would be that the best solution is multi sensorial anyways. So you have GPS, but then you would have an Inertial Measurement Unit, based purely on acceleration and inertia, radar and barometric systems and maybe optical systems for terrain identification and celestial navigation.
I have recently heard an episode of Amp Hour podcast¹ where they talked about using Bluetooth from satellites as a GPS replacement for asset tags. So maybe alternatives to GPS on different bands are on the verge.
Another interesting idea would be to have an antenna array on the plane that captures different radio sources and the angles from which they seem to come from and create a always updating radio activity map that maps a certain "radio landscape" to a position on the planet. AFAIK this is currently being worked on as well under the name "radio-SLAM" (with SLAM standing for Simultaneous Localization and Mapping). This means, jammers could maybe even be used as potential beacons themselves.
Maybe the starlink clocks could continuously calibrate from gps? Clock drift is usually a slow thing (like thermals). Their inter-satellite data links have all sorts of relative clocking info.
IIUC, GPS picks one satellite and 3 more to establish time differences between the first one and the other 3. There's only one point in universe where the set of differences would match what was observed, given the broadcast orbital data of satellites, and that gives the location of the receiver. It's like you are provided radii and center coordinates of 3 circles on a paper that intersect at 1 point and you are to math that point.
So GPS-like systems need precise orbital data and coherent clock from each satellites, or otherwise the receiver cannot figure out those radii.
Presumably you can have each sats reflect a clock from the same master ground station but that's not how GPS is implemented currently.
Coherency is a state. My question is if that state can be achieved by just tuning the individual clocks/locations based on GPS as a master. And, there's other tricks, with all the inter-satellite communication [1].
And, there's a contextual difference here between civilian GPS and military GPS. Much of the precision in the existing GPS satellites is to support the military use of GPS, with intentionally error injected into the civilian side.
And, by the nature of the hostile environment the military operates, military craft don't have as strong of a reliance on something that's ephemeral, like a GPS signal on a battlefield.
What you describe is very normal in telecom and has been for a long time. 4G, 5G, etc are all totally dependent on every device syncing to the one higher up in the chain. These days typically a digital control loop is used to tune a high frequency voltage-controlled oscillator which feeds a counter. An event (like GPS PPS, or an ethernet frame arriving on the cable) is extracted from the signal and the counter generates a timestamp. The chips to do this are incredibly cheap and the performance is hard to comprehend.
The orbit accuracy of starlinks are (to me) and unknown. I would suggest that as they are on a lower orbit, they decay much faster.
However, because of the way they communicate node to node, they do need to know where their nearest neighbour is to a very high degree. So relative position is probably easy to obtain.
But if you're gonna do that, you're better off with 4g/5g masts. there are loads of them and they don't generally change position that much. They are clow enough together to do odometry.
GPS is fantastic and as a primary source everyone loves the benefits it brings. However, in a century of aviation, everyone has kinda sorted out that you can't have too many backups to critical items... unless you let the accountants and bureaucrats run the show.