Some reporting I did on the topic a few years back: https://chicagoreader.com/news/many-on-house-arrest-in-cook-...
GPS glitched across the US by as much as 33 feet (sciencealert.com)
151 pointsby thread_ida day ago93 comments
chaps 9 hours ago
This is exactly the sort of thing that leads to people getting reincarcerated after their electronic monitoring devices mistakenly report them leaving their homes.
cromka 7 hours ago
Sounds like you could be a convict, have your gang interfere GPS on a local level but enough to have other people in the neighborhood notice, make a quick run for whatever you'd need to, and still claim plausible deniability.
john01dav 7 hours ago
Or you could cause such localized things more so to try to get a person sent back to prison or otherwise penalized intentionally
cromka 7 hours ago
Also true.
dataflow 6 hours ago
> This
As in, GPS being off by 33 feet?
> leads to people getting reincarcerated
I'm reading your article and it seems to be about police coming to check and realizing that the reporting of being miles away was in error, not people getting reincarcerted because of GPS incorrectly reporting they moved ~30 feet too far? One of these is much more insane than the other.
chaps 5 hours ago
Not sure where your confusion is coming from. Have you never used GPS in a place with bad signal? Where it gets confused and you miss your turn? It's the same failure mode. Just the repercussions are getting arrested instead of getting off on the wrong exit.
Also wrote this one, hopefully it helps your confusion: https://chicagoreader.com/news/false-alarms/
dataflow 5 hours ago
You're claiming that erroneously reported movements by ~33 ft lead to incarceration, yet the article you're linking to reports that erroneously reported movements by ~1mi+ lead to police coming to double-check what's going on. Am I incorrect and do you therefore have examples of ~30ft errors actually leading to their incarceration, or are you struggling to see the disconnect between what you're claiming vs. what is actually happening?
chaps 5 hours ago
If someone's home is 30' by 30' feet, yes, that person's device will report them having left their home and they'll get a visit if GPS suddenly shifts 30 feet to the left.
I'm talking about the class of problem. It happens in normal weather conditions and things like this exacerbate the problem.
Really not sure what the hostility's about.
saaaaaam 5 hours ago
As a neutral participant I think “the hostility” is probably because of your condescending tone when you said “hopefully it helps your confusion”. Your initial article really didn’t say anything about people being re-incarcerated for these sorts of errors. I really want to say “maybe you got CONFUSED and posted the wrong one!” but that would be snotty and condescending too.
But maybe you posted the wrong one to begin with.
Let’s all be friends and nice to each other.
chaps 5 hours ago
That's totally fair, I shouldn't have said it like that!
dataflow 5 hours ago
Do you have any examples of someone getting thrown behind bars due to a GPS error on the order of ~30ft like you're claiming? That's all I'm asking. Really don't understand why you keep avoiding it. You've essentially claimed police/judges are doing something rather stupid and terrible. I'm asking you to show example(s) of what you claimed (~30ft error → thrown behind bars), not something significantly different (~1+ mi error → only-police-visit-but-not-thrown-behind-bars).
chaps 4 hours ago
"You've essentially claimed police/judges are doing something rather stupid and terrible."
Yes, yes I am. You really need to realize just how unbelievably fucking arbitrary the criminal courts are. At every level. They're not tech people.
To answer your question, I have examples of motions by prosecutors to suspend people from the EM program where they specifically cite specific data points just barely out of their homes, where video footage provably shows they were inside the whole time. They're not mine to share.
Btw, the courts are public and you should try some criminal courts watching. Pick a random day and just go.
dataflow 4 hours ago
> To answer your question
Looking forward!
> motions by prosecutors
Motion ≠ incarceration. So, not an answer (yet?)...
Did defendants get incarcerated as a result of these motions, or no?
chaps 4 hours ago
They did, yes. I hope you have a better day.
dataflow 43 minutes ago
Okay thanks, so here's what I don't understand. When there is video evidence that someone was in their house the whole time, and given that everyone who has used a GPS knows it can easily be inaccurate by 30ft, why in the world would the judge trust the ankle monitor accuracy over video footage they see with their own eyes that directly contradicts it?
Something about this just doesn't seem very plausible, especially if this kind of thing is happening more than once in a blue moon. Either the video evidence is not as convincing as we're being led to assume (thus leaving more doubt in the judge's mind than ours, even if the person is innocent), or there should be some other explanation.
chaps 7 minutes ago
I don't know what to tell you besides yes, it's really that bad. A lot of it is just people passing the buck down the road, a lot of it (at the time) was covid, a lot of it was the difficulty of people on EM to publicly talk about their situation. Defense attorneys just want to get their clients out of jail and a lot of the EM stuff is a distraction to that. It's very much a, "we'll put in a ticket" sort of situation across many moving parts.
Lawyers began telling their clients to film themselves in their homes at all times so that they could prove to judges that they were in their homes when the devices went off. That's advice they give because the risks of not doing that are going back into jail. And to prove all that, you still have to go through all that process, filming yourself, etc. It's truly bad.
porksoda 26 minutes ago
You've kind of spoiled my day with this. Its like an angry chess game when you've got momentum, you POUNCE! But nobody else here appears to have chosen violence. really I get that he "maybe snarked you". But you should be reminded of the beautiful rules of hn discussions.
Dylan16807 2 hours ago
> Btw, the courts are public and you should try some criminal courts watching. Pick a random day and just go.
So you could find an example easily but you don't want to? Or you're suggesting court watching in general and not to find an example?
"I have an example I won't share" is better than nothing, I guess. But you were being snarky before you even said that much, when you were just linking your article and not saying if examples existed at all.
I'm curious why they're not yours to share, though. Did someone give them to you under private circumstances, or can people get there from public information but you think you shouldn't spread the word?
chaps 2 hours ago
It wasn't easy to find, heh. Lots of FOIA requests to even get to a point of where I could go through court records at the Clerk of Courts. Obstinate lawyers for months and months. But after all that, I can find the court documents at the Clerk of Courts. I printed them there and I don't have them anymore. I would need to redact even if I had them. But they were all the same form, but with the actual violations largely hand-written notes written by a prosecutor in a hurry. It was usually a list of like, ten instances that they seemingly picked from random of the GPS rlogs. The best glance I had into what was by FOIA'ing the GPS records of someone who died much later but had a lawyer who fought tooth and nail to show he didn't leave his home. GPS is truly all over the place in these logs. I validate a lot of that against call center records that themselves took months to get but show when alarms and calls are made to the devices, plus when dispatch is done.
A lot of this stuff is hard to talk about because it's about real people in real pending (at the time) cases.. and I'm also not a criminal defense attorney by any means. There's a reason the first article I posted had their identity completely removed. There's weird liability and believe me I want to say more.
Very honestly, I wish I could have done more reporting on how fucked up those devices were. I put a lot of time and effort into reporting that couldn't make it to print, largely because I was doing freelance work at the time. The first offer I got for that piece was $50. Wasn't sustainable.
Do you have any questions about these EM programs?
Aurornis 4 hours ago
GPS is not that accurate even on a good day. A small device attached to someone’s ankle with a small antenna and an obstructed view of the sky will frequently be registering locations outside of the footprint of someone’s house.
A single 33 foot anomaly is already within the geofence buffer zones used. In dense urban areas the geofence buffer allowance may be 10X that already.
brewdad 22 minutes ago
It should be treated that way but we’re talking about elected prosecutors who are itching for an excuse to put people in prison rather than looking “soft on crime” by allowing people to live under house arrest.
avazhi 5 hours ago
Well, don’t commit crimes and you won’t get a gps tracker attached to you, I guess. So that’s easy.
Not really sure what it has to do with tech though. That’s a problem with how certain humans behave, not ancillary technology.
boomboomsubban 5 hours ago
The trackers are part of CCSO’s pretrial house arrest program, so presumably a number of them are attached to people who did no crimes.
zx8080 5 hours ago
Don't live in the US.
avazhi 5 hours ago
> so presumably a number of them are attached to people who did no crimes.
You and I have very divergent views of the type of person likely to be charged or prosecuted for a crime or indeed to participate in a diversion program that involves GPS trackers, my friend.
ryandrake 4 hours ago
Right, because only guilty people become involved with the justice system.
crtified 4 hours ago
Not at all. They're remarkably similar. What differs is our cost:benefit analysis of closing off this errata.
Yours (inferred) is "Too Much : Zero".
Given that those are terminal values it would challenging for literally-any-other-view not to diverge from yours, on this.
rafram 3 hours ago
They are innocent until they’re proven guilty.
qlte 2 hours ago
Your heuristic is flawed. The pool of defendants with pending charges who were granted pre-trial home detention are going to be disproportionately non-violent, first-time offenders compared to those denied bail or with prohibitively large amounts.
Meaning an error like this harms innocent people wrongly accused more than the stereotypically hardened repeat offenders you imagine to be undeserving of your sympathy and presumed guilty.
avazhi 2 hours ago
> non-violent, first-time offenders
Non-violent, first-time offenders don't get ankle monitors. But thank you for highlighting my point.
openasocket 2 hours ago
They absolutely can, depending on the jurisdiction. I saw plenty of cases out of Atlanta with drug possession cases with no prior history getting an ankle monitor, back when they were live streaming bond hearings from Rice street.
avazhi an hour ago
You do not get an ankle monitor for misdemeanour possession as a first time offender, in Atlanta or anywhere else.
Like, actual lol mate.
saaaaaam 5 hours ago
I think the confusion is probably coming from the fact that the one reference to people being potentially re-incarcerated is buried right down the story.
> Harkins said that since last October, she has seen 30-40 people ordered back into custody based on these progress reports. When asked what can be done to improve CCSO’s EM program, Harkins said that the sheriff’s office needs to “properly and fully” investigate alleged violations before writing reports or taking actions.
chaps 5 hours ago
Ah. That's probably just a result of the pasted article being part of a series.
To be explicit: yes, many, many, many people were reinarcerated because of faulty GPS.
saaaaaam 5 hours ago
FWIW I enjoyed your articles. You write very clearly.
iamshs 5 hours ago
The story is about GPS errors which end up making life hell for people under monitoring since they end up generating a Police response. If 80% alerts were false, then an error like this can result in a surge of these alerts. It is an important story to tell.
zamadatix an hour ago
The monitoring device story seems important to tell but the problem seems to be related to monitoring devices, which are supposed to be accurate to 30 meters 90% of the time, being miles off or even reporting the wrong state. That kind of systematic failure does not make a few meters of displacement any more relevant. Saying so really makes it sound like the problem is something significantly less egregious than the devices being incomprehensibly inaccurate - as if we have to solve space weather prediction to a new level of science to not have them report someone in Chicagoland as being in Delaware.
leptons 4 minutes ago
>This
You're cherry-picking a single word to start a pointless internet argument? Calm down. Nobody claimed this exact event lead to problems for anyone.
This is what OP actually said:
>>This is exactly the sort of thing
Yes, this absolutely is the sort of thing. No, 33 feet isn't "the thing" but GPS glitching is the sort of thing at a different increment.
Aurornis 4 hours ago
Geofence monitors have buffer zones because a small GPS on someone’s ankle inside a house, potentially a high rise, isn’t going to be that precise.
33 feet is well within the allowances IIRC. Varies by state.
chaps 3 hours ago
That might be true for a place in the middle of nowhere with no features, the same device model managed by the same company and the same call center person and same sheriff and judges and prosecutor. But with GPS in an urban environment where many homes are over 100 years built from many different materials, 33 feet is a lot.
Aurornis 3 hours ago
Urban environments have larger buffer areas due to multipath interference.
The story you linked has examples about false alerts triggering in a different state.
Do you have any actual examples of 33 feet getting someone arrested? I’ve worked enough with GPS that I could tell you that any system with thresholds that low would result in everyone getting false triggers
chaps 3 hours ago
Yes, everyone on the devices received false alarms. It's why there was a call center that managed the false alarm flows. I highly suggest you read the articles I've shared.
I fought tooth and nail to get access to the data showing how often devices reported "invalid" status (where the device's specs said it was something like, within 150 feet of inaccuracy it reported "invalid"). They initially told me they couldn't get me that data because it'd crash their database running that query. Basically threatened to sue them and over the next 6mo or so I got two 100GB MSSQL dumps. Hundreds of millions of rows of devices reporting back that their GPS signal was invalid. It would take a lot of time to make it sharable because they included the actual GPS location infos.
qlte 2 hours ago
According to this firm selling them, this model has <15ft error, usually 3-4 ft. Given the permissible area is entirely up to what the judge orders, it seems eminently plausible that someone confined to their apartment could be placed outside with a 33ft jump causing a false alarm.
At the very least, it's not enough evidence provided to claim the scenario is inherently impossible based on undefined "allowances" without any actual numbers for something completely dependent on the jurisdiction, monitoring software implementation, and attitude of the police officer/judge.
https://www.youtube.com/shorts/o0sPXL9C53g
I also found Reddit anecdotes of getting alert texts stepping outside onto the street from their yard, or false alarms while still inside but not enough details were provided to be sure they were actually GPS and not RF so not as helpful.
chaps an hour ago
It happened with both RF and GPS! The Cook County Sheriff's Office moved between the two at one point so it was possible to see the effects over time as folk moved over. Some people got more false alarms when moving to GPS, some got fewer. IIRC it was about even.
userbinator 2 hours ago
I misread that as "people getting reincarnated" and was momentarily very confused.
wetoastfood an hour ago
Probably more beneficial than "people getting reincinerated" which is how I almost read it but thought that couldn't possibly be it.
MarkusQ 6 hours ago
TFA article claims: "Indeed, the solar storm of May 2024 is estimated to have cost the US agricultural industry $500 million due to disruptions in precision navigation."
This seems hard to believe. The link provided hand waves through how it maybe could happen, and ends with an Economist just SWAGing that it could have been maybe a dollar an acre or so.
"He estimates that affected farms lost about $1 per acre. Nationally, this equates to an estimated half-billion dollars in lost earning potential, Griffin says."
So yeah, hard to believe was the right call. Very few farms were affected, but if we assume half of the total 900 million acres of farm land were, add 10% to get a round number, and guess it at a dollar per acre, we get the quoted big number.
sen 6 hours ago
That’s how all “estimated damages” work. It’s all just made up by some random person who guesstimated a number then multiplied it by the maximum possible people effected, realistic or not.
Waterluvian 6 hours ago
There’s probably a reasonable methodology but it has a lot of multiplications and some base parameters that make the whole thing wildly sensitive.
qlte an hour ago
Yes, it's actually based on an economic model seemingly developed with a detailed understanding of modern farming practices and historical effects of timeline disruptions on yields. It may be approximate but isn't just some random dude guesstimating based on vibes or something.
https://farmdocdaily.illinois.edu/2024/05/how-will-the-gps-o...
CGMthrowaway 4 hours ago
Tell me "it's different" with this one please https://sustainability.stanford.edu/news/study-links-past-em...
gm678 24 minutes ago
Related:
https://news.ycombinator.com/item?id=49500504 Cheap GPS jammers are filling the world with navigation dead zones (wsj.com) 3 days ago
https://news.ycombinator.com/item?id=48606271 Satellite reveals immense scale of GPS signal tampering (space.com) 3 months ago
f1ay an hour ago
It seems worth mentioning that the last time GPS had issues like this afaik, it was due to the US utilizing 'selective availability' due to national security issues. We used it somewhat famously during the 9/11 attacks.
Also the page 404's via archive.org, but information formerly from gps.gov can still be found here: https://archive.ph/trwOo
Edit: The selective availability page moved to https://www.gps.gov/selective-availability
nwbear90 6 hours ago
An interesting warning as Austin is blanketed by Cybercabs. I think a town full of golden driverless cars 33 feet from where they think they should be is terrifying.
John7878781 6 hours ago
I would be shocked if Cybercabs solely rely on GPS for positioning. I imagine they look at their surrondings, etc.
1e1a 9 hours ago
Wouldn't RTK help mitigate this, as the base stations would also be affected by the atmospheric disturbance?
jplona 4 hours ago
I think so, yes. That's the main purpose of RTK
entrope 3 hours ago
Yes. WAAS should also help a lot, or at least tell you that you can't get a good position.
ashleyn 8 hours ago
Somehow, Selective Availability returned.
Quibblingeek 5 hours ago
Maybe not intentionally (at least by the U.S. Government, but the consequences are far greater now.
Now that GPS has subsumed a large part of air navigation as more VOR and ILS approaches at non-towered airports are being left to fall into disrepair, it forces more and more aircraft operating in instrument conditions to fall back to a navigation system that may not operate to the tolerances that you need to shoot an approach to LPV minimums (which puts you close to 200 feet or so above the ground) or LNAV minimums (higher, but still pretty low to the ground). If you lose signal integrity, you abort the approach. You don't want to break out at minimums only to find that the runway is 33 feet to the left or right of the centerline when it's not a circling only approach where you're expecting it. If that field only has RNAV (GPS) approaches, you have to divert. Hopefully the alternate airport you filed has a ground based approach plate available that is authorized .
When SA was around, we had more VORs, more VOR and ILS approaches, and while NDBs (decades old radionavigation beacon that operated on the AM band) were on their way out, they were still around.
ashleyn 4 hours ago
One of the reasons SA was discontinued was that if you knew the coordinates of a base station, you could correct for the error. Are airports not doing this already?
Quibblingeek 3 hours ago
Actually, several airports are. So you have normal WAAS and it has a ground segment. Most (new) IFR certified GPS receivers installed in airplanes today are WAAS capable.
To that, you can augment something called GBAS (we used to call it Local Area Augmentation System but switched over to the ICAO name for it). It’s basically a ground station that sits at an airport and corrects for GPS errors where you need it most (airports definitely qualify here and that’s where you need the best GPS performance).
Ultimately, the FAA is going to go 100% GPS and it’ll fall back to the minimum number of land-based sources for redundancy. It’s about cost. The issues with GPS may cause them to push the timeline back and develop plans to mitigate the issue, but they’re too far down this path to stop now.
entrope 3 hours ago
To meet its integrity budget, a GBAS needs at least triple redundancy, not just a single station.
If one has a lot of patience, one can read the details of how it works at https://elibrary.icao.int/product/299828 (along with SBAS, the ICAO name for what WAAS does; the constellations they can augment; GRAS, a weird hybrid between GBAS and SBAS that apparently went nowhere; ILS; VOR; DME; and VHF marker beacons). Annex 10 Vol I probably doesn't say explicitly that triple redundancy is required, because if you had a single receiver with an MTBF of something like 10 billion hours then you could use just that one.
This particular 30-foot error should go away almost entirely for future receivers that implement DFMC SBAS or ARAIM.
nubinetwork 2 hours ago
The gps on my phone drifts by a few city blocks all the time...
NooneAtAll3 10 hours ago
were other satellite positioning systems affected as well? was it checked that it wasn't just a misconfig on US command side?
mcpherrinm 9 hours ago
The linked actual research article did look at multiple GNSS systems. GPS is presumably being reported out of familiarity https://doi.org/10.1029/2026GL124645
Insimwytim 9 hours ago
TL;DR:
1. Not today. November 2025. They don't make it clear.
1.1. One would expect that to be put in the title, when the article is written almost a year after that.
1.2. No, the article doesn't state that in the first paragraph. Posting in it full seem to be the only way to fend off lazy comments:
In November 2025, Earth was buffeted by several massive eruptions of solar material that slammed into the magnetosphere.
2. It is unclear what is the source of the claim.
UPD: Commenter below found the source, but TFA doesn't make a good job of presenting that as a source of the 33 feet glitch claim, but rather posts another citation from it.
They should've just quote this and be done with it:
During the storm main phase, an east–west elongated band of intense auroral brightness expanded equatorward, producing a horizontally extended strip of strong density gradients that favored the development of small-scale irregularities. These small-scale irregularities triggered pronounced amplitude scintillations (S4 > 0.5) and caused significant positioning errors exceeding 10 m—large enough to disrupt GNSS-dependent applications across CONUS for hours, particularly precision agriculture and autonomous transportation industries. [1]
4. The author has articles all over the topics. Everything is sensationalized.
5. Every presented link seem to have this awful Cloudflare js challenge.
Bad article overall.
P.S. Response to the false claim below - no, there's nothing about GPS in "the first 3 words of the article".
[1] https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2026GL12...
Elidrake24 9 hours ago
> Not today. November 2025. They don't make it clear.
My brother in christ it's the first 3 words of the article.
gerdesj 6 hours ago
Their first line is TL;DR!
nomel 8 hours ago
> 2. It is unclear what is the source of the claim.
7th paragraph of the article states the source, and contains a link to research paper where they go into great detail.
kbelder 8 hours ago
>P.S. A separate comment would be too much to respond to the false claim below - no, there's nothing about GPS in "the first 3 words of the article".
He's talking about your first (incorrect) claim. The article begins "In November 2025"
ck2 10 hours ago
not just space weather, I am pretty sure the atmosphere is being disturbed by toxic human activity both by accident and on purpose
I've run outside every morning for decades and used Garmin with GPS since 2015, I've never had so much trouble
Garmins finally supports dual-band (L1+L5) and BeiDou on newest models but it's still not enough to compensate, I have to use a Stryd footpod to guarantee accurate distances/speed
btw BeiDou GPS blows away US GPS for number of sats and precision but despite Russia's GPS being legal in US, technically BeiDou is not
* https://warontherocks.com/a-signal-point-of-failure-integrat...
* https://payloadspace.com/gps-faces-growing-competition-from-...
* https://en.wikipedia.org/wiki/BeiDou#GPS_vs._BeiDou_Capabili...
z2 9 hours ago
For resiliency reasons alone, it's been proposed to allow (stop software-disabling) BeiDou in the US, but the last time this was brought up, the administration ran out of time. It's a shame, because when there is precise coverage, it enables things like lane-level city navigation.
https://warontherocks.com/a-signal-point-of-failure-integrat...
ck2 9 hours ago
yup that was my first link
but of course this administration is never going allow China-anything like low cost electric cars even if it means saving people a fortune in gas
so hopefully in 2029 it can be looked at again
or maybe Dems will also get the Senate too and can be petitioned if they run out of corruption investigations before 2029
it is critical to explain to everyone who isn't technical that GPS is "read only" and no data is being sent to China via BeiDou (just like it's not sent to Russia's Glonass) because that apparently is some kind of weird misunderstanding
on that same note if they somehow made Starlink into a lower resolution GPS backup I'd even trust that too despite personally being anti-Musk everything
bigfudge 6 hours ago
Read only or not, relying on Chinese or Russian gps data for increased accuracy even in civilian increases dependency no?
That seems like a bad idea to me. I’ve thought for about 20 years that increasing our dependence on anti democratic regimes was a risk, no matter what the benefits for global development etc. It’s not isolationist to want to only rely on your friends and not enemies.
(Which is why Europe needs to disentangle itself from the US as fast as possible).
z2 5 hours ago
It decreases dependency on GPS and thus it decreases risk because if there's an attack on a GPS system by an adversary, it's likely that they will not be attacking their own system at the same time. I'd rather have my enemy's open source, perfectly functional clock in my back pocket than be without the time. Enjoying that the competing clock has a sub second display is optional.
blululu 2 hours ago
If you are relying on it without a graceful fall back then it would be a problem. If you take it as a boost on top of existing capabilities then it is a pure win. In practice it can be hard to parse the two scenarios. People get used to thinking that something is available and it turns out that it was always leverage against them.
z2 4 hours ago
Whoops I don't know how I missed that in your links!
The weird part to me is that Glonass was not approved either, yet it's as openly as allowed as Galileo that went through a tedious FCC approval process that was invented later. Grandfathered or not, we're clearly not in a worse place using Russian tech for the last 15 years.
arijun 9 hours ago
> BeiDou GPS blows away US GPS for number of sats and precision
Both BeiDou and GPS are supposed to be limited to 10m precision for civilians.
But, going down a rabbit hole, I found that BeiDou is way more accurate than 10m--3.6m and 2.6m outside and inside the Asia-Pacific region, respectively [1]. And that's with the most simple algorithm. It goes down to ~2.5m using dual-frequencies to account for atmospheric error.
myself248 8 hours ago
> Both BeiDou and GPS are supposed to be limited to 10m precision for civilians.
Maybe in 1998, but Selective Availability was turned off in 2000, and here in the modern era: https://rtklibexplorer.wordpress.com/2024/04/28/dual-frequen...
arijun 8 hours ago
Oh interesting, I think I conflated two things; Selective Availability and the military-only bands on GPS. While it's true that access to the military band could improve accuracy with dual-band receivers, consumer devices can use dual-band as well (as your link shows), since there are multiple unencrypted bands from navigation satellites nowadays.
I am not sure how to interpret your link, though. It doesn't measure accuracy on it's own, and doesn't include a number for precision between the two units.
hylaride 8 hours ago
My understanding is that the encrypted military-level GPS is much more robust against jamming and spoofing. There's probably a lot of capabilities we don't know that's classified, too.
arijun 7 hours ago
That is my understanding as well. But having another band allows for error correction as a side benefit.
IndySun 7 hours ago
Called M-Code.
Has a fascinating history.
https://odimpact.org/case-united-states-opening-gps-data-for...
entrope 2 hours ago
M-Code is a new encrypted signal that is not fully operational yet. OCX was supposed to enable it.
GPS has always had the P(Y) code, which is an encrypted signal, at a higher chip rate than the C/A code that has been broadcast on L1 forever. The P(Y) code has its own interesting history of (semi-)codeless processing. If the US military is lucky, the history books will close on that by 2030: https://www.gps.gov/codelesssemi-codeless-gps-access-commitm... . (Currently, 21 GPS satellites broadcast L5.)
pkaye 7 hours ago
I think the new US GPS satellites have ability to do a spot beam of increased strength in a region of interest.
myself248 4 hours ago
There are a couple of points:
The L1 signal had selective availability in the 90s, and it no longer does. SA had the real-world accuracy around 30m in practice; the "up to" 100m rarely happened. Without SA, plain L1 with good skyview (low DOP) gets you around 10m, or down under 5m if WAAS is having a good day.
Availability of multiple civilian bands, and cost-effective dual-band receivers, allows an ionosphere-free (no need for WAAS) solution in the 1-2m range, with a single receiver.
Ubiquity of internet connections makes it trivial to apply RTK correction data without having to set up your own base station, and/or, availability of affordable medium-range data radios makes it easy to set up your own base station. In either case, it's easier to run RTK than ever before, and dual-band RTK gets you down into the low centimeters, limited mostly by how accurately your base station was surveyed-in.
And none of that touches the military signals.
rcxdude 5 hours ago
With a base station in a known position you can get to cm resolution with somewhat pricey but decidedly hobbyist-accessible gear: https://www.sparkfun.com/sparkfun-gps-rtk-sma-breakout-zed-f...
arijun 8 hours ago
It seems that all the device manufacturers (they mention Apple, Samsung, and Google) are using BeiDou anyway. I don't know if that includes Garmin watches, though.
https://www.reuters.com/technology/us-agency-probes-risks-fo...
rzzzt 7 hours ago
AGPS also helps if you have cell towers nearby.
askvictor 5 hours ago
This is only to make getting a fix faster - A-GPS downloads the almanac data via the internet (fast, but needs internet) instead of directly from the GPS satellites (slow but works everywhere). It doesn't make it any more accurate.