it did cover large populations centres, as well as major marine shipping and air traffic routes. Another reason why it didn't the coverage wasn't global was a lot of transmitters (e.g., the continents of South America and Africa).
With regards to timing accuracy, a paper fro 1992:
> The accuracy of time comparison by means of the reception of common time signals, namely, LORAN-C and Global Positioning System (GPS), is examined. Differences of UTC from the emission times of these two radio wave signals measured at four Japanese stations and one U.S. station are analyzed and compared with the clock comparison reports published by the Bureau International des Poids et Mesures. The accuracy of the time comparison via LORAN-C signals of the northwest Pacific chain is about 3.61 µs and that via GPS is 0.14 µs. The accuracy analysis reveals the existence of systematic errors at the individual stations. In LORAN-C, the most serious error comes from those in the delay time calibration of receiving instruments and estimation of the propagation delay over land. Calibration of the clocks by a portable clock improves the accuracy of the GPS method to the accuracy of the GPS signals.
> IIRC, GPS satellites have 25W transmitters, that's insanely powerful, but they're 20000Km away
Both numbers are correct, but 25W isn’t “insanely powerful”. The spec for GPS satellites lists the antenna gain at 13dBi. 25W fed through a 13 dBi antenna is just shy of 500W EIRP. Sitting near me is a 2W 2.4ghz amp and a 24 dBi parabolic antenna. That combo is just over 500W EIRP. The reason you stated (distance) is the big factor, over 12,500mi, the free space path loss is huge (182dB assuming flat gains). 500W is just shy of 27 dBW, so 27-182 is -155, which is a very very very weak signal.
Determining the angle of a microwave signal requires a pretty big receiver dish (or an array of receivers, at least).
The wavelength of the GPS L1 band is 19cm, you'd need (roughly) something of at least 10x that size to get a good sense of where a signal is coming from, so on the order of 2m.
A single good antenna and sensitive amplifier gets you a fix on more satellites, which provides good accuracy with less space and hardware used.
>The most accurate GNSS fixes are obtained from offline solvers that analyze long-duration (e.g. 24 hour) recordings of the GNSS data streams and use computer models to determine the most likely solution accounting for many types of error.
I actually have a receiver that does exactly this! It is a Trimble Resolution T, available for very cheap (I believe they're from old cell towers), and it's intended exclusively as a time reference. You first run a 24 hour data collection, and it precisely determines your location before storing it. From then on, the receiver only calculates the time. Very cool stuff, I'm planning on building a GPS-disciplined rubidium clock and stratum 1 NTP server with it.
>Still, I suspect inexpensive consumer receivers are probably calculating a fix per constellation and then combining those.
This sounds like a reasonable solution now that you mention it, for an inexpensive device. Part of my confusion is based in the fact that the constellations actually use different reference datums [0]. Wouldn't the different constellations give meaningfully different fixes even if the solution was "perfect"?
I mentioned in another comment that I pretty much forgot that this could be a political thing. That's a very good point and I don't know why I didn't think of it. Especially with selective availability, which as far as I understand, the US government could just turn on again at a moments notice.
GPS signals come from satellites. AFAIK there are about 20-40 GPS satellites. Let's say each covers at least 1/50 of the Earth's surface area, about 500 million km^2. That's 10 million km^2 per satellite.
What's the max power generation you can fit on reasonable-sized satellite that's going to be in orbit for decades? I'm guessing not more than 10 kW. So 10 kW / 10 million km^2 means you have .001 watt per km^2.
GPS signals being easy to jam isn't surprising. What's surprising is that you can detect them with equipment that's small and cheap enough to fit in a cellphone!
>> Can other GNSS give usable timestamps? Seems like it'd be tricky for a jammer to target all of them
Actually the opposite; GNSS systems are all purposely designed to operate at virtually the same frequency (check out this figure [1]) while cleverly not interfering with each other. There are sub-bands within each constellation too (L1,L2,L5 etc) but it's very easy to pump out wideband noise across all the GNSS bands.
> These days though, a more popular idea seem to be to piggy back onto GNSS signals, which are also broadly available (no idea how well they compare in practice to low frequency time beacons), and for which importantly many mobile devices already have a receiver built-in
GPS chips are very very power intensive as the compute power involved to decode a signal out of something that would normally be way below the noise threshold is still very high, despite some two decades worth of optimization. And they're pretty useless outside of direct line of sight towards the sky.
> In fact, I can't imagine many events other than weather (which is usually localized) that warrant such a low-entropy signal on a nationwide basis, and everything I can imagine has very limited actionability. ("Asteroid inbound, prepare to hide in the basement for a few thousand years"?)
It's a trigger signal for a low-power receiver that can then turn on a higher-bandwidth (and thus, higher power) device to detect what is actually going on. Say you're a country at war like Ukraine - have the "emergency bit" on nationwide during Russian air raids so that receivers can listen for area-specific signals and blare horns if affected.
No matter what, we have to prepare for war, and that includes having robust technology that is hard to take out on the sender side (which cellphone stations aren't, they're easy targets in a cyber war!) and, most importantly, can be stored in everyone's garden shed and live on a single battery for years.
> You need a precise clock for any form of GNSS[0] to function
How precise? I assume that inaccuracy in the clock produces imprecision in location - how much time inaccuracy produces how much location inaccuracy?
> And anyway, modern GNSS chips are.. $1? Cheaper? No way you wouldn't use GNSS already deployed and working (and you are not paying for it) in a commercial product.
Rather than using the Starlink satellites as an alternative to GPS - get a location fix using both Starlink and GPS, and compare them. Even given the former is going to be significantly less accurate - suppose GPS is accurate to within 5 metres and Starlink is accurate to within 5000 metres - well, if the two differ by 50,000 metres, you know something is going wrong, and the terminal might decide to disable itself in such a situation.
>It's amazing what's connected to GPS that ought not to be.
Is there a practical alternative for high-accuracy synchronisation? You could make the case for a backup shortwave receiver, but I'm not sure that's any more reliable than GNSS.
This has not been true for quite a while. The equipment used on surveying, farming, and construction equipment is much more accurate thanks to the availability of multiple GNSSs (e.g., GPS, GLONASS, Galileo, BeiDou) and terrestrial base stations to remove error (WAAS, DGPS).
I would like to point out the insanely good design of the GPS radio layer (the L1+L2 signals).
Even 46 years on, the radio layer is fully forwards and backwards compatible, and a bunch of important metrics like time to first fix and user equivalent range errors have both improved by factors of 10-1000, with no incompatible change needed to the protocol.
The total RF transmit power to provide service to the whole earth is less than the electricity consumption of a typical US house (far less than 5G or TV or AM/FM radio), and well below the noise floor.
The design has allowed frequency-sharing with competing systems (eg. Galileo) - you don't see mobile phone networks doing that!
The actual signal sent has allowed things like carrier phase decoding, due to the locking of the phase between the modulated data and the carrier, which in turn gives far better pseudoranges and accuracy.
Overall, the designers either had incredible forethought, or incredible luck, or some combination of the two.
>GPS is one of the most reliable systems there is. There are redundant constellations of satellites run by different companies in different countries.
1. GPS refers specifically to the system run by the united states government. The generic term is GNSS
2. Even though there are multiple GNSS systems out there, and modern phones support multiple, it's unknown whether the same applies to other systems. Supporting multiple systems costs more money, so I suspect hardware vendors might skimp on that, especially when the benefits are so marginal.
> One of the most accurate solution is to keep a GPS receiver on a well known location.
I wonder if a network of connected devices with a GPS-disciplined SDR receiver and a regular GPS one could work both as this project does plus as passive radar like the software that was recently taken down. The purpose would be to have much wider coverage along with redundancy and error correction.
You jam L1 (1575.42 MHz) and you take out a whole bunch of services out.
It's one of the reasons why there's L2, L5/E5, and E6. But everything is in the Aeronautical Radio Navigation Service and Radio Navigation Satellite System bands.
> There are also military specific parts of GPS that civilian receivers can't access. I don't know if military receivers are ignoring civilian signals, though.
For all intents it's a different system, the packets are encrypted, the right receiver hardware gives you vastly superior accuracy. There's civilian hardware which can use the packets without decrypting them for better accuracy, there's some patents on doing this amusingly.
> Any time one of your devices contacts a GPS satellite, it's telling a system of satellites owned and maintained by the US government exactly where you are.
GPS receivers are just that. They do not transmit anything to the satellite.
GPS L1 power when received is -128.43 dBm (-158.43 dBW):
* https://apps.fcc.gov/els/GetAtt.html?id=110032&x=
* https://support.spirent.com/index?page=content&id=FAQ14116
* https://support.spirent.com/SC_KnowledgeView?Id=FAQ14116
Meanwhile (e.g.) Loran-C is -60 dBm:
* https://www.qsl.net/df3lp/projects/lfscan/index.html
So GPS signals are below the noise floor, but Loran-C are 50 dBm are above it:
* https://www.prc68.com/I/A2100F.shtml
So while something like Loran-C didn't have global coverage while it was still active:
* https://en.wikipedia.org/wiki/File:NGA-Atlantic_Loran.png
* https://en.wikipedia.org/wiki/File:NGA-Pacific_Loran.png
* https://en.wikipedia.org/wiki/File:Loranstationscrkl.jpg
* https://timeandnavigation.si.edu/multimedia-asset/loran-day-...
it did cover large populations centres, as well as major marine shipping and air traffic routes. Another reason why it didn't the coverage wasn't global was a lot of transmitters (e.g., the continents of South America and Africa).
With regards to timing accuracy, a paper fro 1992:
> The accuracy of time comparison by means of the reception of common time signals, namely, LORAN-C and Global Positioning System (GPS), is examined. Differences of UTC from the emission times of these two radio wave signals measured at four Japanese stations and one U.S. station are analyzed and compared with the clock comparison reports published by the Bureau International des Poids et Mesures. The accuracy of the time comparison via LORAN-C signals of the northwest Pacific chain is about 3.61 µs and that via GPS is 0.14 µs. The accuracy analysis reveals the existence of systematic errors at the individual stations. In LORAN-C, the most serious error comes from those in the delay time calibration of receiving instruments and estimation of the propagation delay over land. Calibration of the clocks by a portable clock improves the accuracy of the GPS method to the accuracy of the GPS signals.
* https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/92RS...
* https://doi.org/10.1029/92RS01010
eLoran can get to the 100s- to 10s-of-ns range:
* https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7697629/
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