GPS vs. Glonass vs. Galileo

Fair question - I’m familiar with the theory of operation and I’ve implemented toy receivers (on the software side). I will say that even in the basic designs I worked with it made sense (and was easier) to solve the system of equations and not do something special like assign a “time satellite”. It’s a much easier way to describe the system to someone without a linear algebra background, so I bet that’s the author’s intent.

As someone who did location work based on timing in another context - picking 3 satellites straight above seems weird - we had better accuracy when beacons were right angles - basically if you have a timing error and sats are near each other - the intersection of the two lines moves hugely

There's pretty much never a reason (...normally, but see below) you'd use a separate satellite for the time -- you do need four satellites to solve for X/Y/Z/time, but you solve them together. Basically, there's only one combination of satellite positions and (pseudo)ranges that will produce a solution, and one of the outputs of that solution is the time (the inputs are just time offsets between the various signals, based on the receiver's non-precision local clock).

The see below part: There are timing receivers that will do a long "survey" to figure out their exact location (or as close to it as they can), and once they have that they can use a single satellite to determine the current time, since they already have most of the needed equations "solved" when the receiver already knows its own (static) location. This is sometimes preferable, depending on one's application, because it makes for less jumpiness in the time solution as new satellites go into and out of view (since the changing geometry of the constellation will make for slightly different solutions every time it changes)

It's physically impossible to dedicate one satellite to time synchronization. The satellites broadcasts a time signal, but that's the time at the moment of transmission, not the moment of reception. Since the receiver by definition doesn't know where it is yet, it doesn't know the propagation delay from the satellite, and it can't calculate the time at the reception site (to the accuracy needed for positioning, anyway).

IMO the secret sauce wasn't necessarily the precision but the ability, granted by technology outside the GPS system, to put a dot on a digital map versus your PLGR giving you a pair of coordinates that you had to look up on a paper map.

the 12 number probably assumes the middle of a flat plain with no tall structures nearby. buildings, hills etc will reduce the number.

The cool part for me was realizing that the three WAAS satellites not only broadcast the correction signals but also act as position / constellation satellites! This adds an additional 2-3 satellites to the "currently visible" mix and helps ensure high availability location data at all times over the continental US.

WAAS (and other SBAS constellations) are used for RNAV/RNP LPV approaches indeed (for both lateral and vertical navigation), but WAAS is only used to obtain atmospheric corrections for the GPS signals, the WAAS ground stations do not transmit atmospheric pressure. Furthermore, unlike GBAS (Ground Based Augmentation Systems), SBAS ground stations are not located at the airport, they can be dozens or hundreds of kms away making the atmospheric pressure from there not very useful.

Good article on the topic by Airbus: https://aircraft.airbus.com/en/newsroom/news/2022-06-satelli...

Only North America for flying … a bit odd. Really?

Here is an article explaining the 2019 outage and the issues with the satellites: https://berthub.eu/articles/posts/galileo-accident/

The article argues that the Galileo project has a bit too many participants in the development. Europe does have some hugely successful multi-participant international projects like Airbus or CERN but it is indeed more challenging to run projects funded by 30 countries each having different culture, language and interests.

It's really a re-occurring theme with no easy fix. The European countries are too small to do such large projects by themselves and our multi millennial history is about fighting each other, so it's not always a smooth sail.

Thanks. I've tried a phone with support for both L1 and L5 (and other systems) and I don't notice any improvements over my old phone. In fact it seems to take longer to get a fix. I would love to hear if anyone has found a phone that actually gets significantly better performance with L5.

Not all of the satellites have L5 either. As of today only 16.

This may be differential gps with a local reference

RTK GPS can be 1cm accuracy with sub second time.

Around 250 bucks for those wondering. Bit heavy for a hobby project but less than expected

You should also have a look at the much better performing Septentrio Mosaic series of GNSS receiver boards. There are dev kits available for a dual antenna heading RTK receiver for around €750. And €645 for single antenna. Dizzyingly cheap compared to what this class of receivers cost just a few years back.

These support all GNSS constellations and signals and do a real 50 Hz PVT in RTK mode. And they even come with a built in, quite useful web interface.

From Garmin, though without a breakdown of proportion of devices:

In addition to GPS, Garmin products utilize other global navigation satellite systems (GNSS) including the Russian Global Navigation Satellite System (GLONASS), the European Union Galileo system (Galileo), and the Chinese BeiDou Navigation Satellite System (BDS), and satellite based augmentation systems (SBAS) including the U.S. Wide Area Augmentation System (WAAS), the Japanese MTSAT-based Satellite Augmentation System (MSAS) and Quasi-Zenith Satellite System (QZSS), and the European Geostationary Navigation Overlay Service (EGNOS) aviation Safety of Life (SoL) service.

Though to the extent this is a US-centric site, the other reason for not caring much about Beidou is that the FCC still has a geofence block for Beidou, so that no signals may be used in US territory: https://www.gps.gov/spectrum/foreign/. It's as if Beidou doesn't exist in the US, and even a receiver that supports it will only start using the signal once it first confirms through other GNSS's that it's not located in US. (Example: most phones made in the last 3-5 years)

There will be soon if not already, but they won't be accessible without an Asian-specific variant of the device and outside of specific Asian countries.

literally the next blogpost on that blog is "FCC approves use of Galileo in the US" ;)

https://www.euspa.europa.eu/newsroom/news/fcc-approves-use-g... is from 2018, before your date of 2019.

But maybe the receivers need updated programming.

It was important for USSR/Russia to have positioning service available at high latitudes. Thus, higher inclination.

Do polar explorers use some other navigational aides then? I wonder how inaccurate is the GPS really close to the poles.

To expand on this for @causi: The largest thing is the earth's atmosphere, because changes in the ionosphere (and a lesser degree, the troposphere) cause the signals to get refracted differently from moment to moment. Part of the data coming from the constellation includes general ionospheric correction data (and WAAS can provide data that covers smaller (but still kinda large) areas, which allows the receiver to adjust for those refractions to some degree, but there's only so much it can do -- conditions change quickly, and in areas smaller than the correction data can accommodate, so there's still variance that can't be completely removed -- it's not a lot, but at one foot per nanosecond of delay, it can still make a significant difference.

At least, that was generally the case until recently. There is actually a way to almost completely remove the effects of both the ionosphere and troposphere: Those things affect signals differently if you have multiple signals at different frequencies. And, as it happens, GPS does actually have multiple frequencies (L1 and L2, and now L5 as well), and for a fairly long time there have been receivers that could listen to signals from the same satellite but different frequencies, and based on the delay difference between those signals, know exactly what the atmosphere was doing at that exact moment, and dial out the influence of the atmosphere almost completely. You can start getting pretty durned precise once that's not a factor.

The problem is that multi-frequency receivers used to be expensive. Like "started at $10k for the cheap stuff" expensive, even within the last decade. Only in the last few years have inexpensive (under $100) chips become available for doing multi-frequency GNSS. And those can get down into the "under 1 meter in realtime" range trivially, and better than that for a fixed-location station. Phones are getting these now, so things should start getting more accurate, though not that much more accurate.

Multipath is also a big problem in "the urban jungle", but chips are getting better at discriminating, and unless you're just utterly surrounded by skyscrapers, usually isn't too big a deal.

Orbital calculations are also another cause of loss of precision -- the orbits are calculated pretty precisely, but for various reasons the ephemeris data sent down from the satellites doesn't actually represent exact orbital data, but represent data that's "good enough" over the couple of hours the ephemeris data is valid. This can be worked around with patience -- there are ground stations around the world with exactly surveyed locations, which monitor the satellites and calculate the exact orbital paths the satellites actually took, and publish that data (though it takes several weeks to get the "final" data). A typical surveying technique is to record several hours of data from an antenna at a survey location, and then when the precision orbit data is published, post-process that recorded data to remove both orbital and ionospheric effects. This can get you down into the sub-cm range, with enough care.

And then there's also a range of other factors, like solid earth tides, which cause the land masses of the earth to rise and fall by up to a meter(!!!) over time, and when you're trying to figure out exactly where a given point in space is on this big rock ball, that matters!

But, yeah, pretty much it's atmosphere, unaddressed multipath, and orbit precision that makes the difference, and the above is how those are usually dealt with.

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wondering is there any upper bound on the error, e.g. is it possible to ever have a timing error of say 100 microsecond seconds.

Yes. Those ones. Damn, thanks for telling me. Do you have a good alternative recommendation for the gas diagnostic function?