GPS vs. Glonass vs. Galileo

[rzimmerman]

The Position Calculation and Satellite Selection sections are a little over-simplified. It's not true that one satellite is used for time synchronization and three are used for trilateration. In a four-satellite case, all four are used to solve the four dimensional problem of "where am I in time and space?" In reality, upwards of 12 satellites are used to find a solution to this problem and adding more data improves accuracy.

It's true that satellites overhead provide better more vertical position information and that satellites at low elevation are more impacted by the atmosphere. But the math isn't that simple - satellites aren't used for specific purposes. They all contribute to a position solution and useful parameters like vertical and horizontal uncertainty.

[avianlyric]

Out of interest are speaking from a position of authority as someone who’s implemented GPS receivers, or as someone who understands the theory of GPS receivers?

From my reading of the article, it sounded like the author was commenting on an interesting implementation detail of GPS receivers, and how they might deviate from their expected theoretical implementation. Rather than describing how an ideal GPS receiver is implemented.

[oddlama]

If anyone wants to learn about GPS from the ground up, there's an excellent website with interactive explanations by Bartosz Ciechanowski that I can wholeheartedly recommend [1].

His blog also covers other topics in a similar style - it's a real treasure trove.

[1] https://ciechanow.ski/gps/

[ThePowerOfFuet]

I knew that name sounded familiar!

https://ciechanow.ski/mechanical-watch/

https://news.ycombinator.com/item?id=31261533

[rzimmerman]

Interestingly despite becoming "fully operational" in 1995, GPS was in use in the 1980s and played a huge part in the Gulf War in 1990/1991. "Fully operational" probably means 12 satellites in view at all times for any point on the earth between 55°N/55°S, but it's still very useful even at lower levels of service.

It's pretty amazing that a system like that could be envisioned in the 1970s and be fundamentally life-changing by the 1990s. Truly a modern marvel of engineering that we rely upon for precise timing, power grid synchronization, navigation, and a lot more.

[dweekly]

Fully operational in 93, but IMO the real slam dunk was disabling of Selective Availability in 2000, which made GPS significantly more useful. WAAS has been amazing for aviation, allowing for RNAV instrument precision approaches to airports with no RF equipment installation required onsite. And there are still innovations getting rolled out, including L2C, L5 and more!

https://www.gps.gov/systems/gps/modernization/civilsignals/

[tzs]

> "Fully operational" probably means 12 satellites in view at all times for any point on the earth between 55°N/55°S

Whenever I've checked how many are visible here (around 48°N) I most commonly see 8 or 9, with 7 or 10 next most common.

[throw0101c]

If anyone wants to get into the nitty-gritty details of GPS / GNSS, there's a good series of lectures (course) available from Standford University; playlist:

* https://www.youtube.com/watch?v=o1Fyn_h6LKU&list=PLGvhNIiu1u...

Topics include navigation message structure, signal encoding, error budgets, Keplerian parameters, path loss/antenna gain/link budget, plus orbital details of GLONASS/QZSS/BeiDou/Galileo.

[JCM9]

GPS can be much more accurate than what the article says. For example, over the continental US the extra WAAS satellites provide accuracy down to more like 3-4 feet for GPS based aircraft navigation. These satellites broadcast correction signals to allow receivers to adjust for small fluctuations in the standard GPS signal. Using that system aircraft can navigate in 3D down to about 200 ft off the ground for a landing approach without the need for any ground-based equipment or transmitters at the airport.

[mastazi]

To add to this, WAAS uses a combination of ground based equipment and geostationary satellites.

The ground based equipment is responsible for detecting the necessary corrections, which are then sent up to the WAAS satellites which will in turn broadcast those corrections.

Unlike GPS satellites, WAAS satellites are always located over North America due to the fact that they are in a much higher, geostationary orbit.

Edit - WAAS also provides GPS-based vertical navigation (e.g. descent profile in an approach) - IIRC this is due to the fact that the ground station transmit accurate atmospheric pressure readings up to the geostationary satellites, then the GPS receiver in the aircraft can use those to adjust altimeter readings. GPS-based vertical navigation is a big deal, because many airports don't have systems such as ILS.

[kentrf]

My first years as a software developer, I worked with gathering data from GPS, Glonass, BeiDou and Galileo for use in a DGNSS product. It's been a while since that time, but I remember implementing RTCM-v3 encoder and decoder, gathering signals from all around the globe, and general TCP/IP/networking to ensure high availability and so on. Oh the memories.

But the thing that taught me most about orbital mechanics, is still KSP (Kerbal Space Program).

[tdeck]

If you're interested in this, you might be interested in the terrestrial radio navigation systems that predated GPS:

https://en.wikipedia.org/wiki/Omega_%28navigation_system%29

https://en.wikipedia.org/wiki/LORAN

https://en.wikipedia.org/wiki/Loran-C#e_LORAN

https://en.wikipedia.org/wiki/Hyperbolic_navigation

[smartmic]

Unfortunately, as it is with many European projects, Galileo is over-promised but yet under-delivered. Currently only 22 satellites are usable: https://www.gsc-europa.eu/system-service-status/constellatio...

[wewxjfq]

Care to enlighten us how the satellite failures of Galileo are worse than the satellite failures of the other systems?

[dataflow]

How did L5 get enabled and still leave us with an accuracy worse than 3.5m? Wasn't it supposed to give us 30cm? What happened to that? https://en.wikipedia.org/wiki/Global_Positioning_System

[killingtime74]

Don't quote me but I believe you have to use both bands to get that accuracy. Some devices support it. https://support.garmin.com/en-NZ/?faq=9NWiPDU4gM0JWMfdWFol7A.

Real life test https://www.dcrainmaker.com/2022/04/garmin-vertix-accuracy.h...

[macropin]

GPS used in survey equipment is accurate to 2cm with ~5 second acquisition time.

[jonathankoren]

According to this paper, if you hook in all four sat nav systems, you can achieve 10 cm accuracy in minutes, 5cm in 30 minutes, and millimeter accuracy in a few hours.

https://www.nature.com/articles/srep08328

[dismalpedigree]

The Ublox ZED-F9P receiver is capable of leveraging dual band across all 4 main GNSS systems to create its solution. It is very accurate. If you combine it with RTK (either local, SBAS, or NTRIP) you easily get sub 5cm accuracy in realtime with sub 30 startup times. (Longer if its a really cold start)

[hugh-avherald]

I'm somewhat curious how one would determine the true position to mm accuracy so as to compare. Put another way, if something says you're at these coordinates, how do you conclude "it should be 1 mm to the north"?

[ummonk]

Kind of weird not to describe BeiDou as well...

[microtonal]

GPSrChive is very much focused on Garmin GPSr and I don’t think there are any Garmin devices that support BeiDou.

[ck2]

Also interesting is the American extension to GPS called WAAS which is used for aircraft precision.

https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System

[ars]

GPS and Galileo use the exact same frequencies, but call it different things?

  L1 (1575.42 MHz) and L5  (1176.45 MHz)
  E1 (1575.42 MHz) and E5a (1176.45 MHz)

Is there a difference I'm not aware of?

[ck2]

Another interesting tidbit is apparently Galileo cannot be "seen" in the USA, not because of same frequency but because the FCC forbids its license?

https://galileognss.eu/why-galileo-is-not-seen-in-united-sta...

https://barbeau.medium.com/where-is-the-world-is-galileo-6bb...

At least as of 2019 but maybe has changed as we approach 2023?

Firing up GPStest on various phones to see for myself.

[dismalpedigree]

I was wondering the same.

[dale_glass]

Those maps are interesting.

So GPS doesn't work well in Greenland or a good chunk of Russia?

[rzimmerman]

You should still have (oblique) views of the constellation above 55°N, partly due to the large altitude of the GPS satellites. But you're not guaranteed to have 12 satellites in view, so your solution may not be as accurate. There are polar regions where you probably do lose service. It's interesting (though not surprising!) that GLONASS has a slightly higher inclination.

[causi]

What are the sources of positional uncertainty? Is it mainly uncertainty of the exact position of each satellite at a given time? It's interesting how slowly positioning accuracy has improved. For something that first reached global coverage in 1995 I would've expected four or five orders of magnitude improved precision by now.

[adgjlsfhk1]

your main sources of error will be from the Earth's atmosphere and signal bounced which both change the time signals take to reach you. there are pretty hard lower limits on the amount of error. gps already has to deal with relativistic effects to get as accurate as it is.

[AviationAtom]

Mandatory mention of a cool related project: https://play.google.com/store/apps/details?id=com.iiasa.cama...

[ggm]

on Android, the "locus" mapping app includes a really good diagnostic on GPS reception which I still refer to in the edge cases (inside trains, planes, where even with a window seat reception can be marginal) to understand whats in the sky above me right now

[ThePowerOfFuet]

If you mean the ones made by Asamm Software, unfortunately they contain privacy-concerning trackers. :(

[airswimmer]

You missed the "Beidou" positioning system, your news is too out of date.

[pingiun]

does this mean norway cannot receive GPS?