[HN Gopher] GPS vs. Glonass vs. Galileo
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GPS vs. Glonass vs. Galileo
Author : cokernel_hacker
Score : 147 points
Date : 2022-11-27 17:51 UTC (11 hours ago)
(HTM) web link (www.gpsrchive.com)
(TXT) w3m dump (www.gpsrchive.com)
| oddlama wrote:
| 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 wrote:
| I knew that name sounded familiar!
|
| https://ciechanow.ski/mechanical-watch/
|
| https://news.ycombinator.com/item?id=31261533
| rzimmerman wrote:
| 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
| 55degN/55degS, 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.
| tzs wrote:
| > "Fully operational" probably means 12 satellites in view at
| all times for any point on the earth between 55degN/55degS
|
| Whenever I've checked how many are visible here (around 48degN)
| I most commonly see 8 or 9, with 7 or 10 next most common.
| jhugo wrote:
| the 12 number probably assumes the middle of a flat plain
| with no tall structures nearby. buildings, hills etc will
| reduce the number.
| dweekly wrote:
| 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/
| 13of40 wrote:
| 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.
| smartmic wrote:
| 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 wrote:
| Care to enlighten us how the satellite failures of Galileo are
| worse than the satellite failures of the other systems?
| mrtksn wrote:
| 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.
| throw0101c wrote:
| 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.
| AviationAtom wrote:
| Mandatory mention of a cool related project:
| https://play.google.com/store/apps/details?id=com.iiasa.cama...
| rzimmerman wrote:
| 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 wrote:
| 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.
| rzimmerman wrote:
| 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.
| onphonenow wrote:
| 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
| dataflow wrote:
| 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
| macropin wrote:
| GPS used in survey equipment is accurate to 2cm with ~5 second
| acquisition time.
| onphonenow wrote:
| This may be differential gps with a local reference
| killingtime74 wrote:
| 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...
| dataflow wrote:
| 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.
| pifm_guy wrote:
| Time to get a fix is mostly what clever techniques the
| receiver uses. There are various papers demonstrating
| getting a fix that is probably correct with just a few
| milliseconds of signal and then lots of processing.
|
| You also have to cut corners to get a (probably correct)
| fix fast. Things like assuming the almanac hasn't changed
| since last time, the user hasn't moved more than a few
| hundred miles, the system clock hasn't drifted by more than
| a second or so, and no satellites have become unhealthy.
| chrisfosterelli wrote:
| I've always noticed my Garmin watch gets a fix extremely
| fast except for the first time I use it after traveling
| somewhere new. That first one always takes several
| minutes.
| eru wrote:
| > Things like assuming the almanac hasn't changed since
| last time, the user hasn't moved more than a few hundred
| miles, the system clock hasn't drifted by more than a
| second or so, and no satellites have become unhealthy.
|
| If you are using a smartphone, you could probably
| validate those assumptions via your other sensors?
| sorenjan wrote:
| Google hosted the Google decimeter challenge, which used
| post processing of data from dual band phones to try and
| get as good accuracy as possible. The top submissions got
| an accuracy of just over one meter.
|
| I have a phone with dual band GNSS, and it's by far the
| best location accuracy I've had in any device. I can see
| which side of the cycle path I've been on, and it regularly
| works indoors. I don't live where there are tall buildings
| though, but even at fairly open space my older devices gave
| worse results. And I get a fix within seconds thanks to
| A-GPS.
|
| https://www.kaggle.com/competitions/smartphone-
| decimeter-202...
| reportingsjr wrote:
| My pixel 6 pro has much, much better GPS accuracy than my
| pixel 3 had.
|
| Here is a comparison between my pixel 6 and my S.O.'s pixel
| 2, from a hike we did recently: https://imgur.com/a/5VibEUw
|
| This makes it pretty obvious how much the L5 band can
| improve GPS. These were good conditions in general, but
| just looking at the bottom right portion alone shows how
| good L1 + L5 in a phone can get. I'm sure devices with
| larger antennas (bike computers, larger handheld GPS units,
| etc) can do even better in worse conditions as well.
| dataflow wrote:
| Interesting. Did you try comparing it against anything
| other than a Pixel 2 by any chance (to rule out the Pixel
| 2 just being worse than better L1 phones)? And have you
| tried it in the city to see if L5 is any help there?
| trollerator23 wrote:
| Not all of the satellites have L5 either. As of today only
| 16.
| JCM9 wrote:
| 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 wrote:
| 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.
| nufflee wrote:
| 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...
| mastazi wrote:
| Thank you for clarifying, I wasn't sure about that part.
| Good link! The diagram is very clear. Basically WAAS allows
| us to triangulate the vertical position of the aircraft. So
| as you pointed out, it's not related to pressure readings.
| dweekly wrote:
| 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.
| kentrf wrote:
| 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).
| jonathankoren wrote:
| 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 wrote:
| 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)
| Havoc wrote:
| Around 250 bucks for those wondering. Bit heavy for a hobby
| project but less than expected
| edb_123 wrote:
| 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 EUR750. And EUR645 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.
| tdeck wrote:
| 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
| ggm wrote:
| 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
| dale_glass wrote:
| Those maps are interesting.
|
| So GPS doesn't work well in Greenland or a good chunk of Russia?
| rzimmerman wrote:
| You should still have (oblique) views of the constellation
| above 55degN, 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.
| ck2 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| detaro wrote:
| literally the next blogpost on that blog is "FCC approves use
| of Galileo in the US" ;)
| ars wrote:
| 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.
| dismalpedigree wrote:
| I was wondering the same.
| ummonk wrote:
| Kind of weird not to describe BeiDou as well...
| danieldk wrote:
| GPSrChive is very much focused on Garmin GPSr and I don't think
| there are any Garmin devices that support BeiDou.
| izzydata wrote:
| 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.
| z2 wrote:
| 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)
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(page generated 2022-11-28 05:00 UTC)