GLONASS and GALILEO usage

Discussion related to the Garmin GPSMAP 66s/st GPSr
User avatar
GPSrChive
Site Admin
Posts: 3881
Joined: Fri Jan 20, 2017 11:29 pm

Re: GLONASS and GALILEO usage

Unread post by GPSrChive »

Spiney wrote: Sun Jan 10, 2021 5:08 pm This post seeks to address the second and third of gpsrchive’s questions regarding the availability of evidence relative to claims of single GNSS superiority in either mountainous terrain or urban canyons. :geek:

......

I can’t claim that this was an exhaustive search, but I was unable to find any academic research that demonstrated superior single constellation accuracy of GLONASS in mountainous areas or GALILEO in urban canyons, but multi-GNSS was generally better than single GNSS.

Thank you Spiney!

I do not dispute that using multiple GNSS systems is generally better than using only one - and like many others here, I have found that Galileo provides better results than GLONASS when used as my second GNSS constellation. But that is simply my experience, and yours, or others experience may be different.

What I am researching now is why some suggest GLONASS is better than Galileo at higher latitudes, and based on the information I have thus far, I am not convinced this is correct. My research is still in the beginning stages, however.
asprin624
Posts: 143
Joined: Sun Dec 29, 2019 11:34 am
Location: Belgium

Re: GLONASS and GALILEO usage

Unread post by asprin624 »

I have asked Roger Timbrook in regards to the posting "GPS Vs. GLONASS Vs. Galileo: What’s The Best GNSS?"

As of yet I have not received any feedback from him about the statement he made in his articular "It’s recommended to use GLONASS in mountainous regions and higher latitudes, while Galileo improves accuracy in urban environments."

I will post his response as soon as he answers me.
User avatar
GPSrChive
Site Admin
Posts: 3881
Joined: Fri Jan 20, 2017 11:29 pm

Re: GLONASS and GALILEO usage

Unread post by GPSrChive »

asprin624 wrote: Mon Jan 11, 2021 7:21 am I have asked Roger Timbrook in regards to the posting "GPS Vs. GLONASS Vs. Galileo: What’s The Best GNSS?"
I do find it interesting that test results he used for the article (GSA Test) showed:
  • Using either Galileo or GLONASS with GPS is more accurate than using GPS alone.
  • Using GPS + Galileo is always much more accurate than using GPS + GLONASS.
  • Every time GLONASS was added to GPS + Galileo, the accuracy was worse than GPS + Galileo alone.
GPS vs GLONASS vs GALILEO Accuracy Charts.png

None of this was surprising to me, as Galileo is far more precise than GLONASS, which arguably has the poorest accuracy of any stand alone GNSS constellation.

asprin624 wrote: Mon Jan 11, 2021 7:21 am As of yet I have not received any feedback from him about the statement he made in his articular "It’s recommended to use GLONASS in mountainous regions and higher latitudes, while Galileo improves accuracy in urban environments."
To add further confusion, just a few lines from that quote he stated:
Galileo satellites are supposed to offer better accuracy than both GLONASS and GPS
This statement is correct, but he then goes on to contradict his original statement two lines later:
European satellites also offer better positioning services at higher latitudes compared to both GPS and GLONASS, which is one of their main advantages.
Thus far I am unable to find any documentation to support one system being more capable than the other "in mountainous regions and higher latitudes."

asprin624 wrote: Mon Jan 11, 2021 7:21 am I will post his response as soon as he answers me.
Thank you!
You do not have the required permissions to view the files attached to this post.
Spiney
Posts: 65
Joined: Thu Apr 04, 2019 9:35 am

Re: GLONASS and GALILEO usage

Unread post by Spiney »

gpsrchive wrote: Mon Jan 11, 2021 12:07 am
What I am researching now is why some suggest GLONASS is better than Galileo at higher latitudes, and based on the information I have thus far, I am not convinced this is correct. My research is still in the beginning stages, however.
This recent article entitled "Comparative Study on GNSS Positioning Systems for Autonomous Vessels in the Arctic Region" should be of interest to your research: http://ceur-ws.org/Vol-2626/paper12.pdf
User avatar
GPSrChive
Site Admin
Posts: 3881
Joined: Fri Jan 20, 2017 11:29 pm

Re: GLONASS and GALILEO usage

Unread post by GPSrChive »

Spiney wrote: Mon Jan 11, 2021 11:33 pm This recent article entitled "Comparative Study on GNSS Positioning Systems for Autonomous Vessels in the Arctic Region" should be of interest to your research: http://ceur-ws.org/Vol-2626/paper12.pdf
That is an interesting GNSS simulation study.

I have also been researching GNSS availability using various satellite prediction services. I wanted to know which GPS, GLONASS and Galileo satellites would be visible, where in the sky they would be located, and the path they would be following relative to multiple latitudes in the northern hemisphere. I recorded results for latitudes of N 5°, N 25°, N 45°, N 65°, and N 85°, all at a longitude of E 25°. The results can be seen in the charts below.

While the number of visible satellites for each constellation fluctuated slightly from one hour to the next, the total number of available satellites at each latitude did not vary significantly. This did not surprise me as all constellations tested provide global coverage.

Over the 24 hour period tested (one full rotation of the Earth), GPS always provided the most visible satellites, while GLONASS and Galileo both maintained similar visibility levels, usually within one or two satellites of each other. Never were there an insufficient number of satellites available. Choosing GPS + GLONASS or GPS + Galileo always yielded a similar number of available satellites.

The charts also revealed how the position of those satellites in the sky changed at each latitude. As the latitude increased, fewer satellites were available directly overhead. Not the total number of available satellites, mind you. Just the number of satellites located directly overhead.

I do not see how this creates any issues, as satellites located at higher elevations in the sky are best suited only for GPS elevation calculations. Conversely, satellites located at lower elevations in the sky (nearest the horizon) are considered least suitable for any position calculations, while those located midway in the sky are best suited for calculating latitude and longitude coordinates. This is where the majority of the satellites appeared in each chart.

E 25° N 5°
GNSS View LAT 05.png

E 25° N 25°
GNSS View LAT 25.png

E 25° N 45°
GNSS View LAT 45.png

E 25° N 65°
GNSS View LAT 65.png

E 25° N 85°
GNSS View LAT 85.png
You do not have the required permissions to view the files attached to this post.
Spiney
Posts: 65
Joined: Thu Apr 04, 2019 9:35 am

Re: GLONASS and GALILEO usage

Unread post by Spiney »

Thanks for posting the interesting plots from the Trimble on-line GNSS Planning tool.
gpsrchive wrote: Tue Jan 12, 2021 3:07 pm The charts also revealed how the position of those satellites in the sky changed at each latitude. As the latitude increased, fewer satellites were available directly overhead. Not the total number of available satellites, mind you. Just the number of satellites located directly overhead.
These observations tie in with the satellite orbital inclination angle which is the highest latitude that a satellite can be observed in the zenith direction (i.e. directly overhead). For GPS and GALILEO this is 55° and 56°respectively. For GLONASS it is 64.8°. North of these latitudes, no overhead satellites can be observed. The further north you go, the lower the elevation angles of the observed satellites, which is true for all GNSS.
gpsrchive wrote: Tue Jan 12, 2021 3:07 pm I do not see how this creates any issues, as satellites located at higher elevations in the sky are best suited only for GPS elevation calculations. Conversely, satellites located at lower elevations in the sky (nearest the horizon) are considered least suitable for any position calculations, while those located midway in the sky are best suited for calculating latitude and longitude coordinates. This is where the majority of the satellites appeared in each chart.
I agree with this and had expected to see that GLONASS had better dilution of precision (DOP) figures than GPS or GALILEO simply because of its satellites being higher in the sky due to its higher orbital inclination angle, but the data does not appear to show a difference.

However, the data we have looked at using the Trimble tool is with the Elevation cutoff set to zero i.e. perfect conditions. If one simulates more difficult conditions with satellite obstruction by incrementally raising this elevation mask, with a single GNSS, one soon gets to a situation where the constellation has a geometry considered poor for an accurate position fix for a period of time during the day. This is where the geometric dilution of precision (GDOP) exceeds 20. On 2021-01-09 at 85°N 25°E, for GALILEO this starts at just 9°. For GPS it is much higher at 21°and for GLONASS slightly higher at 24°. GPS and GLONASS may thus out-perform GALILEO in high-latitude mountainous areas where obstruction and multipath issues exist?
User avatar
GPSrChive
Site Admin
Posts: 3881
Joined: Fri Jan 20, 2017 11:29 pm

Re: GLONASS and GALILEO usage

Unread post by GPSrChive »

Spiney wrote: Wed Jan 13, 2021 5:08 pm However, the data we have looked at using the Trimble tool is with the Elevation cutoff set to zero i.e. perfect conditions. If one simulates more difficult conditions with satellite obstruction by incrementally raising this elevation mask, with a single GNSS, one soon gets to a situation where the constellation has a geometry considered poor for an accurate position fix for a period of time during the day. This is where the geometric dilution of precision (GDOP) exceeds 20. On 2021-01-09 at 85°N 25°E, for GALILEO this starts at just 9°. For GPS it is much higher at 21°and for GLONASS slightly higher at 24°. GPS and GLONASS may thus out-perform GALILEO in high-latitude mountainous areas where obstruction and multipath issues exist?
I had considered this, so I also ran simulations with a 25° Elevation cut off for the same locations, but chose not to include them in my prior post as one can simply look at the 'SkyView' diagrams and count only those satellites that are above a desired elevation threshold. But, since I have them, I will post them below (with the hourly charts).

Certainly, near the highest latitudes, each of the GNSS systems show periods during the day where they do not provide sufficient visibility when used independently, which makes the case for Multi-GNSS receivers especially at these higher latitudes.

Perhaps, in the small regions beyond the peaks of the GLONASS orbits, where there does exist any natural or man made obstructions (tall mountains, deep canyons, tall buildings, etc.), then GLONASS alone may perform better than GPS alone or Galileo alone during a portion of any given 24 hour period. But this is a worst case scenario I suspect is not very likely to unfold - Why would anyone limit themselves to only one GNSS while navigating in these regions when most receivers are capable of working with three or more?

I would love to see some 'real-world' examples from users with boots on the ground in these regions!



E 25° N 5° - Above 25° Elevation

GNSS View 05 - 25.png


E 25° N 25° - Above 25° Elevation

GNSS View 25 - 25.png


E 25° N 45° - Above 25° Elevation

GNSS VIew 45 - 25.png


E 25° N 65° - Above 25° Elevation

GNSS View 65 - 25.png


E 25° N 85° - Above 25° Elevation

GNSS View 85 - 25.png
You do not have the required permissions to view the files attached to this post.
Spiney
Posts: 65
Joined: Thu Apr 04, 2019 9:35 am

Re: GLONASS and GALILEO usage

Unread post by Spiney »

gpsrchive wrote: Wed Jan 13, 2021 5:24 pm Perhaps, in the small regions beyond the peaks of the GLONASS orbits, where there does exist any natural or man made obstructions (tall mountains, deep canyons, tall buildings, etc.), then GLONASS alone may perform better than GPS alone or Galileo alone during a portion of any given 24 hour period. But this is a worst case scenario I suspect is not very likely to unfold - Why would anyone limit themselves to only one GNSS while navigating in these regions when most receivers are capable of working with three or more?

I would love to see some 'real-world' examples from users with boots on the ground in these regions!
I agree that nowadays anyone navigating in the higher latitudes is almost certainly going to use multi-GNSS, since that is going to be the best option (and confirmed by the literature). If one was tied to either GPS+GLONASS or GPS+GALILEO, then the former is likely to provide better results the further north you go, especially if an elevation mask was needed to remove low-elevation noisy signals which are more prone to multipath errors.

As you say it would be great to get some direct feedback on this.
mpistora
Posts: 14
Joined: Tue Sep 17, 2019 8:22 pm

Re: GLONASS and GALILEO usage

Unread post by mpistora »

I would return to the first question of the topic:
987 wrote: Sat Jan 02, 2021 7:06 am If one selects to use GPS+GALILEO or GPS+GLONASS how does the unit actually use the signals from GALILEO or GLONASS?
Can it use them in combination with GPS? Meaning that just one or a few GALILEO or GLONASS signals will make the positioning better, or
Does it require an independent full reading of GALILEO (minimum 4 satellites) or GLONASS to be able to calculate a better position than with GPS only?
gpsrchive wrote: Sat Jan 02, 2021 7:30 am The GLONASS or GALILEO satellites (as configured) are used in combination with the GPS satellites.
Can you write it in more detail, or write a link to an article?
Is it really enough for a GPS unit to receive a signal from only two GPS satellites and only two GALILEO satellites and use them in combination to determine its position? So far, I have assumed that the signals are evaluated and the calculations are performed for each system independently, and at the end some weighted average position is calculated.
User avatar
GPSrChive
Site Admin
Posts: 3881
Joined: Fri Jan 20, 2017 11:29 pm

Re: GLONASS and GALILEO usage

Unread post by GPSrChive »

mpistora wrote: Tue Jan 26, 2021 10:02 pm
gpsrchive wrote: Sat Jan 02, 2021 7:30 am The GLONASS or GALILEO satellites (as configured) are used in combination with the GPS satellites.
Can you write it in more detail, or write a link to an article?
I am working on some additional pages to cover this type of information in more detail, so stay tuned. You might find additional useful information here.

mpistora wrote: Tue Jan 26, 2021 10:02 pm Is it really enough for a GPS unit to receive a signal from only two GPS satellites and only two GALILEO satellites and use them in combination to determine its position? So far, I have assumed that the signals are evaluated and the calculations are performed for each system independently, and at the end some weighted average position is calculated.
Absolutely, yes.

For each satellite, regardless of which GNSS they belong, the GPSr only needs to know when the signal was transmitted (time stamp) and where that satellite was supposed to be at that time (ephemeris). The GPSr then calculates the distance to that satellite by subtracting the time the signal was sent from the time the signal was received and multiplies that answer by the speed of light.

This is repeated for four or more satellites from any combination of constellations to determine your position using a technique called trilateration.

Because a clock error of a few billionths of a second can result in a location error of several hundred feet or meters, and since the GPSr does not include an atomic clock with the required accuracy, one additional satellite clock signal is reserved by the GPSr exclusively for the purpose of synchronizing all of the other signals.

The GPSr does not care which constellation each satellite is part of, so long as the required information is transmitted, the GPSr will calculate its location using the satellites that are best positioned in the sky for doing so. Satellites directly overhead or near the horizon are not used when a sufficient number of remaining satellites are present.
Post Reply

Return to “GPSMAP 66s/st”