GLONASS and GALILEO usage

[987]

Hi.
On the GPSMAP 66
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?

Does anyone know about such technical details?

And home come GALILEO and GLONASS cannot be used without GPS?

[GPSrChive]

The GLONASS or GALILEO satellites (as configured) are used in combination with the GPS satellites.

[asprin624]

GLONASS is generally more precise in mountainous regions, while Galileo offers better accuracy in urban environments. When you combine either of these two systems with GPS, your receiver will usually be dead on about your location.

There are many sites that go into details about this.
You can also see by using an app called GPSTest for android to show just what GPS Sat. are in your area at any given time.

[GPSrChive]

GLONASS is generally more precise in mountainous regions, while Galileo offers better accuracy in urban environments.

I have never heard anything like this before...

There are many sites that go into details about this.

Care to share some of them?

[asprin624]

Here is one of them.

Look at the section

"What Is The Benefit Of Using More Than One GNSS?"

https://expertworldtravel.com/gps-vs-gl ... s-galileo/

Here is a Dutch site (You can use the translate on both these sites)

https://www.gps.nl/blog/gps-glonass-galileo.html

another dutch site

https://www.sporthorloge365.nl/gps-glon ... es-uitleg/

and here:

https://link.springer.com/chapter/10.10 ... 23220-6_35

Like I said there are lots of sites that give details on the subject......

If you need more you can always google it.....

[GPSrChive]

Thank you, Asprin=624.

I read through all the links you provided, and a few I found myself. Unfortunately, all I find is the often repeated "GLONASS is superior to GALILEO at higher latitudes' and 'GLONASS is better for mountainous regions while GALILEO is better in City regions', but never do any of the authors provide any evidence as to why any of these statements may be true. It can only be read as opinion when no supporting documentation is provided.

One of the pages you linked to states:

European satellites also offer better positioning services at higher latitudes compared to both GPS and GLONASS, which is one of their main advantages.

before contradicting themselves with

It’s recommended to use GLONASS in mountainous regions and higher latitudes

So, which is it?

Often stated in one form or another is:

Galileo is actually more reliable in urban environments, where tall buildings can easily block satellite signals.

and

GLONASS is generally more precise in mountainous regions, while Galileo offers better accuracy in urban environments.

I am left wondering how one system is better than the others when used in urban environments or near tall buildings.

AFAIK, GPS, GLONASS and GALILEO all offer global coverage, with their respective satellite constellations orbiting the Earth in evenly spaced configurations. The US GPS constellation is configured such that a minimum of six satellites are always visible from any point on earth, yet that same article states:

There should ideally be four such GPS satellites visible to the person on the ground using GPS at any one time.

The author also makes a big deal out of the amount of time it takes a satellite from each system to complete a full orbit around the Earth, yet this has no bearing on the accuracy of each system and is rather a simple function of the altitude at which they orbit.

So, I am still left with the same questions I had before. I just want to know why any one system is better than the others at:

A. higher latitudes,
B. in mountainous regions, and
C. in urban environments?

[Spiney]

Since we are back in lockdown and it's snowing outside, I spent a bit of time trying to track down some academic evidence to address gpsrchive's challenge, beginning with the "use at high latitudes" question.

For GNSS, performance in high latitude regions is reduced compared to the performance obtained by users at mid-latitudes. The reasons are mainly the satellite-receiver geometry and the ionospheric effects on the satellite signals. The four GNSS constellations have satellite orbits with different inclination angles to the astronomical equator and are at different altitudes above the earth. These two factors account for additional benefits for GLONASS and GALILEO for users at high latitudes such as areas in the North of Canada, Alaska, Northern Europe, and Russia as well as Antarctica compared to GPS alone. Users are likely to see improvements in positional accuracy using multi-GNSS location compared to single constellations at high latitudes.

NB The following academic papers are consistent in their commentary on the use of GNSS at high latitudes. These selected extracts are a relatively small sample of the literature and this does not therefor purport to be an exhaustive or systematic review.

PennState College of Earth and Mineral Sciences, Department of Geography
On-line Course GEOG 862: GPS and GNSS for Geospatial Professionals: Lesson 10.
Link: https://www.e-education.psu.edu/geog862/home.html

GPS
• 55 degree inclination angle
• altitude 20,200 km
Galileo
• 56 degree inclination angle
• altitude 23,616 km
GLONASS
• 64.8 degree inclination angle
• altitude 19,100 km
BeiDou
• 55 degree inclination angle
• altitudes 38,300, 21,200 km


Cherniak I and Zakharenkova I
New advantages of the combined GPS and GLONASS observations for high-latitude ionospheric irregularities monitoring: case study of June 2015 geomagnetic storm. Earth Planets Space 2017, 69, 66. https://doi.org/10.1186/s40623-017-0652-0

A significant advantage of the GLONASS, as compared to the GPS, is that the GLONASS has an orbit inclination of ~65°, that is ten degree higher than the GPS orbit inclination. This feature is important for the high-latitude region, where a multi-system GNSS receiver can track the GLONASS navigation signals for much longer time and with higher elevation angles than GPS ones.

Kees de Jong, Matthew Goode, Xianglin Liu, and Mark Stone.
Precise GNSS Positioning in Arctic Regions. This paper was prepared for presentation at the Arctic Technology Conference held in Houston, Texas, USA, 10-12 February 2014.
https://www.researchgate.net/publicatio ... ic_Regions
As the name implies, Global Navigation Satellite Systems (GNSS), such as the US GPS and Russian GLONASS, provide worldwide coverage for position determination. However, especially in arctic regions, there are a number of issues to be resolved or taken into account. The most important ones are:
- Satellite geometry may not be as good as at lower latitudes, due to the satellite orbits. For example, for GPS the orbits have an inclination with respect to the equator of 55 degrees. As a result, satellites can be seen at 90 degrees elevation only for latitudes equal to or below this inclination. For arctic latitudes, the maximum elevation is much lower, leaving a hole in the sky above these regions in which no satellites are visible. In particular the height component determined from GPS is worse than at lower latitudes.
- Ionospheric effects are more severe than at mid-latitudes (but in general not as severe as in equatorial regions). Part
of the problem can be solved by using multi (dual or triple) frequency GNSS receivers, another part by using more
systems: not only GPS, but a combination of e.g. GPS and Glonass and in the future the European Galileo and
Chinese BeiDou as well.

An image comparing 24 hour plots of GPS satellites visible from Copenhagen in Denmark to Longyearbyen in Svalbard can be viewed here: https://mycoordinates.org/challenges-fo ... he-arctic/
Horizontal position accuracy is in many cases reduced because there is a higher noise level in the observations, caused by the large number of more noisy low elevation satellite signals. The low elevation of the satellites further worsens the ionospheric effect on satellite signals.

Eissfeller, B, Ameres, G, Kropp, V, Sanroma, D.
Performance of GPS, GLONASS and Galileo, University of Stuttgart, 2001.
https://phowo.ifp.uni-stuttgart.de/publ ... feller.pdf
Users in higher latitude areas, such as Canada, obtain better GLONASS derived dilution of precision (DOP) than users of GPS. This is due to the high inclination angle of GLONASS: 64.8 degrees compared to 55 degrees for GPS.

Stelian Cojocaru, Eugen Barsan, Ghiorghe Batrinca and Paulica Arsenie
GPS-GLONASS-GALILEO: A Dynamical Comparison.
Maritime Transport & Navigation Journal, 2009; 1(1): 1-13
https://www.researchgate.net/publicatio ... comparison
A constellation of 30 satellites, 27 active and 3 spare, will populate the spatial segment of GALILEO Satellite System. The satellites will be deployed in 3 circular orbits with radii equal to around 29,600 km, inclined with 56° on the equatorial reference plane. Having ten satellites quasi-equally distributed in each of the three circular orbits, flying at an altitude of around 23,222 km with an orbital period of 14 sidereal hours, the constellation will ensure a global coverage of the Earth. As soon as Full Operational Capability is achieved, the designed space segment will provide 6 to 8 visible Galileo satellites from any terrestrial (or near-terrestrial) location. The combination of the orbital inclination and the flight altitude of the satellites will considerably increase the coverage of the polar regions, not so well achieved by GPS.

[987]

Hi.
All those links were interesting.

Related to GPS and units that have only GPS (not Galileo, Glonass) and high latitudes.
I live around 60 degrees north (in Sweden), and travel very frequently 59-62 degrees (about 400 km north-south distance) and frequently 50-65 degrees (50 is about mid of Germany, Czech republic, south Poland, north France, etc., 60 is about the cities of Stockholm, Oslo, Helsinki, 65 is about 3/4 north of Sweden, mid of Norway or about Iceland).
Looking back a some years, it was very clear that the further north one went the longer it took for the unit to lock the satellite signals and the accuracy reported by the unit was often 8-12 m at 62 degrees, around 5-9 meters at 60 and 3-5 meters in south of Sweden or in central Europe.
Since a few years, can't say how many, i am having the feeling that the unit locks the signals faster even on more northern latitudes, i also observe a slightly better accuracy at those latitudes. This means that i now frequently get accuracy at about 6 meters or better everywhere.
Maybe recent upgrades have made the latitude problem smaller!? Does anyone else have the same observation?

Observations for Glonass and Galileo (on a GPSMAP 66st).
The unit locks Glonass signals just as fast as GPS signals.
The unit locks Galileo signals slower than GPS signals.

Accuracy is about the same, but it seems that GPS+GALILEO gives the best accuracy over time and GPS+GLONASS slightly worse.
The best accuracy i have ever observed is 2.4 meters (about 8 feet). is this a theoretical limit of GPSMAP 66 chipset?

[Spiney]

Your observations regarding historic GPS satellite lock-times and accuracy vs. latitude are probably explained by the explanations regarding satellite coverage and atmospheric phenomena.

Finnish Ministry of Transport and Communications
Challenges in Arctic Navigation and Geospatial Data: User Perspective and Solutions Roadmap
Publications of the Ministry of Transport and Communications 2020:1
https://julkaisut.valtioneuvosto.fi/handle/10024/161989

In the Artic itself, this report published last year concluded:
“Currently, GNSS is utilized as a preferred navigation method in the Arctic. However, due to the low elevation angles and absence of satellites overheading the Arctic, the coverage of GNSS constellations is suboptimal in the area. Despite the developments of multi-frequency and multi-constellation GNSS and entailed improvements in the continuity and reliability of positioning, the increased ionospheric activity sets some limitations at the high latitudes.”

Assuming the same GPSr device, the anecdotal improvements in lock times and accuracy you have noticed are possibly due to the GPS modernisation programme for space and ground segments. Space segment modernisation includes improvements in atomic clock accuracy, satellite signal strength and reliability. Control segment modernisation includes improved ionospheric and tropospheric modelling and in-orbit accuracy, and additional monitoring stations.

I live in the UK and routinely use GPS + GALILEO with SBAS enabled and also find that the best accuracy value displayed by my 66St is 2.4m. My understanding that this figure is about the best accuracy that can be achieved by consumer-grade GPSr devices using the standard positioning service (SPS) for multi-GNSS with SBAS: it is not a limit of the M5 (3313) chipset used in the 66st.

[Spiney]

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.

Satnav use in these areas is related since the problems are the same for both:
• signal obstruction (also called masking or shadowing) causing a reduction in the number of observable satellites in direct line-of-sight (LOS) of the sat-nav device, which may also be in a less favourable geometry with worse dilution of precision (DOP) figures. In cities these are caused by tall, high-rise buildings creating urban canyons. Away from cities, it is generally terrain features i.e. canyons, gorges, bowls, deep valleys, mountains, hills and cliffs etc, which reduce the number of directly observable signals.
• multipath signal reflection errors where terrestrial objects close to the receiver (such as tall buildings, walls, cliffs, bodies of water, the ground etc) reflect the GPS signal and thus prolong its travel time from the satellite to the receiver. Errors caused by echo signals may be very significant.

There is a reasonable body of evidence demonstrating improvements with the use of multi-GNSS compared to using a single constellation. These benefits accrue from both the increase in the number of satellites from the different GNSS constellations in direct LOS for the sat-nav device to select from, and from improved satellite geometry.

One example looking specifically at difficult environments is:

Rx Networks Inc and the European GNSS Agency (GSA) 2014
The Results are In: Galileo Increases the Accuracy of Location Based Services
https://www.gsa.europa.eu/newsroom/news ... 20launched
The results showed that using Galileo with one or more other GNSS constellations provides significantly more accurate location fixes compared to GPS alone, when indoors or in urban canyons.

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.