Best setting for GPS fix


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    Thierry CHARLÈS

    Hello Marek,

    Selecting multiple constellations improves two things:

    - The speed of the first FIX,
    - Reduces inaccuracy when reception conditions deteriorate. Don't delete it, it's better.

    For what ?

    Because the number of visible satellites is multiplied by the number of constellations. In the case of GPS + Beidou (check on the internet) it must be 32 + 20 roughly, i.e. 24 active for GPS and XX active for beidou. GPS + Glonass + Gallilea will be 3 x 32, and 3 x 24 active second or third generation satellites (more precise, especially for Glonass), Beidou is the first generation and there is not much information on precision!

    Beidou is a Chinese constellation, I haven't read any information on the subject but it is probably optimized to cover Asia (Glonass is optimized for northern latitudes favoring "Russia"), the GPS memory is optimized between latitudes 80° N/S, etc.

    The GPS constellation remains the most precise, according to an independent study the precision is better than 4 m 95 fix out of 100. Glonass and Galliléo I have not found independent studies but it is circulating on the internet 5 m for galliléo (Excluding official presentation which gives more precise values but this would be reserved for certain paying users -The discussion would be ongoing to know who will pay-) Glonass WiKi does not give a value but considers that 5 m could be taken for the northern latitudes, given that the second generation of satellites is deployed (the first did not work...).

    In good weather (blue sky) and full visibility towards the horizon, GPS alone remains the most precise (< 4m). As soon as the reception conditions deteriorate, the view of the sky towards the constellations is reduced (masks, canopy, crossing of clouds, etc.) the association of Glonass and Galliléo will reduce the imprecision (See another post on this forum which presents a comparison), why? By increasing the number of satellites visible to the receiver in the reduced sky view, the receiver has a greater choice and will find some at the edge of the reduced view area. The larger the surface of the polygon formed by the tracked satellites, the better the precision will be.

    Then MutiConstelaltion in Europe the ideal is GPS, Gallileo, Glonass which are constellations optimized to cover this area.

    The SBAS EGNOS of CROSS Plus, TERRA, AVENTURA 2 and ROC corrects some of the errors due to crossing the upper layers of the atmosphere, so it is better with Galliléo, GPS and Glonass.

    Dual frequency (No dual frequency GPS at TwoNav) corrects the same thing as EGNOS and corrects errors due to multipath reception therefore improves accuracy in difficult reception conditions, undergrowth, urban canyons, valleys, near walls or walls . Only the GPS constellation has this deployed and operational capacity, Galliléo is gaining momentum and nothing of this operational level for the other constellations.

    A test which is relatively simple to do and within everyone's reach, allows you to get a very good idea. Start the GPS with GPS + Galliléo + Glonass (Not being in Asia we forget Beidou to begin with, moreover this constellation is not yet fully operational, satellites are missing) wait for the FIX and for the precision to drop below 5 m in an open area. Then go stand at the foot of a high wall and if possible close to a closed angle, note the precision (GPS page), then play to remove Glonass, then Galliléo, put them back alone or in pairs.. The same thing without the GPS.. In short, test all the cases (Beidou now if you want) you will see which configuration gives the best precision at the foot of this wall.

    Then, at the moment given the conflict between Russia and Ukraine being close to these countries there may be deliberate damage led by one or the other of the belligerents (jamming, etc.). In this case Beidou could be an alternative.

    The test against the wall I did as soon as my GPS was received, since then I have never touched the setting resulting from this test.

    Best regards.


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  • Avatar
    Thierry CHARLÈS


    For the duration of the FIX itself, on my GPS Trail 2 (GPS + Glonass + Galliléo) it is around 1 minute 30 seconds to 1 minute 40 seconds. On the TERRA which has AGPS it is super fast 10 to 20 seconds maximum after the end of the BOOT (screen with a map). Evadéo before was much longer, roughly 3 minutes. Without AGPS, to avoid having long FIXs, GPS reception must be cut off inside a building before a first FIX, to prevent the receiver from erasing or invalidating the satellite Almanac'h. If he cannot make a fix he will invalidate the Almanac'h and go back to "searching in the sky" which can take up to 15 to 20 minutes in the worst case.

    Best Regards

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  • Avatar
    Marek WEGLORZ

    Hi Thierry,

    can you give it a try with GPS + Beidou only?

    If I am right that it could be something peculiar in my device or generally a feature of TwoNav settings, that only these two systems together gives the best results... I am not considering details, which you wrote above, just GPS fix in quite hard conditions: inside and on a window's sill.

    Edit 1: I was unable to get GPS fix in any other combination of the sattelites systems (GPS, Galileo, Glonass, and their possible combinations). What's more, when I switched to 'GPS + Galileo + Beidou', I cannot get a fix for several dozen on minutes still in the same conditions.

    Edit 2: Finally I also got (inside and on the same window's sill) a fix with GPS + Glonass, but with a little bit lower accuracy declared. 

    Edit 3: my unit catches GPS fix outside immediately, doesn't matter of the chosen constelations (even with pure GPS), as it was repaired somehow by TwoNav Team due to factory fault for the early production phase units.


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    Thierry CHARLÈS

    The FIX happens in the GPS chipset, so the antenna and the chipset are impacted. The only consequence of a repair fault would be a fault in the antenna or its connection but this fault will not only affect a FIX it will also affect all reception.

    The FIX or the reception, if it is not done outside in conditions where the GPS antenna can see the sky, and not only vertically, we cannot draw any conclusions.

    THE GPS SYSTEM HAS NOT BEEN DESIGNED OR DESIGNED TO OPERATE IN A PREMISES OR TO STICK TO THE GROUND. It is designed to work with an external antenna on the upper top of an aircraft or missile.

    The use has been extended (so much the better), notably for hiking, but close to the ground and at very low speed a certain number of "collateral" effects disrupt operation.

    If you try the first FIX in a room, a window edge or this type of situation it can only go badly.

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