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I need to integrate some sort of satellite-based positioning for a mobile robot project. I am at the very beginning - I got 2 u-blox GNSS evaluation boards (these NEO-M8Ts) - I chose an integrated antenna and ground plane to get started faster (no external antenna hassle, ground-plane as RTK-ready). I used the u-center software to do some very simple initial tests both measuring stationary and moving positions - I basically only recorded the position from the sensors to a .ubx log file and then played it back.

I am trying to implement this for my mobile robot and I realize that a smartphone-grade GPS (Samsung J5) gives me better preliminary results than an u-blox eval board - I wonder why, I guess Android may fuse the IMU and have better readings even with worse antenna?

What is the correct way of evaluating GPS performance in terms of position accuracy/precision? I know this will depend on the location and its sky clearance (number of sats used, multipath errors etc.) but is there a rule of thumb? I guess I can only evaluate in stationary position since I will have no correct "ground truth" for mobile testing?

Also, I am interested in RTK performance, but that is probably a candidate for another question.

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If it's possible, use your GPS unit to get position & elevation of a known benchmark and then compare that with the official values.

Here is a picture of a USGS benchmark. If you can't find the locations of any near you, just contact your USGS local office.

If you cannot access benchmarks, see if there is some structure that you can get precise position data on. If there is any new type of infrastructure project -- levee, bridge, maybe even a culvert -- the designs & survey work should be open data.

Finally, if none of that works, you can try getting points from some obvious feature -- like a curb or crosswalk markings -- and then overlaying that on imagery. But the precision of your imagery will be your limiting factor there.

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On top of what Ezra mentioned, you need to measure the repeatability of the receiver. This can be done by placing the device in a static position and recording the position over a certain amount of time. This can help you have a scatter plot of positions and will give you an idea about the positioning error.

If you want better accuracy, you can enable the SBAS (Satellite Based Augmentation System) option on the receiver as it is off by default. For SBAS to work, you need to be in an area covered by one of the available systems.

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Precision of a GPS device depends on how it evaluates the position.

  • A-GPS(Assisted Global Positioning System) Devices :

    • Usually mobile phones.
    • Uses cellular network as well as satellites to triangulate current position.
    • Some Devices even show you altitude with latitude and longitude.
  • GPS (Global Positioning System) Devices:

    • Usually proprietary devices.
    • Only uses satellites to triangulate your position.
    • even works in very remote location like deep Forrest.
  • GNSS GPS with RTK:

    • Precision, survey grade technology. The GNSS refers to units that use both American and Russia GPS satellites. This effectively doubles there number of satellites available for triangulation and thus roughly doubles the precision. The RTK refers to Real Time Kinematic corrections, which is a (usually subscription based) data stream that corrects for atmospheric distortions for the region where you're working.

Articles (to read):

App:

  • GPS Status & Toolbox (Android) { Shows details about your gps sensors on device)

Needs further info, ask in comments. Thanks.

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    Thanks for your answer, but I am asking about position evaluation (precision+accuracy) and not how GPS works technically. I have also a very concrete HW (that u-blox board) which I need to evaluate (though I am too interested about localization on Android smartphones). – Kozuch May 16 '16 at 8:51
  • @Kozuch i am searching for a gps device that sends coordinates to my server directly. All products that i found sends coordinate data to manufacturer server and then i have to buy license to get that data from their api. Stuck at this point. – devprashant May 16 '16 at 10:05
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    @Kozuch Comparison of precision between Neo-M8N and Neo-6M Modules youtube.com/watch?v=FmdG66kTfsI – devprashant May 16 '16 at 10:49
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    @devprashant The use of additional satellite constellations such as GLONASS does not double the precision. It just gives you more satellites to track which can be very handy in difficult environnements such as urban canyons. This might increase the accuracy but not double it. On the other hand, although RTK can be subscription based, it's quite common that you setup your own base station to transmit raw GPS observations to the mobile (rover) unit. You can even have a miniature RTK GPS system such as the Piksi www.swiftnav.com/piksi.html with 2 receivers, 1 acting as reference and 1 as rover – Techie_Gus May 29 '16 at 14:20
  • Actually has part in answer is edited by Ezra Boyd. You can edit the answer appropriately. Thanks for correct info. @techie_gus – devprashant May 29 '16 at 15:25
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Some apps also help us to evaluate performance of GPS Unit. I have mentioned some in this link

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The accuracy of accuracy GLONASS over GPS In terms of accuracy, both have a different perception. It can be used best on high latitudes (north or south), where GPS signals cannot be reached. Talking about the accuracy, GLONASS is much preferable than GPS due to its orbital position. So even when you are at a place where GPS does not work properly. GLONASS will track you accurately and sometimes vice-versa.

In low latitude, GPS performs better and gives much better accuracy than the GLONASS, because of its high coverage over the globe. But putting them into use together both Global Navigation Satellite System and GPS gives a much accurate result than using individually, which are used in cities where getting the position of the person gets difficult due to high rise buildings. Source - https://www.ytechb.com/what-is-glonass/

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