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goGPS-Project / goGPS_Java

goGPS Java is a GNSS observation processing library.
http://www.gogps-project.org
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Weight Matrix #20

Open ZiglioUK opened 8 years ago

ZiglioUK commented 8 years ago

Hi everyone, quick questions:

  1. what's the range of expected values for the Weight Matrix based on satellite elevation?
  2. when you add a height constraint, using a DTM, how do you weight that contribution?

Thank you as usual

ZiglioUK commented 8 years ago

For example, a satellite with a very low elevation (2.5), leads to a weight of: weight = 1 / Math.pow(Math.sin(elevation * Math.PI / 180), 2) = 517.7

Why 1/...? shouldn't weights be bigger when elevation is higher? it seems counterintuitive

ege010 commented 8 years ago

Hi @ZiglioNZ!

What is used there is actually a cofactor matrix, the inverse of which is used to weight observations in the least-squares solution. Here you can find the corresponding code in the MATLAB version: http://sourceforge.net/p/gogps/code/HEAD/tree/trunk/positioning/least_squares/LS_SA_code.m#l104

DTM values are introduced as "pseudo-observations", with their own observation error (line 699 in this section: http://sourceforge.net/p/gogps/code/HEAD/tree/trunk/positioning/kalman/goGPS_KF_DD_code_phase_loop.m#l665). You can check also equations (22) and (30) here: https://www.researchgate.net/publication/257836193_goGPS_Open_Source_Software_for_Enhancing_the_Accuracy_of_Low-cost_Receivers_by_Single-frequency_Relative_Kinematic_Positioning

ZiglioUK commented 8 years ago

I've actually already ported the Matlab code for DTM to Java and have been using it for a few months. I've used it both with the Google Maps Elevation API (that is probably not precise enough for your needs) and without just to add a soft constraint when I only have 4 satellites. I haven't found necessarily an increase in precision but when you use the DTM it converges much more quickly. I've found at times with 5 or 6 satellites I need to run up to 400 iterations before the update is smaller than 1m.

See https://github.com/Sirtrack/goGPS/blob/snapshotGPS/src/main/java/org/gogpsproject/ReceiverPosition.java (line 1302)

// Add height soft constraint
double lam = Math.toRadians(this.getGeodeticLongitude());
double phi = Math.toRadians(this.getGeodeticLatitude());
double hR_app = this.getGeodeticHeight();
...

I'd like to merge my contributions into the main project here (thank you for upgrading my access rights). The problem with this file (ReceiverPosition.java) is it's just too big. That's why https://github.com/goGPS-Project/goGPS_Java/issues/17 In order to foster innovation (!) we should be able to experiment without risking to break things for everyone.