A GPS Question

[ceaser]

Hi Forum

A question to all the GPS experts in this forum. I have managed to read in coordinates from a GPS and then use these coordinates to log tachy data or do setting out in the field.

But, if you have a look at the software from companies like Trimble, Leica, Topcon, etc. there is a correction that gets applied to the coordinates that comes from the GPS.

What we do is to set up our base station on a known point and then with the Rover we calibrate on a minimum of 2 known points. The program then calculates a correction which gets applied in that area to all the GPS coordinates.

Does anyone know the formula or the method to be used which will calculate this correction?

When using a Total Station, the correction that is applied depends on the distance away from the setup point. We use the Q-Point method. The intersecting rays from the beacons to the Total Station form a polygon and the most accurate result is the centroid of this polygon. Does this also apply to a GPS?

Thanks
Michael

 

[derez]

Not including myself in the "GPS expert" group, and with no idea what the companies are doing, this is what I understand:

If you have a known point (Total ?) which coordinates are known to high level of accuracy, you can measure the difference between these and the GPS reading at this point. You can assume that this delta is valid for all area in the vicinity of the known point, so by adding or subtracting the differences, you'll get coordinates with accuracy which is very close to that of the known point. If the known station transmites the difference continuously and you use the real time difference, it is called DGPS (Differential GPS).

It will be smart to use the correction only partially as you go away from the station, but I don't know the ratio.

Also I don't know for how long the difference is valid, if you do not update it in real time.

and there are many other things that I don't know...

 

[ceaser]

Hi Derez

Thanks for the reply. What you are saying is correct and I am sure that is what guys from Trimble, Leica, etc. are doing.

What we do is to set up the base station at a known point and then we read in at least 2 more known points. These 2 points are normally further away from the Base station than what the work is. Using these 3 points a correction is calculated.

That is not my problem. My problem is that I need formulae that I can incoporate into my program.

Thanks
Michael

 

[agraham]

I am not an expert on GPS but I am afraid the the simple method of comparing the GPS coordinates of a GPS at a known position and one roving does not work in practice. See here An Introduction to Differential GPS - O'Reilly Media

In order for corrections to be made it is apparently necessary to know some raw data such the time of the readings and the psuedo range measured for each satellite used.

Appendix B of this Trimble manual is an interesting, if succinct, read ftp://ftp.trimble.com/pub/mpc/placer/pla400-Inst&OpsManual.pdf

 

[ceaser]

Hi Agraham

Thanks for the info. The more I look at it, the more complicated it gets!

Regards
Michael

 

[agraham]

The more I look at it, the more complicated it gets!

Life tends to be a bit like that