# What is causing this shift in coordinates?

I'm working on taking raw GPS measurements and getting a position from them. It involves double differencing measurements from a fixed base station (with a known position) and a rover (in my case, an airplane). I'm following the procedure explained by Geoffrey Blewitt in Basics of the GPS Technique: Observation Equations, starting on page 36.

My problem is, the position I get from this solution is very consistently off by 15 meters or so, with X, Y, and Z coordinates all off by a set amount (5, 10, and 9 meters, respectively). This happens even if the plane is turning.

I've tried accounting for the ionosphere by using an ionosphere-free combination of L1 and L2 measurements. The troposphere shouldn't be an issue due to the short baseline in my case (~5km). And since the measurements are double differenced, I shouldn't have to account for satellite or receiver clock delays.

This leads me to my question: What other sources of error exist in GPS that could cause a translational shift like the one I'm experiencing? Did I miss something? And if so, how do I correct it?

• How do you know you're off? – Kirk Kuykendall Aug 22 at 2:33
• I have processed positions from professional software (Novatel Waypoint). – Jeff Aug 22 at 15:08
• Check all the settings in the Novatel Waypoint software. You're looking for anything that might modify the coordinates...like combined scale factor/grid-to-ground/reduction to surface/etc. – mkennedy Aug 22 at 17:01
• The logs I'm working with are directly from the Novatel GPS, I'm just using Waypoint to plot it. It doesn't look like any scaling or coordinate modification is being done. – Jeff Aug 22 at 18:07
• Just out of curiosity, what is the distance that NADCON shifts between NAD27 and WGS84 points in your study area? gisgeography.com/geodetic-datums-nad27-nad83-wgs84 – Kirk Kuykendall Aug 22 at 18:46