# Looking for a Solution to GPS Challenges DSIAC needs your help in coming up with a solution to two very complex questions, both centered around the way we use the Global Positioning System (GPS) today and the challenges that we can encounter.

1. We would like to understand the basic signal flow and functional processes that occur in getting position location information (PLI) into the Joint Battle Command-Platform (JBC-P). For example, suppose my platform fuses inertial measurement unit (IMU)/GPS receiver navigation solutions and uses this to generate the PLI it sends out to the JBC-P. At the simplest level, we would like to understand how this PLI is aggregated with input from other users and used to form the Common Operating Picture (COP). Some important issues are:

• basic waveforms, data rates, and signal flow
• understanding where the processing is done,
• how is user data validated--e.g. someone spoofs my GPS and I send out erroneous PLI. How is this detected, and what information does the processing node use to update my position in the COP?
• what kind of sensor fusion /tracking is being done at the processing nodes?

2. How is GPS position/time estimate error reduced when there are > 4 visible satellites?  Our current thinking on how to analyze GPS error (in the user location/time estimate) is pretty simple, and pretty common I think. We assume that the GPS receiver uses linearization to solve the pseudorange equations, and break the error into a User Equivalent Range Error (UERE) piece and a Dilution of Precision (DOP) piece to determine the variance of the error, which is treated as a zero-mean Gaussian random variable. There are two problems with this simple approach.

• Not all receivers use linearization/least squares techniques to solve the (non-linear) pseudorange equations. Kalman filter solutions are of particular concern.
• Even if linearization is used, one could develop heuristic methods that capitalize on the fact that the pseudorange system of equations are overdetermined and thereby improve the accuracy of the basic solution.

Any light you can shed on this will be greatly appreciated.  Please reach me at daniel.whipp@dsiac.org, 443-360-3600.

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