[RightWhale]

As far as GPS is concerned, they have picked a limited selection of spheroids and insist you use them. Some spheroids, they claim, are closer to the actual shape of the earth than the one you were using, which means you have to adjust their data to match your maps. Earth is very lumpy and the gravity field also, but if it is changing all the time in new ways, we have a lot of remodelling work ahead.

[r9etb]

As far as GPS is concerned, they have picked a limited selection of spheroids and insist you use them.

I'm talking more about the broadcast message and how it's built.

The message is constructed by a series of least squares fits over a long-term prediction of the satellite trajectory. If the gravity model is significantly flawed the prediction may be off by meters per day.

In real life, some of is mitigated by the operators, who keep an eye on the accuracy of the broadcast message; however, it increases workloads, and if there are any delays in sending new orbit information to the satellites, the world will see additional errors in their solution.

Not too big a deal for most, but probably rather upsetting for a GPS-guided bomb.