# Calculating gravity in rotating frame(ECR)

1. Dec 12, 2003

### Banjo

In reading through URL=ftp://164.214.2.65/pub/gig/tr8350.2/wgs84fin.pdf]this[/URL] pdf I came across the potential described on pg 51. I have no problem with the spherical harmonic expansion of the gravitational force, however I'm a bit confused about the phi potential added to account for the Earth's rotation. I think this only gives the the "centrifugal" force. Assuming the gravity vector that results is in ECR (Earth Centered Rotating, if I was going to rotate the acceleration given by this formula into ECI (Earth Centered Inertial)(from ECR), wouldn't I also need to add a coriolis term? or at least not add the coriolis term when doing the rotation? I have been told otherwise which is why I ask. BTW the position must be in ECR as the formula for V, the gravitational potential, is in terms of geodetic lattitude and longitude.