Hello,

I am having problem interpreting the main VTEC equation (eq.#2, please see S.Schaer's paper 'How to use CODE's global ionosphere maps', May 1997, for your convenience, the paper is enclosed with this post). Specifically, I am not sure if I interpret the arguments 'beta' and 's' in the function E(beta,s) when the SH expansion is given in the geomagnetic (GM), not geographic, frame.

1) If the SH expansion E(beta,s) is given in the usual "geographic" frame (say, WGS84 for definiteness), the arguments 'beta' and 's' look quite straightforward to me:

- 'beta' is the latitude of the IPP in WGS84;

- 's' = lambda(IPP) - lambda(Sun), where 'lambda(IPP)' is the longitude of the IPP and 'lambda(Sun)' is the longitude of the Sun (both in WGS84) at the specified epoch. Moreover, the paper gives a very simple relationship for 'lambda(Sun)': = 3.1415 - UT ( in rad) (*)

2) Now, how shall I interpret 'beta' and 's' in case the expansion E(beta, s) is given in the geomagnetic frame?

I guess 'beta' simply means the latitude of the IPP in the geomagnetic frame?

Does 's' still mean the difference between the longitude of the IPP (in the GM frame) and longitude of the Sun (in the GM frame) at the specified epoch? Now, to the remaining stumbling block: how can I calculate the longitude of the Sun in the GM frame at a specified epoch? What does the counterpart of (*) for the GM frame look like?