[IGSMAIL-7035] Extending the CODE orbit model

[Stefan Schaer]

Author: CODE Analysis Center

Dear colleagues,

We would like to inform you that GNSS orbit products generated at CODE
(starting with January 4, 2015, or GPS week 1826) are computed based on
an extended version of the empirical CODE orbit model (ECOM).

The changes with respect to the ECOM version previously used include:

- two additional harmonic terms for the direct solar radiation pressure
   (SRP) component: D2 (twice-per-rev) and D4 (four-times-per-rev)

- modified angular argument for the harmonic terms, namely changed from
   u_Sat to u_Sat - u_Sun

Spurious effects from SRP modeling (in particular from GLONASS
satellites) on EOPs and geocenter coordinates are reduced by this
particular orbit model update.

Best regards,

The CODE AC Team