We use the DEOS fortran program getorb for obtaining the precise orbits. (This program has to be installed separately, see http://www.deos.tudelft.nl/ers/precorbs/ or [14].) This step actually is only a system call to getorb, and converts the output to a 4 column table: secofday,x,y,z.
It requires the Orbital Data Records (ODR files) to be in an archive directory. The arclist file (which should be downloaded together with the ODR archives) has to be present in this directory. The ODR files have to be untarred and unzipped as from version 2.5.
This step introduces a section in the result file where the ephemerides are placed, and it deletes the ephemerides from the SLC leader file, obtained by the processing step M_READFILES (if there was such a section). The ephemerides (x,y,z) span the time 4 seconds before the first line and 4 seconds after the last line by default. The time is, and should be, in seconds of day. The time interval is 1 second by default. Natural cubic splines are used for interpolation, and the boundary conditions may affect the interpolation if only a few datapoints are used, e.g., 5 points with a time interval of 30 seconds. We nowadays use a time interval of 30 second, and approximtely 21 points. This implies that only spline (degree 3 piecewice polynomial) is used for the whole image, which gives better results for, e.g, reference phase computation. The interpolation errors in Doris are probably due to interpolation of interpolated values of getorb, which output format is in 3 digits.
If you want to use other ephemerides you can simply insert them in the result file in the format described in section 4.2. You will have to correct the number of POINTS in the result file. Note that the orbit system is WGS84 (only).