I’m looking for information behind the “Apply Orbit File” operator in addition to what is given in the documentation. Under “Major Processing Steps”, we’re given two pieces of information alluding to how the operator improves the orbit state vectors (OSV), say, with precise orbit ephemerides (POE). I quote:
3. For each orbit state vector, get its zero Doppler time;
4. Compute new orbit state vector with 8th order Lagrange interpolation using data for the 9 nearest orbit positions around the zero Doppler time.
OSVs in the product annotations are given with UTM timestamps (presumably not zero Doppler corrected; Sentinel-1 Product Specifications 3.0 does not explicitly say these are zero Doppler) at which the OSVs apply. Point 3 suggests it is deriving zero Doppler time rather than just reading it from some element tag. Is it wrong to think zero Doppler time is derived by retrieving the “zeroDopMinusAcqTime” [s] tag element and subtracting that from the OSV timestamp?
Then, moving on to point 4, the operator generates a Lagrange interpolation function with the 9 nearest OSVs in UTM for each position and velocity element from the POE file around the zero Doppler timestamp, derives a new OSV record for that zero Doppler timestamp, and then replacing the old one.
Basically, I’d like to know if this is in fact how “zero Doppler time” is obtained or if I’m missing something here.
Thanks in advance