S-1 orbit number formulae

markcorbin · May 21, 2025, 9:46am

Hi. We are having more success with SNAP12 but there’s one detail we are missing.

We were looking at this article: S1 Products
and it says the relationship between relative and absolute orbit depends on the unit and is given by:

S1-A: Relative Orbit Number = mod (Absolute Orbit Number orbit - 73, 175) + 1

S1-B: Relative Orbit Number = mod (Absolute Orbit Number orbit - 27, 175) + 1

Does anyone know what the calculation for S1-C should be please?

markcorbin · June 18, 2025, 10:46am

Hi @diana_harosa. Do you happen to have the S1-C Relative Orbit formula please or know when it will be published as I still can’t see it on the S1 Product wiki. Many thanks.

diana_harosa · June 18, 2025, 11:28am

@jun_lu can you help?

mengdahl · June 18, 2025, 11:35am

@peter.meadows Would surely know, but he hasn’t been active on the forum for awhile.

mengdahl · June 19, 2025, 12:07pm

I got this info from the S-1 MPC, the orbit formula for S1C is:

cycleNumber = int ( (absoluteOrbitNumber - 172) / 175)) + 5

Relative Orbit Number = mod(Orbit Number - 172,175) + 1

markcorbin · June 19, 2025, 1:42pm

Many thanks @mengdahl. Is there a link to the source or did they provide in response to a question you asked them?

mengdahl · June 19, 2025, 1:56pm

I got this information from the S-1 Data Quality Manager, you can contact her via muriel.pinheiro@esa.int.

piyushrpt · June 19, 2025, 4:01pm

github.com

isce-framework/s1-reader/blob/f1c8aa305d025cff339e7cca0065844a81aebb5e/src/s1reader/s1_burst_slc.py#L1176


      
              """
              if self.burst_eap is None:
                  raise ValueError(
                      "burst_eap is not defined for this burst."
                      f" IPF version = {self.ipf_version}"
                  )
          
              return self.burst_eap.compute_eap_compensation_lut(self.width)
          
          @property
          def relative_orbit_number(self):
              """Returns the relative orbit number of the burst."""
              if self.platform_id == "S1A":
                  orbit_number_offset = 73
              elif self.platform_id == "S1B":
                  orbit_number_offset = 27
              elif self.platform_id == "S1C":
                  orbit_number_offset = 172
              else:
                  raise ValueError(f"Unknown platform_id: {self.platform_id}")
              return (self.abs_orbit_number - orbit_number_offset) % 175 + 1

You can back this out by looking at the ascending node information in the orbit files corresponding to Track 1