Heading attribute of InSAR products expressed clockwise instead of counter-clockwise for descending scenes

[asjohnston-asf]

Originally reported by @yunjunz at insarlab/MintPy#540 (comment)

Sentinel-1 headings are expressed in degrees counter-clockwise from north in the annotation xml files provided with SLC products (around -12 degrees for ascending scenes and -168 for descending scenes).

For descending pairs, the heading attribute for InSAR products is expressed in degrees clockwise from north in the .txt metadata file (around 193 degrees).
descending.txt

GAMMA's SLC_copy_S1_TOPS program appears to be the culprit, an input .par file with a -168 heading generates an output .par file with a positive heading.
https://github.com/ASFHyP3/hyp3-gamma/blob/develop/hyp3_gamma/insar/ifm_sentinel.py#L335
https://github.com/ASFHyP3/hyp3-lib/blob/8594f68c4496c9c0ad1fcdc704c3c8a59f90cc70/hyp3lib/SLC_copy_S1_fullSW.py#L31

This does not appear to impact InSAR products generated from ascending pairs; the heading is still expressed as ~-12 degrees.
ascending.txt

[asjohnston-asf]

This impacts MintPy metadata when importing InSAR products from HyP3 around https://github.com/insarlab/MintPy/blob/main/mintpy/prep_hyp3.py#L118

[yunjunz]

Hi @asjohnston-asf, -168 and 192 are equivalent to each other with 360 deg difference. It was my mistake. The 193 value is correct and fine.

[asjohnston-asf]

prep_hyp3.py may set the ORBIT_DIRECTION attribute incorrectly as a result, might need a heading = (heading % 360) - 360 at
https://github.com/insarlab/MintPy/blob/main/mintpy/prep_hyp3.py#L120 or
https://github.com/insarlab/MintPy/blob/main/mintpy/prep_hyp3.py#L129

[yunjunz]

Yes, I will adjust this value in prep_hyp3.py to make it around -168.