ZERO_REPAIR_PPP.md

Zero repair (-z) to obtain valid PVT solutions

One application of the -z zero repair flag is to correct invalid observations produced incorrectly by low quality setups or receivers.

The CRNX/V3/NYA100NOR_S_2024128 daily observations are incorrect and encoded zeros from time to time in the C5X pseudo range code. This is illegal. Any ppp solving attempt will rapidly panic with physical errors.

One example would be C5X SPP:

rinex-cli \
	-P GPS \
	-P C5X \
	--fp data/CRNX/V3/NYA100NOR_S_20241280000_01D_30S_MO.crx.gz \
	--fp data/NAV/V3/NYA100NOR_S_20241280000_01D_GN.rnx.gz \
	ppp -c examples/CONFIG/Static/gpst_spp.json

[...]
panic: Physical non sense - rx=2024-05-07T12:30:30 GPST prior tx=2024-05-07T12:30:30.000123693 GPST

Or any dual signal technique involving C5X, for example C1C+C5X CPP:

rinex-cli \
	-P GPS \
	-P C1C,C5X \
	--fp data/CRNX/V3/NYA100NOR_S_20241280000_01D_30S_MO.crx.gz \
	--fp data/NAV/V3/NYA100NOR_S_20241280000_01D_GN.rnx.gz \
	ppp -c examples/CONFIG/Static/gpst_cpp.json

[...]
panic: Physical non sense - rx=2024-05-07T12:30:30 GPST prior tx=2024-05-07T12:30:30.000171848 GPST

You can also see that C1C SPP gives correct results, because that signal was correctly encoded:

rinex-cli \
	-P GPS \
	-P C1C \
	--fp data/CRNX/V3/NYA100NOR_S_20241280000_01D_30S_MO.crx.gz \
	--fp data/NAV/V3/NYA100NOR_S_20241280000_01D_GN.rnx.gz \
	ppp -c examples/CONFIG/Static/gpst_spp.json

To fix that, simply request the zero repair operation with -z. We now obtain valid PVT solutions. The C5X signal is quite often lost, so we also add a narrow timeframe to illustrate we can now obtain valid solutions (other problem that is out of scope here):

rinex-cli \
    -z \
	-P GPS \
	-P C1C,C5X \
	-P ">2024-05-07T12:30:00 GPST" \
	-P "<2024-05-07T13:15:00 GPST" \
	--fp data/CRNX/V3/NYA100NOR_S_20241280000_01D_30S_MO.crx.gz \
	--fp data/NAV/V3/NYA100NOR_S_20241280000_01D_GN.rnx.gz \
	ppp -c examples/CONFIG/Static/gpst_cpp.json