preproc focused

Focused Preprocessing Operations

RINEX-Cli allows forming complete GNSS context with its file loader, mostly used in Precise Point Positioning.

It is most convenient to combine some preprocessing to a post processing algorithm like PPP. RINEX-Cli allows doing so, as previously stated, one can combine -P to any algorithms. At the same time, it is rarely intended to rework (preprocess) the whole context, most of the time we're interested in reworking the raw GNSS signals (so called "observations") and preserve all orbital information (usually packed in NAV RINEX format).

To answer this need, the Preprocessor (-P) allows a special prefix in the filter descriptor, to focus on a specific RINEX format.
Currently, RINEX-Cli only allows one special prefix and does not know how to handle several of them.

Typically, when a PPP compliant context is formed, you only want to preprocess the GNSS carrier observations described by the OBS RINEX file.
Let's perform -p position solving on this context, but reduce the observations quantity

rinex-cli \
    -f test_resources/CRNX/V3/ESBC00DNK_R_20201770000_01D_30S_MO.crx.gz \
    -f test_resources/NAV/V3/ESBC00DNK_R_20201770000_01D_MN.crx.gz \
    -P "obs:decim:1 min" \
    -p --spp

In this example we specified that the resampling should only impact the signal observations.
When resampling to intervals that are very close to the original, like in this example, the navigation filter is almost not impacted and still coverges rapidly. Decimation is one way to reduce the number of PVT solutions the solver will generate, but you have to be knowledgeable enough, as you increase the decimation factor, how it will impact the navigation filter.

Record decimation is completely incompatible with the cggtts opmode, at least when using basic setup.
Indeed, CGGTTS implies continuous observation of a signal carrier (usually phase, but we also support PR code), for a 16' period. In this case, record decimation becomes disruptive and will break the default sampling recommendations.
You need to move on to a customized signal tracking strategy to combine special CGGTTS PVT formatting, and record decimation.

Read more about CGGTTS special mode in its dedicated page