GNSS Factors

This fork ships a minimal, loose-coupled GNSS prior injector so that a pre-projected position stream (ENU or any local Cartesian frame) can be attached to the global mapping factor graph without tight receiver-level integration.

Scope

This is intentionally the simplest usable GNSS path:

Loose coupling only. One gtsam::GPSFactor prior per submap pose, with per-axis or isotropic sigma taken from the CSV row. No velocity coupling, no lever-arm, no antenna offset, no bias modeling.
Local nav frame only. The CSV must already be in a local Cartesian frame (ENU, UTM-local, site frame). This fork does not perform latitude/longitude projection; do that at the writer side.
One prior per submap. A GNSS observation is matched to the nearest submap origin stamp within time_tolerance; consume_once=true prevents the same GNSS observation from constraining two submaps.

The module is disabled by default, so existing GLIL runs do not change unless it is explicitly loaded via the extension_modules list in config_ros.json.

CSV Schema

Two row shapes are accepted. Header row, blank lines, and # comment lines are skipped when skip_header=true (default) and skip_invalid_rows=true.

Per-axis sigma:

stamp,x,y,z,sigma_x,sigma_y,sigma_z
12.3,100.0,50.0,2.0,0.05,0.05,0.10
12.4,100.1,50.0,2.0,0.05,0.05,0.10

Isotropic sigma:

stamp,x,y,z,sigma
12.3,100.0,50.0,2.0,0.10

column	meaning
`stamp`	sensor timestamp in seconds, must be ascending
`x`, `y`, `z`	position in the local navigation frame, meters
`sigma_*` or `sigma`	standard deviation in meters (positive, finite)

Rows with a zero/negative/non-finite sigma, non-finite stamp, or non-finite position are rejected by GNSSObservation::valid().

Config

config_gnss.json lives in the same directory as the rest of the GLIL config and is loaded by the gnss_prior_injector extension module:

{
  "gnss_prior_injector": {
    "enabled": true,
    "csv_path": "gnss.csv",
    "time_tolerance": 0.05,
    "consume_once": true,
    "skip_header": true,
    "skip_invalid_rows": true,
    "fail_on_load_error": false,
    "min_sigma": 0.001
  }
}

key	default	meaning
`enabled`	`false`	master switch; when `false` the module only logs and returns
`csv_path`	`""`	CSV path, resolved relative to the config root
`time_tolerance`	`0.05`	max `\\|obs.stamp - submap.stamp\\|` in seconds
`consume_once`	`true`	if `true`, each GNSS observation can attach to only one submap
`skip_header`	`true`	skip the CSV header row automatically
`skip_invalid_rows`	`false`	keep loading after parse errors
`fail_on_load_error`	`false`	disable injector if any parse errors occurred
`min_sigma`	`0.001`	floor applied to each axis of the noise model

Wiring Into A Run

Place gnss.csv and config_gnss.json next to the other config_*.json files in your config root.
Edit config_ros.json and add the shared library to the extension list:

"extension_modules": ["libgnss_prior_injector.so"]

Start GLIL as usual. The injector logs GNSS prior injector loaded N observations on startup and GNSS prior injection poses=... inserted=... every time a submap update binds new priors.

The bundled fixture config/sample_gnss_observations.csv is used by the test_gnss_observation CTest target to exercise the CSV parser and factor construction end-to-end.

What This Is Not

Not a replacement for tight GNSS-IMU coupling. Position-only priors help constrain global drift but cannot recover heading from a single receiver.
Not a geodetic-to-local projector. Latitude/longitude-to-meters conversion is out of scope and must be done before writing the CSV.
Not evaluated against a published dataset here. The module is functional and tested at the CSV + factor level; real-run accuracy numbers are a follow-up.