MRU Transform

This ROS node listens to Motion Reference Unit topics and publishes transforms to /tf. Odometry is also published.

Multiple sensors can be configured in a priority list and the node can fallback to secondary sensors if the primary one stops providing data.

This node is designed for integrated sensors that combine a GPS with an IMU to provide complete position and attitude solutions.

Sensor topics

Each sensor is expected to publish on three topics:

• position: sensor_msgs/msg/NavSatFix
• orientation: sensor_msgs/msg/Imu
• velocity: geometry_msgs/msg/PoseWithCovarianceStamped

Only the linear component of the velocity message is used. It describes velocity of the sensor in an ENU frame. The frame_id should reflect the sensor's frame.

Frames

A map frame is created centered on the first position fix. An earth frame is also created and a transformation from ECEF to ENU used by the map frame is published. A null transformation from map to odom is also published. Finally, a transformation from odom to base_link is published with each sensor update.

Additional frames are published which may be useful in special cases, such as a sensor that publishes data that is already corrected for pitch and roll. The base_link_north_up represents a frame at the base_link location without any orientation applied. A level frame with only heading applied is also published as base_link_level.

Parameters

~sensors

A list of sensors which will be used in order. Each sensor entry has a name and a map of topics.

/**:
  ros__parameters:
    # you must "forward declare" the sensors so they can declare 
    # their sub params
    sensor_names:
    - posmv
    - gps
    # then configure the actual sensors
    sensors:
      posmv:
        topics:  
          position: 'sensors/posmv/position'
          orientation: 'sensors/posmv/orientation'
          velocity: 'sensors/posmv/velocity'
      gps:
        topics: 
          position: 'sensors/gps/position'
          orientation: 'sensors/heading' 
          velocity: 'sensors/gps/velocity'

If no sensors are found, a default sensor will be created with topics position, orientation and velocity.

~base_frame, ~map_frame and ~odom_frame

Override the frame_id's which default to base_link, map and odom.

~odom_topic

The topic used to publish odometry messages. Defaults to odom.

chart_datum_node

chart_datum_node publishes the map → chart_datum (MLLW) and map → chart_datum_mhhw TF transforms — the static vertical offset between the WGS84 ellipsoid (map) and the chart datum at the boat's current position. It resolves the datum from several sources so the boat works on and off NOAA VDatum coverage (e.g. inland lakes).

Resolution order

At each recompute the datum is resolved by this precedence (first match wins; within a config pass, the first matching entry in file order wins):

1. lake_datum param (if set) — wins outright everywhere.
2. config entries with override: true whose polygon contains the boat.
3. VDatum (PROJ + NOAA grids) where it has coverage.
4. config entries with override: false (the default) whose polygon contains the boat — fills VDatum gaps.
5. Nothing matches — chart_datum is not published; navigation continues on the ellipsoidal map / map_tide, and a warning is logged. (chart_datum is intentionally optional, per the marine TF frame design.)

The active source is logged on change and published on the latched datum_source topic (std_msgs/String: vdatum, polygon:<name>, param, or none) so consumers and operators can tell a surveyed datum from none.

Parameters

Parameter	Default	Description
geoid_grid	""	PROJ geoid grid (.tif) for ellipsoid → NAVD88. Needed for VDatum.
vdatum_grid_dir	""	Directory of VDatum *_mllw.gtx / *_mhhw.gtx grids. Empty disables VDatum (non-fatal).
datum_config_path	""	Path to a polygon→datum YAML (see config/datum_polygons.example.yaml). Empty = none. A malformed file fails on_configure.
lake_datum	NaN (unset)	Fixed chart_datum height (m, rel. ellipsoid) that overrides VDatum/config everywhere. For quick one-offs/testing.
lake_datum_mhhw	NaN (unset)	Optional fixed MHHW height to accompany lake_datum.
recalc_interval	60.0	Seconds between position-based datum recomputes.
publish_rate	1.0	Hz at which cached transforms are republished.

Config file

See config/datum_polygons.example.yaml for the schema. Heights are signed metres relative to the WGS84 ellipsoid (negative when the datum is below it), matching the published map → chart_datum translation. Deployment-specific polygons (e.g. a real Lake Massabesic datum) belong in the platform/site config repo, not this generic package — this package ships only a synthetic example.

An entry that sets chart_datum_z but omits mhhw_z publishes chart_datum without chart_datum_mhhw. Downstream, sea_surface_estimator's tide plausibility bound needs both frames, so it stays disabled (no-op) for such an entry — an acceptable degradation, but worth knowing.

Credits

Originally Developed by: Roland Arsenault, University of New Hampshire Center for Coastal and Ocean Mapping

ROS2 migration: Dr. Kristopher Kransosky, Seaward Science