# Broadcast RTCM coordinate system messages

A Network RTK provider can configure a VRS network to broadcast RTCM coordinate system messages that include some of the coordinate system definition parameters. When the Broadcast format is set to RTCM RTK in the Rover options screen of the survey style, and the RTCM messages are broadcast by the VRS network, Trimble Access can use this to provide the datum and ellipsoid definition for a job. See Coordinate system.

Trimble Access supports a subset of the RTCM transformation parameters, as shown below:

Message | Details | Supported
--- | --- | ---
1021 | Helmert/Abridged Molodensky (Control) | Yes
1022 | Molodensky‑Badekas Transformation (Control) | Yes
1023 | Ellipsoidal Datum Shift Grid Residual | Yes
1024 | Plane Grid Residual | No
1025 | Projection | No
1026 | Projection Lambert Conformal Conic Two Parallel | No
1027 | Projection Oblique Mercator | No
1028 | Local Transformation | No

The broadcast RCTM message must contain either the 1021 or the 1022 control message. This defines what other messages will be present. All other messages are optional.

Datum shift grid values are broadcast at fixed time intervals for a grid surrounding the area you are working in. The size of the grid that is broadcast depends on the density of the source grid data. To perform coordinate system transformations, the grid file that is built by Trimble Access must include shift grids covering the location of points you are transforming. When you move to a new location, a new set of datum shift grid values is broadcast and there may be a slight delay until the appropriate values are received from the VRS network server.

The broadcast transformation messages include a unique identifier for the broadcast parameters. If the broadcast parameters change, the identifier changes, and the Trimble Access creates a new grid file to store the new datum grid shift values. A message warns if the broadcast RTCM transformation changes and you are prompted to continue. If you select:

- Yes the system creates a new grid file, or, if it exists, uses another grid file that matches the newly broadcast transformation. If you change grid files, the new grid file may not cover the same area as the old grid file, so Trimble Access may not be able to transform points where there are 'holes' in the grid file.
- No you cannot continue surveying. Create a new job and start the survey again. If you need access to data in the old job, link that job.

If you copy a job that is defined to use a Broadcast RTCM datum onto a different controller, you should copy the appropriate grid file so that the software can transform grid coordinates on the other controller.

When a job with broadcast RTCM data is exported as a DC file, the GNSS observations are output as grid positions.