# GPS Search

During a robotic survey, if the instrument loses lock to the target and the software is connected to a GNSS receiver, you can use the GPS/GNSS receiver to help point the instrument to the target.

By default, GPS Search is enabled when Trimble Access is:

- connected to a Trimble GNSS receiver and running an integrated survey
- running on a controller with internal GPS

When using a controller with internal GPS, a connected GNSS receiver is always used in preference to the internal GPS.

To connect the controller to a third-party auxiliary GNSS receiver, see Auxiliary GPS settings.

If the total station is set up in terms of the defined projection and datum, then GPS Search is ready as soon as the station setup is complete. When GPS Search is ready, the message GPS Search ready appears in the status line, and the target icon displays a satellite icon above the prism .

If you do not have a fully define coordinate system, or if you are using a custom auxiliary GNSS receiver, you will need to configure GPS Search before you can use it. You can use GPS Search when connected to an auxiliary GNSS receiver that is outputting NMEA GGA messages at 1 Hz through a serial or Bluetooth port to the controller.

To configure GPS search settings

1. Start a robotic survey.
2. Tap and select Instrument / Target controls.

3. In the GPS Search group, set the On switch to Yes.

4. Set the Enable 3D check box as required.

   - If 3D is enabled, a 3D GPS Search position is computed, and the instrument can turn to the point both horizontally and vertically.

     If the connected GNSS receiver is initialized in an RTK survey, or if SBAS is available, you can enable 3D because the GNSS heights from the receiver should be accurate enough to turn the instrument's vertical angle.

   - If 3D is disabled, the instrument can only turn horizontally to the GPS Search position.

     If the connected GNSS receiver is producing autonomous positions, or if SBAS is not available, Trimble recommends that you disable 3D to prevent incorrect GNSS heights causing inaccurate turning of the vertical angle.

   In an integrated survey, the Select data source is automatically set to Trimble GNSS and the 3D check box is selected by default.

5. Make sure the value in the Select data source is correct. If the software is connected to:

   - A Trimble GNSS receiver, select Trimble GNSS.
   - The controller's internal GPS receiver, select Internal GPS.
   - Another type of GNSS receiver, select Auxiliary GPS.
6. Make sure the value in the Receiver type field is correct. If it is not, tap the Aux softkey and then configure the Auxiliary GPS settings for the internal or custom receiver. See Auxiliary GPS settings.
7. Tap Accept.

GPS search is now configured. The relationship between the GNSS positions and local positions must be resolved before you can use GPS search.

Resolving the relationship between GNSS positions and local positions

If you have a fully defined coordinate system, then an accurate relationship between the GNSS positions and local positions already exists using the coordinate system definition. The software assumes that the total station is set up in terms of the defined projection and datum, and GPS Search is ready as soon as the station setup is complete. If your total station is not set up in terms of the defined coordinate system then using GPS Search will cause the total station to turn incorrectly.

If you do not have a defined coordinate system, you must resolve the relationship between GNSS positions and local positions before GPS Search is ready. Once a station setup is complete, the Trimble Access software uses the NMEA positions from the GNSS receiver and the angles tracked by the robotic instrument to determine the relationship between the two positioning systems. GPS Search computes the relationship independently of the job's coordinate system settings.

To determine the relationship, make sure that the GNSS receiver has a clear view of the sky and then, with the instrument locked to the prism, move the pole around the instrument until the relationship between the GNSS positions and local positions is resolved. A minimum of five positions at least five meters apart and at least ten meters from the instrument are required. If the geometry and GNSS positional accuracy is poor, you need more than five positions to resolve the relationship. Poor GNSS positional accuracy may result in an inaccurate relationship being computed.

If you are going into a bad GNSS environment for an extended period of time, tap pause to pause GPS Search and stop new positions being added into the GPS Search solution. Tap play to resume GPS Search and start adding points into the GPS Search solution again.

- To view the GNSS status, tap GPS on the Target controls screen. Alternatively, from the GNSS status screen, tap and hold on the target icon.
- When GPS search has good data, it can detect bad data and exclude it from its computations. However, if there are more bad positions than good positions, it is difficult for GPS Search to detect and exclude bad positions. Too much bad data in the computations may prevent GPS Search from becoming ready. If this occurs, move to a better GNSS environment and then tap Reset to restart the GPS Search.
- If you perform a calibration or change the coordinate system settings, the existing relationship between the GNSS positions and local positions is lost and must be recomputed.

Using GPS Search

The software automatically uses GPS Search when searching for the target. If GPS Search is ready, the instrument turns to the GPS Search position. With a good GNSS position, for example from a Trimble R12 receiver with an RTK fixed solution, and when snap is enabled, the instrument should snap to the target immediately. If the instrument does not snap immediately, it performs a search before locking to the target.

When using GPS Search with a Trimble receiver, a cross indicates the position of the GNSS receiver. When using any other receiver and a GNSS position is available, a satellite icon appears in the map. If a GPS Search solution is available, a black satellite icon appears . If a GPS search solution is not available, a red satellite icon appears . To turn to the GNSS position in a conventional survey, make sure that there is nothing selected in the map and then tap and hold briefly in the map. From the menu that appears, select Turn to GNSS to turn the instrument horizontally to the GNSS position.

Tap Search in the Joystick screen to perform a normal search even when GPS Search is ready. Use this when you need to search for a target without using the GPS Search position, such as searching for a backsight target.

To perform a GPS‑aided search from the Joystick screen, tap .

As soon as the instrument locks on to the target, the Joystick screen closes.

To perform a standard search throughout the Trimble Access software, pause GPS Search at any time.