The objective of this project was to explore different laser-based positioning technologies and evaluate their performance when applied to handheld technology integration and limited line-of-sight environment surveying. Laser positioning systems have been in use by land surveyors for many years. Geophysical surveying has used the systems in limited capacities, which include surveying near wooded boundaries, buildings, along cleared wooded paths with heavy canopies, or other areas where GPS does not work effectively. Laser-based positioning technology has a positional accuracy in the millimeter range and a small spherical error, both of which provide the positional accuracy needed for anomaly parameter recovery through inversion techniques.

An initial laser assessment was conducted and significant progress made to modify and deploy the technology. Comparative testing of three different laser-based systems with an industry-standard satellite-based position system was conducted at the Engineer Research and Development Center (ERDC) Test Stand in Vicksburg, Mississippi. After this comparative testing, issues with the Leica instrument’s timing became apparent. The multi-gun control software underwent further development, modification, and testing. Lastly, one of the Leica instruments was integrated with an industry-standard electromagnetic (EM) sensor designed for handheld deployments.

Instrument testing shifted from Leica to the more stable and better performing Trimble platform. Utilizing lessons learned from the previous years, a characterization study measured the performance specifications of the Trimble platform. During the evaluation, several issues were identified and presented to Trimble engineers. These issues included prism list addition and several man-machine interface command problems.

Several systems by two different manufacturers (Leica and Trimble) were tested. As the project evolved, it became clear that the Leica TPS1200 system could not provide a positioning solution at either the speed or with the timing consistency as the newer Trimble SPS930 unit.

The Trimble was a superior instrument for the following reasons:

• Latency and Dither: The Trimble SPS930 has a small dither (4 ms) that makes correcting the timestamp of the positions a simple calculation.
• Faster Update Rate: The SPS930 has a 10 Hz sample rate with a possibility of 20 Hz in the future.
• Active Prism: The flashing light-emitting diode (LED) of the prism provides a glow that the gun identifies and tracks even when multiple prisms are in use. Also the LED provides a glow that the gun uses to locate the prism after the line-of-sight has been interrupted.
• Firmware Stability: The firmware of the SPS930 was more stable and reliable when compared to the firmware of the TPS1200. However, some glitches were noticed during the wooded tests. The SPS930 is a new instrument so some glitches are to be expected.

For handheld sensor integration, the dither and sample rate are the key points. Sensor integration tests with the Leica system did not provide the necessary positional data quality needed for successful inversion. The Trimble provided the necessary positional quality for the reliable inversion of geophysical sensor data, making it the instrument of choice for sensor integration.

The Trimble SCS930 provides an accurate and reliable positioning sensor suitable for deployment in open and wooded areas with tree densities less than 6 trees per 25 m². Above this density, accurate positions can only be obtained over a small fraction of the site. Therefore, laser systems are a partial, but not complete, solution for surveying in wooded areas.