GPS World Applications
GPS World, Vol. 11, No. 8, August 2000, p. 13
Reach the inaccessible
Sometimes, it is impossible to survey a point with a GPS antenna mounted on a vertical pole. To resolve this problem, we have developed a patented GPS surveying probe consisting of two collinear GPS antennas that can be used in an inclined position to measure vertically inaccessible points.
To operate, one first places the probes tip in physical contact with the point to be located. The device can be inclined at any elevation angles (in upward or downward positions), because the antenna brackets can be kept horizontal. This fact and because its length may be extended allows the probe to reach more distant places as well as those that could not be reached with a conventional vertical surveying pole. In addition, the material of the pole section supporting the antennas does not interfere with the GPS electromagnetic waves.
Once the tip of the surveying probe is held in physical contact with the point of unknown coordinates, the coordinates of the two GPS antennas are determined with respect to a GPS reference station. The knowledge of the coordinates of the two collinear GPS antennas then allows the computation of the unit vector defining the direction of the axis passing through the GPS antennas and the tip of the surveying probe. Therefore, knowing the distance between the two GPS antennas and the distance separating one of these antennas from the probe tip, one may readily obtain the surveyed points coordinates through vector calculations.
Field tests have shown that the coordinates of the probes tip can be determined with an accuracy of a few centimeters. We obtained these results using 12-channel, C/A-code units with L1 ambiguity fixed solutions and a reference station located less than 1 kilometer from the probe. We have used the surveying probe, for example, to determine the three-dimensional coordinates of benchmarks embedded in walls. We have also developed an alternative system that replaces the GPS antennas with two corner cubes used in association with a total station.
Dr. Rock Santerre, Dr. Michel
Boulianne, and Ms. Stéphanie Bourgon
For its most recent tests, the Université Laval team employed the Marconi Star Box GPS receivers from Canadian Marconi Company (Ville St-Laurent, Quebec, Canada). The group has been awarded U.S. Patent Number: 6,008,757 for the probe device.