+PFF Posted December 5, 2008 Share Posted December 5, 2008 This should have been an ideal GPS site! I was searching for a triangulation station and two reference marks. I was at a large farm, on top of a low ridge, and the view to the horizon was excellent. Moreover, each reference mark has its own PID, with adjusted coordinates. After obtaining permission from the owner, I set off for the area near the barn. As I approached the main station, the pointer on my GPS unit would suddenly swing in a different direction. It's straight ahead 34 feet. No, it's to the left 51 feet. Nope, it's 27 feet to the right. The same thing happened when I looked for RM2, which was described as being 67 feet north of the barn. However, the GPS unit behaved properly while looking for RM1. The barn is a cinder block structure with a large tin roof. The roof slopes toward the north, in the direction of the station and RM2. [see photo, below.] I believe it was acting as a large "satellite disk", reflecting signals directly at the ground in the area of the disks. RM1 was at a sharp angle to the roof, and was a greater distance away. This might account for the stability of the GPS while searching for RM1. Eventually, I was successful, even though all three disks were recessed 1 to 3 inches. But it left me with some questions: 1. Have the professionals in our Forum ever experienced something like this? 2. Could it have been the barn roof? (A GPS receiver determines its position by measuring time differences from satellites. A multipath reflection certainly would throw off the timing--much like seeing a 'ghost' on a television which is hooked to an outside antenna. 3. Would it be helpful to note this in my report? Clearly, the site qualifies as being suitable for satellite observations, based upon the unobstructed view of the horizon. However, I suspect that GPS readings would be "all over the board", even when averaged over many hours. Oddly, I had a similar experience earlier in the day. I was at a station with Scaled coordinates, and I attempted to average readings over several minutes for inclusion in the NGS log. However, it would not pull in any tighter than 16 feet. I was at a bridge. At each end, there was a large, flat, vertical metal warning post. I moved to a position at a right angle to the post and the reading came down to 10 feet. As with the barn incident, I attribute this to reflected signals. -Paul- Quote Link to comment
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