# Accuracy Test: Topside GPS
If you are looking for absolute ROV position, the Topside GPS sets the fundamental lower limit to system accuracy. The green dots in the image below are the plotted position of the Topside GPS. During the approximately 26-minute test, the Topside position wandered about 6 meters. The ROV position is calculated as an offset to the Topside GPS, and the GPS error is thus manifested directly as an error in ROV position.
The positions are quantized due to precision limits in the map message that displays latitude and longitude. So, although the map only shows about 20 green dots, there were actually 1588 observations made and 1588 points plotted in this map. Each point seen below is overlaid many times.
The magenta dots are positions of the transponder as it was dropped through a hole about 4 meters from the transceiver hole as we were making sure everything was working. The scatter of these points is almost almost entirely due to the GPS error, although there was a small actual change in slant range during the test as the transponder was lowered into the water.
Map at transceiver location
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