GPS Satellite Horizon Experiment

I have permanently mounted a GPS (global positioning system) antenna on my ham radio tower and have been capturing data to determine how much of the GPS satellite constellation is visible from the antenna's location and what effect the nearby trees have on the satellite horizon.

The tower is mounted a few feet to the north of a copse of tall trees that runs west to east and is perhaps 100 feet deep. The tower is triangular and is 18 inches wide on each face. The antenna, a Motorola "hockey puck," is side mounted on the north corner of the tower and is fed with about 45 feet of RG-58 coax. The receiver is a Motorola Oncore PVT-6, running in 2-D mode. The software used to generate the horizon data is Stan Huntting's SA Watch.

Here is the first data plot, made in December when nearly all leaves were off the trees:

GPS horizon in winter

Note the sharp spikes to very high elevations on the south side of the plot. These are probably the result of shielding by the tower members. Otherwise, the plot doesn't seem too remarkable.

Now, here's the same test run during early summer, when the leaves were full and moist on the trees.

GPS horizon in summer

This is strange -- in a lot of ways, this plot looks better than the winter one! The horizon is smoother, and shows fewer spikes. Perhaps the leaves have the effect of softening an interference pattern caused by the tree trunks?

Note, though, that the actual positioning performance is worse than in the winter. The average HDOP is 2.1, compared with 1.8 in the winter. But, one percent of the time the HDOP was below 1.0, while in the winter it never reached that level.

And, during this 7+ day run, there were 6 hours, 24 minutes of time when the GPS was generating bad fixes. During the winter, that effect wasn't noticed at all. Apparently, there were significant times when the receiver wasn't seeing enough satellites to achieve a 2-D solution.

The bottom line appears to be that, although the satellite horizon doesn't get worse when the leaves are on the trees, the quality of the signal certainly does.

Coming next: I'm going to replace the ~45 feet of RG-58 feedline between the antenna and the GPS receiver with much lower-loss 9913 coax. Another seven-day run will then show us what happens when the feedline attenuation is reduced from 10.5dB to 2.8dB.