Random GPS Stuff

Apartment x-ray (88,890 Bytes)

This plot is something like an "X-ray" photograph of my apartment. It is actually a plot of signal/noise for each GPS satellite shown vs. position in the sky over a span of about three days. The receiver was located inside my apartment near a bedroom window. The enclosing building attenuated the signal in different parts of the sky depending on how much material the signal had to pass through. Areas with a strong signal are shown in lighter shades. Regions where there was a weak signal are shown in darker shades.

The left-hand image shows the data without annotation. Since GPS satellites always repeat their path across the sky there were many gaps in the coverage. To make up for this somewhat, each data point was assumed to span a relatively large portion of the sky. Where points overlapped their effects were combined using a simple average. Note that the attenuation pattern was very repeatable on all three days implying the effect seen was truly due to attenuation by the building and not some random effect.

The right-hand image shows a few structures I could tentatively identify. What appears to be the window frame is visible with its rough position shown in red. The bedroom portions of the building are set back from the living room areas so the latter extend into the field of view as partially outlined very roughly in green. Since the apartment is south-facing, there is a relatively clear horizon to the south between these protrusions so there is little attenuation of the signal until close to the horizon. Looking east, west and north involves looking through the bulk of the building so the attenuation is stronger near the horizon. The gap to the north is due to the roughly 55 deg inclination of the GPS satellites. Finally, the blue spots show the locations of the two geostationary satellites used for WAAS. No signal was received from either during the first day so the WAAS option was turned off the other two days to free up two receivers.

Houston-Fort Worth altitude (10,547 Bytes)

Altitude between Houston and Fort Worth as measured with GPS. There are ten curves corresponding to ten trips, five in each direction. All ten were taken after SA was turned off.

There are spikes at Fairfield, Buffalo, Centerville and Madisonville. These are where I stopped for lunch on various trips. They possibly occur because the GPS receiver is not able to detect reflected signals etc. as easily when it isn't moving.

The messy, overlapped lines just before Fort Worth are where I was travelling on I-20 at an essentially constant latitude.

Position error w/SA (2498 Bytes) Position error w/o SA (1336 Bytes)
Altitude error w/SA (2853 Bytes) Altitude error w/o SA (1640 Bytes)
Here's the effect of selective availability on the accuracy of GPS readings. Both sets of plots contain the same number of samples at the same sample rate. The "exact" location is taken as the average of all the points in a given plot. The values plotted are thus the difference between each reading and this average.
Sonic velocity route (2208 Bytes)

The route taken by Jay and Christian on the science fair project trying to measure the speed of sound.

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