GPS Increasing Accuracy

I currently have one of Parallax's GPS units. I worked through the example GPS code and 4,200 samples later, the average (median, which is more accurate than the mean) is still in my neighbor's yard. I haven't tried testing it outside yet since my intended application is indoors. Somehow I was expecting greater accuracy, I could probably work with 3-5 meters of error, but there are some data points going 20 meters down my block.

I look at car GPS units which track your motion down the street. Do these units acquire more data to calculate position than what the Parallax GPS unit acquires? I figure they lock your position to being on the street, which probably helps with their perceived accuracy. They also probably have IMUs and do some filtering (Relative Kinematic Positioning even goes so far as Kulman filtering), but is that really enough to distinctly say you're stopped at a stop light and not on the other side of the intersection moving slowly? I'm curious if there's anything I can do with say, $150, to increase the accuracy of my current setup - such as hacking a used car GPS unit or getting a better antenna. I was hoping to be able to distinguish which room of the house I was in in a reasonable time frame (like 5 minutes if data averaging is needed).

GPS does not tend to work well (if at all) inside a building. Try your unit outside and see if you get better accuracy.

I can see a few meters as normal but 20m is too much. What source did you use when determining on the 20m? Another GPS or google map? I would take the setup outside, and compare it with a commercial gps. If you used google map, maybe the map is off. ;)

The car units typically lock your location to the street map which is part of the reason the accuracy looks better. I believe most of the newer units do have accelerometers built in to help.

When using a GPS (with an integrated patch antenna) indoors a key factor to look at is the sensitivity in the specs measured in -dBm. The larger the value the better the reception (ie -165dBm is better than a -159dBm).