Outside Stationary GPS Data Logging
I have written previously about logging data from a West facing window. After emailing and comparing notes with a friends Garmin GPS readings, I thought it was time to gingerly take the GPS-RaspberryPi outside for a real world test run. Luckily the external antenna comes with a generous 5m long cable, so I could put the unit in the shade and the antenna up on the fence line.
The results where better than I expected:
• The GPS unit started off at 1 (the West facing window)
• Was physically placed at the ‘Outside GPS' location, some 5.3m from where it thought it was (point 2)
• It took 1.5 minutes (~90 readings at 1 per sec), to get from point 2 to the elbow
• It spent the next 30 minutes working its way to be ~ 2.1m off the actual location at point 3.
• 84% of the total number of 2,302 readings are displayed between points 2 and 3 - most are just overlaid on top of each other.
• I then walked from point 3 to point 4
• Then walked to point 5 roughly following the fence boundary
• I paused there for 13 seconds (tight set of grey dots). Point 5 is only ~ 1.5m out!
• Walked form point 5 to around point 6, where I entered the house.
• The readings quickly made their way East - some 88m down the street, while I walked through the house back to the West window - point 7
• The brown dots show the signal locking back in, but then the wandering begins.
As if this page isn't colourful enough, here is another plot of the above data, this time the plot colours are set by the number of satellites used to make the GPS reading:
The above legend shows the number of satellites used for each reading. You can see that 84% of the total readings (all plots between points 2 and 3),have a satellite count between 8 and 10.
The correlation between the used Satellites and HDOP is:
| Satellite Count | HDOP | Reading Count | Percentage of point 2-3 readings |
| 8 | 1.01 - 1.08 | 1087 | 56.5% |
| 9 | 0.96 - 1.0 | 602 | 31.3% |
| 10 | 0.8 - 0.83 | 233 | 12.1% |
Lets have a close up look at the Fence GPS Data. Note: the GPS graphics below show just a few of the 1915 data points, as a lot of them lie on top of each other.
The above graphic is a close up showing the number of satellites per GPS reading.
The above graphic sections the fence data into 383 sequential data points divided up by time (6m 23 sec each). It's another look that shows how GPS readings move over time.