Following on from the GPS accuracy tests carried out on the roof of the OU Biology building, we’ve been doing a set of tests on the balcony of the Knowledge Media Institute (KMI). We’ve tried as far as possible to do a comparable set of tests. The KMI balcony is on the top floor of the building and faces south(ish), the wall of the building blocks part of the sky, but only part of it.
Figure 2. KMI balcony from below showing secured shelf during testing.
Figure 1. Secured shelf on the KMI balcony showing marked positions for devices.
An upturned bookshelf is g-clamped to the railings of the balcony and nine positions (10 cm apart) are marked along the middle of it. The GPS devices can then be placed on any of the nine positions and are held in place either by an elastic shock cord or gaffer tape. The Android phones are put in ‘Aquapac’ waterproof bags which are also tied to the cords on the shelf in case they fall.
We repeated a set of tests on two consecutive days to see the effect of using having an internet connection. On both days we used five phones (three HTC Desires and two HTC Wildfires). The phones had a WiFi connection to a local wireless network and logged their GPS location once per minute to a local web server. On the first day the network was not connected to the internet, on the second day a 3G dongle (connected to the three network) was plugged into the WiFi router and provided internet access for all five phones.
The device positions and configurations were as follows:
- Position 4 – HTC Wildfire (id-6) using internal GPS
- Position 5 – HTC Wildfire (id-7) using internal GPS
- Position 6 – HTC Desire (id-5) using internal GPS
- Position 7 – HTC Desire (id-2) using internal GPS
- Position 9 – HTC Desire (id-4) using external GPS
The tests ran for about 3 hours 20 minutes on both days. Although the data patterns vary across the phones, the accuracy is comparable to the previous sets of tests. The phones’ GPS readings are generally within 2 or 3 meters of their actual position. The phones seem to lock onto a value for a while, so the plots look like they are stepped. The external GPS (a Nokia LD-3W) although comparable to the internal GPS for most of the time, had an unexpected spike on both days.
Here are the plots for the first day (no internet connection).
Figure 3. Position 4 - HTC Wildfire (id-6) using the internal GPS no internet access (click on a plot to view the full size image).
Figure 4. Position 5 - HTC Wildfire (id-7) using the internal GPS no internet access.
Figure 5. Position 6 - HTC Desire (id-5) using the internal GPS no internet access.
Figure 6. Position 7 - HTC Desire (id-2) using the internal GPS no internet access.
Figure 7. Position 9 - HTC Desire (id-4) using an external GPS (Nokia LD-3W with a bluetooth connection) no internet access.
The following day we repeated the tests, but included internet access (i.e. a 3G mobile broadband dongle connection) over the local network.
Figure 8. Position 4 - HTC Wildfire (id-6) using the internal GPS with internet access.
Figure 9. Position 5 - HTC Wildfire (id-7) using the internal GPS with internet access.
Figure 10. Position 6 - HTC Desire (id-5) using the internal GPS with internet access.
Figure 11. Position 7 - HTC Desire (id-2) using the internal GPS with internet access.
Figure 12. Position 9 - HTC Desire (id-4) using an external GPS (Nokia LD-3W with a bluetooth connection) with internet access .
The plots are surprisingly similar across the two days. For each phone the trends are generally comparable. It may be that the internet access used by the AGPS service is not so crucial. At the moment, we don’t know for sure if the phones on the first day (that did not have internet access) were able to use a previously cached AGPS dataset. If anyone knows for sure please let us know.
To conclude, more replication is required to see if the results hold. However, so far an accuracy of two to three meters seems like a reasonable expectation. One of the Wildfire phones (id-6) dropped a lot of data on both tests, it could be that it needs a judicious kicking, but we’ll first try checking for application upgrades and validating the settings (its already running the current version of Android).
Good to know that you can do it without internet access. Now all that’s left is to get a solar powered one that way you don’t have to worry about running out of batteries.
Hi,
I just came across your GPS accuracy test, and thought that I should warn you: The A-GPS function allows the GPS satelite almanac to be pre-loaded via Internet. There may also be an update to the real-time-clock (using NTP via Internet, this is optional).
The net result is that the GPS receiver will be able to immediately interpret the signal from any satelites that it can receive, knowing already there exact positioning information. So that there is no need to first download this “satelite almanac” information (from the best available signal) – which is the normal “cold start” procedure. Having a (reasonably) accurate time can also help, marginally, although of course the GPS receiver will end up calculating the really-really-really-accurate time during its “fix” procedure.
So the bottom line is: using A-GPS should drastically reduce the time-to-first-fix, meaning that the time-to-fix will be both short and predictable. Thats all. The accuracy is really effected, at least not once the fix has maintained for a short time already.
Other systems allow to improve on GPS accuracy, both by correcting actual positioning errors of the GPS satelites themselves, and also with some information about environmental propogation (air conditions), although this is locally variable and so the relevance of those propgation informations depends on proximity to the correction source.
DGPS (Differential GPS) uses ground based long-wave transmitters to broadcast correction information around a local area (ocean coastline typically), meaning that a seperate and very different antenna is required (large physically, due to “long” radio waves, as there are no miracles involved).
WAAS (Wide Area Argmentation System => North American) and EGNOS (European Geostationary Navigation Overlay Service => European, like it says) relay the correction information via geo-synchronous satelites, and thus also use similar frequency bands to the GPS satelites themselves, meaning that WAAS-enabled and/or EGNOS-enabled GPS receivers receive the correction information with the same antenna, but need to dedicate some reception channels just for that (such fancy GPS receivers may advertise 50-60 channels, or more, not 8-10!!!). And of course it must be purpose-built for this functionality – apparently the SirfStarIII chipset has it built-in, for example. And the software must manage all of that, of course!
I have also heard that the WAAS/EGNOS correction information can also be downloaded via Internet. This is similar in principal to A-GPS, but especially for correction information, and not the basic GPS satelite almanac. Of course, those functions could be cumulative, with both the almanac (A-GPS) and also correction information (EGNOS, but via “SISNet” infortaion via Internet, and via satelite). I am speculating here, as I have not yet found any device that uses these 2 possibilities together.
Hope that helps!
Please, if you find any information about EGNOS-enabled Android devices, either “standard” via satelite or “cheating” via Internet: please let me know. I’m hoping to find a reasonably priced solution, with good accuracy, so that I can do good mapping with just my own telephone (which is always in my pocket, and so always available…).
Best regards,
Neil Dewhurst.