When we started geocaching in 2006, one of the first things we did was purchase a handheld GPS receiver (GPSr). At that time, a GPSr was essential for storing coordinates of geocaches waiting to be found and for marking the coordinates of our own cache hides. Even without a thorough understanding of how GPS receivers work, visiting online forums and attending local events netted advice, tips, and tricks on how to improve the accuracy of the coordinates we posted for our new cache placements. Particularly for urban hides, we followed the then-prevailing practice of including a comment like “accurate to +/- 3 m” in the cache description. And it was a milestone day when we figured out a way to download cache descriptions and past logs into our Palm devices so we didn’t have to carry reams of geocaching printouts with us all the time.
Boy have times changed! From what we observe, many geocachers today use GPS-enabled smartphones, Google Earth and other tools while on the hunt. Smartphone applications make geocaching more accessible to more people. We’ve used Mr. GeoKs’ iPhone to find a cache once or twice, but for the most part, we’re “old school”, preferring to use our dedicated GPS receivers, updated to more current versions that also contain the cache description, hint (if needed) and previous logs.
In the interim, we took the time to learn how a handheld GPS receiver works. If you’re not familiar with the term “trilateration”, consider running a quick internet search on “how do gps receivers work” or clicking on this link to the kowoma.de website (source of the images for this post) and then spending 10 minutes or so gaining a basic understanding. Whenever we’re teaching middle school students about geocaching (we usually do this twice per year), we squeeze in at least a 90 second overview of how the technology works. A couple of years after we started geocaching, our Oldest GeoKid’s science fair project was an experiment designed to answer the question “Which Variable Has the Greatest Impact on the Auccracy of GPS Coordinates?”
I mentioned this science fair project in a tweet last spring, and geocacher The Bad Cop replied, asking if I could post more detail. So consider this Part 1 of a 4-part series of blog posts outlining his background research, the design of his experiment, his results and the practical implications for geocaching.
CAVEAT: Please keep in mind that this experiment was designed and carried out by a 10 year old, with a little help from his parental units.
THE TASK: The purpose of this science fair experiment was to try to figure out which variable has the single largest effect on the accuracy of GPSr coordinates.
IDENTIFYING VARIABLES FOR TESTING: After doing some background reading, Oldest GeoKid identified the following specific variables for investigation:
- Atmospheric conditions – Certain conditions in the ionosphere can reduce the speed of the radio waves transmitted by GPS satellites. Any such slowing will result in an error in the position calculated by civilian-grade GPSrs. NOTE: In some locations, this can be compensated for by making use of the Wide Area Augmentation System (WAAS).
- Dilution of precision (aka DOP) – DOP refers to the geometric strength of the GPS satellite configuration. When visible satellites are close together in the sky, the satellite geometry is weak and the DOP value is high. When the satellites are far apart, the satellite geometry is strong and the DOP value is low. Several internet resources provide DOP information for a given location at any given time. Opinions vary on what constitues a “good” DOP vs a “marginal” DOP, but generally seem to average out that anything below 5 is pretty good, anything between 5 and 10 is marginal and anything over 10 is poor and highly likely to cause a degradation of accuracy.
Good geometrical alignment of two satellites (low DOP) - note the small area of uncertainty around the calculated location
Bad geometrical alignment of two satellites (high DOP) - note the larger area of uncertainty around the calculated location - GPSr warm-up time – Oldest GeoKid noticed when we were out searching for geocaches that we seemed to have better success if our GPS was turned on and had some time to “warm-up” beforehand, so he wanted to investigate whether accuracy topped out after 5 minutes, 10 minutes or something longer.
- Wide Area Augmentation System (WAAS) – Originally developed to improve the accuracy of GPS-based aircraft navigation, the WAAS system in North America is comprised of 25 ground stations covering the U.S.A. and parts of Canada and Mexico. On an on-going basis, these stations compare their calculated GPS location to their known location. When there is a difference, they calculate a correction factor which they send to a master WAAS station. The master WAAS station prepares a correction message and sends it to the WAAS satellites. These are not the same things as the GPS satellites. At the time of this experiment, there were 3 fully-operational geostationary satellites in the WAAS system, with a 4th satellite in test mode. The 4th “test mode” satellite was the only one reaching our home location in Calgary, Alberta, Canada. When WAAS is “enabled” on your GPSr, it looks for one of the WAAS satellites and incorporates the correction information into its location calculations.
- Weather – Oldest GeoKid wondered whether he’d see the most accurate GPSr locations in sunshine, rain, clouds or snow.
- Proximity of buildings, hills, canyons, trees, etc. – This is commonly referred to as multipath error and is caused by the reflection of GPS satellite signals off nearby objects. The reflected signal takes more time to reach the GPSr than a direct signal. The resulting time difference can result in a location calculation error of anywhere from a few meters to 10 or more meters. This type of signal bounce is often reflected on your GPSr by a larger than usual accuracy range (i.e. +/- 25 meters vs the more usual +/- 4 or 5 m).
Example of multipath error, caused by GPS satellite signal reflection
Watch for Part 2 (experiment design and data collection) to come out in the next few days. As always, comments and questions are welcome.