One of the key components of the Autonomous Underwater Explorers (AUEs) currently under development in our lab is a GPS receiver for locating, tracking, and recovering the AUEs after a deployment. Because the AUEs will be floating right at the surface of the water, there are problems acquiring satellites and getting reliable GPS fixes using conventional receivers.
On a recent cruise, we had the opportunity to test the GPS receiver of the iPhone 3G. The test consisted of putting together a small plastic housing for the iPhone that was weighted to ensure the iPhone was at the surface. Below is a photograph of the components of the housing:
The housing consisted of a set of large washers for ballast, some padding to fill the empty space in a 1 L plastic jar, electrical tape to fix a line to the jar, and the iPhone. The iPhone was then powered on, and a GPS application was started to acquire and log GPS fixes. The parts were then assembled:
We then deployed the iPhone off the stern of the ship, payed out about 200 feet of line, and then towed the package behind the ship for a few minutes. After recovering the package, downloading the GPS data and plotting using GPSVisualizer, we obtained a nice GPS track:
Note that this was recorded by the iPhone while it was partially underwater, bobbing up and down as it was towed behind a ship!
Many GPS receivers fail to work at all when used right at the surface of the ocean. Yet the iPhone not only worked, it worked well! There are several factors to this performance. Perhaps one of the most significant is that the iPhone uses the cellular network to acquire GPS almanacs and therefore can rapidly initialize and start computing GPS coordinates. Of course without the cellular network, this performance would be reduced, but none the less, the iPhone does provide a solution for acquiring GPS fixes at the surface (perhaps even slightly under water) of the ocean!