Now that cell phone technology has become ubiquitous in the new generation of smart mobile phones, the question on everyone’s mind is whether you can now get rid of your separate hand-held GPS receivers. If your primary use is highway navigation, the answer is a qualified “Maybe,” but if you use your GPS at all in the outdoors, the answer today is still “No.”
This might surprise some people. “My iPhone has a true GPS receiver just like a Garmin, so why should I need anything else?” they ask. The answer comes from design tradeoffs made by mobile phone designers. For the moment, don’t even worry about the fact a Garmin outdoor receiver is much more rugged than an iPhone. The fact is, the iPhone (and other GPS-enabled smartphones) won’t always find your position when you are far in the wilderness. I know. I’ve tried it, and it doesn’t always work. And if you do any amount of research on the Internet, you’ll come across many other people who have had similar experiences.
Before you say something like, “Steve, you must have a bad iPhone,” hear me out. The issue comes from a technology called “Assisted GPS” or simply “A-GPS.” It’s a great technology that allows the GPS in your mobile phone to find it’s position very rapidly. I won’t go into all the details of A-GPS here, I’ll just point out one tradeoff mobile phone designers typically make.
You probably know that a GPS receiver uses signals from satellites overhead to find its position. In order to do so, it needs to know precisely where in the sky each satellite is located any given instant. For this, it uses two sets of data stored in its memory about satellite orbits: almanac data and ephemeris data. Both almanac and ephemeris data need to be updated regularly because satellite orbits are never truly stable; there is always a small amount of variation as their orbits slowly decay. Also, new satellites are occasionally added to the system and old ones retired. The almanac data stored in your receiver’s memory remains valid for a few months, but ephemeris data needs to be updated every day.
The satellites themselves broadcast the almanac and ephemeris data for your GPS receiver to update its memory, but this is a slow process. Each satellite can only transmit the data at 50 bits per second (!). At this rate, it takes a minimum of 12.5 minutes to update almanac data and around 2 minutes to update ephemeris data. This would be a serious limitation for the typical mobile phone user, who expects to know his position within a few seconds.
Mobile phone designers realized they didn’t need to depend on the satellites to update the almanac and ephemeris data. Because their receiver is also a phone, they can simply program it to call up and get the current data from the network, where they’re not limited to a 50 bps transfer rate. This allows the GPS-enabled mobile phone to find its position much more rapidly than a dedicated GPS receiver, which can’t call up a network to get the data. (But beware, it also means you are transmitting and receiving data, for which you could be charged depending on your rate plan.)
This all works fine as long as you are within the coverage area of a WiFi or cellular network. But if you’re not, you could have real trouble. Here’s an example from my own experience. I was out on the Sonoma Coast with my iPhone, on a hiking trail well outside the range of a cellular network. I hadn’t used its GPS feature in a few days. When I turned it on, I waited and waited, and it eventually said it couldn’t find its position. I tried again and again, with no success. I finally gave up and drove to the nearby town of Bodega Bay, where I tried again. This time it had no trouble finding its position. Later, I went back to the same trail, and to my surprise this time the iPhone again had no trouble finding its position.
Here’s the explanation. When I first turned on the iPhone out on the trail, the ephemeris data was invalid because it was several days old. But since I was not in range of any kind of network, the iPhone had no way to get updated information, so it gave up. When I drove to Bodega Bay, I was again inside AT&T’s coverage area, so the iPhone downloaded the new ephemeris data and easily found its position. Then, when I went back to the trail, it already had current ephemeris data so it didn’t need to connect to the network to find its position.
The moral of this story is that if you’re going to use your GPS-enabled smartphone in the wilderness, make sure you give it a chance to acquire valid almanac and ephemeris data from a network before you head out. I haven’t done enough experimentation to know whether the iPhone will keep the ephemeris data updated from the satellites once it has found its position. This would have implications if you were planning to use it on a multi-day backpacking trip. I’d be interested in hearing from readers about their own experiences with GPS-enabled smartphones in the outdoors.
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