For outdoor adventurers who hike, fish, kayak, cross-country ski, or mountain bike in the backcountry, a GPS receiver can help them reach their destination & return safely — but only if they know how to use it! Here is the guide to getting the most out of a GPS receiver, from basic consumer advice to advanced techniques. It even includes fun solo & team games that utilize GPS. Starting with essential definitions & moving on to creating waypoints, & using your GPS with a computer, this succinct book teaches the basics of navigation & outdoor GPS use. Advanced techniques are covered, such as creating custom maps, & new technologies are discussed, including using GPS-enabled mobile phones, & how to use GPS with Google Earth & Google Maps. With years of experience as a GPS instructor, Hinch is well-versed in all aspects of navigation & GPS use, & he covers them in a jargon-free, easy-to-follow style.
The Quest to Improve Acquisition Time
The Achilles heel of GPS has always been how long it takes to find your position when your receiver hasn’t been recently used. If it hasn’t been turned on in a few days, it could take several minutes, & if it hasn’t been turned on in several months, it could take 15 minutes or more. Manufacturers are constantly striving to improve this performance, & while they can’t change the basic design of the GPS system, they have continued to improve their receivers. Three recent improvements are particularly noteworthy.
First, manufacturers continue to increase the number of parallel channels in their receivers. Until recently, 12-channel receivers were the norm. Today, it’s not unusual for receivers to have as many as 32 channels. Since there can be as many as 32 active GPS satellites at any time, you might think each channel is dedicated to a specific satellite, but this isn’t the best way to use all 32 channels. Instead, several channels may be devoted to tracking a single satellite at different frequencies. The satellites are in constant motion either toward or away from you, & this causes their signals to experience a slight change in frequency known as a Doppler shift. The amount of the shift depends on the apparent speed & direction of the satellite at any given moment. By devoting parallel channels to a single satellite, your receiver can search simultaneously over several frequencies to make acquisition faster.
To overcome the delay associated with downloading a full almanac, many receivers now come with a default almanac stored permanently in memory. That way, even if the current almanac gets out of date, the receiver can usually still get a position fix without waiting for a whole new almanac to be downloaded. Another improvement is the use of a technology called “Long Term Orbits” to predict satellite ephemeris data up to 7 days in advance. This technology was first developed for GPS-enabled smartphones, where the data is stored on the cellular network & accessed by the phone whenever necessary. A similar technology is now starting to find its way into dedicated GPS receivers, although it isn’t exactly the same because a dedicated receiver doesn’t include a phone that can call up a cellular network to get the data. Manufacturers don’t publicize how their software actually works, but you can usually identify such receivers because they are advertised as being able to find their positions in less than a minute under all conditions.