Handheld GPS Units

A handheld GPS (Global Positioning System) is a pocket-sized handheld navigational device that provides very precise positioning/location data by using 24-30 Navigation Satellite Timing and Ranging (NAVSTAR) satellites. These satellites were originally developed in 1973 by the United States Department of Defense for military purposes. These orbiting satellites complete one Earth orbit every 12 hours. They are positioned at an altitude that allows them to cover the same ground once every 23 hours and 56 minutes.

Each satellite carries an atomic clock,that transmits its position and a time signal, which a GPS receiver picks up. They are positioned so that at least four can be seen from any spot on the Earth. Just like your home TV satellite, the unit will work best when it can get an unobscured view of the sky. This means that a handheld GPS unit is receiving data from multiple satellites all it once and each one has a different perspective on the distance. This requires the GPS to perform mathematical calculations to resolve all of this input. This allows it to come up with a position fix to within 50-100 feet. Some of the factors that can influence the accuracy include weather conditions, buildings, tunnels and heavy foliage/tree cover. Newer technology has allowed GPS receivers to become more precise and more sensitive. I'm sure we'll see this improve even more over time.

Most GPS units also use Wide Area Augmentation System (WAAS). This system performs the same type of calculations I was just describing, but on a grander scale. This system is based on a network of 25 ground-reference stations. Each station receives GPS satellite signals, makes corrects for any errors, and then forwards these corrected signals to a Wide Area Master Station, which makes some additional calculations and uploads the newly fine-tuned corrected data to a satellite. The message is then broadcast and picked up by GPS receivers capable of reading WAAS signals and within the broadcast coverage area of the WAAS. All of these extra calculations can fix your position to less than 10 feet. This is a significant difference on a handheld GPS.

You're probably wondering what handheld GPS units are used for. They are great for common outdoor activities such as hiking, biking, camping, hunting, boating, or my new favorite past-time, geocaching.

As with almost any gadget, you're going to find models that have more bells and whistles than others. You're typically going to pay extra for those features. Why pay extra for features you will not need or use? When considering a purchase of a GPS handheld system, think about how you plan to use it. Some factors to consider and why they are important are:

• Will you be using it primarily for occasional hiking and backpacking? Geocaching? Boating? How rugged does it need to be to withstand how often you'll be using it?
• Will you be using it at or near water such as the case with boating, canoeing or even hiking? Most handheld units are waterproof. Make sure the one you choose is. The IPX-4 standard on a GPS unit means the unit will stand up to water splashed on it from any angle. An IPX-7 standard means it can withstand accidental dunks into water. They are not designed for swimming.
• How detailed do the maps need to be? Determine the map type and degree of accuracy you'll need. Some units come with the software, and some offer it as an option. Most handheld GPS units only come with base maps. Consider mapping vs. non-mapping. The units internal memory will determine how much map data you can download onto it. Any Garmin model with “X” on its name means it will accept SD memory cards for unlimited expansion).
• Twenty (20) route memory is standard. Some models have up to 50 routes. If the model has a memory-card slot you can add routes.
• Some units have a black and white screen and some have color. Personally, I wouldn't waste my time with black and white.
• Make sure the unit you're considering has a good battery life particularly if you'll be using for long periods at a time.
• If you'll be primarily using your unit in heavily wooded areas, or even areas with lots of tall buildings, make sure the unit accepts an external antenna. Consider a unit that uses a 12-channel parallel receiver system or better.
• Get a minimum of at least 500 user-entered waypoints and make sure the unit points the direction to the next waypoint.
• Consider the size and weight. This is a device you'll be carrying around with you. You want to make sure it's not too bulky or too heavy. Make sure you can read the screen. Some are smaller than others.