Choosing the Right GPS Receiver

One of the most often asked questions that I receive from a new prospective gps user is: "What unit would you recommend and why?" I generally refuse to recommend any particular unit since there are many fine units on the market and what you would like or need may be very different from my needs. What I would rather do is to teach you how to make a good evaluation so that you can select a good unit the first time you try.

The Basics

A gps receiver is a unit that computes your location using triangulation techniques. It gets information from 3 or more satellites and computes a position and as part of that solution it can also derive speed and direction data. (For more details in exactly how this is accomplished click here.) It can save a certain amount of the data that it collects and/or computes for future use. Finally it offers a display that presents this data to the user. (Some units use a pc or a palm as the display device.) The computation is based on a set of moving satellites that circle the world twice a day so it can be used anywhere on the planet and up to 60,000 feet above the planet, but just a thin film of water blocks all signals so it can't be used under the water. The method of display and the kind of data it saves differs from unit to unit and is one of the things that separates units.

The user display often mimics devices that the user may already be familiar with which could fool a new user into thinking those devices are in the unit. For example a speedometer display matching the one on your car or a compass display that looks like a standard compass. Neither device is actually in a gps receiver but are computed based on your current position and doppler data available as part of that solution. So if you are moving the unit can look like it has these devices. It will also compute an altitude which causes some folks to wonder if there is an altimeter inside. Again this is a computed output and the unit does not have a built in altimeter. By the way, due to the satellite position the altimeter reading isn't very good and can be up to 30 meters off and swinging wildly. Both a compass and an altimeter are good backup devices for a gps when hiking.

Having said that, there is no reason why a unit couldn't have these extra sensors inside and indeed the Silva gps and some Garmin models do have a compass built in (and even an alitmeter) as do some survey grade receivers but in general inexpensive gps devices do not. A gps usually does have a built-in clock to help with its initial startup but after a position is computed the display of the time is based on data that comes from the satellites and not from the built-in clock. Using the satellite data for a clock provides you with a very accurate clock. Note that the behavior of these extra built-ins are different from the simulated displays in a standard gps. For example the altimeter does not need a fix to work but may be calibrated using a gps fix later. The compass can be used while the unit is stationary, unlike the simulated compass display, and even know which direction you are holding the unit in while the computed compass knows the direction the entire unit is moving in. Often these are entirely different things.

The Features

At one time all gps units attempted to cater to everyone. As units progressed and needs become more focus manufactures have begun to produce units that are specific to an application. While there are still universal units in the market place there are more and more specialized units available. For this reason you really need to understand exactly what you intend to do with a gps before you buy one. By the same token once you have a unit you will probably find uses for it that you didn't consider earlier so you will want a few features that are beyond the basic things you can currently think of. You may want to spend some time reading my short article on what can you do with a gps. You may also want to read Joe's and Jack's articles on what you really need in a gps based on application. They have What features do I need for hiking? and What features do I need for driving? If you need more technical background then I suggest Joe's site, Peter's site, and if you have a Macintosh then Karen's site. Each of those sites has references to other sites with specific data that you may find valuable.

A GPS receiver represents a quantum leap in technology over most items that you may have purchased. You need ot spend some time understanding the technology behind the gps system in order to determine if it will do what you need and to help you understand what to look for in your purchase.

First some recommendations of my own. Unless price is really an important consideration go for a 12 channel parallel unit. If you want the receiver to work without waiting around for 2 or more minutes with a clear view of the sky or to use it in applications where there is not always a clear view of the sky then get a 12 channel unit. Next, if you own a computer, then get one with a computer interface. Finally I would recommend a unit that can work standalone unless your application always needs to have a computer along. A standalone unit can work with a computer but a unit without a display won't work without one.

Here are a few things to help with your evaluation: I think the best thing you can do is to spend some time working with the proposed unit in your application. Or if that is impossible the see if you can try it out in the store. Familiarize yourself with the interface and decide which is more comfortable for you.

The questions

Questions that you may want to ask. To get answers to these questions you can ask the salesman, but you may not be able to trust the answer unless you get one that understands gps receivers. A better route is to go to the manufacturer's web site and look up the data. They often have manuals available on-line so you can download and read about the unit you are considering. If you have friends with gps receivers that talk to them but learn a little first so you can speak the same language. Many gps users claim that the unit they decided on is the best. This may be true, but filter out the reasons and make sure that it is the best for what you need.

There are also some questions that you don't really need to ask:

Your applications

Be sure and align your expectations with what a gps receiver can really deliver. It cannot reliably deposit you at your front door but can locate your yard. It will not replace a compass in all applications but can if you are driving in your car. It could replace your speedometer on your car but may not be as accurate at low speeds like hiking or on your sailboat. Newer units do better in this regard.

Typical applications for gps include automotive, RVing, hiking, biking, aviation, motorcycles, and marine. Other categories include skiing, snowmobiling, and off-road driving. You need to decide how many of these you intend to use the unit for and what is really important. Many units have been specifically targeted at customers in specific categories. If you have specific mounting requirements then check into this as well.

A navigation receiver is not a surveying device but it is likely more accurate than most of the digital maps you use with it anyway. For more accuracy you can add a separate beacon receiver to supply correction data to the gps (some receivers have a beacon receiver built in but it still needs a separate antenna) or you might buy a unit that has WAAS capability. WAAS can provide corrections similar to a beacon receiver and uses the same antenna as your gps receiver. It, however, is line of site and does not offer as good a reception as a beacon receiver in areas covered by beacon receiver. In addition WAAS depends on geo stationary satellites which may be fairly low in the sky depending on your latitude so they may be blocked by buildings, trees, or hills. Both solutions can get your accuracy down to 3 meters or so, which is still not survey accuracy.

Most of all, enjoy your search. Welcome to the world of gps.

1999/12/27 original release.
2000/1/1 revised to include more basic information.
2001/4/19 minor changes, added dgps paragraph.
2001/5/22 added discussion of real compasses and altimeters.

Still have questions? Post to news:sci.geo.satellite-nav or send them to me at: Dale DePriest