The $3,000 Club

Today you can walk into any marine retail superstore, plunk down a few hundred bucks, and walk out with a GPS receiver that fits in your shirt pocket. Some of the fixed units sell for just a few hundred more. So when you consider the fact that three GPS manufacturers--Trimble, Northstar, and Leica--are still selling fixed units with list prices in excess of $3,000 (a common price circa 1989!), you've got to wonder how these units can command that kind of scratch. All GPS receivers use the same satellites, right? All of them give you lat/lon, speed/course over ground, and cross-track error, right? They all store waypoints and can string them together in routes, right? So why spend all the extra dough? We brought three of these receivers together and got the answers during a two-day, head-to-head performance shootout.

Before we get into the details, consider the evolution of the marine GPS receiver. Initially it was a clunky, fixed unit--somewhat confusing to operate but still better than the older lorans. Then came the handheld units--the first ones were about the size of a small brick and were limited to four-line alphanumeric displays. But as time went on, displays improved, graphics took over, and sizes, as well as prices, came down to earth.

But there was still the issue of S/A, that annoying error the Federal Government introduced into GPS signals. If you didn't have a special military-type receiver that was immune to S/A, your signals were semi-scrambled, and position accuracy was only good to +/- 100 meters, though my testing showed it was usually better, more like +/- 30 meters. But still the error remained until Differential GPS (DGPS) arrived on the scene.

Differential GPS was introduced to eliminate S/A errors in the United States. It works by sending radio signals with error-correcting messages out to GPS receivers from land-based reference stations, and by counter-acting the effects of S/A, DGPS improves position accuracy to +/- 10 meters. Reception range is limited to about 150 miles offshore. But to get a differential signal, you had to have GPS receiver that could accept it (so-called "differential ready"), and a separate black box (the "beacon receiver") and antenna to receive it. That was, until today.

The three receivers in this test are not differential "ready." They are Differential GPS receivers, which means they not only have the ability to handle the DGPS signal, but the beacon receiver is built-in. There's your first distinction that separates these units from the lower-priced crowd. In addition, all three units come with, or have as an option, a combination GPS/DGPS Beacon antenna, so you only need to install a single antenna to get DGPS accuracy. With a minimum of six and a maximum of 12 channels, they can track up to 12 satellites simultaneously, and two of the three offer electronic charting either builtin (the Northstar 951XD) or with an extenal chart-card reader (the Trimble NT 200 DGPS). All offer extensive graphics displays, a wide variety of alarms, and in the case of Northstar and the Leica MX 400 Professional, additional non-GPS data like tide predictors and sun/moon almanacs. And all of them are extremely sensitive and highly accurate. But the question is, how accurate, and furthermore, is there an accuracy difference between one DGPS and another?

The Proving Grounds

To find out, I took all three units to a First-Order National Geodetic Survey land monument in Margate, Florida. It's coordinates are known and published to the nearest 3/4-inch! I took ten samples of each unit's lat/lon position and speed (if any) over a one-hour period, averaged them, and then compared them to the monument's exact coordinates. All three units tuned automatically to the differential beacon tower in Miami, Florida, and the results appear at the bottom of the accompanying table.

Next, I spent several hours with each unit out on the water, evaluating the units from your perpsective. How is the screen display? Is the unit easy to use? How hard is it to enter and recall waypoints? Is it easy to create a route? How much memory does each unit have? How responsive is the receiver to speed and course changes? These questions are answered in the results below.

Results--General

As a group, these DGPS units lived up to the government's accuracy claims of +/- 10 meters (33 feet). In fact, the Trimble's position varied just four feet during my sampling period, and all units’ static positions on average were closer than seven feet to the mark! If that can't get you through the channel at night, maybe it's time to take up golf. Sitting still, all units showed speeds of less than 0.2 knots (non-DGPS speeds can vary by as much as 2.0 knots). So as expected, accuracy is not a variable you can use to choose one unit over another.

Results--Specific

What you can use to pick a unit is its ease of use, screen visibility, and operating procedures. Here's how they each fared.

Trimble NT 200 DGPS. The NT 200 DGPS I tested is the third version of this unit, and a ninth-generation receiver from Trimble Navigation. As such, Trimble has built-in much of its experience into the NT 200 DGPS. The unit has a good-sized monochrome display screen, backlit keypad, and six dedicated menu keys with five multipurpose soft-keys for menu-driven operation. While both its nighttime and daytime visibilities are fine, the screen itself is incorrectly polarized, so you cannot read it while wearing polarized sunglasses. Buddy Morgan, Trimble's southeast sales manager, says units with new, correctly polarized screens should ship by the end of March, 1997.

The NT 200 has two "Nav" screens, toggled back and forth with the NAV key. The first shows Trimble's oft-copied 3-D steering "highway" steering with user-selectable data like COG/SOG/XTE and heading, and the other screen shows a steering compass to your next waypoint and cross-track indicator along the bottom. Both displays are useful. The unit also offers three user-customizable nav data screens, capable of showing up to four lines of information. While this is a good feature, surprisingly the unit does not have a dedicated lat/lon position key. To see your lat/lon, you have to select it from a menu and add it into one of the user screens, along with whatever other data you wish. The good news is you only have to do that once, then push the USER key to access it. And since you'll probably be using this unit as an electronic chart display, you'll rarely need the actual lat/lon numbers, anyway.

You can enter waypoints either from the cursor's position on the chart display or by pressing the WAYPT key. This brings up a waypoint library and you can manually type in the coordinates and assign it on of six symbols. The NT 200 gives you several ways to recall the waypoint using various search criteria, one of Trimble's stong suits. Yet, use of the waypoint screen is not intuitive. The unit has a unique >> key used to move the cursor around on some displays, but when entering/recalling routes and waypoints, cursor movement is often restricted and the process can be frustrating. Operation is not intuitive, but the manual is well-written, and I especially appreciated the SAVE key that automatically stores your present position in the next available waypoint slot.

Performance-wise, average time to redraw a chart onscreen was about 10 seconds--moderately slow. But on the other hand, the NT 200 was the fastest both to acquire satellites, lock-on to a position, and to update speed and course data. I also appreciated the fact that if you add another control head as a second station, waypoint/route data will be automatically transferred between the two units.

Northstar 951XD

Like the Trimble, the Northstar--with its easy-to-read, high-contrast LCD display--has six primary-function keys and five softkeys along the side of the screen for making menu choices. The POSITION key is