cture. At least, that's the promise. We evaluate two PC Card solutions to compare the promise to reality.
A GPS receiver provides position information (i.e., latitude, longitude, and altitude) by repeatedly making comparisons on the arrival times of digitally tagged radio signals from four (out of 24) GPS satellites. Successive readings allow easy calculation of travel speed. Small hand-held receivers are useful for hikers, boaters, pilots, and forest rangers. The same electronics go into highway navigation systems. These were originally for truckers and emergency services, but they are now appearing in cars.
Trimble vs. Rockwell
Trimble Navigation's Mobile GPS PC Card 110 resembles a modem card with a tiny coaxial-cable socket. On one end of the 2-meter cable is a mating coaxial plug; on the other end is an antenna module the size of a hockey puck with a magnetic mount that clearly identifies the target market as cars.
Rockwell Semicond
uctor Systems' NavCard is 2 inches longer than a normal PC Card, with an antenna connector on the center of the protruding section. A small, squarish, black plastic antenna clips onto this protrusion. An optional remote kit has a coaxial plug on one end; a combination coaxial socket/magnetic mount is on the other end.
You unclip the antenna from the NavCard's protruding section and snap it onto the socket/mount. The end result is the same as with Trimble's product, but Rockwell's mounted antenna is smaller and neater.
The
two cards are marketed
differently. Trimble sells the Mobile GPS PC Card 110 as a consumer product, complete with its WinMobile software. Rockwell sells the NavCard primarily as an OEM product. Companies such as Liikkuva Systems International (Cameron Park, CA) place their own label on the card and sell it with value-added software as a complete package. Some dealers sell the NavCard in single-unit quantities, and applications software to use with it is
available.
Not Quite Plug-and-Plot
We used each card in a Hewlett-Packard OmniBook 430, a 486SX/25 with 6 MB of RAM. We also tested the cards in a Toshiba Portég7eacute; P90 with 8 MB of RAM. We tested using Windows 3.1, though both products work with Windows 95.
One or 2 W may not seem like much power. However, it represents a significant drain on the OmniBook's battery. With either GPS card inserted and running, the OmniBook's normal 2-hour battery life drops to little more than half an hour. Similarly, the Portégé's 4-hour battery life is cut to approximately 3 hours. Extended road use requires some form of power adapter. Rockwell has a new version, called the NavCard LP, that consumes only 0.6 W.
We had serious problems swapping the Trimble card in and out of the OmniBook, which has no true on/off switch and just powers down, ready to resume work in its previous state. Removing or inserting the Trimble card would often initiate a reboot. Wors
e, the OmniBook would then forget it had a PC Card hard disk, and boot from its default ROM. We had to temporarily remove and reinsert the hard disk card. We didn't have this problem with the NavCard or with a PC Card modem.
Look! Up in the Sky! It's a Satellite!
The 24 GPS satellites are constantly changing their position in the sky. Thus, a GPS receiver will obtain an initial fix most quickly after power-up if it knows its approximate location and time of day, and if it has a recent almanac table stored in its memory. It then knows which of the 24 possible GPS signals to search for first. Otherwise, a cold start just runs through all the possible satellite numbers or signal codes in succession, and it's potluck when it will chance upon a signal. In practice, this initialization process can take up to 45 minutes.
The two GPS cards were roughly comparable in initial (i.e., warm-start) acquisition. Both use the same algorithm. In the almanac, they look up the satellite that's
highest in the sky and have all receiving channels try to acquire that satellite. After some time, if that one isn't found, they try the next lower satellite (typically, there are five to eight in the should-be-available list).
Although the Rockwell card's five channels will, on average, find a signal more quickly than the Trimble card's three channels, this difference is small compared with the time lost if, for some reason, that particular signal is unavailable because it's blocked by a tree or a building. Once any channel finds a satellite signal, it locks onto it and the remaining channels hunt in parallel for other signals.
The NavCard's five channels appear to give it a slight edge in performance while on the move. When some GPS signals are temporarily blocked, it helps to be tracking as many satellites as possible and to be on the lookout for others that the almanac says should be available but aren't at the moment.
Multiplexing lets the Trimble receiver track up to eight signals
in its three hardware channels, while the Rockwell receiver can track up to nine. But multiplexing involves some performance loss in both sensitivity and acquisition time. With only three channels, the Trimble receiver must always multiplex, whereas the Rockwell receiver can track four satellites and use the fifth channel in a multiplexing mode to scan for others.
In typical situations, the Rockwell receiver reported two to four GPS signals (two when heavy foliage caused brief outages), with a few short bursts of five. When turning into a residential street, we found that one or two satellites could be obscured and would drop out, but they were almost instantly replaced by others, with no net loss of vehicle tracking. In static tests with the antenna indoors and only part of the sky visible, the Rockwell NavCard would usually track four GPS satellites while the Trimble card struggled to hold onto three.
Good search and tracking algorithms can compensate for some of these factors, and Trimble's W
inMobile is particularly clever in balancing different settings. For example, there are Heavy Foliage and Urban Canyon settings, as well as choices about whether the user is fixed or moving slowly or quickly. No such control seems available in the software we had for the Rockwell card. Still, we believe most users prefer not to fiddle with settings, and the Rockwell defaults work well.
37 degrees 55.382' N, 83 degrees 11.565' W: Huh?
To simplify integration, both manufacturers make their cards behave like serial interfaces to an external GPS receiver. The computer sends commands to, and receives data from, the receiver in a serial bit stream. The data includes what satellites are being received, signal quality, and position information. The computer can display this on-screen and/or log it to disk for later playback or processing.
This isn't a lot of functionality, but it lets you record property boundaries, for example, or the location of a secret treasure trove or favored
fishing spot. (But see the sidebar
"How Accurate is GPS?"
.)
Rockwell doesn't supply user software directly, but a complete set of technical development and testing software is available from distributors and other sources. We used the DOS program Labmon and a rudimentary program written to illustrate use of the APIs. While Labmon isn't intended for nontechnical, real-life use, it's a powerful, complete program that lets you control all aspects of receiver operation. However, neither WinMobile nor Labmon is much use on the road, so most GPS users will want to develop or purchase additional software. Both Rockwell and Trimble sell software development kits.
Microsoft owns NextBase, a U.K. company that produces Automap (called AutoRoute in the U.K. and Europe) software. We used the CD-ROM version of AutoRoute Pro, which can be linked to a variety of GPS receivers. Curiously, the U.K. version has options for both Trimble and Rockwell cards, while the U.S. versions (downloadable from Microso
ft's World Wide Web or FTP sites) have drivers for Sony, Rockwell, and some other units, but not for the Trimble card. You'll also need the manufacturer's proprietary Windows DLL. Trimble's TGPSDLL.DLL ships with the PC Card bundle, while Rockwell's NAVCORE.DLL is available on CompuServe.
The two cards work somewhat differently with the three applications. Both Automap (or AutoRoute) and the NextBase GPS Windows application program must be opened. With a Trimble card, the application program automatically opens WinMobile, with all the latter's controls available. The
screen shows all three
of the applications open. For the NavCard, an invisible binary-mode reader communicates with the GPS card, so only two windows appear. In our tests, the OmniBook didn't have enough memory to run AutoRoute and either of the GPS card interface programs simultaneously.
Other mapping and position-display programs are available for use with these two GPS receivers. City Streets, from Road Sch
olar Software (Houston, TX), and MapExpert, from DeLorme Mapping (Freeport, ME), are two well-known products. These mapping programs are quite resource-intensive, and running them simultaneously with a GPS tracking program probably requires at least 8 MB of RAM and a 486DX processor.
Take Your Pick
The bottom line? If you're going to use commercial mapping software and can find the needed software drivers, and if the way the card sticks out from the PC Card socket won't be a problem for you, the NavCard's superior performance and lower cost make it a good choice. (Alternatively, Liikkuva's bundled package offers a complete, ready-to-run solution.) In principle, combining Automap Pro and the Trimble bundle should offer the same functionality, but Microsoft doesn't seem to offer the necessary Trimble driver. However, the Mobile GPS PC Card 110 is available with other software from a variety of sources.
In the end, both of these units offer roughly equivalent results. If you wa
nt to know where in the world you are, there's nothing like a GPS receiver.
Mobile GPS
Product PC Card 110 NavCard
Manufacturer
Trimble Rockwell Semi-
Navigation conductor Systems
Sunnyvale, CA Newport Beach, CA
(800) 827-8000 (800) 854-8099
(408) 481-7808 (714) 833-4600
fax (408) 481-7781 fax (714) 833-4078
http://www.nb.rockwell.com/
Price
$595, with software for $495-$595, hardware
Microsoft map systems; only; $795, Liikkuva
$649, with so
ftware for unit with software
Road Scholar's
City Streets
Number of
3 5
receiver channels
Number of
8 9
satellites tracked
Card size
Type II; 86x54x5 mm Type II extended;
137x54x5 mm
Power consumption
0.75-1 W
<
2 W;
<
0.75 W average
Antenna
Hockey puck size 48x57x22 mm, attaches
and shape, 65 mm directly to card;
diagonal by 23 mm; optional 6-foot cable
2-meter cable, magnetic and magnetic mount
mount
Included software
WinMobile (Windows 3.x); None
Mobile.exe (DOS)
Other software
Developer's kit Developer's kit;
available
Labmon; downloads
from CompuServe's
sailing forum
Automap/AutoRoute
Yes Yes
link capability
Acquisition times
<
30 seconds/15-45 20-30 seconds/10-30
(initialized/cold
minutes minutes
start)
Inquiry number
1056 1057
photo_link (49 Kbytes)
At the
left,
attached to its round antenna, is Trimble's Mobile GPS PC Card 110.
Next to it
is Rockwell's NavCard, with the optional remote antenna mount below it.
screen_link (44 Kbytes)
The NextBase GPS application, Trimble's WinMobile, and NextBase's AutoRoute Pro are all active. Note the different numerical formats used for position displays. The red stickpin shows the position on a map of London. Weak GPS signals are due to using the unit indoors through a coated window.
George V. Kinal is with Inmarsat, an international consortium in London that pr
ovides satellite-based mobile communications and positioning services. You can reach him on the Internet at
GKinal@nyx.cs.du.edu
or on BIX c/o "editors."
Where on Earth Am I?
