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Exploring GPS

DN Staff

September 22, 2003

12 Min Read
Exploring GPS

Many parents use pagers and cell phones to keep track of their kids. But who would make a kid carry a GPS receiver just to know where they are at all times? Many parents do just that, using a watch-like Personal Locator from Wherify Wireless. Today's small GPS receivers make such devices possible. To track youngsters, an adult logs into the Wherify website and "pings" the watch, which replies with its location. But instead of getting longitude and latitude information, the site superimposes the watch's location on a street map or on a high-resolution satellite image that shows buildings and neighborhoods. A kid-proof lock keeps the watch where it belongs so it can't get left behind at a friend's house while the kids scoot to the local arcade.

The point is today's GPS receivers bear little resemblance to the older bulky devices used in vehicles. In fact, manufacturers make GPS receivers available in everything from chips, to modules, to pocket-size, all-in-one devices. And those don't include the standard devices sold to consumers.

For most applications, the modules offer a nice tradeoff between ease of use, size and cost, and they easily drop into new designs. GPS modules provide an easy way to add position, speed, or timing capabilities to a product. Many manufacturers offer these modules, so users can choose from a spectrum of features.

Vendors also sell development kits that let designers quickly set up a GPS receiver. To get started in GPS, development kits include a GPS module, an antenna, software, cables, and any needed power connections. A typical module communicates position and time information as ASCII characters sent to a computer's serial port. Unless you work with a group of talented engineers who have experience with GHz-frequency circuits, always start with a kit rather than a basic module. To make sure a kit operates properly, vendors include the shielding and power conditioning needed to guarantee success.

"A development kit becomes a golden standard," says Joel Avey, director of marketing, Trimble Navigation." It runs right out of the box, so later you can disassemble it and use the components in a prototype. Then if something in the prototype doesn't work properly, you can rebuild the original kit and check out the GPS components. This lets you see what differs between the kit and the prototype. You also can use the kit as a test bed for new software, different antennas, and so on." Prices for kits start at several hundred dollars and go up to several thousand, depending on the features and capabilities of the module they include. Module prices can go as low as $30 each in OEM quantities.

Standardized Sentences

Data comes out of a GPS module in a format standardized by the National Marine Electronics Association in its NMEA-0183 document. A typical module produces 8-bit ASCII characters at 4,800 bps, so even a dumb terminal can display a GPS module's serial-port data. But don't expect to see a module's messages directly print longitude and latitude. Instead, a module produces information in standardized "sentences," such as this one below, which carries timing data: $GPZDA,161530.5,09,7,2003,,*93&CR>&LF>

By following the standard, programmers can write code to extract the time and date from the string above: 1615 hours, 30.5 seconds, July 9, 2003.

Each manufacturer documents the sentences and data formats its receivers produce. If you plan to use a GPS module to its fullest, get the complete NMEA-0183 document.

In addition to the NMEA-formatted information, manufacturers often provide a second serial port that puts out "raw" GPS information engineers can process as they wish. But this information-pseudo range, phase data, receiver time, and so on-varies from manufacturer to manufacturer. Trimble Navigation, for example, lets users choose either a binary data stream or a proprietary ASCII stream on modules that include a second serial port.

Precise Timing

Although many GPS applications require only position information, GPS satellites also provide time data, accurate to within a few nanoseconds. Many cell-phone services, for example, use GPS timing information to synchronize and interleave transmit and receive operations and to generate call-duration bills. But some timing applications involve unusual measurements.

Pop-Up: When it pops to th esurface, this plankton mimic will get a position fix from GPS satellites and then descend back to its operating depth. The unit saves position daa so researches can track plankton paths in the ocean.

The Met Office in the UK's Ministry of Defence uses precise GPS timing to help it track lightning strikes. Seven monitoring stations, located from Iceland and Finland to Gibraltar and Cypress, detect the unique radio signals produced by lightning. By calculating the arrival-time differences between the signals recorded at the seven stations, and at a central UK station, the Met Office can locate lightning activity. Accuracy ranges from 5 km in the UK to about 100 km at a distance of 8,000 to 10,000 km from the network of stations. Although each receiver relies on a local atomic clock for accurate time data, all clocks synchronize to GPS time every 10 minutes.

Accurate timing occurs only at a fixed position. A fixed timing receiver will gather information over, say, 24 hours, during which it hones its position information and determines satellite timing with great accuracy. But you can't get such precise timing from a mobile GPS receiver. Most GPS modules do end up moving from place to place, often in unusual environments.

Some people want to know where a device has been more than where it is. Tom Wolcott, a professor at North Carolina State University, monitors plankton flow in oceans using a "plankton mimic;" essentially a drifting bottle that moves with the plankton. In its latest version, the mimic-an evacuated fire-extinguisher bottle filled with electronics-will monitor sea conditions at the plankton's drifting depth. (The fire-extinguisher bottle can descend to about 100m.) Every few hours the bottle will swim to the surface, get a "fix" from GPS satellites, and quickly descend back to its drift depth. Early models used an ultrasonic "pinger," but Wolcott and his team had to constantly follow and monitor these devices. The GPS modules will let the mimics operate on their own for long periods. Submerging a GPS antenna shouldn't pose a problem: Wolcott plans to coat the mimic's antenna with wax to resist the effects of salt water.

Antenna Basics

Because antennas "gather" weak satellite signals, they deserve special attention. Trimble Navigation's Joel Avey suggests engineers use an active antenna-one that incorporates a low-noise amplifier (LNA) right in the antenna. Such an antenna improves the signal-to-noise ratio of a GPS system, and it boosts the weak signals a receiver must process. Doug Baker, an applications engineer at Navman says designers need to properly feed power to LNAs. The power travels to the antenna through the same conductor that carries GPS signals to a receiver. "Engineers need to ensure they have a low-noise power source, not just the output from a noisy switching power supply," says Baker. He also notes power connected to an LNA through a GPS module should include some form of current limiting. Without it, a short circuit across the antenna's co-axial cable may destroy the GPS module.

Module vendors and third parties also offer passive antennas-those without a built-in LNA-and they offer antennas packaged and ready to use, or unpackaged for designers to incorporate within a product. The packaged antennas offer an advantage-the manufacturer knows they'll work. Unpackaged antennas rely on the designers' expertise to position and shield them properly.

Avey of Trimble Navigation says that no matter what antenna engineers choose, they should make sure it gets oriented toward satellites. He has seen at least one application in which designers installed an antenna upside down. "Antennas should include a good ground plane. And a shield between the antenna and the receiver will help prevent a receiver from jamming itself."

Tim McCarthy, director of GPS products at Motorola cautions against buying inexpensive antennas. After all, a receiver isn't any good if you can't get a good signal into it. In his opinion, the hollow-tube helix antennas work better than flat patch antennas. He also cautions engineers to test antennas in various orientations because there's no guarantee end users will follow instructions.

Navman's Baker says, "A GPS antenna should match the characteristics of the GPS receiver it connects to. You don't overpower a receiver with too much signal, nor 'starve' it with a low-gain antenna. Good RF engineering techniques are a must."

Up, Up, and Away

Although GPS receivers provide precise position data, not all applications require that sort of information. To gather weather data, for example, a hydrogen- or helium-filled balloon lifts a Vaisala radiosonde to an altitude of about 20 miles. During its ascent, the sonde logs barometric pressure, relative humidity, and air temperature, and it transmits these measurements to a ground station about once a second. Sondes equipped with a GPS module measure only the Doppler shift of GPS satellites' signals to get a velocity measurement. A full-blown GPS module would cost too much for an expendable sonde, which costs a few hundred dollars. Coupled with time measurements, the velocity data produces a position track for the sonde. Because the sonde starts its flight at a known location, it's easy to trace its travel. (Velocity data includes speed and bearing.)

Doppler-shift information has Earth-bound applications, too. Many auto manufacturers rely on a Racelogic VBOX to log vehicle-performance data. Like a GPS-equipped radiosonde, the unit uses the Doppler shift in the signals from four satellites to triangulate and measure velocity. Integrating the velocity data yields the distances driven. But, unlike a radiosonde, the VBOX does rely on standard GPS information for latitude and longitude, and it can supply heading, height, lateral acceleration, longitudinal acceleration, vertical velocity, and radius of a turn, updated 20 times/sec. Unlike earlier optical-, microwave-, and radar-based tracking and timing systems, the VBOX requires no calibration, and a novice can set it up in about three minutes.

Block Interference

Engineers who want to use a GPS module can get lulled into thinking all they need is power, an antenna, and a nearby computer to get useful data. That setup may work on a lab bench, but not in a real-world setting. Motorola's Tim McCarthy compares the power in a GPS signal to standing in Chicago and observing a 100-W light bulb in Japan. So to operate properly, GPS receivers require as quiet an electrical environment as possible. Even a slight bit of interference or attenuation can prevent a GPS receiver from finding satellite signals. Thus, engineers who want to include GPS capabilities in a product must pay careful attention to the design of low-noise circuits that surround a GPS module. And they must keep any nearby radio-frequency (RF) signals to a minimum.

Brad Wiseman, the OEM sales manager at Gramin, recently observed a CD player shut down a nearby GPS receiver. The faulty player swamped the receiver with high-order harmonic RF energy. Wiseman cautions that GPS systems won't work in every location. Some locations suffer from extraneous RF radiation that inadvertently-but effectively-blocks GPS signals.

"To get good results, you have to invest in good RF engineering," says Wiseman.

Motorola's Tim McCarthy says engineers who plan to place other RF equipment, such as a cell phone or pager, in a system with a GPS receiver must ensure transmitters don't interfere with the GPS receiver. He suggests system designers try to avoid receiving GPS signals when another circuit transmits. Baker of Navman agrees. He recently saw a design in which an RF-modem's transmitter signal passed right next to a GPS receiver-with ill effect. "Poor RF-signal routing and noisy power sources are the two problems we see most often," says Baker.

Cameron Henderson, an application engineer with NovAtel says people need to consider where they plan to use a GPS receiver. In one case, racetrack banking tilted cars' antennas away from satellite signals at several places. The cars couldn't "see" satellites. At other times, metal fences and barriers reflected satellite signals and caused cars' receivers to see satellites in the wrong places, which produced incorrect locations. Eventually engineers overcame these problems, but they're typical of what people run into. Henderson adds, "GPS receivers rely on line-of-sight communications, so anything that blocks a signal degrades or blocks satellite signals." He also notes that due to the inclination of the GPS satellite orbits, coverage beyond about 80 degrees north and south latitude is spotty. The Russian GLONASS (GLObal NAvigation Satellite System) satellites orbit the Earth at higher inclinations and offer better coverage over far northern and southern latitudes. But you'll need a GLONASS receiver to acquire their signals.

Joel Avey at Trimble Navigation stresses that engineers often think a GPS receiver will work in every location and under every condition. But it doesn't. Avey says, "With a clear view of the sky, a GPS receiver should work more than 90% of the time. Will it track you in a shopping mall or as you drive through a tunnel? Maybe, but you can't count on it. So if you need a GPS module that will operate deep in a multi-story parking garage or in a mine, talk with vendors to learn how they can help with an application."

Vendors have experience using GPS modules in many situations and can offer suggestions and application information for specific problems. Because vendors adjust GPS modules to cover "average" operating conditions, they have the capability to adjust designs or parameters for unusual operating conditions. Module buyers should always ask the actual operating conditions under which data-sheet specifications such as resolutions, operating speeds, and times to acquire satellites, actually apply. Don't expect "data-sheet" performance-if any-sitting in a subway tunnel.

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