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Cover Story
November 2000
Know
where
The coming explosion in GPS applications.
Terrence P Lynch, Contributing Editor
"If you want to get where you're going, it helps to know where you are," explained the old sergeant in the early 1980s, as he handed this writer a 12-pound GPS kit. He said it would change my foot-soldier's life forever. He was right about the last part. Within a year, that 12-pound unit, added my other 60 or 80 pounds of "lightweight" gear, took a toll on my body and ended my Army career. The sergeant's first statement, then just a truism, is about to change the world of mobile electronics.
Knowing where you are is the gift of the Global Positioning System, a network of US military satellites that continually broadcast their identity, time, and orbital position. GPS receivers use that information to calculate their position, anywhere on or above the earth, with considerable accuracy (for details, see sidebar, "GPS: a primer").
What's more, that considerable accuracy is now better than ever before. For the first 20 years of its existence, a policy called "selective availability" (SA) limited GPS accuracy for non-military receivers to about ±100 meters. This understandable but noisome policy limited GPS's utility in most civilian applications.
But in May, responding to pressure from industry and acknowledging that several technological end runs had made the policy moot, the US government ended selective availability. Standard GPS receivers can now deliver location data accurate to 10 meters or less, opening the way to a host of new position-dependent applications and other devices as well. For example, the high-precision timing available from GPS (sans SA) provides a very inexpensive 20- to 30-nanosecond timing reference, which has benefits for spectrum communications.
“Five years from now, people will wonder why we didn’t have the concept of location intelligence built into all the databases.”
Kanwar Chadha, SiRF Technology |
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Meanwhile, 20 years of progress in electronics manufacturing has shrunk GPS receivers/processors to the point where they can be easily integrated into handheld or even wrist-worn devices. Coupled with similar advances in cell phones and the advent of the wireless Internet, GPS is set to become pervasive. This imminent GPS explosion will not only give users new tools to get where they're going, but also bring new meaning to the old adage that "it's all about location, location, location."
Happy coincidence
The end of selective availability comes at a time when the electronics industry was already gearing up to meet the challenge of another government decision: Phase II of the FCC's enhanced 911 (E911) rules. Under Phase I of the FCC rules, cellular-system operators had to be able to trace a 911 call from a cell phone using existing technology. In essence, it meant that operators had to provide emergency dispatchers receiving a 911 cell-phone call with the phone number of the phoning handset and the location of the cell site or base station handling the emergency call.
Phase II has broader technological implications. Under the current rules, system operators must be able to provide a more precise fix on the 911 caller. As of this writing (milestones have already been moved more than once), operators have until October 1, 2001 to begin providing Automatic Locator Identification (ALI) capabilities in user handsets. By December 1, 2001, 25 percent of all newly activated handsets must be ALI capable. Full penetration of ALI capabilities (defined as 95 percent of subscriber handsets) must be in place by the end of 2005.
To meet those requirements, the industry had looked at a variety of options including network-based sensing systems that would triangulate a phone's location using signal strength or other transmission parameters measured at multiple base stations. Although workable, network-based plans would be hard-pressed to deal with multipath signal interference in cities and the relative sparseness of cell sites in rural areas.
Alternatively, a handset-based ALI system could use GPS, even with selective availability in place. The system would query the handset's GPS remotely and receive position information in return via the cell phone. Chipmakers were already planning to expand production of GPS chips when the Pentagon announced the end of SA.
It was a happy coincidence for manufacturers and cellular-system operators alike. "The industry was lobbying hard for it, but the timing was sooner than we thought it would be," says Ashok Kamath, manager of international operations in the Software and Systems Technology Division of Analog Devices. For once, the industry may not get caught flat-footed as a new electronics market segment gets hot.
Expanded horizons
Applications for the newly unleashed GPS fall into three categories: convenience, safety and security, and information access.
Convenience comprises most of the technology's traditional navigation duties, but with a wider scope and with less cost. For example, in-car navigation systems have thus far been expensive options mostly limited to luxury cars. The reason was that inaccurate GPS signals had to be backed up by a series of dead-reckoning sensors such as gyroscopes and wheel-rotation counters. With improved GPS accuracy, much of this overhead goes away.
“The industry is now providing a GPS solution that is below 1 inch square in footprint with power consumption below 0.5 watts.”
Mario Liegghio, Conexant Systems |
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Safety and security applications range from ubiquitous E911 services to miniature tracking devices for people, pets, and machines. Insurance companies have instituted lower rates for shippers who affix GPS transponders to cargo containers, which can alleviate losses due to theft or misrouting. Similarly, a wearable GPS transponder could be used to keep track of parolees or dependents—children who walk to school or memory-impaired senior citizens.
Information-access services open up an entirely new playspace for GPS technology, says Kanwar Chadha, founder and vice president of marketing at technology manufacturer SiRF Technology. Location intelligence built into a Web-enabled device could act as a filter for incoming information. He imagines a GPS-equipped PDA with wireless Web access that could outfit travelers in a strange city with personalized directions to their hotels or specific ethnic restaurants, or information on sales in local shops. (The prospect of such location-based services has generated so much buzz that we've devoted an entire feature article to it; see "Pinpointing.")
"With the mobile Internet and devices with location intelligence, you can have a content and service provider who can build [location filtering] on the Internet infrastructure," Chadha says. "All these three elements will start developing in parallel instead of one waiting for the other—that's going to create much more momentum behind location types of applications."
“People don’t have to invest their time and effort in [designing] something as fundamental as a GPS anymore.”
Ashok Kamath, Analog Devices |
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GPS is entering some markets that are potentially very large. The cellular platform sells approximately 300 million units a year; forecasts say it may come close to 1 billion units a year in five years. The PDA platform is projected to grow from several million units today to tens of millions of units in two to three years. The automobile platform, currently 50-million-plus units worldwide, won't be growing very much. But GPS penetration in cars is close to zero today and is sure to grow as the cost of GPS systems for navigation and traffic management fall.
As for safety and security and information-access services, there's no telling what the eventual market will grow to, but SiRF's Chadha is highly optimistic: "Five years from now, people will wonder why we didn't have the concept of location intelligence built into all the databases."
Smooth work
The burgeoning of markets for GPS-enabled devices means that many engineers who've never worked with the technology will have to get acquainted with it quickly. Fortunately, chipmakers have considered the problems and taken steps to make GPS integration as simple as possible.
GPS technology has followed the same trends as other branches of the semiconductor industry, says Scott Cooper, platform marketing manager for the Wireless Terminals Business Unit of Texas Instruments. "Several years ago, you might have required the RF section of the GPS done completely with discrete components," he says. "Now you're able to squeeze many of those into a single [silicon] die. It's all silicon technology—both in the processing and the RF technologies."
 EMINENTLY EMBEDDABLE This GPS receiver, based on SiRF Technology’s SiRFstarIIe chip set, shows the progress manufacturers have made in shrinking GPS hardware down to size. |
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Typical GPS products comprise two chips: an RF (radio frequency) downconverter and a baseband-processing unit, which can also serve as a navigation processor. Peripherals include a clock to drive the RF and baseband chips and memory to store the almanac—information that helps the unit predict the orbital positions of the satellites (see sidebar, "GPS: A primer"). Manufacturers have made great strides in reducing both the size and power consumption of GPS chips, according to Mario Liegghio, senior product manager for GPS at Conexant Systems. "The industry is now providing a GPS solution that is below 1 inch square in footprint with power consumption below 0.5 watts, getting down to a 10th of a watt." Further, he explains, there are "snapshot" applications ("query it, it turns on, takes a position snapshot, tells you where it is, then shuts back down again") where "energy consumption is in the millijoule level."
Integration of GPS into a non-traditional application doesn't require any special mindset, says Analog Devices' Kamath. "Philosophically, you have to look at GPS as just another sensor. It's a chip. Put an antenna at one end, get outputs from the other." And the industry, he says, has standardized the outputs. "Position, time, and velocity come out in NMEA 0183 [format]. All the receivers, engines, put it out in that format." GPS chips from Analog Devices and others can supply raw GPS data for applications like some projected information-access services, as well. The point is, says Kamath, "people don't have to invest their time and effort in something as fundamental as a GPS anymore."
“It’s not a trivial exercise to integrate a radio into any application that’s going to use another radio inside the box.”
Scott Cooper, Texas Instruments |
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Other industry observers are less sanguine. TI's Cooper cautions that with devices like multi-channel cell phones, GPS integration can still require specialized expertise. "It's not a trivial exercise to integrate a radio into any application that's going to use another radio inside the box," he says. He imagines a tri-band phone with a Bluetooth radio and GPS—five radios in one phone—and points out that designers will have to be very careful to avoid mutual interference. "It's not magic," he says, "but there's going to have to be some serious applications work."
So engineers looking to add GPS to an existing or emerging design should find a partner, advises Jefferey Wilson, advanced technical marketing manager with the Automotive Business Unit at ST Microelectronics. "Our partner, Sigem, works on making modules of various sorts and embedding GPS functionality," he says. "The advantage to that is that there aren't too many people in these converging fields who know gigahertz RF design." The partnership, he says, "takes some of the pain out of the process for the system designer and allows them to implement GPS as a black-box function within their system."
Location knowledge is power. Although some may fear the Orwellian possibilities of ubiquitous personal tracking devices, others think GPS-enabled devices will make the world safer and less frustrating. The convergence of GPS with cellular phones and mobile information access, these optimists say, will permit devices that are more like digital companions—they won't let you get lost, they'll know where the good restaurants are, and they'll call for help when you're in trouble.
The debate between the two points of view will no doubt go on for some time. But it's clear that the emergence of GPS from behind the curtain of selective availability will soon bring on a host of opportunities for companies and designers willing to adapt the technology to new applications. That's where we are, and that's where we're going.
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New uses
The marriage of low-cost, low-power position calculators and wireless communications has spawned a host of ideas for new GPS-based products. Most of them revolve around the ability to query a device from afar, asking it to identify itself and divulge its location—without human intervention. New products include: A wristwatch “SOS” device for children, the infirm, or military personnel that would call for help at the push of a bu
ton. Defibrillators implanted in cardiac patients could trigger the device automatically.
A wearable remote locator for children, persons with diminished mental capacities, or paroled criminals.
Asset security systems, such as pollable tags for cargo containers, livestock, pets, or anything else that you want to
keep tabs on.
Asset-management tags that allow quick location of needed laboratory or emergency-room equipment.
Widespread vehicular telematics systems for toll-taking, navigation, emergency response, and traffic management.
Authentication systems for IT security, to prevent access to private networks from unauthorized sources or locations.
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GPS: A primer
In essence, a GPS satellite is just a radio transmitter and a highly precise atomic clock whizzing around the planet at a known speed and a known orbit.
In essence, a GPS receiver on earth monitors broadcasts from accessible satellites, notes their positions, computes their ranges based on the time lag between broadcast and reception, and uses all that information to calculate its own position by simple geometry.
In reality, it's a bit more complicated.
The Global Positioning System comprises 24 satellites, a network of ground-based tracking and control stations, and individual GPS receivers. Originally called NAVSTAR (navigation satellite for timing and ranging), the satellite constellation occupies six circular, equally spaced orbits 10,900 miles high. From any point on earth, as many as eight but no fewer than five satellites are "visible" at any time.
Satellite orbits are subject to perturbations by solar storms, non-uniform mass concentrations on the planet below, and other phenomena. Their atomic clocks are precise, but they may not be accurate—the orbital and rotational speed of the earth, and thus the length of the year and day (in other words, the time)—changes as the planet interacts with other celestial bodies. In addition, a host of other vagaries influence the system, including changes in signal propagation speed as conditions fluctuate in the ionosphere, as well as Doppler frequency shifts and relativistic effects as the satellites move toward or away from the receiver.
That's where the ground-based tracking-and-control network comes in. Armed with precise radar-tracking data on each satellite and accurate time information from celestial observations, the control network relays corrections ("ephemeris data") to each satellite's predicted position and propagation-behavior (its "almanac") every 30 seconds. The ephemeris data and almanac become part of the satellites' regular broadcasts and are used by GPS receivers to improve position calculations on the ground.
Each satellite broadcasts its orbital position and a time signal continuously in the L segment of the microwave radio band, specifically at L1 (1575.42 MHz) and L2 (1227.6 MHz). Twenty-four satellites operating on the same frequencies might be a bit confusing, but each modulates its broadcasts on L1 with a unique, 1 MHz-wide, pseudo-random noise (PRN) algorithm, which allows for identification. Ephemeris data overlay that signal in a 50-bit/sec stream. Transmitted in 1500-bit frames, complete ephemeris data sets take 30 seconds to send. Complete almanacs take 12.5 minutes. For civilian purposes, that's all you need to know—broadcasts on the L2 band provide additional precision-enhancing data available only to military receivers.
Most of the new GPS devices are "two-chip solutions," comprising a 12-channel RF section (a receiver capable of listening to 12 satellites simultaneously) and a baseband processor (a DSP for analyzing signals and position-data output). As chip design has grown in sophistication, the DSP sections now routinely incorporate memory, clock, and other devices that previously added additional chips to a design.
Many GPS units record almanac information internally. Knowing where to look, or rather, knowing which satellites it's probably seeing, allows the unit's processor to "acquire" a satellite in as little as 0.2 seconds. Quick satellite acquisition and identification allow a quick-and-dirty position fix—good enough for many non-navigation applications. In difficult navigation scenarios, such as driving through the "urban canyons" of a city, where satellites may only be briefly unobscured by buildings, quick acquisition becomes very important for even minimal system operation. It's a job the newest systems can handle easily.
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Other ways
To date, the global positioning system has garnered the lion's share of infrastructure investment, but several alternative technologies merit consideration for navigation and position-reporting applications.
It's worth remembering that cellular equipment manufacturers spent large sums of money developing network-based position-locating systems to meet FCC E911 requirements before the removal of selective availability made GPS a more attractive approach. That money wasn't wasted says TI's Scott Cooper: "In a properly planned system with enough base stations, then a network-based solution may be appropriate [for some applications]. Signal-strength measurements and triangulation to multiple cellular base stations may outperform GPS, for example, for tracking vehicles in a city.
In addition, other satellite-based alternatives to GPS exist. Recall that GPS was a US defense department project, designed and implemented during the cold war to help troops and autonomous weapons find their targets. Not to be outdone, the old Soviet Union launched its own version of GPS called GLONASS—GLObal'naya Navigatsionnay Sputnikovaya Sistema (global orbiting navigation satellite system).
With the end of the Soviet Union, there was much talk about privatizing the system. At that time, the US government was still committed to the policy of selective availability, and GLONASS offered better accuracy in a civilian handheld device. A number of equipment manufacturers began offering GLONASS or dual-frequency GLONASS/GPS receivers. The inherent redundancy of two separate systems and the effective doubling of the navigation satellite constellation made GLONASS' prospects seem bright.
Unfortunately for the Russians, says Conexants' Mario Liegghio, "with SA off, GPS has shown a little better accuracy than GLONASS." For most system manufacturers with investments in GPS development, it didn't make sense to spend money on another service that was no better than GPS in terms of accuracy or ease of use. Moreover, says Analog Devices' Ashok Kamath diplomatically, "There's concern about the capability of the Russians to replenish the satellites." As a result, interest in GLONASS has dwindled.
At the same time, the European Space Agency (ESA) and a consortium of European aerospace and telecommunications companies are working toward another alternative to GPS, called Galileo, or the global navigation satellite system. Scheduled to begin operations in 2005 with full capability by 2008, Galileo would use a combination of medium-earth-orbit and geosynchronous transmitters. Performance criteria for the system are meant to equal or better GPS.
Although the initial rationale for Galileo was to provide a better, civilian-controlled alternative to GPS, with SA off, the need for the new system appears to be more political than pragmatic. Nevertheless, says SiRF Technology's Kanwar Chadha, "As long as Galileo is compatible with GPS—meaning it supplements rather than tries to fight GPS—we'll support it." After all, he adds, "the more satellites you have, the more reliable location information you're going to get."
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Author information
Contributing Editor Terrence P Lynch is a freelance technical writer based in Newton, MA. He cast a skeptical eye on the hype over 3G in our October issue.
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