Global positioning systems: A 21st-century technology
Global Positioning System (GPS) technology is coming into the mainstream. Once reserved for only high-end geographic applications, this technology is finding its way into K-Mart, Wal Mart, sporting goods stores and mail-order catalogs.
Cutting-edge technology in component development and large-scale integration, as well as new markets, have brought about GPS technology’s move from the public to the private sector.
Satellite-based GPS technology relies on differential satellite positions to pinpoint locations. The navigation satellite timing and ranging (NAVSTAR) system was built and is maintained by the U.S. Department of Defense. The system uses 24 satellites, plus two spares, operating in six orbital planes 20,200km (10,900 nautical miles) above the earth at inclination angles of 55ø with a 12-hour period. The satellites are positioned precisely so that a minimum of four always are in view of the user with a position dilution of precision (PDOP) of six or less.
Each satellite transmits on two L-band frequencies, L1 (1,575.42MHz) and L2 (1,227.6MHz). All satellites use the same L1 and L2 frequencies. Even so, each satellite’s signal is Doppler-shifted by the time it reaches the user. L1 carries a precise (P) code and a coarse and acquisition (C/A) code. L2 carries only the P code. A navigation data message is superimposed on these codes. The same navigation data message is carried on both frequencies. The P code normally is encrypted, so that only the C/A code is available to civilian users. Some information can be derived from the P code. When encrypted, the P code is known as Y code.
To locate positions, readings are taken from different satellites. Three satellites can be used for a two-dimensional position fix, but four are required for height above terrain (HAT). The receiver takes the data from each of the four known transmission sources (the satellites) and determines the time it takes for the signal to arrive at the receiver. This range is called the pseudorange. Solving for the four unknowns, Ux, Uy, Uz, and Cx (clock bias) provides a differential time element based on the time it takes for the signals to reach the receiver, relative to each satellite’s position. The data received determine the device’s position in latitude, longitude, altitude and time. These data are then converted into latitudinal and longitudinal coordinates from which position, speed and time can be determined. Because the elements are time, speed and distance, GPS works in a three-dimensional plane, allowing for land, sea and air applications.
Accuracy levels GPS comes in two flavors: standard positioning service (SPS) and encoded precise positioning service (PPS). The PPS is a non-reduced service, primarily used by agencies that require extremely accurate readings, such as the military, emergency services, public works, fleet vehicle services and U.S. Geological Survey (USGS) mapping. PPS is capable of providing accuracy of 16m or less in positioning, 0.1m in velocity and 90ns-100ns in time.
The SPS commonly is used in the commercial environment and provides position only to an accuracy of 100 meters. SPS signal accuracy is intentionally reduced by the government to protect U.S. national security interests. This reduced accuracy, called selective availability (SA), controls the availability of the system’s full capabilities.
As is often common to encryption or encoding technology, methods soon develop to defeat these schemes. Commercial manufacturers have developed a way to eliminate most of the effect of SA by what is called differential GPS (DGPS). DGPS adds stationary receivers at known points on the ground. The receivers transmit low-rate (100 bits per second [bps]) data at a frequency of about 300kHz. These transmitters relay error-correcting data to the receiver and improve accuracy to 10 meters or better. Accuracy also can be affected by ionospheric conditions and ground clutter such as woods, buildings and multipath signal reflections. Under ideal conditions, GPS receivers are limited to an accuracy of about 2m.
Technology For a long time, commercial GPS receivers were stand-alone components. The user would be required to make the receiver integrate with his application. Today’s GPS systems are becoming more complete. When a GPS system is deployed, it tends to include support data, such as navigation maps and post processing software. For example, a commercial GPS unit sold to the automotive market may contain not only the receiver, but also localized data maps of city streets for the area in which the unit is being sold. It also may contain a car power adapter kit, external antenna and even software to allow the user to modify the database. In this application, the GPS becomes a component of a complete product. This trend also is being seen in commercial markets such as airline navigation and law enforcement.
Greg Turetzky, a product manager at SiRF Technology, a GPS front-end component manufacturer, says the approach that SiRF is taking is to apply very large scale integration (VLSI) techniques to shrink the module and lessen component count. From a design standpoint, the GPS modules of the receiver comprise two chips, an RF converter module, largely made up of bipolar components and the complementary metal oxide semiconductor (CMOS) digital signal processor (DSP). The front-end section is the RF portion that takes the signal, which can be as low as -160dBm at about 1.5GHz, processes it and converts it to digital output of a few hundred millivolts. The digital output is fed to the DSP section. The function of the hardwired DSP is to remove the spread-spectrum encoded portion of the signal and to convert it into basic in-phase and quadrature (I and Q) for running track loops.
This “component only” approach is typical of many manufacturers in many industries in which a standard component approach is being developed. Turetzky points out that SiRF is attempting to make the inclusion of a GPS module easy for the product developer. SiRF’s approach “is to make it easy for people who aren’t skilled in RF design to be able to add this to their product.” By adding a handful of external components, such as filters, reference crystals and bias components, for example, a complete GPS front-end is made available. It is this technology approach that opens doors for commercial GPS.
As this technology develops, it is likely that we will see an integrated RF and DSP unit that will offer advantages such as improved signal-to-noise ratio, simplified interconnect and better immunity to environmental interference.
Applications and markets In the early days of GPS, the principle markets were surveying and timing. Before GPS systems were used for marine navigation, the compass and the Loran coastal navigation system were the only ways mariners could navigate. The compass was not particularly accurate, and Loran only worked in coastal waters. GPS provided a one-stop application for both open seas and coastal navigation and even provided a precise location device for inland water bodies. A secondary benefit was that the user did not have to learn to navigate by stellar. It was necessary only to take a GPS position fix, look at the charts and point the boat to a compass heading. GPS was simpler than previous methods.
GPS is beginning to find its way into people and package tracking. This type of application has a unique set of issues that other tracking objects do not. First, these objects are not always outdoors. Second, the object is not always moving. Once a person or a package is surrounded by massive brick and steel structures, signal reliability drops markedly, and if the object is not moving, there is no (t. The challenges that GPS faces in this arena are to try to improve sensitivity to these minimal signal levels, which may be 2160dBm or greater, and to intelligently sense whether the object has stopped moving. A rather ambitious project would be to attach a GPS receiver to packages delivered by courier services.
Another technology area that is likely to see the implementation of GPS is personal communications. Cellular phones are a prime candidate for integrated GPS. In an emergency, dialing 9-1-1 from a cellular phone does nothing to determine the actual location of the phone (unlike landline phones, which can be traced). According to one industry source, the FCC is prepared to require a certain percentage of cellular 9-1-1 calls to be locatable within a defined area within a set time period. GPS in-building unreliability, blocked signal areas and movement requirements are likely to cause debate over this type of technology.
The obstacles Although there have been a great number of advancements, GPS technology needs further refinements. To use an industry vernacular, GPS signal strength at the receiver is in the mud. Imagine you are an integrated, ultra-sensitive receiver in a cellular telephone looking for a signal at -160dBm. All of a sudden a 3W, 900MHz cellular transmitter, only a few inches from you, fires off. Even though the transmitted freqency is 600MHz away, the power radiated at the proximity of the receiver is close enough to affect and sometimes damage the sensitive front end of the GPS receiver. Even with today’s high isolation and rejection devices, the super-sensitive GPS receiver is always affected by the proximal power.
GPS manufacturers face power consumption also. Today’s GPS receivers have a steady-state power drain of around 750mW. Hand-held GPS receivers usually use 6V battery configurations (1.5V Ý3 4 cells). Some quick math reveals that the current drain is 125mA. Although this does not seem like much of a drain, add in the display (popularly the liquid crystal type) and some other support electronic, such as DSP, and it becomes an issue. To improve on time between batteries (TBB), various power-saving schemes are supported, such as powering down the components when the unit is not in actual receiving mode, or after a specific period of non-use is sensed. Even so, to maintain updated positioning information, the receiver still has to make periodic calculations, even when the unit is in standby mode. Ways to reduce power further are still high on the list of developmental projects.
The sensitive nature of current RF technology still requires careful shielding, grounding and component placement. Although digital components have relative immunity to spurious emissions, high-frequency RF circuitry does not have that luxury. High-speed microprocessors running at 100MHz and higher can wreak havoc with ultra-sensitive RF sections.
The future As was mentioned earlier, most of the movement in the GPS industry is in trying to improve receiver sensitivity and to deal with stationary receivers. This translates into the issue of reliable signal levels. At a certain level, the receiver loses phase lock but not necessarily the signal itself. Although maintaining phase lock is required for guaranteed accuracy, some of the industry designers are looking at methods that can sense signal, even though it is too low for the phase-lock-loop (PLL) circuitry’s threshold. Even at signal levels of -180dBm and below, if the signal can be identified, some information still can be gleaned. If this information can be identified by using digital enhancement technology, it is possible to make it useful in position location. As Turetzky indicates, part of the new technology is a paradigm shift in perception.
If the position fix is not accurate to 1m, but is accurate to 100m, does that make the data unreliable? If you were hiking and got lost in the woods, would you throw away your GPS receiver just because it could fix only to 100m rather than to 10m, or would you rather have the rescuers looking all over the mountain? Even though the information is not 100% reliable, it is not useless. Consider today’s cellular technology. If it had to be 100% reliable, none of us would be using it.
GPS appears to be a means to an end, rather than the end itself. As a stand-alone product, latitude and longitude are not hot items outside of surveying and navigation, but as an integral part of 21st century technology, GPS has a bright future. For example, consider using a GPS receiver to keep your portable computer’s time set or as a small light emitting diode (LED) on your automobile’s instrument cluster that will locate you on any street or highway and then reference the data to the local geography and populous.
As with any high-technology industry, GPS is benefiting from the developments in surface-mount technology (SMT), VLSI, low-power devices and other advancements. Another area of development is in embedded processors and microprocessors in general. It is likely that many of the functions, once exclusively the domain of discrete components, can be emulated and will be provided in a digital format as processor speeds approach 500MHz and eventually break the gigahertz barrier. Faster processors will allow the elimination of much of the frequency manipulation and conversion techniques we now use to bring the frequencies into workable digital range. The future chipsets likely will look like a universal asynchronous receiver and transmitter (UART), just another component that gets called into use when needed. Microsoft is designing a position navigation application program interface (PNAPI), so computers with the operating system can position-tag data if needed, and GPS manufacturers can easily design software driver interfaces for their products.
The final limiting technology in GPS is the satellite system itself. Although it is relatively easy to update earth-bound hardware, to do the same to satellites involves launching new ones (or going aloft and updating the existing ones). Either way, it is an expensive proposition, but the power of the commercial market is one that is hard to ignore.
Low-cost GPS technology may be the next opportunity for consumer GPS systems. At any time, under ideal conditions, a GPS receiver could gather data from 12 satellites. On the other hand, the system is designed to permit precise positioning calculations with data from only four. Somewhat less precise positioning can be obtained using three satellites. The application of digital technology and VLSI, coupled with innovative design, makes it possible to continue calculating position “fixes” with only a single satellite’s signal.
Single-satellite positioning is critical for GPS use in urban areas. Automobiles equipped with GPS-based accessories often travel on narrow streets surrounded on two sides with tall buildings. Such “urban canyons” can obliterate reception of more than one satellite’s signal. With single-satellite positioning, positions can continue to be fixed and are corrected as soon as an automobile enters an intersection, permitting multiple-satellite signal reception.
A key to this capability is the receiver’s ability to quickly access satellite signals while the automobile proceeds through the intersection. As a result of the fast movement of vehicles combined with the canyon effect, acquisition technology must be designed to achieve re-acquisition in as little as 100msec, much faster than the more typical two to three seconds required by other technologies. To achieve such quick re-acquisition time, new technological developments, such as parallel spectrum search involving 20 code samples at a time, are needed, and parallel spectrum search is coming on the market.
Another problem associated with urban areas is multipath errors. Signals bounce off buildings, introducing time delays in reception and positioning errors. Dual-multipath rejection technology eliminates low-level reflected signals and filters higher-level reflected signals. Without multipath rejection, such errors can produce large-scale positioning deviation.
Finally, GPS originally was designed for use with expensive, spread-spectrum receivers. The standard signal threshold is -160dBw. This permits receiving a signal that is much reduced in power. Consumer products with GPS capabilities often will be used in wooded or tree-lined areas, where signal levels tend to fall below this threshold. Therefore, increased sensitivity receivers allowing sensitivity 20dB lower than the threshold are needed for these conditions. Thus, signals that are indistinguishable to other GPS receivers are easily detected.
These issues and their resolutions are only part of the developing market for GPS. The combination of GPS innovations and consumer demand is making it possible to provide fast, accurate, personal positioning information in almost any outdoor location, using hand-held consumer products that sell for less than $200. Greg Turetsky SiRF Technology
Worthman is contributing editor with RF Design magazine.