GPS - Global Poistioning Systems

Times change--so do latitudes, longitudes and altitudes. Communications technology is shrinking the world, making a company's people and assets almost instantly accessible. In the past, companies were content just knowing their resources were "out there" in the field. Now they need to know exactly where every part of their enterprise is in real time. Some companies also need to perfectly synchronize their equipment even if that equipment is dispersed over thousands of miles.

How can these companies ensure their operations run like clockwork? The answer comes from space. No it's not some recently discovered Martian bacteria, it's a network of satellites in orbit at 1/000 miles above the Earth, signaling in perfect unison-the Global Positioning System (GPS). This man-made constellation is guiding vehicles, ships and mobile professionals like the natural constellations once guided sailors. Using atomic clocks, it is also broadcasting a nearly flawless time standard all over the world simultaneously.

From Defense system to consumer product
Consisting of a constellation of twenty-four satellites and a satellite tracking network maintained by the Department of Defense, the GPS system was originally designed in the early 1970s to provide rapid timing and positioning for remote military users such as submarines.

Only a few government investments have paid off big for both businesses and average citizens. The Internet, for example, originally created as a military network that could withstand a nuclear war, now helps businesses create global information enterprises and allows people from all over the world to easily communicate with each other.

GPS too was first conceived of as a strategic tool Consisting of a constellation of 24 satellites and a satellite tracking network maintained by the Department of Defense, the system was originally designed in the early 1970s to provide rapid timing and positioning for remote. With applications for network synchronization, vehicle location, navigation and asset tracking, small, inexpensive GPS receivers are taking advantage of a huge government infrastructure investment. military users, such as submarines.
Since then, GPS receivers have become cheap enough and small enough to be carried by anyone In 1984, a commercial GPS receiver cost ($l40,000-today), GPS receivers are advertised as a $200 Father's Day gifts at discount stores. The receivers are commonly smaller than a credit card and use less than one watt of power. This makes them easy to integrate into hand-held devices and laptop computers. GPS antennas are small too often just thin disks, five centimeters in diameter.

How GPS works
GPS receivers integrate a radio and a navigation computer and can receive the faint, twenty-watt signals coming from the satellites. The computer uses these signals to calculate the distance between the satellites and the receiver. With this information, the computer can further calculate the position and velocity of the receiver.
The number of satellites visible to a receiver constantly varies between four and eleven according to time and location. Each satellite broadcasts a number of unique spread-spectrum codes, but only one, the Coarse Acquisition (C/A) code, is easily accessible for civilian use. The C/A in orbit 11,000 miles above earth, GPS satellites transmit at twenty watts a number of unique spread-spectrum code. The number of satellites visible to a GPS receiver constantly varies between four and eleven according to time and location. Code is effectively a timing signal synchronized to an international time standard-Universal Coordinated Time (UCT). UCT is kept by a world-wide ensemble of cesium and hydrogen maser frequency standard atomic docks. The highest-quality GPS receivers measure the C/A code to better-than- nanosecond precision.

A GPS receiver determines its position by first receiving a broadcast code, which contains the time the code was transmitted from the satellite The receiver then subtracts the transmission time from the reception time (based on the receiver's internal clock) and multiplies the result by the speed of light, The result, or "pseudorange," is a measurement of the distance between the satellite and the receiver and includes whatever errors might occur from receiver cock offset or atmospheric signal delay. With four or more pseudoranges, the receiver has enough information to calculate its "fix" -- determining its latitude, longitude, altitude and clock offset. The fix computation requires precise details such as satellite clock synchronization, accurate satellite orbits and accurate models of atmospheric effects on the speed of light, These details are all supplied by the satellite tracking network and broadcast to the receivers as part
of the GPS signals.

Almost a Fee lunch
The costs to the federal government for GPS system maintenance is more than off-set by the savings provided by the system to government operations. But this lunch isn't completely free. Under specific conditions, GPS will not provide a position. For instance, the 1,542 MHz GPS signal does not penetrate buildings, which makes it difficult to receive signals indoors. Also the signal can be critically weakened by heavy foliage and interfered with by other sources such as poorly maintained television broadcasting equipment.
Paging systems require synchronized broadcast over their coverage area. Time synchronization, provided by GPS signals, of approximately 10 microseconds (transmitter to transmitter) is typically required. Utilizing only one GPS satellite, these systems can achieve synchronization of as little as 100 nanoseconds.

Accuracy is another issue The basic GPS horizontal position accuracy is 50 to 100 meters, and speed accuracy is one meter per second. While these levels are sufficient for many applications, other applications require more precise measurements. A technique called Differential GPS (DGPS), which measures the errors in GPS signals in real-time and broadcasts them to the receiver community is used for higher levels of accuracy.
Currently, DGPS data services are available from private companies and the US Coast Guard and require a second datalink receiver The Federal Aviation Administration is in the process of launching a satellite that transmits a GPS-like signal containing the differential corrections. This will provide the accuracy
improvement through the GPS receiver itself at no extra cost.

Precision is part of the solution
The time accuracy of the GPS solution is just like the position accuracy -- except scaled by the speed of light. For a state-of-the-art GPS timing receiver, time accuracy relative to the UCT is better than 100 nanoseconds--the time it takes light to travel 30 meters. Fifteen years ago, the only way to tie into the UCT network with high accuracy was to fly an atomic clock from one site to another. Now the transfer of time is provided efficiently by GPS.
Beyond providing the to the UCT reference, GPS provides a cost-effective way to synchronize a communications network over a large area lntra-network synchronization is used to coordinate both the timing and frequency of the transmit/receive nodes to accuracy's previously available only through atomic clock technology leader ultra-stable but expensive cesium frequency standards or the less costly and less accurate rubidium cell.

Synchronization solutions for wireless providers
Several wireless applications rely on GPS for synchronization and frequency control and many other wireless technologies are looking very closely at incorporating GPS in next generation systems. Current use of GPS technology include paging systems, CDMA cellular communication systems and mobile platforms such as laptops and PDAs
Paging systems broadcast synchronously within a coverage area. This strategy over-comes the relatively small antenna gain of pagers. Time synchronization of approximately 10 microseconds (transmitter to transmitter) is typically required. This level of accuracy is well within the capability of the GPS system. Even with selective availability (SA) turned on, 100 nanosecond RMS synchronization is achievable using only a single GPS satellite. CDMA cellular communication systems mandate the use of GPS for both time synchronization and frequency control CDMA systems require each cell be within few microseconds of the CDMA system time base (GPS time + the CDMA system time base). Frequency accuracy of parts per billion is required by CDMA. Again, GPS provides a cost effective solution.

GPS improves the accuracy of atomic clocks
Key parts of the infrastructure supporting wireless services like CDMA utilize GPS for time and frequency control. Stratum one clocks often found in a central office or regional switching center, are set by Rubidium cell atomic clocks. These relatively low-cost clocks are very stable but drift on UCT time slightly and GPS is used to constrain (or "discipline") this drift. Even though in the short term GPS is not minutely accurate, in the long term, GPS is never more than 100 nanoseconds off UCT time. Furthermore, Rubidium cell atomic clocks can use their own frequency stability to steer themselves to a long-term average of the GPS. This average is accurate to only a few nanoseconds. Before the deployment of GPS, such performance could only be achieved using high-cost Cesium frequency standard atomic clocks.
GPS references are becoming the standard source of time for stratum one time servers as well. Stratum one time servers are used to synchronize computers attached to a TCP/lP network such as the Internet. Unlike stratum one clocks they do not have a strict time accuracy requirement but they must be tied directly to an absolute time source such as GPS or wide area radio time broadcasts (known as "WWV"). GPS, however, is more available, reliable and accurate than WWV.
TCP/lP networks are becoming popular for communication infrastructure maintenance, as well as, for customer billing logs. Ericsson, a leading provider of wireless infrastructure equipment, will begin the use of TCP/IP time references to log usage for billing purposes in the near future.

GPS to the rescue
GPS receivers are already found in many automobiles. The migration to other mobile platforms is inevitable as prices and power consumption continue to fail. The addition of "location awareness" awakens many possibilities for the mobile user The ability to broadcast location data greatly adds to the utility of mobile communication devices, especially the ability to summon aid. Systems have been developed to automatically contact 911 and provide vehicle location when the airbag deploys after an accident.
Additionally a knowledge of position greatly enhances the ability of wireless services to broadcast a wide range of ancillary data. The location of ATM machines, restaurants, hospitals and other services can be requested by and communicated to the mobile user. The user of a GPS-equipped communication device can have the convenience of custom yellow pages available, as well as, highly developed navigation aids.

Tracking with GPS
The greatest real-time commercial use of GPS is not "Where am i?" but rather "Where are they?" Commercial fleets are finding productivity improvement in GPS. The ability to quickly locate one's mobile assets allows quick and efficient re-deployment under changing conditions. This translates directly into improved productivity per unit. These improvements are especially appreciated. In public safety services, such as police, fire, and towing.
Ambulance operators are adopting GPS as a method of tracking their vehicles. Accurate real-time vehicle tracking enables quick identification of the nearest ambulance to an emergency reducing time to respond to an emergency call. With a large fleet this reduction can be significant enough to reduce the active fleet size while maintaining the desired response time, more than enough savings to offset the cost of GPS.

Not just for latitude and longitude
The GPS receiver position fix provides latitude, longitude and altitude, but people don't think in terms of mathematical models of the Earth's surface. Information must be provided in terms of directions, landmarks and addresses. Detailed map databases are required that describe each feature in detail with its latitude and longitude coordinates. This need has spawned an industry in develop-GPS receivers and the need for detail databases linking map features with precise latitude, longitude ad altitude coordinates have spawned an industry is developing, upgrading and updating maps using GPS receivers to position-tag the data collected in the field. A direct benefit to wireless services is in maintaining maps of their fixed assets, such as broadcast and relay towers, which can be "named" precisely by their GPS coordinates. One of these "fixed assets" is the received power in the distribution area. Wireless services are continuously maintaining the reliability and availability of their signal in their service areas with mobile test equipment position-tagging the collected data with GPS.

The future at GPS
Society has a tendency to take the best technologies and make them indispensable then almost invisible. GPS gives the world accurate time to nanoseconds and accurate position to meters without high-cost precision equipment The ultimate applications are difficult to predict. GPS is already the most reliable and cost-efficient method for creating maps, synchronizing telecommunications systems, positioning car navigation systems and reporting locations to tracking and emergency systems. Shrinking in size, weight power and cost-GPS is following the classic electronics trend and with each step, a new set of users find they can profit from the technology.