GPS and Its Applications
For thousands of years, human beings have utilized various techniques to navigate themselves both in sea and on land. In the ancient times, the need for a consistent and fairly accurate navigation technique would not have been felt any more than by seamen, who have to direct a vessel from one point to another at greater distances in the open sea, often at night when the visibility is poor. According to history, the Pacific people, who began sea exploration as early as 800 A.D., used a device known as Latitude Hook, to travel between islands on the same latitude. However, the "astrolabe" or "star-taker", originated in ancient Greece and was later "refined" by the Arabs into a more sophisticated device, is believed to be the first scientific instrument used for navigation. Using astrolabe in conjunction with the positions of various celestial bodies, seamen calculated the latitude for navigation.
The latest in this trend is the Global Positioning System or GPS, which uses artificial satellites as guides. GPS (the full description is: NAVigation System with Timing And Ranging Global Positioning System, NAVSTAR-GPS) was developed by the U.S. Department of Defense. During its development phase, emphasis was placed on three aspects. First, it has to provide moving and stationary users with the capability to determine position, speed, and time. Second, it had to have a continuous, global, 3-dimensional positioning capability with a high degree of accuracy, irrespective of weather conditions. Last, but not the least, it had to be of potential use for civilians. In this effort, the first satellite was launched to orbit on February 22, 1978. Now, there are 28 satellites orbiting the Earth. Though this system was designed for and is operated by the U. S. military, it is available for civilian use as well. The civil signal SPS (Standard Positioning Service) can be used freely by the general public, whilst the military signal PPS (Precise Positioning Service) is only available for authorized government agencies.
Today, there is a vast use by civilians of this sophisticated technology. There are portable navigation systems integrated with GPS and digital maps, available for automobiles and as handheld devices. The handheld devices are often used for leisure activities, such as trekking, balloon flights, and cross-country skiing. Various companies and government departments with large fleet of vehicles are increasingly utilizing GPS for vehicle monitoring, surveying, and security.
As shown in Figure 1, the Global Positioning System essentially consists of three major components:
- The space component
- The control component
- The user component
Figure 1: GPS System (Picture obtained from UBLOX document)
The space component consists of 28 operational satellites that are orbiting the Earth at a height of 20,180 km on 6 different orbital planes. To ensure maximum precision in 3-dimensional positioning, at least 4 satellites have to be in radio communication with any point on the planet. To achieve this, the orbits at which these satellites spin are inclined at 55 degree to the equator and the 28 satellites are ingeniously distributed along them. Each satellite orbits the Earth in approximately 12 hours and has 4 atomic clocks on board. The information transmitted by the satellite, known as navigation message, contains satellite time; precise orbital data (ephemeris); approximate orbital data for all other satellites (almanac); synchronization signal for satellite time; correction signals for signal transit time; information on satellite health; and data on ionosphere.
The control component encompasses a master control station (located in the state of Colorado, USA), monitoring stations spread all around the globe along the equator, and control stations. The purpose of the control component is to monitor and predict behaviours of all the satellites; synchronize the satellite clocks; perform calculations of orbital data; as well as relaying information about the satellites to other communication centres.
The user component is basically the GPS receiver. It uses the navigation message from the satellites to determine the transmission time of each satellite signal and the exact position of the satellite at the time of transmission. Using the positional information from four different satellites and their range, the receiver then calculates its longitude, latitude, altitude, and time.
To elaborate the above in a bit more detail, the GPS receiver, which is tethered to the subject to be tracked on ground, calculates its position using information received from four different satellites in the vicinity. Thus, the GPS receiver uses the time taken for the signals to travel from these satellites. The underlying principle used in this calculation is that the distance traveled is equal to speed over time (d = v * t). To be specific, the distance between the receiver and each satellite is calculated by multiplying the speed of electromagnetic waves (which is the speed of light, 300,000,000 m/s) by the time taken for the signal to travel from the satellite to the receiver. Interestingly enough, receivers are able to calculate their position accurate to within a range of 20 meters to 1 millimeter and the time is calculated within the accuracy of 60 nanoseconds to 5 nanoseconds (1 nanosecond = 1 second divided by 1,000,000,000).
Why the need for four satellites? To understand why, readers should refresh their understanding of basic algebra. To solve for unknowns (or variables), we need as many equations as the number of unknowns. In 3-dimensional positioning, there are three positional variables: latitude, longitude, and altitude. Hence, three different equations have to be set up using values obtained from three different satellites. The fourth satellite is needed to circumvent the possible error in time (Δt) due to the clock onboard the satellite and the clock in the receiver being out of sync. Though all the satellites in orbit are synchronized by the control component in GPS, there is no way to synchronize each and every clock in thousands of GPS receivers. So, time difference between the atomic clock onboard the satellite and the clock in the receiver is the fourth unknown to be solved for. To position on a 2-dimensional plane, only the latitude and longitude are necessary. For that, signals from three satellites would suffice.
One of the drawbacks in using satellites for navigation is felt quite often in large cities with skyscrapers all around. In such areas, satellite view is often blocked by tall buildings. Also, signals get attenuated to a large degree by them bouncing off buildings. Similar problem is experienced when the receiver is located in underground garages, under big bridges, or highly dense forests. To circumvent this problem in vehicles, nowadays, receivers come with added complexity. Thus, they have a feature known as “Dead Reckoning”, which uses signals coming from vehicle speed sensor and an onboard electronic gyroscope to estimate positions starting from the point where the GPS receiver loses view of required number of satellites.
Do we really need satellites as guides for GPS? Not necessarily. An alternative solution known as GPS One is available for CDMA wireless networks. An example of CDMA networks would be the Bell Mobility in Canada. GPS One is a wireless-assisted GPS. Essentially, it uses cell towers in cellular grid, instead of satellites, as guides. As the position of the cell towers are known, the travel time of signals from these towers allows the GPS One module in the CDMA modems to calculate its position. GPS One would be an ideal alternative to satellite-based GPS in cities, where there are interferences from tall buildings.
GPS is undoubtedly a very well-planned technology and is here to stay for the obvious reasons mentioned above. Its development was driven by the vast potential for its applications. As the size of electronic devices gets smaller every year, so does the size of GPS receivers. Soon enough, it will be possible to plant a GPS receiver as a microchip (or even “nanochip”) underneath human skin for tracking purposes.
Amirthamshan Murugesapillai
November 15, 2007
