Posted: November 30th, 2013
How It Works
GPS refers to a navigation system that uses satellites to give information on timing and positioning at any part of the world for tracking or security purposes. “The Global Positioning System (GPS) is a satellite-based navigation system that was developed by the U.S Department of Defense (DoD) in the early 1970s” (El-Rabbany, p1). The US military base used it in their military ventures but later it was also put into civilian use. Within the GPS, there lies the Initial Operational Capability (IOC), which is a constellation of 24 satellites. Each of the six orbital planes contains four satellites at ten-degree elevation angle, allowing for global visibility. GPS is characterized by three segments namely, the user, space and control segment (El-Rabbany, 2). Each of the 24 satellites in the space segment transmits signals to the receiver.
Accurate atomic clocks control the signals. Components such as a navigation message, carrier frequencies and two digital codes are contained in the transmitted signals. The distance from the GPS satellite to the user’s receiver is determined by the codes and carrier frequencies. As a function of time, the coordinates of the satellites are contained in the navigation message. The worldwide network of trackers and the Master Control Station (MCS) that enable the control segment to track GPS satellites, verify, and forecast satellite location among other satellite system characteristics. It is centered in Colorado in the United States. Through the S-band link, the information obtained from operational control segment is gathered and uploaded into the GPS satellites. Civilian and military users are in the user segment of GPS. A user’s position is determined by the GPS signals received through his or her GPS receiver and antennae. Therefore, GPS helps users know their location in their ventures.
The control segment can track all the visible GPS satellites with the help of the master control stations that have cesium oscillators and GPS receivers of high quality. Information is uploaded to the GPS satellites through the ground antennae positioned at the stations. The master control stations process GPS observations for predicted satellite navigation data. Information such as the satellites position as a function of time, satellite almanac, satellite clock parameters and atmospheric data are predicted. The stations in the control segment monitor the system integrity of the GPS. Microwave radio signals are continuously transmitted by the satellites, and they are obtained by an active GPS receiver through the antennae. The receiver has built-in software, which enables it to process the received signal.
The navigation message expresses the satellite coordinates while digital codes express the distance to the GPS satellites. “If the distances from a point on the earth (a GPS receiver) to three GPS satellites are known along with the satellite locations then the location of the receiver can be determined by simply applying the well-known concept of resection” (El-Rabbany, 8). Determining the location and distance from the satellite is based on a theoretical and practical point of view. The theoretical aspect shows that only three satellites are needed while practically, the satellites should be four as the additional one is for the clock offset of the receiver. If the theoretical aspect is observed, the location of the GPS receiver would be where the three spheres intersect with each centered on a specific satellite. The distance between the satellite and the receiver marks the radius of each sphere. Aside from determining the time and location of the user, his or her velocity can also be measured by GPS through the estimation of the Doppler frequency of the GPS signal received by the user. GPS receivers are useful for time dissemination, navigation, surveying and further research. GPS navigation is applied in vehicles, aircrafts and ships among other features in motion.
How It Changes with Time
There are two distinct GPS positioning services namely, Standard Positioning Services (SPS) and the Precise Positioning Services (PPS). The SPS is used worldwide by civilians with no restrictions while only authorized users can access the PPS with keys and cryptographic equipment. The GPS has its own time system known as the GPS time and it provides the Coordinated Universal Time (UTC) as computed by the GPS receiver. The satellites move relative to users on the ground constantly. Therefore, it is difficult for the GPS satellites to maintain accuracy for 20-30 nanoseconds for precision in determining the location and time. Each atomic clock on the satellite maintains accuracy of one nanosecond (Johnson, p149). Users observe the satellites’ motion relative to them thus; special relativity is applied in prediction. In special relativity, atomic clocks are estimated to be slower than the ground clocks by around seven microseconds per day leading to the dilation effect of time.
In general relativity, it is predicted that atomic clocks with proximity to massive bodies will be slower than the clocks a further distance. The earth’s mass results to space-time curvatures in the orbits containing the satellites located high above the earth. However, in general relativity, observers on the surface of the earth sees the atomic clocks as faster than ground clocks. Hence, the prediction is that onboard atomic clocks on the GPS satellite are faster by 45 seconds per day. Nanosecond accuracy is required in the GPS for high precision. When the two relative effects are combined, the atomic clocks of the GPS appear to tick faster than the ground clocks at the GPS stations. Despite the corrections made by the GPS on various errors, residual errors still exist. Errors are caused by factors such as selective availability, relativistic effects, artificial and natural sources of interference, antispoofing, ephemerisis and clock errors, atmospheric effects and when the precision computation is diluted geometrically.
Observers in relative motion or on different gravitational positions can measure the difference of elapsed time between to actions. The different times shown by the two working clocks illustrate time dilation. The latter depends on space-time rather than the clocks’ technical aspects. The GPS adjusts for time dilation with the help of ground-based devices. The GPS satellites change when they are programmed fro compensation of the time differences or time dilation. The process of programming is determined by gravitational influences and speed. According to Hawkings, a famous physicist, the theory of special relativity makes the GPS relevant (Johnson, p149).
However, Johnson (p149) claims that the theory of special relativity is not fully comprehensible because satellites do not move rectilinearly and time dilation occurs at an extremely small extent. Despite this, special relativity is still evident in the GPS in relation to time. With a closer look, we find that GPS satellites carry identical atomic clocks with the clock rate adjusted at launch to 10.22999999543 MHz from 10.23 MHz, corresponding to 38 nanoseconds per day (Johnson, p149).” Physicists argue that both relativist effects contribute to the adjustment. Johnson (p150) still maintains that time dilation in GPS is negligible and that the relativist effects do not clearly account for it. Other than time dilation, the efficiency and accuracy of atomic clocks on the GPS satellites are affected by the temperature of the sample atoms and the level of electronic transition frequencies.
El-Rabbany, Ahmed. Introduction to GPS: The Global Positioning System. Artech House, 2002. Print.
Johnson, Claes. Many Minds Relativity. Johnson Claes, n.d. Print.
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