The navstar global positioning system (gps)

General Information

GPS is a military satellite navigation system that is owned and operated by the United States Department of Defense. The space segment of this system, consists of at least 24 operational satellites evenly distributed between 6 orbital planes with an inclination to the horizon of 55° to the Equator and altitudes of 20200 km (10900 n miles). This configuration ensures that a minimum of four satellites with suitable elevations is visible to a receiver anywhere on the Earth's surface at any time. GPS will therefore provide continuous, worldwide position fixing. The complete constellation is in place and the US authorities have declared initial operational capability. Subject to a number of constraints, the current constellation of satellites provides useful position information. However, GPS users are cautioned that signal availability and accuracy are subject to change without warning. System integrity is also unreliable because of the current lack of real-time warning to users when satellites malfunction. The United States Department of Defense policy is that GPS satellites transmit signals, which are intended primarily for military purposes and are subject to change without prior notice. The use of GPS signals for positioning, navigation, time transfer, or any other purpose will be at the user's risk. However, whenever possible, advance notice of periods when the GPS satellites should not be used will be provided by the Department of Defense and distributed by the US Coast Guard.

A recent (1998) announcement by the Office of the Assistant Secretary for Public Affairs in Washington DC, USA, stated that two new civilian signals will be provided by the US Global Positioning System (GPS). The second signal, to be made available from the year 2003, will have public safety applications, particularly in international aviation, land transportation and maritime uses. A lead-time of several years is needed because the new signal capability will have to be built into the next generation of GPS replacement satellites. The addition of a second signal will greatly enhance the accuracy, reliability and robustness of civilian GPS receivers by enabling them to make more effective corrections for the distorting effects of the Earth's atmosphere on the signals from space. The addition of a third frequency from the year 2005 will be of particular benefit to scientific users and surveyors, adding to the speed with which users can obtain GPS positions and timing, and improving the overall accuracy and reliability of the system. One of the new signals will be located in the frequency of the current GPS L2 signal, which previously has been used only for military purposes. Separate civilian and military signals can coexist on this frequency, as they already do at the GPS L1 frequency. The frequency for the third civil signal is now known as L5 and will receive the international frequency allocation and protection needed to enable it to meet safety of life requirements.

A GPS position fix is obtained by measuring the ranges from a series of selected satellites to a receiver. Ranges are determined by measuring the propagation time of the satellite data transmissions. However, it is not possible to precisely synchronise the satellite and receiver clocks, the ranges measured are not true ranges, but are termed "pseudoranges" since they contain a receiver clock offset error. In order to achieve a two-dimensional (2-D) fix on the Earth's surface at least three "pseudoranges" must be obtained; the receiver microprocessor can then resolve the three range equations to remove the effects of receiver clock offset error. Similarly four "pseudoranges" would be required to obtain a 3-D fix.

Each satellite transmits data on two frequencies in the L-band: L1 = 1575-42 MHz and L2 = 1227-60 MHz. Both frequencies are integer multiples of the basic 10-23 MHz clock frequency. Dual-channel receivers will be able to use two frequencies to correct for the effects of ionospheric refraction. Data transmitted on the L1 and L2 frequencies is encoded by a Pseudo Random Noise (PRN) modulation. The L5 frequency will be 117645 MHz.

GPS provides two levels of positioning capability: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS). The PPS is derived from the Precise (P) code whilst the SPS is derived from the Coarse and Acquisition (C/A) code. The P code, which is primarily for military use, is transmitted on the L1 and L2 frequencies. The C/A code is transmitted on the L1 frequency only at present but in the future will also be transmitted on the L2 frequency to provide the second civil signal. In order to protect the military use of the L1 and L2 signals, the M code has been developed to be spectrally separate from the C/A code. The M code is an improvement on the P code and will be broadcastable on a regional basis. Investigations into methods to improve fix accuracy have led to the development of Differential GPS (DGPS).

2. Answer the following questions

1. What is the GPS?

2. What does the space segment of this system consist of?

3. What does this configuration ensure?

4. How is a GPS position fix obtained?

5. In what way are ranges determined?

6. What levels of positioning capability does GPS provide?

3. Decipher the abbreviations:

GPS, DGPS, 3-D, L-band, PRN, MHz, SPS, PPS, C/A code, P code

4. Translate the following words and word combinations from English into Russian

Global Positioning System, Department of Defense, operational satellite, orbital plane, inclination, altitude, the Earth's surface, position fixing, constraint, signal availability, accuracy, malfunction, civilian signal, lead-time, distorting effect, allocation, range, the propagation time, satellite data transmission, "pseudorange", offset, dual-channel receiver, Pseudo Random Noise.