Figure 1. GPS satellite constellation (public domain, http://www.gps.gov/multimedia/images/constellation.gif).
Satellite-based positioning includes deriving the location of a user’s receiver based on radio frequency signals transmitted by satellite systems. Current fully-operational Global Navigation Satellite Systems (GNSS) include the US GPS (Global Positioning System) and the Russian GLONASS (GLObalnaja NAvigatsionnaja Sputnikovaja Sistema). Satellite-based positioning is grounded in accurate time determination: the time difference between the transmitted and the received radio signal denotes the signal travel time, which reveals the distance between the satellite and the user antenna. Utilizing distance measurements between the user antenna and four different satellites, the receiver can obtain three-dimensional receiver coordinates in a global reference frame and the time difference between the receiver and satellite clocks.
Satellite-based positioning is undergoing a rapid change. There is a need to reform the GPS and GLONASS systems due to the increasing number of applications, more demanding requirements from users, and the need to mitigate interference and disturbances to the radio signals used by these systems. Both the GPS and the GLONASS systems are being modernized to better serve the challenging applications of today in harsh signal conditions. These modernizations include increasing the number of transmission frequencies and changes to the signal components. In addition, the European Galileo and the Chinese Compass systems are under development. There is a strong intention to design the forthcoming, modernized systems to be resistant to interference, as well as more accurate and available over a wider range of conditions. Also, the use of multiple systems for positioning increases the positioning accuracy and reliability even further.
The GPS is maintained and financed by the US Department of Defense and currently consists of 32 satellites orbiting at an altitude of about 20200 km in six orbital planes, inclined 55 degrees with respect to the equator (US Naval Observatory). These satellites transmit signals primarily at two frequencies, called L1 (1575.42 MHz) and L2 (1227.6 MHz). In the future, GPS satellites will also transmit a signal at L5 (1176.45 MHz). In a GPS signal transmission, two types of pseudorandom codes are modulated onto the carrier signal: a civilian code called C/A used for coarse positioning and another one called P-code for precision positioning. Also, a navigation message is modulated onto the signal, which includes information about the satellites’ orbits, clocks, and health status. Generally, only the C/A-code is utilized in civilian receivers for obtaining satellite to user distances and, thereafter, the user location. The more precise P-code is mostly designed for military purposes. A GPS receiver obtains and processes signals simultaneously from multiple satellites. When the satellite locations can be determined from the navigation message modulated onto the signal, the different satellite to user distance measurements provide the means to calculate the user receiver’s position, velocity, and time.
The accuracy obtainable with GPS ranges from a few millimetres to tens of meters depending on the operating environment, weather conditions, and receiver technology used (e.g. one or dual frequency use, code or carrier phase measurements, one or several receivers). Better positioning accuracy can be acquired using more complicated and thus more expensive technology. Very high accuracy, however, can only be achieved in good, unobstructed signal conditions, such as in open, outdoor areas. Satellite measurements are generally very noisy and are especially erroneous in so-called “urban canyons,” where signals are attenuated (weakened) and reflected due to nearby obstructions such as buildings.
After vigorous maintenance and reformation efforts in recent years, Russia’s satellite navigation system GLONASS is again fully operational. GLONASS differs from GPS especially in the scheme used for signal transmissions; each satellite in GLONASS has its own transmission frequency. GLONASS satellites orbit in three orbital planes with an inclination of 64.8 degrees with respect to the equator and at an altitude of about 19000 km. Currently GLONASS has 24 operational satellites (Russian Space Agency). GLONASS, like GPS, was originally designed for military purposes, but nowadays its applications have spread to, e.g., land and sea traffic, and are gradually spreading as well to other consumer products. GLONASS continues to undergo modernization, and in the future GLONASS will also use code division technology in its signal transmissions like GPS (i.e., all the satellites will transmit on the same carrier frequency). New signals and codes are also being added to GLONASS in order to provide improved accuracy and interoperability with other systems.
Europe’s own satellite navigation system, Galileo, is designed for civilian use and has been under development for almost a decade. Problems in financing, design, and interoperability with, for example, the US GPS have delayed the plans and deployment of the new system. One of the most important motives for developing Galileo has been the goal to be independent from USA’s and Russia’s military systems. Dependency on other systems is regarded to affect European safety and bring about too much insecurity for all the applications and services depending nowadays on satellite-based navigation. Two test satellites (GIOVE-A, GIOVE-B) have already been in orbital operation since 2005 and 2008, respectively, and in October 2011 two of the first in-orbit validation satellites were launched successfully into their planned orbits. The Galileo system will consist overall of 30 satellites in three orbital planes with an inclination angle of 56 degrees with respect to the equator, orbiting at an altitude of about 23000 km (European Space Agency). Galileo is expected to be fully operational around 2018-2020. Galileo satellites will transmit signals on the same frequencies as GPS but modulated with different code techniques.
The Chinese satellite navigation system under development, called COMPASS, is somewhat similar in design to the US GPS. COMPASS will consist of 35 satellites, five of which will orbit in geostationary orbits (Beidou/Compass Navigation Satellite System). The geostationary part of the COMPASS is called Beidou and will transmit differential error corrections related to GNSS signals. COMPASS is estimated to be operational at the latest in 2020.