Location Based Services
Satellite positioning systems

Possibility of using satellites for location has been discovered by a team led by Dr. Richard B. Kershner during monitoring radio transmission of the first satellite - Sputnik. The team observed the frequency of the transmitted signal changes as satellite moves due to Doppler effect. Based on this frequency changes, they were able to determine the position of the satellite. Reversing this approach and knowing position of the satellites, the position of users or objects is known.

The satellite systems for determination of position are based on measurement of a propagation time of a signal transmitted by the satellites. The propagation time, t, is then converted to the distance, s, between the satellite and the user according to formula s=c*t, where c is the speed of light in vacuum (3∙108 meters per second). Knowing the position of the satellite and the time of propagation, exact position is determined as a sphere of points located in the same distance from the satellite position. It means each satellite enables to determine a sphere of potential locations of the user. Exact position of an object is defined by four parameters: latitude, longitude, altitude, and time. For deriving all four parameters, four spheres must be known. Then the user location is represented by intersection of all four spheres. In ideal case with no error in estimation of the distance between the satellite and the user, all spheres intersect in one point. If the distance is not derived precisely, intersection of four spheres defines an area of potential locations of the user. The size of this area is proportional to the errors introduced by distance measurement and it can be reduced by considering information from additional satellites.

Principle of determination of user position

This general principle is exploited by the most common navigation systems such as American GPS system, European Galileo system, Russian Glonass system, or Chinese COMPASS.

## GPS

The GPS is global navigation satellite system developed by United States Department of Defense for military purposes. The GPS project started in 1973 but the first experimental satellite was lunched in 1978. Now, the GSP provides positioning, navigation, and timing services not only for military but also for civilian use all around the world. It is composed of three segments: space, control, and user.

- Space segment: The space segment consists of at least 24 satellites flying roughly 20200 km above the Earth with orbital period of 11 hours and 58 minutes. The satellites are deployed in six orbits with inclination of 55°. The minimum number of satellites is derived from condition to ensure visibility of at least four satellites from each point at the Earth. Those additional satellites can increase accuracy of the system. In 2011, the former constellation of satellites was slightly changed to improve coverage and three satellites were included to the basic constellation of 24 satellites. It means 27 satellites are now considered as a baseline configuration. In last years, 31 actively operating satellites and three or four spare satellites are flying around the Earth. All satellites are transmitting signals with information required for derivation of user position.
- Control segment: The control segment is created of ground facilities for monitoring, controlling and managing the space segment satellites. The core of the control system is a master control station, in Colorado in United States. The master control station is responsible for management of all control procedures. To avoid the system shut down in case of the master control station failure, an alternate one is deployed in California. The master control station receives and analyses information from satellites (e.g., status information, transmitted navigation information, etc.) and navigation information from 16 ground monitor stations. The monitor stations monitor navigation transmission of the satellites and track the satellites. Based on the collected information, the master control station sends commands to satellites to ensure correct and reliable services (e.g., to change the satellite position). The commands, information for the satellites, or other data are sent via ground antennas. In control systems, 12 ground antennas are deployed not only in America but worldwide.
- User segment: The user segment is represented by a GPS receiver, which processes information received from satellites. The GPS receiver contains a GPS chip with radio part, digital signal processor, memory, control part, and interface to a host unit. The GPS provides processed navigation information to the host unit in form of NMEA (National Marine Electronics Association) protocol.

The satellites transmit three types of signal. The first one, P (precision code), is ciphered code with 10.23 MHz pseudorandom code sequence. This one can be replaced by the second one, Y code, in case of need of anti-spoofing transmission mode. The last one is C/A (coarse/acquisition code) used for civil purposes or for acquisition of the P(Y) codes. The C/A is composed of 1.023 MHz pseudorandom non-ciphered sequence. The navigation message, i.e., the message with information for determination of users’ position, is transmitted with speed of 50 bps and it is added to the C/A code and P(Y) code. Both resulting sequences are modulated to so called L1 carrier at 1575.45 MHz. Next, sequence resulting from adding navigation message to a P(Y) code is modulated to L1 and in addition to L2 carrier at 1227.6 MHz. Both outputs L1 and L2 are then transmitted to users' GPS receivers. Besides, additional signals L2C (1227 MHz), L5 (1176 MHz), and L1C (1575 MHz) can be transmitted for minimizing positioning error for commercial, safety-of-life, and interoperability with other systems respectively. Since all satellites transmit at the same carrier frequency, CDMA multiple access technique is considered to enable reception of signal from more satellites.

The overall navigation message is transmitted in frames with 1500 bits length. Each frame is composed of five subframes containing different information such as, satellite position, health, clock correction, Ionospheric delay effect, constellation status or information on satellites orbits. To transmit the whole navigation message, 30 seconds is required (1500 bit transmitted with 50 bps).

As a time required for delivery of complete information on all satellites is too long (i.e., the time between device turn-on and determination of user position), a mobile communication networks can be exploited in combination with satellites navigation systems for delivery of some information by using mobile networks instead of conventional delivery directly by satellites. This approach is known as Assisted GPS. This way, the process of GPS initiation is speed up.

The accuracy of the positioning is influenced by several aspects: measurement of signal arrival time, atmospheric effects (especially ionospheric effects), multipath propagation, satellite position update, clock synchronization and amount of visible satellites. The most significant error is introduced by ionosphere; however, it can be minimized by considering several signals (e.g., L1 and L2) as an impact of ionosphere influences individual frequencies differently. The overall error is in order of meters for the GPS (typically, up to 8 meters). The accuracy can be further enhanced by differential GPS using ground nodes with exactly defined position to precise the position derived from satellites.

Transmitted signals by GPS satellites

## Galileo

The European alternative to GPS is system Galileo. In space segment, 27 operational and 3 spare satellites are assumed to be placed over three orbits with inclination of 56°. The elevation of the satellites is 23 222 km above the Earth with orbital period of 14 hours. Control segment contains two control centers in Oberpfaffenhofen, Germany and Fucino, Italy. Further, two LEOP (Lunch and Early Operations) centers are in Toulouse, France and Darmstadt, Germany. Analogically to GPS, also 5 telemetry, tracking and control stations, 40 sensor stations, and 10 uplink stations for monitoring and controlling the satellites will be deployed worldwide.

The satellites, like in GPS, transmit several navigation signals (E1, E5a, E5b, E6). The first signal, E1, carries 1.023 MHz non-encrypted ranging codes and navigation data modulated at 1575.42 MHz. This signal with 125 bps rate is available for civil usage (denoted as Open services), commercial services and safety-of-life services. The second and third signals, E5a and E5b, are analogical to the E1; however, E5a and E5b both are modulated to 1176.450 MHz and E5b 1207.140 MHz respectively and both are of 10.23 MHz code sequence. The signal E5a carries data for navigation and timing functions. Contrary to E1 and E5b, the E5a uses more robust modulation and thus only 25 bps rate is supported and it is available only for open services. The last signal, E6, is designated for commercial purposes and thus it is encrypted. This signal is carried at 1278.750 MHz with 5.115 MHz of code sequence. The supported bit rate is 500 bps.

## Other navigation system

The principle of the determination of user position is analogical also for other navigation systems such as:

- GLONASS (GLObalnaja NAvigacionnaja Sputnikovaja Sistema) - It assumes 26 satellites placed over three orbits in full constellation. An interesting fact is that the system was not originally compatible with GPS or Galileo due to using of FDM, i.e., each satellite transmits at different frequencies. However, from 2011, Glonass supports also CDMA signals for compatibility purposes. The ground system is located in Russia.
- COMPASS, also known as BeiDou 2, is Chinese navigation system, currently developed for national purposes, but expected to be extended to worldwide navigation (with 35 satellites) in next years.
- IRNSS (Indian Regional Navigational Satellite System) consist of 7 satellites in space segment, 3 in geostationary orbit and 4 in geosynchronous orbit to cover Indian subcontinent.