C.3 Implementation Issues

03.713GPPFunctional descriptionLocation Services (LCS)Release 1999Stage 2TS

If the Timing Advance (TA) to the serving BTS is known, i.e. the mobile station is in active mode, the ring represented by the TA can also be included in location determination. For a sectored serving cell the TA ring will be reduced to a segment of a ring thus improving the location estimate.

The E-OTD calculation process depends on the MS being able to ‘hear’ a sufficient number of BTSs whose timing is known. The ‘hearability’ of the E-OTD location method depends on many factors but in general good hearability results in a system with improved coverage and location determination accuracy.

Both hyperbolic and circular types require a minimum of three spatially distinct BTSs. However use of more measurements brings improved accuracy.

Location is possible when the MS is idle or dedicated (in-call) modes. Continuous location (tracking) or single location can be requested. Continuous location is more feasible in the mobile based architecture, because uplink signaling is not needed at all.

If BTSs transmit their coordinates and RTD values by using a method such as Cell Broadcast (SMS-CB), the MS has sufficient information to calculate its own position when in idle mode. This idle mode location makes possible a very high frequency of measurements, thus allowing use of advanced filtering both in OTD measurements and location estimates.

An implementation of the E-OTD location method is expected to require an LMU to BTS ratio between 1:3 and 1:5.

Annex D (informative):
Description Of Assisted GPS

The Global Positioning System (GPS) provides a means to determine position, velocity, and time around the globe [D1,D2]. GPS uses satellites emitting radio signals to the receiver to determine the position of the receiver, often on the surface of the Earth. A satellite system generally consists of satellites, receivers, and monitor and control stations as shown in Figure D.1.

Figure D.1: A typical GPS positioning system (Source:[D2])

The four satellites shown in Figure D.1 emit radio signals from space. GPS satellites transmit a direct-sequence spread-spectrum (DS-SS) signal at 1.023 Mchip/sec with a code period of one millisecond. All satellites transmit at 1575.42 MHz using code-division multiple-access (CDMA) techniques. Each satellite’s DS-SS signal is modulated by a navigation message that includes accurate time and a description of the satellite’s position. A GPS receiver in the vehicle, connected with an antenna which receives the radio signals to calculate its position. The ground network includes several monitor stations that observe the satellite signals and a master control station that uploads the data to the satellites. The GPS constellation consists of 24 satellites orbiting at an altitude of approximately 20,183.61 kilometers above the Earth’s surface.

Positioning measurement of the GPS receiver is based on the time of arrival (TOA) principle. When 4 or more satellites are in line of sight from the receiver (or receiving antenna), the latitude, longitude, and altitude of the receiver are determined. Standard positioning service (SPS), a grade of GPS service, is available for commercial applications, including the mobile phone location determination. The SPS is deliberately degraded by selective availability (SA) and provides horizontal position accuracy within a circle of 100-meter radius 95% of the time. Much better accuracy can be obtained by utilizing differential correction techniques. Differential GPS (DGPS) can reduce the position error to under 5 meters, while SA and other error factors are in effect. It uses a reference receiver at a surveyed position to send correcting information to a mobile receiver over a communications link.

As mentioned above, GPS is based on the TOA principle. Figure D.2 is used to depict a simplified two-dimensional view of this principle. A TOA system determines the position based on the intersection of the distance (or range) circles. The range is calculated from the signal transmission time, which is derived by multiplying the time by the speed of the signal. Three range measurements determine a unique position. Geometric accuracy is the highest within the triangle formed by the centers of the three circles. The accuracy gradually decreases as one moves away from the triangle. GPS uses the same principle, where the circle becomes the sphere in space and a fourth measurement is required to solve the receiver-clock offset. Because the receiver and satellite clocks are unsynchronized prior to the measurement, the signal transmission time determined by the GPS receiver is not the true transmission time. As a result, the corresponding range measurement becomes a pseudorange measurement.

Figure D.2: Time of Arrival (TOA) positioning system (Source: [D2])

To solve the clock offset between the receiver clock and satellite clock, a fourth satellite is used. Although the satellite clocks are unsynchronized, the individual clocks are modeled to meter-level accuracy by the GPS ground network. As a result, both the receiver position and clock offset can be derived from the equations below [D1].

where (, , ), (, , ), (, , ), and (, , ) are the known satellite positions, , , , and are measured pseudoranges, is the speed of light, are the known satellite clock bias terms from GPS time, and is the unknown receiver clock offset from GPS time. The satellite clock bias terms are derived by the receiver from the satellite navigation message. For simplicity, several error terms have been left out in the above equations. The square-root term represents the geometric range between the satellite and receiver, and all the other terms contribute to the measurement being a pseudorange.

There are four main functions for a conventional GPS receiver:

1) Measuring distance from the satellites to the receiver by determining the pseudoranges (code phases);

2) Extracting the time of arrival of the signal from the contents of the satellite transmitted message;

3) Computing the position of the satellites by evaluating the ephemeris data at the indicated time of arrival.;

4) Determining the position of the receiving antenna and the clock bias of the receiver by using the above data items using an iterative solution.

To reduce the errors contributed from satellite clock and position modeling, ionospheric delay, tropospheric delay, and selective availability (SA), corrections can be done before the Function 4 above. The most important technique for error correction is DGPS.