Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-11-13
2004-08-03
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06771211
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for positioning a receiver based on code modulated signals transmitted by satellites and formed by an individual code for each satellite. The invention equally relates to such a receiver, to positioning means, to a computing server and to a positioning system.
BACKGROUND OF THE INVENTION
A known positioning system which is based on the evaluation of signals transmitted by satellites is GPS (Global Positioning System). The usual constellation in GPS consists of 24 satellites that orbit the earth in 12 hours. This constellation provides between five and eight satellites visible from any point on the earth.
Each of the satellites, which are also called space vehicles (SV), transmits two microwave carrier signals. One of these carrier signals L1 has a carrier frequency of 1575.42 MHz and is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) Code, as which a Gold code is used. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a 1 MHz Pseudo Random Noise (PRN) Code and is spreading the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s, which information comprises ephemeris data, data on clock corrections, and other system parameters. Ephemeris parameters describe short sections of the orbit or the respective satellite. The ephemeris parameters can be used with an algorithm that computes the position of the satellite for any time within the period of the orbit described by the ephemeris parameters.
Receiving means of a receiver of which the position is to be determined, receive the signals transmitted by the currently available satellites. The information in the received signals enables positioning means connected to the receiving means to compute the distance to several satellites and thus the current position of the receiver. The computed distance between a specific satellite and a receiver is called pseudo-range, because the time is not accurately known in the receiver. The pseudo range can be computed based on the reciprocal pseudo propagation delay of signals from the respective satellite. The receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute positions in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
Receiving means and positioning means can be comprised in a single, autonomous electronic device constituting a receiver. Alternatively, the positioning means can be external to the receiver. The receiver can for example have access to a cellular network with positioning means. The receiver then only has to transmit the received data to the network, where the positioning calculations are carried out.
The employed modulation technique enables the receiver to distinguish between the signals transmitted by the different satellites and thus to extract the included information, even though the satellites use the same carrier frequency. To this end, the receiver has to synchronize with the respective channel employed by a satellite, i.e. to detect and track the C/A code in the signal.
For detecting and tracking a code of a received signal, GPS receivers usually use a correlation method by which the codes in received signals are compared with replica codes for each satellite available at the receiver. The receiver, or external positioning means, can either generate the respective C/A code sequence for a specific satellite with a code generator, or store the different C/A codes. Before performing the correlation, the received signals are down converted by a multiplication with an intermediate frequency. Then, the down converted signal is multiplied for the correlation with the replica of one of the codes. The receiver slides a replica of the respective code in time and repeats the multiplication. The result of the respective multiplication is integrated or low-pass filtered. As the code in a signal transmitted by a satellite and the receiver code line up completely, a correlation peak is reached at which the resulting value is the greatest. A channel of a received signal resulting in the correlation with a specific replica code in the highest peak is assumed to be the channel employed by the satellite for which this specific replica code is provided. A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the satellite signal.
The positioning is based on the one hand on C/A code related information, like the C/A code phase, and the number of chips received after the change of the last epoch, and on the other hand on the navigation information included in the signal.
In weak signal conditions, e.g. indoors, a GPS receiver may be able to detect the C/A code in signals, but not to demodulate the included navigation data. Usually, missing navigation data is the key element why positioning cannot be maintained or initiated in weak signal condition for a long period. The receiver thus requires in such conditions assistance for performing a positioning. In case the receiver functions at the same time as mobile terminal, such an assistance may consist in ephemeris data provided over a cellular network to the receiver.
A more sophisticated form of assisting the receiver which also supports the detection of C/A codes is a delivery of the exact GPS time. Exact time is needed e.g. to improve the sensitivity of the receiver. In time recovery methods the accurate GPS time, i.e. the time of transmission of a satellite signal, is not known and is determined indirectly. However, time recovery methods need a reference position of some quality for calculating the accurate position in GPS. A reference position is a known position near to the expected location of the receiver and is needed for calculating approximated geometrical distances between satellites and the receiver. The calculated distances can then be used in the prediction of navigation data bit edges and C/A-code phases, in order to improve the sensitivity of the receiver and to speed up the signal acquisition. If a reference position is not available or if an available reference position is too far from the receiver, the possibilities of assisting the GPS hardware in acquisition and in tracking of C/A codes are decreased, and also the use of some time recovery methods are prohibited in the case that the time assistance is not exact. Time recovery methods have been presented for example in copending US patent applications by the same applicant.
A possibility for determining a reference position is also of particular interest for new GPS signals called L2C, which are presented for example in the documents “The Modernized L2 Civil Signal” in GPS World, September 2001, by Richard D. Fonata, Way Cheung, and Tom Stansell and “The New L2 Civil Signal” by Richard D. Fonata, Way Cheung, Paul M. Novak, and Tom Stansell. These L2C signals comprise a new pilot signal which does not have any data modulation. Therefore, it is not possible to decode the exact time from the satellite signals, if only these pilot signal is measured. In case there is no accurate time assistance from the network available either, the accurate time can only be obtained by the use of a time recovery method, for which a reference position is needed.
In case a receiver has in addition the functionality of a mobile terminal, a reference position can be provided in a network assistance message transmitted by a cellular network. It is assumed that the receiver receiving the assistance message is close to the base station transmitting the message, and that therefore the position of the base station can be used as reference position. For several reasons, such
Kontola Ilkka
Syrjarinne Jari
Valio Harri
Blum Theodore M.
Nokia Corporation
Perman & Green LLP
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