A positioning of a device is supported by various Global Navigation Satellite Systems (GNSS). These include for example the American Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the future European system Galileo, the Space Based Augmentation Systems (SBAS), the Japanese GPS augmentation Quasi-Zenith Satellite System (QZSS), the Locals Area Augmentation Systems (LAAS), and hybrid systems.
A GNSS usually comprises a plurality of satellites that orbit the earth. The satellites are also referred to as space vehicles (SV). Each of the satellites transmits at least one carrier signal, which may be the same for all satellites. Each carrier signal may then be modulated by a different pseudo random noise (PRN) code, which spreads the signal in the spectrum. As a result, different channels are obtained for the transmission by different satellites. The code comprises a number of bits, which is repeated in cycles. The bits of the PRN code are referred to as chips and the time of a cycle is referred to as the epoch of the code. The carrier frequency of the signal is further modulated with navigation information at a bit rate that is significantly lower than the chip rate of the PRN code.
The navigation information may comprise among other information a satellite identifier (SV ID), orbital parameters and time parameters. The satellite identifier indicates the satellite for which data in the navigation information can be applied. It may be for instance an ordinal number. The orbital parameters may include ephemeris parameters and almanac parameters. Ephemeris parameters describe short sections of the orbit of the respective satellite. They may comprise for example a parameter indicating the semi-major axis and the eccentricity of the ellipse along which the satellite currently travels. Based on the ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is located in the described section of the orbit. The almanac parameters are similar, but coarser orbit parameters, which are valid for a longer time than the ephemeris parameters. It might be noted that in the case of almanac, all satellites send the almanac parameters for all satellites of the system, including an SV ID indicating to which the respective almanac parameters belong. The time parameters define clock models that relate the satellite time to the system time of the GNSS and the system time to the Coordinated Universal Time (UTC). Further, they include a time-of-ephemeris (TOE) parameter indicating the reference time for ephemeris, and a time-of-clock-model (TOC) parameter indicating the reference time for the clock model.
In the case of GLONASS, the terms “immediate information” and “non-immediate information” are used instead of the terms “ephemeris” and “almanac”. It is to be understood that any reference in this document to “ephemeris” and “almanac” is used to denote all possible terms that may be used for the same kind of information, including GLONASS “immediate information” and “non-immediate information”.
A GNSS receiver, of which position is to be determined, receives the signals transmitted by the currently available satellites, and it acquires and tracks the channels used by different satellites based on the different comprised PRN codes. Then, the receiver determines the time of transmission of the code transmitted by each satellite, usually based on data in the decoded navigation messages and on counts of epochs and chips of the PRN codes. The time of transmission and the measured time of arrival of a signal at the receiver allow determining the pseudorange between the satellite and the receiver. The term pseudorange denotes the geometric distance between the satellite and the receiver, which distance is biased by unknown satellite and receiver offsets from the GNSS time.
In one possible solution scheme, the offset between the satellite and system clocks is assumed to be known and the problem reduces to solving a non-linear set of equations of four unknowns, namely three receiver position coordinates and the offset between the receiver and GNSS system clocks. Therefore, at least four measurements are required in order to be able to solve the set of equations. The outcome of the process is the receiver position.
In some environments, a GNSS receiver may be able to acquire and track sufficient satellite signals for a positioning based on the PRN codes, but the quality of the signals may not be sufficiently high for decoding the navigation messages. This may be the case, for instance, in indoor environments. Further, the decoding of navigation messages requires a significant amount of processing power, which may be limited in a mobile GNSS receiver.
If the GNSS receiver is included in a cellular terminal or attached as an accessory device to a cellular terminal, a cellular network may therefore be able to provide the cellular terminal via a cellular link with assistance data including parameters extracted from decoded navigation messages. Such a supported GNSS based positioning is referred to as assisted GNSS (AGNSS). The received information enables the GNSS receiver or the associated cellular terminal to obtain a position fix in a shorter time and in more challenging signal conditions. Assistance data is typically provided for each satellite that is visible to the GNSS receiver associated to the cellular terminal. The assistance data may comprise navigation model parameters, which usually include orbit parameters, TOE and TOC parameters and SV ID parameters.
Moreover, an external service may provide long-term orbits, which are accurate substantially longer than the orbit models (ephemeris/almanac) in the SV broadcasts.