Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
  • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
  • G01S5/14—Determining absolute distances from a plurality of spaced points of known location
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/03—Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers
  • G01S19/05—Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers providing aiding data
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
  • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
  • G01S19/258—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS relating to the satellite constellation, e.g. almanac, ephemeris data, lists of satellites in view
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
  • G01S19/27—Acquisition or tracking or demodulation of signals transmitted by the system creating, predicting or correcting ephemeris or almanac data within the receiver

Definitions

• the present invention generally relates to position fixing by means of a satellite positioning system, in particular to a method for providing a mobile station with assistance data that facilitate position fixing of the mobile station.
• GNSS Global Navigation Satellite System
• AGNSS Assisted GNSS
• a server unit which communicates with the mobile station via a communication network, e.g. a cellular communication network.
• a communication network e.g. a cellular communication network.
• the mobile station sends a request for assistance data to the server unit.
• the latter transmits the assistance data to the mobile station.
• assistance data may be demanded by the mobile station, e.g. reference location, reference time, navigation model, ionosphere corrections, differential corrections, almanacs, etc.
• the mobile station Having received the assistance data from the server unit, the mobile station then processes the satellites signal, e.g.
• satellite acquisitions and/or pseudo range measurements There are currently two options for computing the location: either the mobile station performs the necessary computations itself (MS-based mode) or the mobile station transmits the pseudoranges to the server unit, which computes the location and then sends it to the mobile station (MS-assisted mode).
• MS-based mode the mobile station performs the necessary computations itself
• MS-assisted mode the mobile station transmits the pseudoranges to the server unit, which computes the location and then sends it to the mobile station
• receiving the ephemeris data via the cellular network has the advantage that it does not need to demodulate the ephemeris data, which is part of the so-called "navigation message" contained in the signal in space (SIS), which is the signal broadcast by the satellites.
• SIS signal in space
• Certain methods for providing assistance data to a mobile station use the high level layers of the communication network, i.e. the applicative layers.
• the advantage of such a solution is that the applicative layers have a much higher data rate than the control plane layers. Nevertheless, the user can access to this layer only if he has subscribed, which raises the issue of position-fixing related to emergency calls.
• the protocol is standardised in the TS44.031 (RRLP) for GSM and the TS23.371 (RRC) for UMTS.
• control plane implementation An important advantage of the control plane implementation is that data exchange with regard to position is possible, even without a SIM (Subscriber Identity Module) card. As a consequence, emergency calls can be located, even if the user has not subscribed. Another advantage is that the operator is completely in control of the process and can vouch for the service. The main drawback is that the signalling layers have a low data rate, which leads to problems if many mobile stations request for assistance data. One therefore seeks for methods for exchanging GNSS assistance data with reduced data rate.
• SIM Subscriber Identity Module
• a position location server transmits almanac data with a long validity period (e.g. a week) to the mobile station, which stores these data.
• the server transmits a correction vector, which corresponds to the difference between the current ephemeris data and the current almanac data, which are both received from the satellites in real-time. Because of the long validity of the almanacs, these data need not to be retransmitted frequently. As only difference vectors are sent, a smaller number of bits can be used than that would be necessary for sending ephemeris data as broadcast by the satellites.
• a problem that is not addressed by the above document is concerned with "roaming", i.e. when the mobile station asks for assistance data when located in a geographical area that is not covered by its home operator.
• the server unit forwards navigation data broadcast by the satellites, the situation may occur that the satellites visible from the mobile station are not visible from the GNSS receiver of the server unit and that the validity of the last ephemeris data stored at the server unit for the concerned satellites has expired.
• the known solution to this problem is the deployment of a network of fixed reference GNSS receivers disposed around the globe and connected to the server unit. The operator currently has the possibility to build the network on his own, which is very costly, or conclude a service contract with an owner of such a reference network, which makes him dependent from another party.
• a satellite is considered as "visible" from a certain geographical point if it is above the horizon with respect to this point.
• US 2004/0117114 A1 as well as related US 2002/0190898 A1 and US 2002/0188403 A1 describe the use of long-term satellite tracking data in a remote receiver instead of ephemeris data broadcast by the satellites.
• a server Upon request for assistance data by the remote receiver, a server transmits satellite tracking data, which are obtained via satellite orbit predictions and have a validity period of up to four days.
• Satellites of a satellite positioning system broadcast within the navigation signals ephemeris data having a certain period of validity.
• the period of validity may, for instance, be defined as the time interval in which the accuracy of a position fix derived from these ephemeris data meets the specifications, respectively the end user's expectations.
• a mobile station e.g. a mobile phone, a digital camera, a portable computer, a handheld computer or any like device equipped with a satellite-positioning receiver
• ephemeris data are required for position-fixing.
• acquisition of navigation signals emitted by the satellites is facilitated as assistance data are provided to the mobile station.
• a server station e.g.
• an AGNSS server or any like assistance data provider receives the ephemeris data broadcast by the satellites, e.g. by means of a reference receiver connected to it.
• a request for assistance data from the mobile station is received at the server station, which then transmits ephemeris data as part of the assistance data to the mobile station in response to the request.
• the server station upon receiving the request for assistance data issued by said mobile station, the server station decides whether the mobile station could achieve a specified position fix accuracy if the mobile station was provided with the broadcast ephemeris data. In the positive, i.e. if the specified position fix accuracy can be achieved with the broadcast ephemeris data, the server station transmits the broadcast ephemeris data to the mobile station.
• the server station transmits, instead of broadcast ephemeris data, long-term ephemeris data to the mobile station as part of the assistance data requested.
• the long-term ephemeris data are derived from satellite orbit predictions and have a period of validity substantially increased with respect to the ephemeris data broadcast by the satellites.
• the decision step is kept simple. It may e.g. comprise determining whether the broadcast ephemeris data received at the server station are valid at the time of the request.
• the disclosed method reduces the impact on the capacity of a communication channel between the server station and the mobile station while taking into account whether the transmission of the broadcast ephemeris data or the long-term ephemeris data is appropriate.
• updates of the ephemeris data stored at the mobile stations become less frequent. This results in a net reduction of bandwidth, since the overall size of long-term ephemeris data does not exceed the size of conventional ephemeris data.
• this increases the autonomy of the receiver, i.e. the period during which the receiver does not need additional assistance data.
• the period of validity of the long-term ephemeris data is increased by at least a factor of 1.5, more preferably, by at least a factor of 2, and still more preferably, by at least a factor of 4, with respect to conventional ephemeris data.
• the format of the long-term ephemeris data is the same as the format of the broadcast ephemeris data, so as to remain compatible with existing standards.
• An external orbit prediction facility may provide the server station with the satellite orbit predictions, which then derives the long-term ephemeris data therefrom.
• the server station may provide orbit predictions using as input broadcast ephemeris data received at the server station, e.g. by means of an appropriate satellite receiver connected to the server station.
• the satellite orbit predictions are preferably based on a mechanical model of forces acting on the satellites.
• Orbit predictions may be achieved by integrating the fundamental law of mechanics, using known satellite parameters as starting values.
• the server station is equipped with a GNSS receiver, it can acquire and store the ephemeris data broadcast by the satellites as long as these are visible from the location of the receiver.
• the server station may compute the orbit of the satellite based on the stored ephemeris data.
• the server station and its reference receiver are located in Europe, whereas the mobile station requests assistance data for Australia.
• the reference receiver does not provide the server station with current ephemeris data of the concerned satellites, i.e. with those satellites that can be used at the same time for navigation in Australia.
• the server station may then compute long-term orbit predictions for these satellites based on the most recent ephemeris data in memory.
• the server station does not need to be connected to a reference network of receivers distributed around the globe. If placed in a suitable geographical location, a single reference receiver may be sufficient. It is not necessary that the reference receiver is located in the same geographical area as the server station.
• the assistance data transmitted to the mobile station may comprise ionospheric refraction data and/or synchronisation data in addition to the ephemeris data.
• the kind of data to be transmitted can be specified by the mobile station in the request for assistance data.
• Ionospheric refraction data or synchronisation data may further reduce the duration of the position fixing or increasing the accuracy of the calculated position.
• the server station receives with the request for assistance data an initial guess of the location of the mobile station, e.g. the information on the cell of the communication network, in which the mobile station is located. The server station may then optimise the assistance data according to the initial guess. Location-specific optimisation may be achieved in view of saving bandwidth of the communication channel between the mobile station and the server station.
• the ephemeris data sent to the mobile station can be reduced to ephemeris data concerning only those satellites that are currently visible from the cell of the communication network.
• Ionospheric refraction data transferred to the mobile station may be reduced to location-specific ionospheric refraction data.
• Fig. 1 is a block diagram of the components of a communication system using a satellite positioning system to locate a mobile station;
• Fig. 2 is a high-level flow diagram of a method for providing assistance data to a mobile station of a satellite positioning system according to a preferred embodiment of the invention
• Fig. 3 is an illustration of a timeline of events occurring before, respectively after a request for assistance data.
• Fig. 1 shows the components of a communication system 10 using a satellite positioning system (such as e.g. GPS, GLONASS, Galileo or a combination thereof).
• the communication system 10 includes stationary infrastructure, such as the base transceiver stations 12, 14, 16 and a mobile station 18, e.g. the mobile phone shown in Fig. 1.
• the stationary infrastructure further includes a base station controller (BSC) 20, a serving mobile location centre (SLMC) 22 and an AGNSS server 24.
• BSC base station controller
• SLMC serving mobile location centre
• AGNSS server 24 AGNSS server
• the AGNSS server is connected to a reference GNSS receiver 26, which receives ephemeris data broadcast by those GNSS satellites 28, 30, 32 that are visible from the location of the reference receiver 26. Communication between the reference receiver 26 and the AGNSS server 24 may be based on the internet protocol or any other suitable protocol.
• the server 24 receives the broadcast ephemeris data over the communication link 34 from the reference receiver 26 and stores the broadcast ephemeris data in a memory.
• the broadcast ephemeris data have, for example, a period of validity of about 3 hours from the time they have been sent. Out of the period of validity, the discrepancies between the broadcast ephemeris data and the actual satellite orbits significantly increase, so that the specified accuracy of a user's position fix cannot be achieved any more if he uses these ephemeris data.
• Position-fixing may be initiated by the user of the mobile station 18 either intentionally or automatically, e.g. in response to the user starting location- aware browsing on his mobile station. Alternatively, position-fixing may be initiated by an external application, e.g. in response to the user having dispatched a distress message.
• a request for assistance data is sent from the mobile station and forwarded to the AGNSS server 24. Requested assistance data may be chosen from the standardised set of the technical specification 3GPP TS 44.031 issued by the 3 rd Generation Partnership Project.
• This set comprises inter alia reference location (preliminary location generally deduced from cell information), reference time (to synchronise the mobile station to GNSS time), navigation model (ephemeris data), ionosphere corrections, almanacs, etc.
• the AGNSS server 24 then elaborates the requested assistance data, which are transmitted to the mobile station.
• the mobile station uses the received assistance data to acquire the signals broadcast by the satellites and to perform a pseudorange measurement. The details of this operation are well known to those skilled in the art of AGNSS.
• the location can than be computed either in the MS-based mode or in the MS- assisted mode, depending on the configuration of the mobile station 18.
• Fig. 2 illustrates the steps performed for elaborating ephemeris data for the mobile station 18 according to a preferred embodiment of the invention.
• a request for assistance data issued by the mobile station 18 is received at the AGNSS server 24 (step 201 ).
• the request further contains information on the network cell 35, e.g. provided by the SLMC 22, which information relates to the current location of the mobile station 18.
• the AGNSS server 24 determines which satellites may be visible from the user's location (step 202).
• the server 24 then retrieves from memory the most recent ephemeris data broadcast by these satellites and received at the reference receiver 26 and evaluates whether the mobile station 18 would be able to fix its position with the specified accuracy (e.g. 40 m) under the assumption that mobile station 18 was able to use the broadcast ephemeris data (step 203).
• the evaluation may be based on a simple heuristic stating that the broadcast ephemeris data of each satellite may not be used at a time not contained in the validity period of the broadcast ephemeris data.
• the ephemeris data in memory are recent enough, they may be forwarded in to the mobile station 18 as part of the assistance data. This case may e.g. occur if, at the time of the request, the user is in the same geographical area as the reference receiver 26 connected to the AGNSS server.
• the ephemeris data are prepared for the transmission to the mobile station.
• long-term ephemeris data are computed (step 205) and prepared for the transmission to the mobile station (step 206).
• the ephemeris data are transmitted to the mobile station 18 (step 207).
• the server 24 is connected to an external orbit prediction facility 36.
• This function may be assumed by the so-called Orbit and Synchronisation Processing facility (OSPF).
• OSPF Orbit and Synchronisation Processing facility
• the orbit prediction facility 36 may dispatch to the AGNSS server 24 the orbit predictions or even the long-term ephemeris data derived from the orbit predictions.
• the server 24 keeps a history of the position and velocity of a given satellite as long as it receives ephemeris data from this satellite by means of the reference receiver 26. These data are used as input for extrapolating the orbit of the satellite by means of a mechanical model, taking into account the forces acting on the satellite.
• the orbit prediction concept bases on integration of the fundamental law of mechanics, with the known satellites positions and velocities as starting values. Orbit prediction may take into account gravitation of earth, moon and sun, earth's pole rotation, earth's sideral rotation, precession and nutation, solar pressure, tides, etc. Satellite orbits are predicted for a period that substantially exceeds the period of validity of the broadcast ephemeris data used as input. Depending on the orbit prediction algorithm that is used, orbits can be predicted for 24 hours or even more. The predicted orbits serve to derive current long-term ephemeris data.
• the external orbit prediction facility 36 may compute the orbits and/or the long-term ephemeris data in the same or a similar way as described above for the server 24. Internal and external orbit and/or ephemeris computation may be used redundantly, i.e. one serving as backup for the other in case of a dysfunction, or in a complementary way. In particular, the latter possibility may be interesting if the AGNSS server 24 has to deal with different satellite constellations, e.g. GPS and Galileo. For Galileo, an orbit prediction facility 36 is planned, whereas this seems not to be the case for GPS. Ephemeris data regarding GPS satellites could be computed by the server 24 itself, while ephemeris data regarding Galileo satellites might be provided by the OSPF.
• Fig. 3 shows the course of the events on a time line. Time progresses from left to right along the axis 38.
• the AGNSS server receives ephemeris data broadcast by a particular satellite at different times ToE-2, ToE-1 , ToE-O (ToE standing for "time of ephemeris").
• ToR time of request
• the AGNSS server receives a request for ephemeris data for that particular satellite.
• the latest ephemeris data of the concerned satellite with respect to the time ToR were received at a time ToE-O.
• the AGNSS server provides an orbit prediction for a time interval 42 that includes the time ToR.
• the orbit prediction starts at a point in time, for which position and velocity of the satellites are known, i.e. before TE.
• Long-term ephemeris data are derived, from the orbit prediction, for a time interval 44, which has substantially longer duration than the validity period of broadcast ephemeris data, and which includes, preferably at the beginning of time interval 44, the time ToR.
• orbit predictions can be performed in response to a specific request of a mobile station.
• orbit predictions can be continuously updated in a memory and accessed in case of a request of a mobile station to derive the long-term ephemeris data.

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• Engineering & Computer Science (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Computer Networks & Wireless Communication (AREA)
• Position Fixing By Use Of Radio Waves (AREA)
• Navigation (AREA)
• Radio Relay Systems (AREA)
• Mobile Radio Communication Systems (AREA)

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• 2005
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