Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2003-03-27
2004-08-03
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S387000
Reexamination Certificate
active
06771215
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for determining an accurate time of transmission of a signal part of a code modulated signal transmitted by a beacon of a positioning system and received by a receiver of said positioning system, which receiver tracks beacons signals with a tracking unit. The invention relates equally to a corresponding receiver and to a positioning system comprising a receiver.
BACKGROUND OF THE INVENTION
A well known positioning system which is based on the evaluation of signals transmitted by beacons is GPS (Global Positioning System). The constellation in GPS consists of more than 20 satellites employed as beacons that orbit the earth. The distribution of these satellites ensure that usually between five and eight satellites are 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 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. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated 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 in particular ephemeris and almanac data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position and the velocity of the satellite for any time while the satellite is in the respective described section. The orbits which are calculated based on ephemeris parameters are quite accurate, but the ephemeris parameters are only valid for a short time, i.e. for about 2-4 hours. The almanac data, in contrast, contain coarse orbit parameters. The orbits calculated based on almanac data are not as accurate as the orbits calculated based on ephemeris data, but they are valid for a longer time, a week or even more. Almanac and ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission of the ranging code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits.
Based on the time of transmission and the measured time of arrival TOA of the ranging code at the receiver, the time of flight TOF required by the ranging code to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the general GPS time is not accurately known in the receiver. Usually, the receiver calculates the accurate time of arrival of a ranging code based on some initial estimate, and the more accurate the initial time estimate is, the more efficient are position and accurate time calculations. A reference GPS time can, but does not have to be provided to the receiver by a network.
The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute a position of a receiver in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
If navigation data are available on one of the receiver channels, the indication of the time of transmission comprised in a received signal can also be used in a time initialization for correcting a clock error in the receiver. In GPS, an initial time is needed for the positioning. For the initial time estimate, the average propagation time of satellite signal of around 0.070 seconds is added to the time of transmission of a ranging code extracted from the navigation information. The result is used as initial estimate of the time of arrival of a ranging code, which estimate lies within around 20 ms of the accurate time of arrival. The receiver then determines for different satellites the time at which a respective ranging code left the satellite. Using the initial estimate of the current time, the receiver forms pseudorange measurements as the time interval during which the respective ranging code was propagating from the satellite to the receiver either in seconds or in meters by scaling with the speed of light. After the position of the receiver has been calculated from the determined pseudoranges, the accurate time of arrival can then be calculated from standard GPS equations with an accuracy of 1 &mgr;s.
However, in order to be able to make use of such a time initialization, the navigation data from a satellite signal is needed. Currently, most of the GPS receivers are designed for outdoor operations with good signal levels from satellites. Thus, only good propagation conditions ensure that the navigation data required for the described time initialization is available.
In bad propagation conditions, in contrast, it may not be possible to extract the navigation message accurately enough from received satellite signals, since a high bit-error rate and weak signal levels make a robust decoding of navigation bits impossible. Such bad propagation conditions, which are often given indoors, render the time initialization and the pseudorange measurements more difficult.
For those cases in which the standard time initialization methods cannot be applied since the navigation data are noisy, the time initialization process for the receiver can be performed by a time recovery method. Some known time recovery methods are based on the cross-correlation of a part of a tracked signal and an expected signal part to define the time of transmission.
Cross-correlation methods allow for determining the system time with an accuracy of one microsecond. It is a disadvantage of cross-correlation methods, however, that they require raw navigation data and a reference time with an accuracy of 6-30 seconds.
For the case that only submillisecond or sub-20 millisecond measurements of the time of transmission are available, e.g. from epoch and chip counts in the tracking unit of the receiver, it has been proposed to use a search technique over the possible GPS time which minimizes the sum of squared residuals or errors (SSE). Such a method has been presented by J. Syrjarinne in “Time recovery through fusion of inaccurate network timing assistance with GPS measurement”, Proc. 3rd Int. Conference on Information Fusion, Paris, France, July 10-13 2000, Vol. II, pp.WeD5-3-WeD5-10, and in “Possibilities for GPS Time recovery with GSM Network Assistance”, in Proc. ION GPS 2000, Salt Lake City, Utah, USA, Sep. 19-22 2000].
Moreover, a 5-dimensional Least Square approach has been proposed to find the position and the time
Akopian David
Valio Harri
Issing Gregory C.
Nokia Corporation
Ware Fressola Van Der Sluys & Adolphson LLP
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