Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-04-17
2001-05-29
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C701S215000
Reexamination Certificate
active
06239742
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to systems which utilize received signals from satellite positioning systems (SPS) to locate themselves or to determine time-of-day.
SPS receivers such as GPS (Global Positioning System) receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites such as GPS (or NAVSTAR) satellites. These satellites transmit, as part of their satellite data message, both satellite positioning data as well as data on clock timing, so-called “ephemeris” data. In addition they transmit time-of-week (TOW) information that allows the receiver to determine unambiguously local time. Each received GPS signal (in C/A mode) is constructed from a high rate (1.023 MHz) repetitive pseudorandom (PN) pattern of 1023 symbols, commonly called “chips.” Further imposed on this pattern is low rate data at a 50 Hz rate. This data is the source of the above mentioned time-of-week information. The process of searching for and acquiring GPS signals, reading the ephemeris data and other data for a multiplicity of satellites and computing the location of the receiver (and accurate time-of day) from this data is time consuming, often requiring several minutes of time. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications.
In addition, in many situations, where there is blockage of the satellite signals, the received signal level from the GPS satellites is too low to demodulate and read the satellite data signals without error. Such situations may arise in personal tracking and other highly mobile applications. Under these situations it is possible for a receiver to still acquire and track the GPS signals. However, performing location and unambiguous time measurement without such data requires alternative methods.
Tracking the GPS signals without reading the data messages may result in 1 millisecond ambiguities in time, as explained below. Such ambiguities are normally resolved in a conventional GPS receiver by reading the satellite data message, as previously described. At very low received signal levels, the pseudorandom pattern may be tracked, or otherwise used to provide ambiguous system timing by processing many repetitions of this signal (e.g. 1000 repetitions over 1 second). However, unless the signal-to-noise ratio measured over one data period (20 milliseconds) is above about 12 dB, there will be many errors present when attempting to demodulate this signal. The current invention provides an alternative approach for resolving ambiguities in time when such reading is impossible or impractical.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatuses for measuring time related to satellite data messages which are used with satellite position systems, such as GPS or Glonass. A method in one embodiment comprises the steps of: (1) receiving, at an entity, a first record of at least a portion of a satellite data message; (2) comparing the first record with a second record of the satellite data message, where the first record and the second record overlap at least partially in time; and (3) determining a time from the comparing step, where the time indicates when the first record (e.g., the source of the first record) was received at a remote entity. In one example of this embodiment, the remote entity is a mobile SPS receiver and the entity is a basestation which communicates with the mobile SPS receiver through a wireless (and perhaps also wired) link. A method of the present invention may be performed exclusively at the basestation.
An embodiment of the present invention for establishing receiver timing is for the receiver to form an estimate of a portion of the satellite data message and transmit this estimate to the basestation. At the basestation this estimate is compared to a record of the satellite data message received from another GPS receiver or source of GPS information. This record is assumed to be error free. This comparison then determines which portion of the basestation's message most closely matches the data transmitted by the remote unit. Since the basestation has read the satellite data message without error it can associate each data bit of that message with an absolute time stamp, as seen by the transmitting satellite. Hence the comparison results in the basestation assigning an appropriate time to the estimated data transmitted by the remote. This time information may be transmitted back to the remote, if desired.
A variation on the above approach is to have the basestation send a clean record of the satellite data message to the remote plus the absolute time associated with the beginning of this message. In this case the remote compares this record to the estimate of this data which it forms by processing a GPS signal which it receives. This comparison will provide the offset in time between the two records and thereby establish an absolute time for the locally collected data.
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Blakely & Sokoloff, Taylor & Zafman
Blum Theodore M.
SnapTrack, Inc.
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