Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-09-10
2002-10-01
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S378000, C701S213000
Reexamination Certificate
active
06459407
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to global positioning systems, and more particularly to synchronizing the clocks (system time) used by receivers making use of information provided by such positioning systems.
BACKGROUND OF THE INVENTION
In the well known Global Positioning System (GPS), synchronization to system time (also called GPS time) is a key function of any GPS receiver. GPS-based receiver positions are determined by the intersection of a series of simultaneous ranges to orbiting satellite vehicles (SVs). The ranges are established on the basis of the time that elapses between when each SV transmits a ranging signal and when the ranging signal is received by the receiver, multiplied by the speed of light. A ranging signal includes an indication of when, according to system time, the ranging signal was transmitted by the SV. The ranging system is included in what is called the navigation data provided by each SV, which also includes information needed by a receiver to construct satellite orbits (position as a function of system time). The navigation message is a 50 Hz signal consisting of data bits that describe the GPS SV orbits, clock corrections, and other system parameters. If either the SV orbital position data or the GPS time data in the receiver calculations are incorrect or incomplete, as for example when the navigation data provided by a satellite as part of the ranging signal cannot be demodulated correctly due to noisy or weak signal conditions, then substantial positioning errors can result. In such conditions, network-based timing assistance can be used to help reconstruct GPS time information, i.e. to help determine when a ranging signal arrives according to GPS time. GPS time is a “paper clock” ensemble of a Master Control Clock and the SV clocks. GPS time is measured in weeks and seconds from 24:00:00, Jan. 5, 1980, and is steered to (but not synchronized to) within one microsecond of universal coordinated time (UTC); GPS time has no leap seconds and is ahead of UTC by several seconds. What is here called SV time is the time maintained by a satellite, usually using four atomic clocks (two cesium and two rubidium), monitored by ground control stations and occasionally reset to maintain time to within one-millisecond of GPS time; clock correction data bits reflect the offset of each SV clock from GPS time.
The navigation message consists of time-tagged data bits organized into frames of 1500 bits divided into five 300-bit subframes; the time-tagged data bits mark the time of transmission of each subframe. (Each subframe indicates when the first bit of the next subframe is to be broadcast, according to SV time). Since the GPS Navigation Message is broadcast at 50 Hz, a data frame is transmitted every thirty seconds, and a subframe every six seconds. Three six-second subframes contain orbital and clock data. SV clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up what is referred to as a complete navigation message, which is sent over a 12.5 minute period.
The basic operation in GPS based positioning assumes an estimate of a signal receiving time and signal transmitting times from each of several satellites (ideally at least four), which are used to calculate satellite positions and then the ranges from the satellites to the user (GPS receiver). The transmitting time estimates can be obtained from a knowledge of three components, namely, a Time of Week (TOW) component (with a precision of six seconds), a second, millisecond component related to a number of milliseconds that have elapsed since the beginning of a subframe, and a third, sub-millisecond component.
The standard positioning mechanism proceeds as follows. First, from the data message provided by the SV signal, the TOW at which the SV signal was transmitted is found, i.e. the time according to the GPS receiver at which the bits indicating the TOW are received is noted by the GPS receiver. From that time, the GPS receiver counts the number of milliseconds and sub-milliseconds. (The sub-millisecond component is provided based on a correlation of the spread spectrum signal bearing the received data message with a replica, and the millisecond component is provided based on the difference between the latest millisecond count and the millisecond count at TOW. When the satellite is tracked there is an internal millisecond counter that counts from some arbitrary instant of time the number of code epochs, i.e. code periods. Given the millisecond count at the TOW point of a received signal and also the latest millisecond count, both measured from some same point in time, the difference is the millisecond count from TOW.) The time of transmission is then computed as:
t
trans
=TOW+
milliseconds+sub-milliseconds,
and the pseudorange is formed as:
pseudorange=estimate of time of reception−
t
trans
.
The estimate of the time of reception is often computed as the time of transmission for one of the channels plus a nominal time of flight (such as e.g. 70 msec).
As indicated above, in determining the pseudorange according to the standard procedure, the GPS receiver should be synchronized to the GPS time (so as to be able to determine the time that has elapsed since a TOW signal was transmitted by a SV and received by the GPS receiver). Multiple SVs and a navigation solution (or a known position for a timing receiver) permit SV time to be set to an accuracy limited by the position error and the pseudorange error for each SV. After a GPS receiver precisely determines SV time for a satellite, it converts it to GPS time using information provided in the navigation message.
As mentioned, the TOW count is reported in the navigation message every six seconds; however, in weak signal conditions, demodulation of navigation data is sometimes not possible. In such conditions, time assistance from a cellular network (via a base station of the cellular network) accurate to within a few seconds can provide the TOW count, and the sub-millisecond component can be obtained from a receiver using state-of-the-art acquisition and tracking techniques. However, when the navigation message cannot be demodulated and when any network assistance provides insufficiently accurate timing information, the millisecond part of the time must be recovered using other methods (because cellular assistance and state-of-the-art acquisition techniques are of no use in providing an estimate of the millisecond part).
For the millisecond part, cross-correlation is used in case of poor signal conditions to align the earlier-received reconstructed GPS navigation signal subframes or bit sequences with the same information elements received subsequently, but the cross-correlation technique should account for the sinusoidal modulations of the carrier signal remaining after the tracking phase in the GPS receiver, modulation caused by Doppler shifting and by clock drift, what are here called residual sinusoidal modulations. If the residual modulations are not compensated for, modulation of the satellite carrier signal by sinusoidal modulation due to Doppler frequency and clock drift makes conventional cross-correlation to determine the time of transmission ineffective.
The Problem that the Invention Overcomes
The invention is of use in case of a GPS receiver tracking a satellite, but in a situation where the satellite navigation data cannot be decoded (for example because of poor signal conditions).
The invention overcomes the problem of determining GPS time in case of a poor-quality SV signal, poor enough in quality that the navigation data cannot be decoded from the SV signal by the GPS receiver and the tracking component of the receiver does not completely compensate for the Doppler frequency shift and clock drift. The invention improves on the prior art in that it compens
Akopian David
Syrjārinne Jari
Blum Theodore M.
Nokia Mobile Phones
Ware Fressola Van Der Sluys & Adolphson LLP
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