Wireless assisted GPS using a reference location

Details Wireless assisted GPS using a reference location Wireless assisted GPS using a reference location

1999-05-27

2003-03-25

Phan, Dao (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

C342S357490

Reexamination Certificate

active

06538600

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to satellite positioning systems and, in particular, to a telecommunications-based auxiliary system that assists a wireless terminal in determining its position from signals transmitted from one or more navigation satellites.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a prior art satellite positioning system
100
, also referred to herein as a Wireless Assisted Global Positioning System (WAG). The satellite positioning system depicted comprises wireless terminal or WAG client
501
, satellite constellation
503
, auxiliary system or WAG server
505
, and timing source
507
. Satellite constellation
503
is the Global Positioning System (GPS) comprising a plurality of satellites for transmitting GPS signals that convey a variety of location related information including ranging information and satellite ephemeris information. GPS is well-known in the art. It will be clear to those skilled in the art how to make and use embodiments of the present invention that work with other satellite constellations. WAG server
505
and WAG client
501
are operable to acquire GPS signals and process GPS signals (i.e., demodulate and extract) in order to obtain the information being conveyed by the GPS signals.
The principal goal of WAG is to reduce the signal acquisition and signal processing requirements of a conventional wireless terminal so that a wireless terminal in accordance with the illustrative embodiment can determine its location more quickly and with weaker signals than wireless terminals in the prior art. In accordance with WAG, the signal acquisition and signal processing requirements of wireless terminal
501
are reduced at the expense of auxiliary system
505
. In particular, the tasks of signal acquisition and signal processing required for a conventional wireless terminal to determine its position are divided between wireless terminal
501
and auxiliary system
505
.
It will be clear to those skilled in the art how the signal processing task can be divided between wireless, terminal
501
and auxiliary system
505
, as partially processed signal information can be exchanged back and forth between the two through wireless telecommunications link
504
as needed to achieve desirable division of the signal processing task.
It is possible to divide the signal processing task between wireless terminal
501
and auxiliary system
505
because each signal transmitted by each satellite in satellite constellation
503
conveys ranging and satellite ephemeris information that are responsive to independent acquisition and independent processing. Specifically, a GPS signal transmitted from a particular satellite will convey the same satellite ephemeris information to a receiver (e.g., WAG client or WAG server) regardless of which receiver acquires and processes the signal. Basically, the GPS signal is modulated with digital information in a manner similar to how, for example, a cellular telephone's radio signal is modulated with voice data. Such information can be detected and demodulated by any receiver adapted to do so. The information reconstructed by the receiver is an exact replica of the information modulated onto the signal by the transmitter (except for unwanted errors due to noise, distortion, etc.) and is the same for all receivers, regardless of their position.
By contrast, a GPS signal transmitted from a particular satellite will convey different ranging information to different receivers acquiring and processing the GPS signal. The transmitter (i.e., satellite) carefully adjusts the timing of the transmitted signal according to some precise reference, such that the timing of the signal, as received by the receiver, carries information about the distance between the transmitter and the receiver (and, therefore, about the receiver's position). Such information will be different from receiver to receiver, and is only available at the receiver itself.
For example, since each satellite in constellation
503
transmits a signal
502
that contains both kinds of information to both wireless terminal
501
and auxiliary system
505
, some or all of the information about the satellites' ephemeris is acquired by auxiliary system
505
through antenna
553
, even though the ranging information acquired by auxiliary system
505
is relevant to the position of auxiliary system antenna
553
and not to the position of wireless terminal
501
. However, auxiliary system
505
has approximate knowledge of the position of wireless terminal
501
(for example, through knowledge of the cell and sector where the wireless terminal is located); therefore, auxiliary system
505
combines this knowledge with the acquired ranging and satellite ephemeris information to compute an estimate of the ranging information at the position of wireless terminal
501
. This estimate, together with the satellites' ephemeris information, is transmitted, via wireless telecommunications antenna
551
, to wireless terminal
501
to assist wireless terminal
501
in acquiring and processing ranging information.
Once the ranging information has been acquired by wireless terminal
501
, wireless terminal
501
can use the satellite ephemeris information and ranging information to determine its location, or wireless terminal
501
can transmit the ranging information back to auxiliary system
505
so that auxiliary system
505
can determine the location of wireless terminal
501
.
Because wireless terminal
501
is freed from the task of acquiring some or all of the information about the satellites' ephemeris and is advantageously provided with an estimate of the ranging information, it can be fabricated from less expensive technology that need only perform the easier task of acquiring and processing the ranging information with a priori knowledge of an estimated form of that information. Furthermore, because the satellite ephemeris information is modulated onto the same carrier as the ranging information, the provision of the satellites' ephemeris information to wireless terminal
501
enables wireless terminal
501
to remove the satellites' ephemeris information from the satellite signal received through antenna
512
and, thereby, acquire the ranging information even under faded conditions of low signal-tonoise noise ratio that are inadequate for the operation of a wireless terminal in prior art.
FIG. 2
depicts a block diagram of the salient components of auxiliary system
505
, which comprises: timing signal receiver
603
, timing signal antenna
552
, coarse location estimator
601
, telecommunications system manager
617
, GPS receiver
605
, GPS receiver antenna
553
, timing signal calibrator
607
, PRN synchronization estimator
609
, demodulator
611
, satellite visibility estimator
613
, satellite Doppler estimator
615
, telecommunications transmitter
619
and telecommunications antenna
551
.
In general, auxiliary system
505
uses its GPS receiver to obtain from each satellite above the horizon both ranging information and information about the satellite's ephemeris, in well-known fashion using a Coarse Acquisition (C/A) code. It will be clear to those skilled in the art how to make and use embodiments of the present invention that use a P(Y) or P code. In the process of obtaining the ranging and satellite ephemeris information, auxiliary system
505
learns, among other things: (1) the PRN synchronization from each satellite (i.e., the exact timing of the PRN code transmitted by each satellite), (2) the Doppler shift associated with each satellite, (3) which satellites are above the horizon, and (4) the 50 bps modulated bit stream from each satellite. Auxiliary system
505
then transmits to wireless terminal
501
, via a wireless telecommunications channel, for each satellite above the horizon: (1) an estimate of the PRN synchronization, (2) an estimate of the Doppler shift, and (3) the 50 bps modulated bit stream. Collectively, this information will be called “Navigation Message Data.”
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