Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2003-05-05
2004-08-31
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S372000, C701S213000
Reexamination Certificate
active
06784831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates broadly to systems for receiving Global Positioning System (GPS) satellite signals, and more particularly, to GPS receiving systems employing a phased array communication mechanism.
2. State of the Art
GPS is a satellite navigation system funded by and controlled by the U.S. Department of Defense (DOD). While there are many thousands of civilian users of GPS world-wide, the system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. The Space Segment of the GPS system consists of a constellation of satellites that send radio signals from space. The nominal GPS Operational Constellation consists of twenty-four satellites that orbit the earth in twelve hours. There are often more than twenty-four operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each twenty-four hours (four minutes earlier each day). There are six orbital planes (with nominally four satellites in each plane), equally spaced (sixty degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight satellites visible from any point on the earth.
Each GPS satellite transmits a microwave signal L
1
at a carrier frequency of 1575.42 MHz and a microwave signal L
2
at a carrier frequency of 1227.60 MHz. The L
1
signal and the L
2
signal carry the GPS Navigation Message and binary codes, which shift the phase of the L
1
and L
2
signals. The binary codes of the L
1
signal include a C/A Code (Coarse Acquisition code) and P Code (Precise code). The L
2
signal includes only the P Code. The P Code is a repeating 10.23 MHz pseudo-random noise (PRN) sequence. The C/A code is a repeating 1.023 MHz PRN sequence. These noise-like codes provide spread-spectrum modulation of the L
1
and L
2
signals. The P code is intended for military use and is only available to authorized users. Civilian users access the GPS signals through the C/A code. There is a different C/A code PRN for each GPS satellite. GPS satellites are often identified by their PRN number, the unique identifier for C/A code PRN assigned thereto. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. The GPS Navigation Message consists of time-tagged data bits. Such time-tagged data bits provided by multiple GPS satellites are used to determine latitude, longitude, height, velocity and the exact time. Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.
The GPS constellation's design insures that six to eleven satellites are in view from any point on the earth's surface at any given time. Because of the GPS signal design, two-dimensional and three-dimensional positions can be determined with the signals from just three and four satellites respectively. Accordingly, GPS receivers typically have the capability of automatically selecting three or four of the satellites in view based upon their received signal strength and Position Dilution of Precision (PDOP).
A number of undesirable interference sources (e.g., deliberate electronic countermeasures, RF electromagnetic pollution, clutter scatter returns and nature noise) can cause a GPS receiver to be ineffective or unreliable. Since GPS signals are very weak, they can be overcome by interference caused by even low power and low cost transmitters. When close to the interference source, GPS receivers are unable to track the satellite signals, and when further from the interference source, GPS receivers are able to track, but not acquire the GPS signals.
Phased array antenna systems have been predominantly used in military and aerospace applications because of their high implementation costs. In such systems, the signals received from a number of antenna elements are supplied to signal processing channels that provide a variable gain and variable phase shift to such signals. An antenna pattern for the combined receive signal can be formed by a set of specific gain values and phase shift values over the signal processing channels and a specific geometry and placement of the N antenna elements. The set of specific gain values and phase shift values is commonly referred to as “weights” (or “weight vector”) for the phased array antenna system. A unique advantage of the phased array antenna system is that the antenna pattern can be adjusted by changing the “weights” as described above to perform one or both of the following operations:
a) beam steering: steering the beam by adjusting the phase shift values of the pattern for each processing channel; no adjustment to the gain values of the pattern is necessary.
b) antenna null: the phase shift values and gain values of the pattern are adjusted to the suppress signal (i.e., interference) from a specific direction.
U.S. Pat. No. 6,246,369 to Brown et al. describes a phased array antenna system for use in conjunction with a GPS receiver. The phased array antenna system nulls interference sources and/or applies gain through beam steering in the direction of the desired GPS signal sources.
FIGS. 1A and 1B
are block diagrams that illustrate this phased array antenna system, which includes a digital front end (DFE) unit
70
, a digital beam steering (DBS) card
71
and a receiver processor board
72
that reside inside a personal computer
74
and that are controlled by a software program through the computer data bus. A plurality of DFE channels
63
within the DFE unit
70
convert the analog signals output from the antenna elements
18
to a digital sampled signal. Each of the DFE channels
63
operates from a common reference local oscillator (REF LO)
61
and a common sample clock
64
which is synchronized to the local oscillator
61
. The outputs from the plurality of DFE channels
63
are passed to the DBS card
71
where the digital phase shifting is applied. The DBS card utilizes digital signal processing logic blocks
62
to apply complex weights to the input digital signals and form a digital summation to thereby provide composite complex digital output signals to a plurality of channels
73
of the receiver processor board
72
.
The DSP logic blocks
62
operate under control of the personal computer
74
to provide the complex weights to adjust the antenna array pattern in order to track the GPS satellites as they move across the sky, to apply calibration corrections to compensate for offset between the individual antennas and the DFEs, or to apply nulling in the direction of the an interference source.
FIG. 1B
illustrates the circuit components for each DFE channel
63
of the DFE unit
70
, which operates to down-convert the GPS signals from radio frequency (RF) to intermediate frequency (IF) and to sample and convert the analog IF signal into a digital data stream. The GPS signals received at the antenna element
81
are passed through a low-noise amplifier
82
, a ceramic filter
83
and another amplifier
84
for output to a mixer
85
. The mixer
85
mixes this signal with coherent signals generated by a common local oscillator
61
. The mixed and down-converted signals are then passed through a surface acoustic wave (SAW) filter
86
to form the IF signals. The IF signals are then passed through an amplifier
87
and an automatic gain control stage
88
, which is operated under control of computer
74
to set the correct levels for analog-to-digital converter
89
. The output of the A/D converter
89
is a sampled digital data stream that represents the digitized GPS data signals from each antenna element.
One of the drawbacks of
Lin Wen Yen
Wang James June-Ming
Yang Chau-Chin
Blum Theodore M.
Gordon & Jacobson P.C.
Tia Mobile, Inc.
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