Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-06-15
2003-03-25
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S458000
Reexamination Certificate
active
06538601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to communication and navigation systems. More particularly, it relates to mobile communications systems with satellite and terrestrial navigation features.
2. Description of the Related Art
Satellite-based navigation systems, such as the Global Positioning System (GPS), allow for accurate navigation almost anywhere in the world. A constellation of GPS satellites surrounds the earth in three different planes with the satellites orbiting the earth every 12 hours. The orbit of each satellite is closely monitored and its position is precisely known. Each GPS satellite transmits navigation signals, each signal being modulated with at least one pseudo-random noise (PN) code, or a portion of a PN code, unique to the satellite. Each GPS satellite uses a very stable frequency reference for generating the satellite's signals. Also modulated onto the navigation signals is a satellite data message (SDM) with a bit rate of 50 bits/second, which is much slower than the chipping rate of the PN code. The signal structure of the GPS satellites is described in further detail in Spilker, Jr., J. J. “GPS Signal Structure and Performance Characteristics,” Global Positioning System, Papers Published in Navigation, The Institute of Navigation, Washington, D.C., 1980, which is incorporated by reference herein for all purposes.
Using a GPS receiver with an unobstructed view of the sky a person can determine his or her position within a few meters. In order to determine its position in three dimensions, and to determine time precisely, the receiver must receive navigation signals from at least four satellites, as shown in FIG.
1
. Here, a mobile unit (MU) can determine its position (x, y, z) and time by determining its distance from four sources, in this case four GPS satellites (
10
-
1
,
10
-
2
,
10
-
3
and
10
-
4
), at known positions ((x
1
, y
1
, z
1
), (x
2
, y
2
, z
2
), (x
3
, y
3
, z
3
), (x
4
, y
4
, z
4
)). A conventional GPS receiver is shown in FIG.
2
. It includes an antenna
20
, a front end
30
for converting the received signals into digital signals in a form suitable for processing,.an array of delay lock loop (DLL) circuits
40
and a Kalman filter estimator
50
for estimating the receiver's position. Each DLL circuit outputs a pseudo-range measurement (PR
1
through PR
n
, respectively) to a summation unit (units
51
1
through
51
n
) in the Kalman filter estimator
50
. Each summation unit combines the latest pseudo-range measurement from the DLL with the Kalman filter's estimate of the receiver's position x(t) and provides the combined signal to a vector estimation unit
52
. The vector estimation unit
52
uses the outputs of the summation unit to update the Kalman filter's estimate of the receiver's position and outputs a position vector x{circumflex over ( )}(t).
A schematic diagram of each of the delay lock circuits (DLL
1
through DLL
n
) is shown in
FIG. 3
, shown here as a coherent DLL, although a non-coherent DLL also can be used. Each DLL circuit includes a local PN code generator
65
that generates the same PN codes broadcast by the GPS satellite the DLL circuit is tracking. The conventional receiver also includes an early correlator
60
E and a late correlator
60
L that correlate the received signal (i.e., the satellite signal A(t) at t=&tgr;, plus noise n(t)) with an early and late versions of the locally generated PN code S
0
(t). As shown in
FIG. 3
the early version of the PN code is advanced by half a PN chip period T (i.e., +T/2) and the late version is delayed by half a PN chip period (i.e., −T/2). The early and late correlation signals are each filtered by low pass filters
61
E and
61
L, respectively. The filtered signals are combined using combiner
62
and the resultant signal is filtered by loop filter
63
. The output of loop filter
63
is a control signal that controls a number controlled oscillator (NCO)
64
. The NCO generates a number, based on the control signal from loop filter
63
, that causes the PN generator
65
to output its PN sequence either faster or slower depending on which of the early and late correlators outputs a stronger signal. The DLL circuit tracks the code in the received signal and a time measurement, called a pseudo-range measurement, is made. The measured pseudo-range is a measure of the signal propagation time between the satellite and the receiver.
Upon acquiring the satellite signal, the receiver demodulates the SDM from that received signal. The SDM includes satellite ephemeris information concerning the orbits and positions of each satellite, and satellite time model information concerning the satellites' clocks. A receiver uses this information in combination with the measured pseudo-ranges to determine the receiver's position by calculating a navigation solution.
The SDM is 900 bits long and is broadcast every 30 seconds at 50 bits/second. Accordingly, a GPS receiver can receive and store the SDM if it is able to receive a satellite's signal continuously for the amount of time the SDM is broadcast, and during the period that the satellite broadcasts the SDM. Also modulated onto the navigation signal at 50 bits/second is the GPS almanac: a 15,000 bit block of coarse ephemeris and time mode data for the entire GPS constellation. The receiver needs this course data to assist in its acquiring a GPS satellite signal. Receiving the GPS almanac can take at least 12 ½ minutes of listening to a single GPS satellite to receive the almanac data.
Other methods of delivering the SDM and GPS almanac data are known. For example, U.S. Pat. No. 5,365,450, incorporated by reference herein for all purposes, describes delivering the SDM via a cellular telephone system to shorten the time required to acquire a GPS satellite. There, a person wanting to rapidly acquire a GPS navigation signal uses a cellular telephone network to request the SDM. The cellular network stores the current ephemeris and time models of the GPS satellite constellation in a GPS satellite almanac database. Upon receiving the request the cellular telephone system responds to the request by sending the requested data over an independent data link. The receiver then uses that data to acquire a GPS satellite signal. However, this system relies on the receiver having an unobscured view of the GPS satellites to continuously track the satellites. Such an unobscured view is not always available to a user, particularly a mobile user.
Certain environments can obstruct the view of the sky. A prime example is the so-called “urban canyon” where tall buildings densely concentrated in an urban setting obscure significant portions of the sky and block the GPS satellite signals preventing their reception. One way to track GPS satellites in an obscured environment is to use a vector delay lock loop described in U.S. Pat. No. 5,398,034, incorporated by reference herein for all purposes. A vector DLL improves over a conventional delay lock loop by using the receiver's estimated position vector (x, y, z and time) to control the local PN code generator, rather then using the correlator output as in a conventional DLL circuit. The vector DLL can use measurements from a variety of sources in computing the receiver's position vector. These sources could include signals from all GPS satellites visible to the receiver, signals from other visible navigation satellites such as satellites in the former Soviet Union's GLONASS system, and even signals from ground-based navigation transmitters such as so-called “pseudolites” that transmit a GPS-like navigation signal from a known, fixed location. A conventional vector DLL is shown in
FIG. 4
, and is similar to the receiver shown in
FIG. 2
, but also contains a H(x) transformation unit
70
that transforms the estimated position information into control signals for controlling the DLLs.
Other techniques for providing navigation signals in environments with obstructed visibility
Bruno Ronald C.
Schuchman Leonard
Edell, Shapiro, Finnan & Lytle LLC
ITT Manufacturing Enterprises Inc.
Phan Dao
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