Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-05-29
2003-05-27
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06570533
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for determining the phase of information modulated in a code-modulated signal transmitted by a satellite, using a receiver. At least some of the same information is transmitted from at least a first and a second satellite substantially simultaneously and the code-modulated signal transmitted from at least the first and the second satellite is received.
The invention also relates to a positioning system including at least two satellites, a positioning receiver, and means, in the receiver, for determining the phase of information modulated in a code-modulated signal transmitted by the satellites. In the positioning system at least partly the same information is arranged to be transmitted from first and second satellites substantially simultaneously. The receiver includes at least means for receiving the code-modulated signal transmitted from the first and the second satellite.
The invention further relates to a positioning receiver including means for receiving a code-modulated signal transmitted from at least a first and a second satellite, where at least partly the same information is being transmitted in a code-modulated signal from the first and the second satellite substantially simultaneously. The positioning receiver also includes means for determining the phase of information modulated in the code-modulated signal transmitted from the satellites.
The invention still further relates to an electronic device including a positioning receiver for receiving a code-modulated signal transmitted from at least a first and a second satellite, at least partly the same information being transmitted in a code-modulated signal from the first and the second satellite substantially simultaneously. The electronic device further includes means for determining the phase of information modulated in the code-modulated signal transmitted from the satellites.
2. Discussion of the Related Art
One known positioning system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites, of which a maximum of 12 are simultaneously within the sight of a receiver. These satellites transmit e.g. Ephemeris data of the satellite, as well as data on the time of the satellite. A receiver used in positioning normally deduces its position by calculating the propagation time of a signal transmitted simultaneously from several satellites belonging to the positioning system to the receiver. For the positioning, the receiver must typically receive the signal of at least four satellites within sight to compute the position.
Each satellite of the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L
1
. This frequency is also indicated with 154f
0
, where f
0
=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L
2
, i.e. 120f
0
. In the satellite, the modulation of these signals is performed with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. As a result of the modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible in the receiver to distinguish between the signals transmitted from different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L
1
signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G
1
is formed with a polynome X
10
+X
3
+1, and the second binary sequence G
2
is formed by delaying the polynome X
10
+X
9
+X
8
+X
6
+X
3
+X
2
+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to produce different C/A codes with an identical code generator. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the code epoch is 1 ms. The L
1
carrier signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the health of the satellite, its orbit, time data, etc.
During their operation, the satellites monitor the condition of their equipment. The satellites may use for example so-called watch-dog operations to detect and report possible faults in the equipment. The errors and malfunctions can be instantaneous or longer lasting. On the basis of the health data, some of the faults can possibly be compensated for, or the information transmitted by a malfunctioning satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, different satellites can be weighted differently on the basis of the health data. Thus, it is possible to minimize the effect of errors on measurements, possibly caused by satellites which seem unreliable.
To detect the signals of the satellites and to identify the satellites, the receiver must perform acquisition, whereby the receiver searches for the signal of each satellite at the time and attempts to be synchronized and locked to this signal so that the data transmitted with the signal can be received and demodulated. After the acquisition, the receiver attempts to keep locked, or to track the signal of the satellite at least during the time of positioning, but in some cases, the tracking phase can be maintained as long as the receiver receives the signal of the satellite sufficiently strongly.
The positioning receiver must perform the acquisition e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. Such a situation can easily occur e.g. in portable devices, because the device is moving and the antenna of the device is not always in an optimal position in relation to the satellites, which impairs the strength of the signal coming in the receiver. Also, in urban areas, buildings affect the signal to be received, and furthermore, so-called multipath propagation can occur, wherein the transmitted signal comes into the receiver along different paths, e.g. directly from the satellite (direct line-of-sight) and also reflected from buildings. This multipath propagation causes that the same signal is received as several signals with different phases.
The positioning arrangement has two primary functions:
1. to calculate the pseudo range between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudo ranges and the position data of the satellites. The position data of the satellites at each time can be calculated on the basis of the Ephemeris and time correction data received from the satellites.
The distances to the satellites are called pseudo ranges, because the time is not accurately known in the receiver. Thus, the determinations of position and time are iterated until a sufficient accuracy is achieved with respect to time and position. Because time is not known with absolute precision, the position and the time must be determined e.g. by linearizing a set of equations for each new iteration.
The pseudo range can be calculated by measuring the pseudo transmission time delays between signals of different satellites. After the receiver has been synchronized with the received signal, the information transmitted in the signal is determined.
FIG. 1
b
shows, in a reduced principle view, the determination of the code phase and the frequency deviation on the basis of the received signal by dividing a two-dimensional code phase and frequency space into cells C
11
, C
12
, . . . , C
Syrjārinne Jari
Valio Harri
Mull Fred H
Nokia Mobile Phones Ltd.
Perman & Green LLP
Tarcza Thomas H.
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