Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-01-03
2002-07-09
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490
Reexamination Certificate
active
06417800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for determining reference time error as set forth in the preamble of the appended claim
1
, a positioning system as set forth in the preamble of the appended claim
10
, an electronic device as set forth in the preamble of the appended claim
20
, as well as a computing server as set forth in the preamble of the appended claim
21
.
2. Brief Description of the Related Developments
One known positioning system is the GPS system (Global Positioning System), which presently covers more than 20 satellites. These satellites transmit e.g. Ephemeris data as well as data about the time of the satellite. A receiver used for positioning normally infers its location by calculating the time of propagation of a signal to be transmitted simultaneously from several satellites of the positioning system to the receiver. For the positioning, the receiver must typically receive the signals of at least four visible satellites to be able to calculate the position.
Each GPS satellite transmits a so-called L
1
signal at a carrier frequency of 1575.42 MHz. This frequency is also indicated as 154f
0
, in which f
0
=10.23 MHz. Furthermore, the satellites transmit an L
2
signal at a carrier frequency of 1227.6 MHz, i.e. 120f
0
. In the satellite, these signals are modulated with at least one pseudo random noise (PRN) sequence. This PRN sequence is different for each satellite. As a result of modulation, a code-modulated wideband signal is generated. The modulation technique used makes it possible for the receiver to separate the signals transmitted by the 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, the PRN sequence used for modulating the L
1
signal is e.g. a so-called C/A code (Coarse/Acquisition code), as which a Gold code is used. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two binary sequences of 1023 bits. The first binary sequence G
1
is formed with the polynomial X
10
+X
3
+1 and the second binary sequence G
2
is formed by delaying the polynomial 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 that different C/A codes can be formed with a similar code generator. Consequently, C/A codes are binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the epoch of the code is 1 ms. The carrier frequency of the L
1
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.
Satellites monitor the condition of their equipment during their operation. The satellites can use for example so-called watch-dog functions to detect and report failures possibly occurred in the equipment. The failures and functional disorders can be momentary or last a longer term. On the basis of the health data, some of the failures can possibly be compensated for, or the information transmitted by a failed satellite can be totally disregarded. Furthermore, in a situation in which the signal of more than four satellites can be received, the information received from different satellites can be weighted differently. It is thus possible to minimize errors in measurements which are possibly caused by satellites which seem unreliable.
To detect signals from the satellites and to identify the satellites, the receiver must perform synchronization, in which the receiver searches for the signal of each satellite at a time and attempts to be synchronized to this signal, so that the data to be transmitted with the signal can be received and demodulated.
A positioning receiver must perform synchronization e.g. when the receiver is turned on and also in a situation in which the receiver has not been able to receive a signal from 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 optimally oriented with respect to the satellites, which weakens the strength of the received signal. Also in urban areas, buildings affect the signal to be received, and moreover, so-called multipath propagation can occur, in which the transmitted signal enters the receiver via different propagation paths, e.g. directly from the satellite (line-of-sight) and also reflected from buildings. This multipath propagation causes that the same signal is received as several signals with different phases and delays.
The positioning arrangement has two primary functions:
1. to calculate the pseudoranges between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by using the calculated pseudoranges and the position of the satellites. The position of the satellites at a 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 pseudoranges, because the time is not accurately known in the receiver. Thus, the determinations of the position and the time are iterated, until a sufficient accuracy has been achieved with respect to the time and the position. Because the time is not known with absolute accuracy, the position and the time must be determined e.g. by linearizing a set of equations for each new iteration.
The pseudorange can be calculated by measuring the relative propagation delay differences of signals from the different satellites. After the receiver has been synchronized with the received signal, the information transmitted in the signal can be determined.
Almost all known GPS receivers use correlation methods for acquisition and tracking of the code. Reference codes ref(k), ie. the PRN sequences of the different satellites, are stored or generated locally in the positioning receiver. The received signal is subjected to conversion to an intermediate frequency (down conversion), after which the receiver multiplies the received signal with the stored PRN sequence. The signal formed as a result of the multiplication is integrated or low-pass filtered, wherein the result is information on whether the received signal contained a signal transmitted by a satellite. The multiplication to be performed in the receiver is iterated in such a way that each time, the phase of the PRN sequence stored in the receiver is shifted. The correct phase is deduced from the correlation result preferably in such a way that when the correlation result is the greatest, the correct phase has been found. Thus, the receiver is correctly synchronized with the received signal.
The acquisition of the code is followed by fine adjustment of the frequency and by phase locking. This correlation result also indicates the information transmitted in the GPS signal.
The above-mentioned acquisition and frequency adjustment process must be performed for each signal of a satellite which is received in the receiver. In some receivers, there may be several receiving channels, wherein the aim is to synchronize each receiving channel with the signal of one satellite at a time and to find out the information transmitted by this satellite.
The positioning receiver receives information transmitted by satellites and performs positioning on the basis of the received information. To perform the positioning, the receiver must receive the signals transmitted by at least four different satellites, to be able to find out the x, y, z coordinates and the time. The received navigation information is stored in a memory, wherein of this stored information, e.g. the Ephemeris data on the satellites can be used.
So-called differential positioning DGPS has been developed particularly for adjusting the positioning of a mobile receiver. Thus, the positioning
Syrjārinne Jari
Valio Harri
Nokia Mobile Phones Ltd.
Perman & Green LLP
Phan Dao
LandOfFree
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