Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-08-31
2003-06-24
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06583759
ABSTRACT:
The present invention relates to a method for determining a position according to the preamble of the appended claim
1
. The invention also relates to an electronic device according to the preamble of the appended claim
10
, and a positioning system according to the preamble of the appended claim
17
.
One known positioning system is the GPS system (Global Positioning System) which presently comprises more than 20 satellites. Some of these are out of sight, below the horizon, at a time when viewed from a positioning receiver, wherein the signal of these satellites cannot be received in practice. The satellites transmit e.g. Ephemeris data 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 operating in the GPS system transmits a ranging signal at a carrier frequency of 1575.42 MHz called L1. 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 L2, i.e. 120f
0
. In the satellite, the modulation of these signals is performed with at least one pseudorandom sequence. This pseudorandom 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 L1 signal, the pseudorandom 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 G1 is formed with a polynomial X
10
+X
3
+1, and the second binary sequence G2 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 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 iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the “health”, orbit, time data of the satellite, 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 the satellite 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 may easily occur for example 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 reduces the strength of the signal coming to 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 pseudorange between the receiver and the different GPS satellites, and
2. to determine the position of the receiver by utilizing the calculated pseudoranges 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 pseudoranges, 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 pseudorange 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.
After the code acquisition has been completed, the next steps are frequency tuning and phase locking. This correlation result also indicates the information transmitted in the GPS signal.
The above-mentioned acquisition and frequency control process must be performed for each signal of a satellite received in the receiver. Some receivers may have several receiving channels, wherein an attempt is made on each receiving channel to be synchronized 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. For the positioning, the receiver must receive the signal transmitted by at least four different satellites to find out the x, y, z coordinates and the time data, if none of this information is available for use by the receiver in a sufficiently reliable way. In some cases, it is possible to transmit, e.g. from a base transceiver station, the height data of the base station, wherein for the positioning it is sufficient that the receiver receives the signal transmitted by three satellites. Inaccuracies of a few meters in the height direction do not significantly impair the positioning accuracy. The received navigation information is stored in a memory, wherein of this stored information e.g. satellite position information can be used.
FIG. 1
shows, in a principle chart, positioning in a positioning receiver MS by means of a signal transmitted from four satellites SV
1
, SV
2
, SV
3
, SV
4
. In the GPS system, the satellites transmit Ephemeris data as well as time data, on the basis of which the positioning receiver can perform calculations to determine the position of the satellite at a time. These Ephemeris data and time data
Pietilä Samuli
Syrjārinne Jari
Issing Gregory C.
Nokia Mobile Phones LTD
Ware Fressola Van Der Sluys & Adolphson LLP
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