Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-11-13
2003-11-11
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S464000
Reexamination Certificate
active
06646596
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for positioning a receiver based on code modulated signals transmitted by a plurality of transmitting units, which code modulated signals are formed by an individual code for each of said transmitting units. The invention equally relates to such a receiver, to positioning means and a computing server for positioning a receiver, and to a positioning system comprising a receiver and a computing server.
BACKGROUND OF THE INVENTION
A known positioning system which is based on the evaluation of signals transmitted by satellites as transmitting units is GPS (Global Positioning System). The constellation in GPS consists of more than 20 satellites that orbit the earth in 12 hours. The distribution of these satellites ensure that usually between five and eight satellites are visible from any point on the earth.
Each of the satellites, which are also called space vehicles (SV), transmits two microwave carrier signals. One of these carrier signals L
1
has a carrier frequency of 1575.42 MHz and is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L
1
carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) Code, as which a Gold code is used. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a 1 MHz Pseudo Random Noise (PRN) Code and is spreading the spectrum over a 1 MHz bandwidth. The C/A code is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L
1
signal is further modulated with navigation information at a bit rate of 50 bit/s, which information comprises in particular ephemeris data and data on clock corrections. Ephemeris parameters describe short sections of the orbit or the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section.
Receiving means of a receiver of which the position is to be determined, receive the signals transmitted by the currently available satellites. The information in the received signals enables positioning means connected to the receiving means to compute the distance to several satellites. The computed distance between a specific satellite and a receiver is called pseudo-range, because the time is not accurately known in the receiver. The pseudo range can be computed based on the reciprocal pseudo propagation delay of signals from the respective satellite. The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver. The receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute a position of a receiver in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
Receiving means and positioning means can be comprised in a single, autonomous electronic device constituting a receiver. Alternatively, the positioning means can be external to the receiver. The receiver can for example have access to a cellular network with positioning means. The receiver then only has to transmit the received data to the network, where the positioning calculations are carried out.
The employed modulation technique enables the receiver to distinguish between the signals transmitted by the different satellites and thus to extract the included information, even though the satellites use the same carrier frequency. To this end, the receiver has to synchronize with the respective channel employed by a satellite, i.e. to detect and track the C/A code in the signal.
For detecting and tracking a code of a received signal, GPS receivers usually use a correlation method by which the codes in received signals are compared with replica codes for each satellite available at the receiver. The receiver, or external positioning means, can either generate the respective C/A code sequence for a specific satellite with a code generator, or store the different C/A codes. Before performing the correlation, the received signals are down converted by a multiplication with an intermediate frequency. Then, the down converted signal is multiplied for the correlation with the replica of one of the codes. The receiver slides a replica of the respective code in time and repeats the multiplication. The result of the respective multiplication is integrated or low-pass filtered. As the code in a signal transmitted by a satellite and the receiver code line up completely, a correlation peak is reached at which the resulting value is the greatest. A channel of a received signal resulting in the correlation with a specific replica code in the highest peak is assumed to be the channel employed by the satellite for which this specific replica code is provided. A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the satellite signal.
For illustration,
FIG. 1
shows a situation in which the described positioning of a receiver can be employed. A receiver
1
is located at a position at which it is able to receive signals from at least four GPS satellites SV
1
-SV
4
, as indicated in the figure by arrows. The receiver
1
comprises receiving means
2
for receiving the signals via an antenna, positioning means
3
to which the received signals are forwarded for performing the correlation procedures and for determining the position of the receiver. Storing means
4
provide the positioning means
3
with the replica codes required for the correlation procedures.
In order to enable a positioning in weak signal conditions, an assistance can be provided for recovering the positioning capability. The simplest form of GPS assistance is to deliver navigation data comprising ephemeris data over a cellular network to the receiver. Usually, missing navigation data is the key element why a positioning cannot be maintained or initiated in weak signal condition for a long period.
Assistance data can also be used to improve the sensitivity of a receiver, i.e. to make it capable to acquire and track satellite signals, which are considerably lower than nominal signals. A sufficient sensitivity is a necessary condition for satellite-based positioning in weak signal environments, e.g. within a building.
A high sensitivity, however, has a disadvantage in case strong and weak satellite signals are received at the same time by a receiver. This might be the case for example in a windowed room, in which the signals of one or two satellites are received directly through the window without a significant attenuation, while the signals of all other satellites penetrate the building and are thus attenuated significantly. A cross-correlation of a received strong signal with a wrong replica code may lead to a higher correlation peak than a correlation of a weak satellite signal with a correct replica code. The performed correlation may therefore lead to the detection of signals which are not originating from the satellite of which a replica code is currently used in the correlation procedure. Such erroneously detected signals can either be signals from other satellite signals or noise. These spurious signals may prevent a successful positioning, since even a single spurious signal employed in position calculations may cause the position calculation to fail and in the worst case to destabilize the positioning means for a while.
The attenuation of a signal resulting from a building penetration can be up to 20-30 dBs. A modern GPS receiver with very good sensitivity is able to acquire and track signal that are attenuated up to 30 dBs from the nominal signal level. At the same time, the correlation properties of the C/A or Gold codes in the satellite signals are very poor. The difference resulting between the correlation of a satellite signal with a wrong replica signal compared to the correlation of the same satellit
Syrjarinne Jari
Turunen Seppo
Mull Fred H
Perman & Green,LLP
Tarcza Thomas H.
LandOfFree
Method, system and devices for positioning a receiver does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method, system and devices for positioning a receiver, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method, system and devices for positioning a receiver will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3179963