Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-12-06
2003-12-16
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S445000
Reexamination Certificate
active
06664921
ABSTRACT:
This invention relates to apparatus for receiving ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system such as a satellite positioning system, for example, the Global Positioning System (GPS).
The original concept of GPS envisaged user equipment with an antenna having a largely clear view of the sky, whether mounted on aircraft, ships, land vehicles or in portable applications. Increasingly, however, there are requirements for GPS receivers capable of operating in poor signal conditions and which operate at higher than previously accepted accuracies. Weak signal conditions in ranging systems such as the GPS satellite-based system, are usually caused by obstructions or multipath propagation. Such reception conditions often occur inside buildings, but also outdoors in densely built-up areas or under foliage. The region over which a GPS receiver can be expected to operate can be improved by maximising the received power-to-noise ratio for each satellite signal.
Signal reception of GPS inside buildings is characterised by a transmission model that has no dominant mode since most signals arrive at the receiver after one or more reflections. This transmission model implies that the signal has Rayleigh fading statistics. Such statistics are generally found when there are multiple signal paths to the receiver, each model allows the Central Limit Theorem to be invoked to describe signal amplitude. The magnitude of the resulting signal, free of signal phase measurements, may then be established as a two-dimensional normally distributed random variable. The magnitude is a first order &khgr;
2
distribution with the characteristic Rayleigh probability density function:
P
(
R
)=(
R/&sgr;
2
)exp(−
R/
2&sgr;
2
)
The direction of the signal arrival is not well defined in this transmission model and may come from any direction. In reality, this will be constrained by physical factors such as nearby windows capable of GPS signal transmission. For the most part, it will also not be possible to determine the polarisation of the signal. GPS satellite transmissions are right-hand circularly polarised but become circularly or elliptically polarised on reflection from metallic objects.
In the circumstances described above, the signal field can be subject to deep fades, similar to those experienced by FM radio receivers in automotive environments.
Another difficulty is the shrinking physical size of GPS receivers bringing, amongst other problems, the danger of self-jamming due to electromagnetic noise from small devices with large gate counts exceeding local thermal noise levels. In addition, the external electromagnetic environment is deteriorating as a result of increasing congestion in the electromagnetic spectrum. As an example, digital broadcast television, which is due to expand rapidly in coming years, has relatively high out-of-band emissions compared with previous systems. Other satellite communication services are being allocated spectrum space in bands adjacent GPS frequencies.
This invention aims to counter such difficulties by providing a receiver system having a plurality of antennas. According to a first aspect of the invention, apparatus arranged to receive ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system comprises a radio receiver unit having a plurality of radio frequency (r.f.) input connections, and a plurality of spaced-apart antennas each connected to a respective said input connection, wherein the receiver unit includes: an antenna signal processing circuit responsive to the ranging signals for producing output signals including components each individually representative of at least one characteristic of the ranging signals received at a respective antenna; and a signal combiner configured to receive the output signals, to monitor, for each of a plurality of the said remote transmitters, at least one characteristic of the ranging signals from that remote transmitter via the different antennas, and to weight one or more of the said components to yield a range estimate.
According to a second aspect of the invention, the apparatus has a receiver unit with a signal selection arrangement configured to receive the output signals, to monitor, for each of a plurality of the said remote transmitters, at least one characteristic of the ranging signals from that remote transmitter via the different antennas, and to select one or more of the said components to yield a range estimate.
The signal combiner or selection arrangement is preferably configured to weight or select the components according to the signal-to-noise ratios of the ranging signals received from at least one of the transmitters, advantageously from each of them. Alternatively, or in addition, the signal combiner or selection arrangement may be configured to weight or select the components according to the amplitudes of the ranging signals whether from one or several of the transmitters.
Typically, the antenna signal processing circuit includes at least one code-tracking loop for tracking code modulation of the ranging signals when received by the antennas. The selection may be performed periodically so as to select one of the ranging signals the amplitude of which is greater than the others, and then to apply the selected signal to the code tracking loop. There may be a number of code tracking loops, the signal selection arrangement being configured periodically to select a number of the ranging signals by weighting the magnitudes according to a predetermined weighting scheme which is dynamically variable, being adapted to the changing electromagnetic conditions as happens, for instance, when a motor vehicle containing the apparatus passes through a built-up area.
In the last-mentioned situation, the changing electromagnetic environment is largely due to varying intensities of signals reflected from adjacent buildings. Thus, the signal combiner or selection arrangement may be specifically configured to weight or select the components due to signals from the different antennas according to their multi-path content.
As an alternative to using a code-tracking loop for tracking code modulation of the ranging signals, a parallel correlator structure may be used.
Although, in the case of GPS, code-tracking may be relied upon to provide position fixes, improved accuracy may be obtained by measuring carrier phase. In the context of the present invention, a receiver unit including a carrier phase tracking loop may include a phase-shifting arrangement operable to apply a selected phase shift to a selected or weighted signal, the magnitude of the phase shift being dynamically adjusted according to the antenna via which the selected or weighted signal is received. Accordingly, it is possible to configure the phase-shifting arrangement to adjust the phase shift in a manner that maintains coherent phase matching between ranging signals received via the different antennas to improve phase tracking accuracy. Coherent phase tracking may be maintained in order to allow switching between different selected ranging signals from different antennas.
Phase and amplitude taper multiplication coefficients may be applied to the ranging signals from the different antennas using, for instance, an adaptive equaliser. Phase taper, then, can cause the ranging signals received at different antenna elements from a particular direction to reinforce each other, giving a means of selecting signals from a particular direction (e.g. from a particular transmitter), and for rejecting signals received from other directions, e.g. due to unwanted reflections or other sources of interference. An amplitude taper can act as a spatial filter altering the combined beam pattern of the antennas as an array.
According to a third aspect of the invention, apparatus arranged to receive ranging signals produced by a plurality of remote transmitters which form part of a universal ranging system comprises a radio receiver unit having a plurality of r.
Parthus (UK) Limited
Phan Dao
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