Pulse or digital communications – Receivers
Reexamination Certificate
1999-02-23
2002-03-26
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
C342S357490, C455S012100, C701S207000
Reexamination Certificate
active
06363123
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to Global Orbiting Navigation System (“GLONASS”) signal receivers. More particularly, the present invention relates to a novel technique for calibrating GLONASS receivers for use in making survey measurements with sub-centimeter accuracy. GLONASS is a global navigation system developed in the former Soviet Union to perform the same functions as the Global Positioning System (GPS) developed in the United States. Receivers are being developed to process signals from both systems of satellites. Having more satellites available to a receiver results in a faster convergence on an accurate position result and, if visibility is limited by geographic or architectural obstructions, may provide for location determination that would not have been obtainable if only one set of satellites were used.
There are, however, design differences between GPS and GLONASS that have an impact on high accuracy applications. The present invention addresses a significant problem arising from one of these differences, as will be discussed below after first providing some background on GPS.
Overview of GPS:
The global positioning system (GPS) may be used for determining the position of a user with a GPS receiver located on or near the earth, from signals received from multiple orbiting satellites. The orbits of the GPS satellites are arranged in multiple planes, in order that the signals can be received from at least four GPS satellites at any selected point on or near the earth.
The nature of the signals transmitted from GPS satellites is well known from the literature, but will be described briefly by way of background. Each satellite transmits two spread-spectrum signals in the L band, known as L
1
and L
2
, with separate carrier frequencies. Two signals are needed if it is desired to eliminate an error that arises due to the refraction of the transmitted signals by the ionosphere. Each of the carrier signals is modulated in the satellite by at least one of two pseudorandom noise (PRN) codes unique to the satellite, and transmitted as a spread spectrum signal. This allows the L-band signals from a number of satellites to be individually identified and separated in a receiver. Each carrier is also modulated by a slower-varying data signal defining the satellite orbits and other system information. One of the PRN codes is referred to as the C/A (clear/acquisition) code, while the second is known as the P (precise) code.
In the GPS receiver, signals corresponding to the known P code and C/A code may be generated in the same manner as in the satellite. The L
1
and L
2
signals from a given satellite are demodulated by aligning the phases, i.e., by adjusting the timing, of the locally-generated codes with those modulated onto the signals from that satellite. In order to achieve such phase alignment the locally generated code replicas are correlated with the received signals until the resultant output signal power is maximized. Since the time at which each particular bit of the pseudorandom sequence is transmitted from the satellite is known, the time of receipt of a particular bit can be used as a measure of the transit time or range to the satellite. Because the C/A and P codes are unique to each satellite, a specific satellite may be identified based on the results of the correlations between the received signals and the locally-generated C/A and P code replicas.
Each receiver “channel” within the GPS receiver is used to track the received signal from a particular satellite. A synchronization circuit of each channel provides locally generated code and carrier replicas, which are synchronous with each other. During acquisition of the code phase within a particular channel, the received satellite signal is correlated with a discrimination pattern comprised of some combination of “early” and “late” versions of the channel's locally generated code replica. The resultant early-minus-late correlation signals are accumulated and processed to provide feedback signals to control code and carrier synchronization.
Although there are several ways to create a spread spectrum signal, the one most often used is “direct spreading” with a pseudorandom code, which is the technique used in GPS. A direct sequence spread spectrum signal is normally created by biphase modulating a narrowband signal with a pseudorandom code. Each GPS satellite normally transmits three spread spectrum navigation signals. One is on the L
2
carrier signal and is based on the P code from a P code generator, and two are on the L
1
carrier signal and are based on the P code from the P code generator and the C/A code from a C/A code generator, respectively. To accomplish this, the L
1
carrier signal is first divided into two components that are in phase quadrature. Each of these components is individually modulated with navigation signals before being combined, amplified, and transmitted.
The frequency spectrum resulting from this process is one in which the original carrier frequency at F
0
is suppressed, and the total signal energy is spread over a bandwidth around F
0
of plus and minus the code clock frequency to first nulls. Spectral components outside this bandwidth also are created, but at ever lower amplitude with frequency separation.
The key functions of each satellite are all driven by a single clock with a frequency of 10.23 MHz . The L
1
carrier frequency of 1575.42 MHz is obtained by multiplying 10.23 MHz by 154. The L
2
carrier frequency of 1227.6 MHz is 120 times the clock. The P code rate is 10.23 MHz and is obtained directly from the clock. The C/A code rate is one tenth the clock frequency and is obtained through a frequency divider. Even a 50 bit per second data rate used to retrieve data from a memory is derived from the same clock. It can be said that all of these signals are coherent because they are derived from a single clock.
In a typical GPS receiver, a single antenna collects all available signals, which are processed through a filter, an amplifier and a downconverter to obtain a lower intermediate frequency (IF) for further processing. Only then is the composite signal digitally sampled, to facilitate further processing of the signals in digital form.
A key aspect of the GPS design is that each satellite uses the same L
1
and L
2
carrier frequencies but pseudorandom code sequences (P-code and C/A-code sequences) that are unique to the satellite. In other words, the satellites are identified in a receiver by their unique pseudorandom code sequences.
The GLONASS Approach, and the Problem:
Each satellite in GLONASS uses the same pseudorandom code sequences but uses unique L
1
and L
2
frequencies. Thus, a GLONASS receiver identifies satellites by their carrier frequencies and not by their pseudorandom code sequences. Specifically, each satellite in GLONASS transmits on a frequency in the bands 1,597-1,617 MHz for L
1
and 1,240-1,260 MHz for L
2
. The channel center frequency spacing is fixed at 0.5625 MHz for L
1
and 0.4375 MHz for L
2
.
As originally conceived, both systems were designed to compute receiver locations for navigation purposes, based on measurements made of the arrival times of the pseudorandom code sequences from each satellite in view of the receiver. It was later found that receiver locations could be determined much more accurately by using measurements of the carrier phase. Receivers using carrier phase for location determination form a distinct and increasingly important class of positioning receivers referred to as kinematic processing receivers or survey receivers. These highly accurate receivers find application in survey work, in aircraft landing systems and in earth moving or landscaping machines.
A difficulty arises in processing GLONASS carrier frequency signals simply because they are different for different satellites. Each received carrier signal can be identified and isolated either with a separate, narrow bandpass filter for each satellite channel, or accounted for in digital data processing. In the preferred embodi
Chin Stephen
Heal Noel F.
Jiang Lenny
Leica Geosystems Inc.
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