# US 5,347,546 A · Method and apparatus for prefiltering a global positioning system receiver

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- Number: US 5,347,546 A
- Document ID: US5347546A
- Status: Granted
- Kind: A
- Application number: 07/874,795
- Filed: 1992-04-28
- Granted: 1994-09-13
- Earliest claimed: 1992-04-28
- Assignees: Ashtech Inc.
- Inventors: Kamran K. Abadi; Steven C. Evans
- Web page: https://patentgrep.com/patent/US5347546A

## CPC

- G01S 19/36: relating to the receiver frond end
- G01S 19/21: Interference related issues {; Issues related to cross-correlation, spoofing or other methods of denial of service}
- H04B 1/18: Input circuits, e.g. for coupling to an antenna or a transmission line (coupling networks between antennas or lines and receivers independent of the nature of the receiver H03H)
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- H04B: TRANSMISSION

## Abstract

A method and apparatus for pre-filtering a global positioning system receiver, which includes the steps of: receiving a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites; splitting the plurality of radio frequency signals into at least a first L1 radio frequency signal and a second L2 radio frequency signal; passing the first L1 radio frequency signal to a first radio frequency receiving section through a L1 signal passage defining a path from a common junction point to the L1 radio frequency receiving section, while simultaneously passing the L2 radio frequency signal to a second radio frequency receiving section through a L2 signal passage defining a path from the common junction point to the L2 radio frequency receiving section; filtering the L1 radio frequency signal from the L2 signal passage and filtering the L2 radio frequency signal from the L1 signal passage, while substantially preserving the L1 radio frequency and the L2 radio frequency signals; and feeding the L1 and the L2 radio frequency signals to the first L1 radio frequency receiving section and the second L2 radio frequency receiving section, for determining position information.

## Claims

### Claim 1

A method for prefiltering a global positioning system receiver, comprising the steps of:
a) receiving a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites;
b) splitting said plurality of radio frequency signals into at least a first L1 radio frequency signal and a second L2 radio frequency signal;
c) passing said first L1 radio frequency signal to a first radio frequency receiving section through a L1 signal passage defining a path from a common junction point to said L1 radio frequency receiving section, while simultaneously passing said L2 radio frequency signal to a second radio frequency receiving section through a L2 signal passage defining a path from said common junction point to said L2 radio frequency receiving section;
d) filtering said L1 radio frequency signal from the L2 signal passage and filtering said L2 radio frequency signal from the L1 signal passage, while substantially preserving the L1 radio frequency and the L2 radio frequency signals and matching the lengths of said L1 passage and said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to a center frequency of the L1 radio frequency signal and the impedance at the end of said L1 passage is substantially infinite with respect to a center frequency of the L2 radio frequency signal; and
e) feeding said L1 and said L2 radio frequency signals in step d to said first L1 radio frequency receiving section and said second L2 radio frequency receiving section, for determining position information.

### Claim 2 (depends on 1)

The method of claim 1, further comprising the steps of developing intermediate frequency signals in the first L1 radio frequency receiving section and the second L2 radio frequency receiving section; and processing said intermediate frequency signals for determining information.

### Claim 3 (depends on 1)

The method of claim 1, wherein the steps (a) to (d) occur before the signals reach the L1 or L2 radio frequency receiving section.

### Claim 4 (depends on 1)

The method of claim 1, wherein said fed signal of said L1 radio frequency receiving section includes the step of filtering the first L1 radio frequency receiving section from other radio frequency signals including said L2 radio frequency signals in the L1 radio frequency receiving section, and further includes the step of filtering the second L2 radio frequency receiving section from other radio frequency signals including said L1 radio frequency receiving section.

### Claim 5 (depends on 1)

The method of claim 1, wherein said filtered L1 radio frequency signal of said L1 passage includes a loss of less than 0.5 dB, and said filtered L2 radio frequency signal of said L2 passage includes a loss of less than 0.5 dB.

### Claim 6 (depends on 1)

The method of claim 1, wherein said global positioning system receiver has a noise figure of less than about 3 dB.

### Claim 7 (depends on 1)

The method of claim 1, wherein said filtering step includes fixing the lengths of said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to said L1 radio frequency signal, and said filtering step further includes fixing the length of said L1 passage such that the impedance at the end of said L1 passage is substantially infinite with respect to said L2 radio frequency signal.

### Claim 8 (depends on 1)

The method of claim 1, wherein said L1 frequency is about 1575 MHz and said L2 frequency is about 1227 MHz.

### Claim 9 (depends on 1)

The method of claim 1, wherein said L1 passage includes a micro-strip comprising a conductive material of about 2 inches in length, and said L2 passage includes a microstrip comprising a conductive material of about 3 inches in length.

### Claim 10 (depends on 1)

The method of claim 1, further comprising the step of processing the signals from said first and second radio frequency receiving sections to calculate a relative phase for each signal and end quantities of position, distance, velocity, time or the like.

### Claim 11 (depends on 1)

The method of claim 1, wherein said filtering step includes simultaneously, substantially minimizing unwanted reflection of the L1 and L2 radio frequency signals along the L2 and L1 paths, respectively.

### Claim 12 (depends on 1)

The method of claim 1, wherein said first length of the L1 passage is defined by ##EQU4## and said second length of the L2 passage is defined by ##EQU5## wherein f.sub.1 and f.sub.2 are the L1 and L2 frequencies, Z.sub.o is the transmission line characteristic impedance, c is the propagation speed and X.sub.f is the reactance at the f.sub.2 and f.sub.1 frequencies.

### Claim 13

A method for pre-filtering a global positioning system receiver, comprising the steps of:
(a) receiving a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites;
(b) splitting said plurality of radio frequency signals into at least a first L1 radio frequency signal and a second L2 radio frequency signal;
(c) passing said first L1 radio frequency signal to a first radio frequency receiving section through a L1 signal passage defining a path from a common junction point to said L1 radio frequency receiving station, while simultaneously passing said L2 radio frequency signal to a second radio frequency receiving section through a L2 signal passage defining a path from said common junction point to said L2 radio frequency receiving section;
(d) isolating said L1 radio frequency signal from the L2 signal passage and isolating said L2 radio frequency signal from the L1 signal passage, while substantially preserving the L1 radio frequency and L2 radio frequency signals;
(1) fixing the lengths of said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to said L1 radio frequency signal;
(2) fixing the length of said L1 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to said L2 radio frequency signal;
(3) matching the lengths of said L1 passage and said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to the L1 radio frequency signal and the impedance at the end of said L1 passage is substantially infinite with respect to the L2 radio frequency signal; and
(4) simultaneously, substantially minimizing unwanted reflection of L1 and L2 radio frequency signals along the L2 and L1 paths, respectively;
(e) feeding said L1 and said L2 radio frequency signals to said first L1 radio frequency receiving section and said second L2 radio frequency receiving section;
(f) developing intermediate frequency signals in the first L1 radio frequency receiving section and the second L2 radio frequency receiving station; and
(g) processing said intermediate frequency signals for determining a relative phase for each signal and end quantities of position, distance, velocity, time or the like.

### Claim 14 (depends on 13)

The method of claim 13, wherein said global positioning system receiver has a noise figure of about 2.5 dB or less.

### Claim 15 (depends on 13)

The method of claim 13, wherein said fed signal of said L1 radio frequency receiving section includes the step of filtering the first L1 radio frequency receiving section from other radio frequency signals including said L2 radio frequency signals and wherein said fed signal of said L2 radio frequency receiving section includes the step of filtering the second L2 radio frequency receiving section from other radio frequency signals including said L1 radio frequency signals.

### Claim 16 (depends on 13)

The method of claim 13, wherein said isolated L1 radio frequency signal of said L1 passage includes a loss of less than 0.5 dB and said isolated L2 radio frequency signal of said L2 passage includes a loss of less than 0.5 dB.

### Claim 17

An apparatus for prefiltering a global positioning system receiver, comprising:
means for receiving a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites;
means for splitting said plurality of radio frequency signals into at least a first L1 radio frequency signal and a second L2 radio frequency signal;
passing means for passing said first L1 radio frequency signal to a first radio frequency receiving section through a L1 signal passage defining a path from a common junction point to said L1 radio frequency receiving station, and passing said L2 radio frequency signal to a second radio frequency receiving section through a L2 signal passage defining a path from said common junction point to said L2 radio frequency receiving section;
means for filtering said L1 radio frequency signal from the L2 signal passage and filtering said L2 radio frequency signal from the L1 signal passage, while substantially preserving the L1 radio frequency and the L2 radio frequency signals, said filtering means includes a first length defined as the length of said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to said L1 radio frequency signal, and a second length defined as the length of said L1 passage such that the impedance at the end of said L1 passage is substantially infinite with respect to said L2 radio frequency signal.

### Claim 18 (depends on 17)

The apparatus of claim 17, wherein said filtering means includes matching the lengths of said L1 passage and said L2 passage such that the impedance at the end of said L2 passage is substantially infinite with respect to a center frequency of the L1 radio frequency signal and the impedance at the end of said L1 passage is substantially infinite with respect to a center frequency of the L2 radio frequency signal.

### Claim 19 (depends on 17)

The apparatus of claim 17, wherein said L1 frequency is about 1575 MHz and said L2 frequency is about 1227 MHz;
said L1 passage includes a micro-strip comprising a conductive material of about 2 inches in length;
said L2 passage includes a micro-strip comprising a conductive material of about 3 inches in length;
said filtered means L1 radio frequency signal of said L1 passage includes a loss of less than about 0.5 dB; and
said filtered means L2 radio frequency signal of said L2 passage includes a loss of less than 0.5 dB.

### Claim 20 (depends on 17)

The apparatus of claim 17, wherein said first length of the L1 passage is defined by ##EQU6## and said second length of the L2 passage is defined by ##EQU7## wherein f.sub.1 and f.sub.2 are the L1 and L2 frequencies, Z.sub.o is the transmission line characteristic impedance, c is the propagation speed and X.sub.f is the reactance at the f.sub.2 and f.sub.1 frequencies.