Global positioning system receiver with improved radio frequency and digital processing

• US 4,928,106 A
• Filed: 07/14/1988
• Issued: 05/22/1990
• Est. Priority Date: 07/14/1988
• Status: Expired due to Term

• Current Assignee: MiTAC International Corporation (MiTAC Holdings Corp.)
• Original Assignee: Ashtech Telesis, Inc.
• Inventors: Lorenz, Robert G., Ashjaee, Javad, Sutherland, Robert A., Helkey, Roger J.
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US07/219,353
• Time in Patent Office: 677 Days
• Field of Search: 342/352, 342/356, 342/357
• US Class Current: 342/352
• CPC Class Codes: G01S 19/32 Multimode operation in a single same satellite system, e.g. GPS L1/L2
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: G01S 19/43 using carrier phase measurements, e.g. kinematic positioning; using long or short baseline interferometry

Abstract

A global positioning system (GPS) receiver having a common radio frequency section and a separate digital signal processing channel for each of a plurality of satellite signals which are simultaneously being received and processed by the receiver in order to calculate the position, velocity or other desired parameters of the receiver. The radio frequency section receives and processes both of the standard satellite signals on different frequency L1 and L2 carriers in order to provide the multi-channel digital section signals from which the relative phase of the carriers from each of the plurality of satellites may be determined. Particular mutually coherent local oscillator and digital clock frequencies are selected in order to minimize the complexity of the receiver without creating any undesired side effects. A high resolution relative phase measurement is made in each digital section by averaging the number of phase shifting pulses over a period of time that are generated by a phase locked loop that is locked onto an L-band carrier frequency, that average incremental phase being added to a base phase measurement obtained simultaneously in each of the digital processing channels.

Claims

1. In a global positioning system receiver having a radio frequency section adapted for receiving from an antenna a L1 radio frequency signal having a nominal carrier frequency of 1540f₀, where f₀ =1.023 MHz., and for delivering an intermediate frequency signal to a digital section that processes the intermediate frequency signal in a manner to extract desired phase information therefrom, an improved receiver section, comprising:

a first radio frequency stage connectable to said antenna and having an output,

a first mixer receiving said first radio frequency stage output and responsive to a first local oscillator signal for reducing a signal output of the first radio frequency stage to a first intermediate frequency signal,

a second radio frequency stage receiving the first intermediate frequency signal and having an output,

a second mixer receiving the second radio frequency stage output and responsive to a second local oscillator signal for reducing the first intermediate frequency signal to a second intermediate frequency signal,

a third radio frequency stage connecting said second intermediate frequency signal to an input of said digital section, and

means responsive to a common precise reference oscillator for generating said first and second local oscillator signals and clock signals for said digital section that are all mutually coherent with each other, said reference oscillator having a frequency of substantially 20f₀, said first local oscillator signal being substantially 1372f₀, wherein said first intermediate frequency signal is substantially 168f₀, and said second local oscillator signal being substantially 171.5f₀, wherein said second intermediate frequency signal is substantially 3.5f₀, whereby said digital processing section can be easily process said second intermediate frequency signal.

2. The improved receiver according to claim 1 wherein said second radio frequency stage include a band pass filter.

3. In a global positioning system receiver having a first radio frequency section adapted for connection to an antenna to receive an L1 radio frequency signal and a second radio frequency section adapted for connection to said antenna for receiving an L2 radio frequency signal, said L1 and L2 signals having first and second spaced-apart carrier frequencies, each of the first and second radio frequency sections connected to deliver individual intermediate frequency signals to a digital section for processing said intermediate frequency signals in a manner to measure the phases of each of the L1 and L2 radio frequency signals, the improvement comprising:

said first radio frequency section having a first mixer connected to receive the L1 radio frequency signal,

said second radio frequency section having a second mixer connected to receive the L2 radio frequency signal, and

local oscillator means connected to each of said first and second oscillators for simultaneously delivering thereto a singular fixed frequency signal intermediate to that of said L1 and L2 radio frequencies, thereby simultaneously developing a first intermediate frequency signal at the output of the first mixer and a second intermediate frequency signal at the output of the second mixer, with a singular, intermediate local oscillator signal.

4. In a global positioning system receiver having a first radio frequency section adapted for connection to an antenna to receive an L1 radio frequency signal and a second radio frequency section adapted for connection to said antenna for receiving an L2 radio frequency signal, said L1 and L2 signals having first and second spaced-apart carrier frequencies, each of the first and second radio frequency sections connected to deliver individual intermediate frequency signals to a digital section for processing said intermediate frequency signals in a manner to measure the phases of each of the L1 and L2 radio frequency signals, the improvement comprising:

said first radio frequency section having a first mixer connected to receive the L1 radio frequency signal,

said second radio frequency section having a second mixer connected to receive the L2 radio frequency signal, and

local oscillator means connected to each of said first and second oscillators for delivering thereto a fixed frequency signal wherein said frequency is substantially 1372f₀, wherein f₀ =1.023 MHz, thereby to develop a first intermediate frequency signal at the output of the first mixer and a second intermediate frequency signal at the output of the second mixer, with a common local oscillator signal.

5. The improved receiver according to claim 4 wherein said first radio frequency section includes a third mixer connected to receive the intermediate frequency signal output of said first mixer.

6. The improved receiver according to claim 5 wherein said local oscillator means generates a second local oscillator signal coherent with said first signal and which is applied to said third mixer, said second local oscillator signal being substantially equal to 171.5f₀.

7. The improved receiver according to claim 6 wherein said second radio frequency section includes means receiving said intermediate frequency output of said second mixer for autocorrelating said intermediate frequency signal, and a fourth mixer receiving the autocorrelated intermediate frequency signal, said local oscillator means providing a third local oscillator signal to said fourth mixer that is coherent with the first and second local oscillator signals.

8. The improved receiver according to claim 7 wherein said third local oscillator signal is substantially equal to 343F₀.

9. In a global position electronic receiver having a radio frequency section connected to receive a radio frequency signal from an antenna and deliver an intermediate frequency signal to an input of a digital processor that derives therefrom information of the relative phase of said received radio frequency signal, an improved relative phase measuring portion of said digital processor:

a phase-locked loop circuit connected to receive and lock upon said intermediate frequency signal, said phase lock loop circuit including a multibit counter whose most significant bit is utilized in the phase locked loop, said counter being characterized by shifting its output an amount of phase in response to an input pulse developed as part of said phase locked loop in order to keep said loop locked onto said intermediate frequency signal, counter

means connected to the output of said for periodically latching all bits of its output in response to a periodic clock signal, thereby to obtain a base measurement of relative phase of the intermediate frequency signal,

means receiving and counter phase shifting pulses for periodically determining the average number of said pulses over a fixed period of time, thereby to provide an incremental phase measurement, and

means receiving both the base phase measurement and the incremental phase measurement for summing said measurements, thereby to obtain the relative phase of said received radio frequency signal.

10. A method of determining the relative phase of a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites, comprising the steps of:

receiving said plurality of radio frequency signals and reducing them by a common radio frequency down-converting system to a plurality of intermediate frequency signals,

digitally identifying each of said plurality of intermediate frequency signals by their unique modulation,

simultaneously locking a separate phase locked loop electronic circuit onto each of said plurality of signals,

simultaneously determining a base relative phase of each of said plurality of intermediate frequency signals,

determining separately for each of said plurality of intermediate signals from a quantity that responds to their relative phase an incremental relative phase having a higher degree of resolution than the base relative phase determinations, and

arithmetically combining the base and incremental phase values for each of the intermediate frequency signals, whereby the relative phase of each of the radio frequency signal is determined.

11. A method of determining the relative phase of a plurality of L-band radio frequency signals having unique modulation and originating in a plurality of global positioning system satellites, comprising the steps of:

receiving said plurality of radio frequency signals,

mixing said plurality of radio frequency signals with a local oscillator frequency of substantially 1372f₀, where f₀ =1.023 MHz., thereby to reduce the frequency of the radio frequency signals,

mixing said reduced frequency signals with a local oscillator frequency of substantially 171.5f₀, thereby to obtain a plurality of intermediate frequency signals,

digitally identifying each of said plurality of intermediate frequency signals by their unique modulation.

simultaneously locking a separate phase locked loop electronic circuit onto each of said plurality of signals,

simultaneously determining a base relative phase of each of said plurality of intermediate frequency signals,

determining separately for each of said plurality of intermediate signals from a quantity that responds to their relative phase an incremental relative phase having a higher degree of resolution than the base relative phase determinations, and

arithmetically combining the base and incremental phase values for each of the intermediate frequency signals, whereby the relative phase of each of the radio frequency signals is determined.

12. In the global position electronic receiver having a radio frequency section connected to receive a radio frequency signal from an antenna and deliver an intermediate frequency signal to an input of a digital processor that derives therefrom information of the relative phase of said received radio frequency signal, an improved relative phase measuring portion of said digital processor comprising:

means for generating a clock signal of a given period;

means responsive to said clock generating means for determining a base measurement of the relative phase of said intermediate frequency signal to an accuracy of the period of said clock signal;

means responsive to said base measurement means and said clock means for determining an incremental phase measurement over a series of clock periods with an accuracy that is better than that of said base measurement; and

means responsive to said base and incremental phase measurement means for summing said base and incremental phases, thereby deriving a very accurate relative phase measurement.

13. The improved digital processor of claim 12 additionally comprising:

a phase locked loop circuit connected to receive and lock upon said intermediate frequency signal, and

means for transferring the signal of said phase locked loop to the base measurement means.

14. The improved digital processor of claim 13 wherein said phase locked loop includes means for locking onto said intermediate frequency signal, generating its own in-phase signal, and transferring this signal to said base phase measurement means, thereby significantly reducing the signal to noise ratio in the relative phase measurement means and creating a more accurate relative phase measurement.

15. The relative phase measurement means of claim 12 wherein the clock means has a frequency of substantially 34.3F₀.