Method and system for determining the phase and amplitude of a radio occultation signal

• US 6,731,906 B2
• Filed: 04/23/2001
• Issued: 05/04/2004
• Est. Priority Date: 04/23/2001
• Status: Active Grant

• Current Assignee: University Corporation For Atmospheric Research
• Original Assignee: University Corporation For Atmospheric Research
• Inventors: Sokolovskiy, Sergey V., Rocken, Christian
• Primary Examiner(s): Maung, Nay
• Assistant Examiner(s): TRINH, TAN H
• Application Number:
  US09/840,261
• Publication Number:
  US 20020155809A1
• Time in Patent Office: 1,107 Days
• Field of Search: 455/12.1, 455/24, 455/65, 455/3.2, 455/52.2, 455/15, 455/311, 455/20, 455/21, 455/22, 455/40, 455/67.6, 455/115.1, 455/118, 455/131, 455/189.1, 455/226.1, 455/355, 455/505, 455/306, 342/357.12, 342/358
• US Class Current: 455/12.1
• CPC Class Codes: G01S 19/14 specially adapted for specific applications
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01W 1/02 Instruments for indicating weather conditions by measuring two or more variables, e.g. humidity, pressure, temperature, cloud cover or wind speed G01W1/10 takes precedence
• CPC Class Codes: G01W 1/08 Adaptations of balloons, missiles, or aircraft for meteorological purposes; Radiosondes transmission of measurements G01D; transmitting circuits H04B; balloons in general B64B; aeroplanes in general B64D; missiles in general F42B15/00

Abstract

The present system for determining the phase and amplitude of a radio occultation signal modifies the traditional Open Loop tracking process to maximize the signal-to-noise ratio, minimize the sampling rate, and also preserve the structure of the radio occultation signals. A radio occultation system includes a transmitter system, a receiver system, and a post-processing system. The receiver system receives the radio signal that is transmitted by the transmitter system through the earth'"'"'s atmosphere, where it is occulted, and down converts the received radio occultation signal to generate a down converted signal based on a phase model that uses a refractivity climatology. The receiver system then low pass filters the down converted signal and samples the in-phase and quadrature components of the down converted and low pass filtered signal. The post-processing system receives the in-phase and quadrature components of the down converted and low pass filtered signal, determines the mean residual frequency, and performs additional down conversion of the sampled signal to eliminate aliasing. The post-processing system then re-samples the down converted signal at a higher sampling rate and calculates the accumulated phase and amplitude from the re-sampled signal.

Claims

1. A method for measuring phase and amplitude of a radio occultation signal, where the radio occultation signal is transmitted from a satellite-based transmitter to a receiver through an atmosphere, the method comprising the steps of:

receiving the radio occultation signal;

down converting the received radio occultation signal based on at least one of a phase and frequency model to generate a down converted radio occultation signal;

low pass filtering said down converted radio occultation signal;

sampling in-phase and quadrature components of said down converted and filtered radio occultation signal; and

determining the phase and amplitude of the radio occultation signal based on said sampled in-phase and quadrature components.

2. The method of claim 1 further comprising the step of:

determining at least one of a phase and frequency model of the radio occultation signal prior to an occultation.

3. The method of claim 2 wherein said step of determining at least one of a phase and a frequency model comprises:

the use of predicted positions and velocities of said transmitter and said receiver and account for refraction of radio waves in the atmosphere by use of refractivity climatology.

4. The method of claim 3 wherein said step of determining at least one of a phase and frequency model comprises:

computing said at least one of a phase and a frequency model using a model of the radio signal bending angle as a function of the height of the ray asymptote over the reference ellipsoid.

5. The method of claim 3 wherein said step of determining at least one of a phase and frequency model comprises:

computing said at least one of a phase and a frequency model using refractivity climatology which depends on latitude and season.

6. The method of claim 3 wherein said step of determining at least one of a phase and frequency model comprises:

computing said at least one of a phase and a frequency model using a refractivity climatology that is independent of latitude and seasons.

7. The method of claim 1 wherein said step of determining the phase and amplitude comprises:

an additional down conversion of said sampled radio occultation signal which reduces its mean residual frequency to zero and thus eliminates aliasing.

8. The method of claim 7 wherein said step of additional down conversion comprises:

determining a model of the mean residual frequency of said sampled radio occultation signal based on the sliding window spectral analysis.

9. The method of claim 8 wherein said step of determining the model of the mean residual frequency comprises:

computing said model of the mean residual frequency using a model of the shape of a radio occultation signal spectrum.

10. The method of claim 7 wherein said step of determining the phase and amplitude further comprises:

re-sampling said additionally down converted radio occultation signal at a higher rate and determining the accumulated phase by recurrent comparison of raw phases for adjacent samples.

11. The method of claim 1 wherein said radio occultation signal is a GPS signal, said step of receiving the radio occultation signal comprises:

demodulating with the aid of said at least one of a phase and frequency model the GPS radio occultation signal under conditions of low signal-to-noise ratio.

12. A radio occultation system for measuring a phase and amplitude of a radio occultation signal, where the radio occultation signal is transmitted from a satellite-based transmitter to a receiver through an atmosphere, the radio occultation system comprising:

a receiver system comprising;

means for receiving the radio occultation signal;

means for down converting the radio occultation signal based on at least one of a phase and frequency model to generate a down converted radio occultation signal;

means for low pass filtering said down converted radio occultation signal;

means for sampling in-phase and quadrature components said down converted and filtered radio occultation signal; and

a post-processing system, responsive to said sampled in-phase and quadrature components of the radio occultation signal, for determining the accumulated phase and amplitude of the radio occultation signal.

13. The radio occultation system of claim 12 further comprising:

means for calculating at least one of a phase and a frequency model of a radio occultation signal prior to an occultation.

14. The radio occultation system of claim 13 further comprising:

means, responsive to predicted positions and velocities of said transmitter and receiver for taking into account refraction of radio waves in the atmosphere by use of refractivity climatology.

15. The radio occultation system of claim 14 wherein said means for taking into account refraction of radio waves comprises:

means for computing said frequency model using a model of the radio signal ray bending angle as a function of the height of ray asymptote over the reference ellipsoid.

16. The radio occultation system of claim 14 wherein said means of determining at least one of a phase and frequency model comprises:

means for computing said at least one of a phase and a frequency model using a refractivity climatology which depends on latitude and season.

17. The radio occultation system of claim 14 wherein said means for determining at least one of a phase model and a frequency model comprises:

means for computing said at least one of a phase and a frequency model using a refractivity climatology that is independent of latitude and season.

18. The radio occultation system of claim 12 wherein said post processing system comprises:

means for an additional down conversion of said sampled radio occultation signal which reduces its mean frequency to zero and thus eliminates aliasing.

19. The radio occultation system of claim 18 wherein said means for the additional down conversion comprises:

means for computing a model of the mean residual frequency of said sampled radio occultation signal based on sliding window spectral analysis.

20. The radio occultation system of claim 19 wherein said means for computing a model of mean residual frequency comprises:

means for computing said model of the mean residual frequency using a model of the shape of the radio occultation signal spectrum.

21. The radio occultation system of claim 12 wherein said post processing system further comprises:

means for re-sampling said additionally down converted radio occultation signal at a higher rate and computing the accumulated phase by recurrent comparison of raw phases for adjacent samples.

22. The radio occultation system of claim 12 wherein said radio occultation signal is a GPS signal, said means for receiving the radio occultation signal comprises:

means for demodulating with the aid of said at least one of a phase and frequency model the GPS radio occultation signal under conditions of low signal-to-noise ratio.