Method and apparatus for calibrating the ionosphere and application to surveillance of geophysical events

• US 4,463,357 A
• Filed: 11/17/1981
• Issued: 07/31/1984
• Est. Priority Date: 11/17/1981
• Status: Expired due to Term

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: MacDoran, Peter F.
• Primary Examiner(s): Blum, Theodore M.
• Application Number:
  US06/322,317
• Time in Patent Office: 987 Days
• Field of Search: 343/5 W, 343/460, 343/378, 343/357, 343/422, 343/352
• US Class Current: 342/460
• CPC Class Codes: G01S 11/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G01S 19/072 Ionosphere corrections
• CPC Class Codes: G01S 19/14 specially adapted for specific applications
• CPC Class Codes: G01S 3/023 Monitoring or calibrating

Abstract

The columnar electron content of the ionosphere between a spacecraft (10) and a receiver (12, 14) is measured in realtime by cross correlating two coherently modulated signals transmitted at different frequencies (L₁, L₂) from the spacecraft to the receiver using a cross correlator (16). The time difference of arrival of the modulated signals is proportional to electron content of the ionosphere. A variable delay (21) is adjusted relative to a fixed delay (20) in the respective channels (L₁, L₂) to produce a maximum at the cross correlator output. The difference in delay required to produce this maximum is a measure of the columnar electron content of the ionosphere. A plurality of monitoring stations (31, 32, 33) and spacecrafts (GPS satellites 34, 35, 36) are employed to locate any terrestrial event that produces an ionospheric disturbance.

Claims

1. A method of calibrating the electron columnar content in the space between a transmitter on a spacecraft and a receiver on Earth comprising the steps of:

transmitting from said transmitter two broadband signals separated in frequency, each coherently modulated by pseudorandom noise;

receiving in a direct path from said transmitter both of said signals at a receiver having two channels for obtaining said modulations on said two signals; and

cross correlating the modulation of one received signal to the modulation of the other received signal to determine the phase difference between the modulation of said one received signal to the modulation of said other received signal as a measure of electron content in the space between the transmitter and receiver.

2. A method as defined in claim 1 wherein said calibration is carried on continuously in real time to monitor the electron columnar content for any change therein.

3. A method as defined in claim 2 wherein said spacecraft is at a known position relative to the Earth and said receiver is at a known position on Earth and changes in the electron columnar content between the Earth and said spacecraft is indicative of a terrestrial event of significance, including the step of detecting any sudden change in electron columnar content in the known path between said transmitter and said receiver.

4. A method as defined in claim 3 used a plurality of receivers and transmitters, each transmitter on a different spacecraft of known positive relative to the Earth, and each receiver being in a known position on the Earth for monitoring the columnar electron content between receivers and transmitters, each receiver operating the same as all other receivers with signals received from a transmitter, including the step of locating the source of sudden change in the electron content in the space between said plurality of transmitters and receivers in two dimensions from the known paths of the receivers to the transmitters.

5. A method as defined in claim 4 wherein a plurality of spacecraft transmitters are in a position to be in line of sight communication with each receiver at all times.

6. Apparatus for calibrating the electron columnar content between a transmitter in space and a receiver on Earth comprising

means for transmitting from said transmitter two broadband signals separated in frequency, each coherently modulated with pseudorandom noise,

means at said receiver for receiving said transmitted signals along a direct path from said transmitter,

means for cross correlating said modulation of one received signal with the modulation of the other received signal at said receiver, including

means for separating said two received signals into two channels converted to baseband,

means for adjustably delaying the baseband signal in one channel relative to the other,

means for cross correlating said delayed baseband signals, and

means for determining the delay of said first channel signal producing the maximum output of said cross correlating means as a measure of the columnar electron content between said transmitting means and said receiving means.

7. A system for detecting the occurrence of a terrestrial event that disturb the ionosphere by monitoring electron content of the ionosphere using a plurality of stations and spacecrafts at known locations, each station operating the same as all other stations with signals received from a spacecraft, comprising

means at each spacecraft for transmitting two signals at different frequencies coherently modulated with pseudorandom noise,

means at each station for receiving said two signals in a direct path from a spacecraft,

means for converting each of said two signals received to baseband,

means for adjustably delaying one baseband signal relative to the other,

means for cross correlating said two baseband signals as said one is delayed relative to the other to determine the delay of said one relative to the other which produces a maximum cross correlation output as a measure of said electron content, and

means for locating the position of the source causing disturbance of the electron content of the ionosphere from the known position of stations and spacecrafts detecting a disturbance.