SIMULTANEOUS ACQUISITION OF TRACKING DATA FROM TWO STATIONS

• US 3,860,921 A
• Filed: 11/15/1973
• Issued: 01/14/1975
• Est. Priority Date: 11/15/1973
• Status: Expired due to Term

• Current Assignee: Gordon E. Wood, James C Fletcher Administrator of The National Aeronautics and Space Administration
• Original Assignee: Gordon E. Wood, James C Fletcher Administrator of The National Aeronautics and Space Administration
• Inventors: Wood, Gordon E., James C. Fletcher, Administrator of the National Aeronautics and Space (N/A)
• Primary Examiner(s): Wilbur, Maynard R.
• Assistant Examiner(s): Montone, G. E.
• Application Number:
  US05/416,135
• Time in Patent Office: 425 Days
• Field of Search: 343/6.5R,6.5LC,6.8R,6.8LC,17.5,9,8,1ST
• US Class Current: 342/109
• CPC Class Codes: G01S 13/84 for distance determination by phase measurement

Abstract

A method and apparatus is disclosed for obtaining simultaneous tracking data from two ground stations relative to a spacecraft, and in particular for obtaining two-way range and doppler measurements with respect to the spacecraft using only one transponder on the spacecraft. The technique employs simultaneous transmission from two stations to produce a return signal with upper and lower sidebands resulting from the interference of the two transmissions. A transponder transmits the upper and lower sidebands centered about a carrier received by both stations. One station tracks the carrier and the other tracks a sideband aided by the carrier.

Claims

1. A method for obtaining simultaneous tracking data from two ground stations relative to a spacecraft or aircraft carrying only one transponder, said stations transmitting independently at difFerent stable frequencies spaced apart sufficiently to allow said transponder to track the frequency of a signal received from one station despite interference from the other station, interference of said transmitted signals produced a beat frequency signal at said transponder, said method comprising the steps of tracking said transmitted signal from said one station in said transponder to produce a coherent reference signal, transmitting a coherent carrier signal from said transponder to said ground station at a frequency that is a predetermined constant, G, times the frequency of said reference signal, phase detecting the signals received by said transponder with said reference signal, phase modulating said carrier signal transmitted by said transponder with the output signal of said phase detection, thereby providing a carrier signal with upper and lower sidebands, acquiring said carrier signal and performing doppler extraction at said one station as a measure of craft velocity relative to said one station, and acquiring one of said modulated sidebands and performing doppler extraction at said other station as a measure of craft velocity relative to said other station.

2. A method as defined in claim 1 wherein at least one of said stations modulates the signal transmitted to said craft with a binary valued ranging signal, whereby said output signal of said phase detection includes said ranging signal.

3. A method as defined in claim 1 wherein each of said stations modulates the signal transmitted to said craft with an independent binary valued ranging signal, whereby said output signal of said phase detection includes a ranging signal combined with said beat frequency signal, and said beat frequency signal is modulated with said ranging signal from said other station.

4. A method as defined in claim 3 wherein said doppler extraction in said other station from a sideband of said carrier is accomplished by tracking at said other station the carrier transmitted by said transponder, coherently multiplying the tracked carrier frequency by a constant, (G-1)/G, and adding the product of that multiplication to a coherent signal at the frequency of the signal transmitted by said other station thereby providing a reference signal at said other station for doppler extraction.

5. A method as defined in claim 3 wherein said one station transmits at a frequency f1 and the other station transmits at a frequency f2, whereby the beat frequency signal is at a frequency f2D2s - f1D1s, where D1s is the doppler shift from said one station to said craft and D2s is the doppler shift from said other station to said craft, the transmitted sidebands are at frequencies G D1s f1 + or -(f2D2s - f1D1s) and the upper sideband fUSB is selected for acquisition at said other station, said upper sideband being at a frequency (f1 (G-1)D1s + f2D2s) Ds2, said signal transmitted by said one station is received by said other station at a frequency G D1sDs2f1, said doppler extraction at said other station is accomplished by coherently multiplying said received signal at frequency G D1sDs2f1 by (G-1)/G to obtain a signal at frequency (G-1) D1sDs2f1, adding that signal to a signal at frequency f2 to obtain a coherent signal fc f1 (G-1)D1sDs2 + f2 for use as a reference in doppler extraction, subtracting said reference signal from said sideband to obtain the difference therebetween, and detecting the difference with phase resolution to obtain doppler frequency shift of the signal transmitted by said otheR station at frequency f2 as a measure of craft velocity relative to said other station.

6. A system for obtaining simultaneous tracking data from two ground stations relative to a spacecraft or aircraft carrying only one transponder comprising separate means at each of said stations for transmitting independently at different stable frequencies spaced apart sufficiently to allow said transponder to track the frequency of a signal transmitted by one station despite interference from the signal transmitted by the other station, interference of said transmitted signals producing a beat frequency signal at said transponder, means for tracking said transmitted signal from said one station in said transponder to produce a coherent reference signal of the same frequency, coherent multiplying means for multiplying said coherent reference signal by a predetermined constant, G, to produce a carrier signal to be transmitted by said transponder, means for phase detecting signals received by said transponder using said reference signal, means for phase modulating said carrier signal with the output signal of said phase detection means before transmission by said transponder, thereby providing a carrier signal transmitted by said transponder with upper and lower sidebands, means for acquiring said carrier signal at said one station, means for forming doppler extraction at said one station as a measure of craft velocity relative to said one station, means acquiring one of said modulated sidebands at said other station, and means for performing doppler extraction at said other station as a measure of craft velocity relative to said other station.

7. A system as defined in claim 6 wherein at least one of said stations includes means for modulating the signal transmitted to said craft with a binary valued ranging signal, whereby said output signal of said phase detection means in said transponder includes said ranging signal.

8. A system as defined in claim 6 wherein each of said stations includes means for modulating the signal transmitted to said craft with an independent binary valued ranging signal, whereby said output signal of said phase detection means in said transponder includes a ranging signal combined with said beat frequency signal, and said beat frequency signal is modulated with said ranging signal from said other station.

9. A system as defined in claim 8 wherein said means for doppler extraction in said other station from a sideband of said carrier includes means for tracking said carrier transmitted by said transponder, means for coherently multiplying the tracked carrier frequency by a constant, (G-1)/G, and means for adding the product of that multiplication to a coherent signal at the frequency of the signal transmitted by said other station, thereby providing a reference signal at said other station for doppler extraction.

10. A system as defined in claim 8 wherein said one station transmits at a frequency f1 and the other station transmits at a frequency f2, whereby said beat frequency signal is at a frequency f2D2s - f1D1s, where D1s is the doppler shift from said one station to said craft and D2s is the doppler shift from said other station to said craft, the transmitted sidebands are at frequencies G D1s f1 + or - (f2D2s - f1D1s) and the upper sideband fUSB is selected for acquisition at said other station, said upper sideband being at a frequency (f1 (G-1)D1s + f2D2s) Ds2, said signal transmitted by said one station is received by said other station at a frequency G D1sDs2f1, and wherein said doppler extraction means at said other station includes means for coherently multiplying said received signal at frequency G D1sDs2f1 by (G-1)/G to obtain a signal at frequency (G-1) D1sDs2f1, means for adding that signal to a signal at frequency f2 to obtain a coherent signal fc f1 (G-1)D1sDs2 + f2 for use as a reference in doppler extraction, means for subtracting said reference signal from said sideband to obtain the difference therebetween, and means for detecting the difference with phase resolution to obtain doppler frequency shift of the signal transmitted by said other station at frequency f2 as a measure of craft velocity relative to said other station.