COHERENT PULSE DOPPLER ALTIMETER

• US 3,739,379 A
• Filed: 02/03/1971
• Issued: 06/12/1973
• Est. Priority Date: 02/03/1971
• Status: Expired due to Term

• Current Assignee: Gould NSD Incorporated
• Original Assignee: Donovan C. Davis
• Inventors: Davis, Donovan C.
• Primary Examiner(s): Hubler, Malcolm F.
• Application Number:
  US05/112,295
• Time in Patent Office: 860 Days
• Field of Search: 343/7.3,7.5
• US Class Current: 342/88
• CPC Class Codes: G01S 13/18 wherein range gates are used
• CPC Class Codes: G01S 13/53 performing filtering on a single spectral line and associated with one or more range gates with a phase detector or a frequency mixer to extract the Doppler information, e.g. pulse Doppler radar G01S13/5244 takes precedence
• CPC Class Codes: G01S 13/70 for range tracking only
• CPC Class Codes: G01S 13/882 for altimeters measuring height using barometric means G01C5/06

Abstract

A coherent pulse doppler altimeter system employing a single antenna wherein the transmit signal is a generated series of pulses the pulse width and repetition frequency of which are varied keeping the average power high although peak transmitter power can be low enough to permit utilization of all solid state components. The ground return pulse will have a doppler shift caused by the vertical component of the aircraft velocity and a doppler spread due to the horizontal velocity. The receiver is tuned to receive the first side band of the return signal. The receiver is range gated by a gate pulse which is varied as the pulse repetition frequency (PRF) is varied so that just the leading edge of a return pulse passes through the gate. Variations in PRF are controlled by the tracking of that leading edge by the range gate. The PRF is then counted to give altitude.

Claims

1. A coherent pulse doppler altimeter system, comprising:

transmitter means for transmitting a series of pulses at a predetermined pulse repetition frequency;

receiver means for receiving transmitted pulses after said pulses have been reflected, said receiver means having detecting means for detecting only signals from said antenna reflected from surfaces having a relative velocity with respect to said transmitter, said receiver means including a first and second mixer, said first mixer including a first local oscillator and said second mixer including a second local oscillator at a frequency of the pulse repetition frequency of the transmitter means, the output of said first mixer being electrically coupled to the input of said second mixer and the input of said first mixer being adapted to receive the signals detected by said detecting means; and

a single antenna coupled to said receiver means and said transmitter means.

2. A system as in claim 1 including a voltage-controlled-oscillator means electrically coupled to said transmitter means for varying the pulse repetition frequency and pulse width of the transmitted pulses.

3. A system of the type described in claim 1 wherein said receiver means includes gate means electrically coupled between said antenna and said input of said first mixer for passing oNly the leading edge of the received transmitted pulses.

4. A system as in claim 3 wherein the gate means is terminated at a fixed fraction of the inter-pulse interval.

5. A system as in claim 4 wherein the fixed fraction is equal to one-half of the inter-pulse interval.

6. A coherent pulse doppler altimeter system comprising:

a single antenna;

transmitter means coupled to said antenna for transmitting a series of pulses;

voltage-controlled-oscillator means coupled to said transmitter means for varying the repetition frequency and width of said pulses; and

receiver means coupled to said antenna for receiving said pulses after they have been reflected from a remote surface, said receiver means having detecting means operative to only permit the passage of pulses from said antenna reflected from surfaces which have a relative velocity with respect to said antenna, range gate means coupled to said voltage-controlled-oscillator means for passing said reflected pulses from said antenna, said range gate means being operative to only pass the leading edge of said reflected pulses occurring within a time indicative of a maximum preselected range, said receiver means including a first and second mixer, said first mixer including a first local oscillator and said second mixer including a second local oscillator at a frequency of the pulse repetition frequency of the transmitter means, the output of said first mixer being electrically coupled to the input of said second mixer and the input of said first mixer being adapted to receive the signals detected by said detecting means.

7. An altimeter system set forth in claim 6 wherein said transmitter means comprises an rf oscillator means having an rf frequency said rf oscillator being at the frequency of said first local oscillator, switch means coupled to the output of said rf oscillator means, the aforesaid voltage-controlled-oscillator means being coupled to said switch means to actuate said switch means to provide said transmitted pulses having a variable repetition frequency and pulse width.

8. The system set forth in claim 7 wherein an isolator is interposed between said rf oscillator means and said switch means.

9. The system set forth in claim 7 wherein said receiving means includes track detector means coupled to said voltage-controlled-oscillator means for varying the pulse repetition frequency and pulse width thereof when t he output of said second mixer is above a predetermined value.

10. The system set forth in claim 3 wherein said receiving means further includes threshold detecting means coupled to said track detect or means, said threshold detecting means being responsive to reflected pulses passed by said receiving means at the output of said second mixer, said threshold detecting means allowing said track detecting means and said voltage-controlled-oscillator means to vary the pulse repetition frequency and the pulse width thereof.

11. The system set forth in claim 6 wherein said receiver means further comprises:

a first i-f amplifier coupled to said output of said first mixer, said first i-f amplifier having a pass band which passes at least the first sideband of said rf frequency and which does not pass signals at said frequency, said input of said second mixer coupled to said first i-f amplifier;

a second i-f amplifier coupled to said output of said second mixer, said second mixer having a pass band the lower frequency limit of which is greater than zero and and the upper frequency limit of which is somewhat greater than t e maximum expected doppler frequency shift; and

second detector means for detecting the output of said second i-f amplifier coupled to the output of said second i-f amplifier.

12. The system set forth in claim 11 wherein said voltage-controlled-oscillator is electrically connected to a noise input of said i-f amplifier said noise input being operative to permit passage of pulses through said first i-f amplifier during only a portioN of the time when said range gate means is operative to pass reflected pulses.