Title:
Frequency-modulated radio altimeter
United States Patent 2257830


Abstract:
543,943. Radio echo apparatus condensers. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 31, 1940, No. 13698. Convention date, Sept 1, 1939. [Class 37] In a frequency modulated radio altimeter the modulation band is made inversely proportional to the altitude so as to improve the ratio of signal to amplifier noise by decreasing the modulation band width as the signal strength diminishes. In the arrangement shown in Fig. 1 a heterodyne receiver 1 is connected through an amplifier 3 to a frequency meter 5 of the electronic type. The frequency meter output is connected to a D.C. amplifier 7 which controls the gain of a low frequency amplifier 9. The input of the low frequency amplifier is connected to a low frequency oscillator 11 and the amplifier output is applied to a frequency modulator 13 which is connected to a radio transmitter 15 and an A.C. voltmeter 17. The frequency modulator 13 may be of the variable capacitor type shown in Fig. 3. A pair of stationary capacitor plates 19, 21 is secured by insulating posts 39 to a frame 23 on which is mounted an electromechanical motor 25. The movable element of the motor reciprocates vertically and carries a movable capacitor plate 29. The resulting variation in capacitance is applied to the transmitter to vary its instantaneous frequency. Alternatively inductive variation may be used and may be applied by means of a motor or electronically. The altitude is measured in terms of the A.C. voltage or current applied to the frequency modulator. The A.C. voltmeter 17 may be calibrated directly in altitudes. Voltage or current corresponding to the altitude may alternatively be measured at other points in the system such as the controlling bias or voltage.



Inventors:
Irvin, Wolff
Sanders Jr., Royden C.
Application Number:
US29312839A
Publication Date:
10/07/1941
Filing Date:
09/01/1939
Assignee:
RCA CORP
Primary Class:
Other Classes:
342/122, 361/289
International Classes:
G01S1/02; G01S19/40; G01S19/48; G01S19/49
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Description:

This invention relates to frequency-modulated radio altimeters and more particularly to a frequency-modulated altimeter in which the effective range is increased by narrowing the modulation band width as contrasted with the constant band width devices.

In a frequency-modulated altimeter, the instantaneous frequency of the outgoing wave, which is varied, is compared to the frequency of the reflected wave. The difference in these two frequencies is proportional to the altitude. In this type of altimeter, it has been customary to employ a modulation band of considerable width constant with respect to altitude notwithstanding the facts (1) that the maximum receiver band width is required at the maximum altitude-the point of weakest signals-and (2) that one of the limiting factors of amplification is the noise arising within the amplifier. These two factors bear a relation which is also well known 1. e., the noise voltage in the output of an amplifier is proportional to the square root of the amplified frequency band and directly proportional to the amplification.

In other words, if the frequency band being amplified were reduced to 1/nth of its original value, the amplification could be increased VF times the original value and the original noise level would be unaltered. While these factors may not be of outstanding importance for short ranges or low altitudes, they are of great importance at the long ranges or high altitudes, especially with a low power transmitter.

It is one of the objects of the invention to provide a frequency-modulation distance measuring device of increased range and decreased band width. Another object is to provide means in a radio altimeter for making the modulation band approximately inversely proportional to the altitude. An additional object is to provide means for improving the ratio of signal to amplifier noise by decreasing the modulation band width as the signal strength diminishes.

The invention will be described by referring to the accompanying drawing in which Figure 1 is a schematic diagram of one embodiment of the invention; and Figure 2 is a graph illustrating the operation of the invention.

Referring to Fig. 1, a radio receiver I is connected through a low frequency amplifier 3 to a frequency meter 5 of the electronic type. One form of frequency meter is shown in the copending application Serial No. 248,577, filed December 30, 1938, by Royden C. Sanders, Jr., and entitled "Frequency meter." The frequency meter output is connected to a D. C. amplifier 7, which controls the gain of an A. C. amplifier 9, in the manner of the well known automatic volume control connections which supply automatically biasing potentials to the amplifier to be controlled. In the present arrangement the A. V. C. control opposes the normal amplifier bias, as is also known in the art. The input of the A. C. amplifier is connected to an A. C. power source II and the amplifier output is applied to the motor 25 of a frequency modulator 13 which is connected to a radio transmitter 15. An A. C. voltmeter 17, calibrated as an altimeter, is connected to the power leads to the motor 25.

All of the foregoing components, with the possible exception of the frequency modulator 13, are well known to those skilled in the art and, therefore, do not require a detailed description.

The frequency modulator may be of the variable capacitor type driven by a plunger type motor 25. A pair of fixedly mounted capacitor plates 19, 21 is secured to a frame 23 on which is mounted an electromechanical motor 25. The movable element of the motor moves back and forth as indicated by the arrow 27. A movable capacitor plate 29 is fastened to the movable element. As the movable plate 29 is vibrated back and forth by currents applied to the motor from the low frequency oscillator II through the low frequency amplifier 9, the capacity of the fixed plates 19, 21 is varied. While the rate of variation of the capacitor is determined by the applied frequency, the variation in capacity is directly proportional to the applied current, and inversely proportional to the effective dielectric between the capacitor plates. This capacity variation is applied to the transmitter by connecting the capacitor 19, 21 across the resonant circuit of the transmitter to vary its instantaneous frequency. It should be understood the inductive variation may be used and such variation may be applied by means of a motor or electronically.

If an alternating current of a constant peak amplitude were applied to the motor 25, the movable plate 29 attached to the motor shaft would vibrate at the frequency of the applied current and between limits determined by the amplitude of the current. Such movement would result in a variation of capacity of the plates 19, 21 at a fixed rate and between fixed limits. Since the capacity between the plates 19, 21 is part of the resonant circuit of the transmitter 15, a frequency variation or modulation of constant band width would be produced. This would be the case if the D. C. amplifier 7 and connections thereto were omitted from the circuit of Fig. 1.

However, the D. C. amplifier 7 is included in the circuit and is supplied with D. C. from the frequency meter from which unidirectional or rectified currents are obtained. These currents are amplified by the D. C. amplifier 7 and are applied to the low frequency amplifier 9 so that the gain of the latter amplifier 9 is reduced. The reduced gain of the low frequency amplifier reduces the amplitude of the currents supplied from the low frequency oscillator II, through the amplifier 9, to the motor 25 (see Fig. 3) of the frequency modulator 13. Since currents of lesser amplitude are applied to the motor, the pistonlike excursions 27 of the motor shaft will be reduced. This in turn reduces the movements of the plate 29 and hence reduces the capacity variation of the fixed plates 19, 21 from what would be obtained from the application of currents of larger amplitude. In turn,, the frequency modulation of the transmitter covers a narrower band width because the fixed plates are connected to the resonant circuit of the transmitter, as previously mentioned. The relation between the frequency modulation band of the transmitter and the altitude or distance from which measurements are to be made will be hereinafter described.

The several factors determining the applied currents are indicated by the formula Received heterodyne frequency= (Mod. freq.) (Altitude) (Band width) where the heterodyne and modulation frequencies are expressed in cycles per second, the altitude in feet, the band width in megacycles per second, and K=250. From the formula, which 40 has been determined mathematically and experimentally, it follows that for a given modulation frequency and altitude range the amplifier band width is proportional to the frequency modulation band width. However, the trans- 45 mitted band width must be large enough to obtain the desired accuracy at the lowest height to be indicated. While the lowest modulation frequency should not be less than 15 cycles per second, 120 cycles per second is preferred. 50 In the prior art radio altimeters, the usual procedure has been to adjust the modulation band width for the desired accuracy at the low altitudes and to maintain this band width for all altitudes and to accept the aforementioned dis- 55 advantages. In the altimeter of the present invention, the method is to adjust the modulation band width just wide enough to obtain the desired accuracy at the lower altitudes as indicated by the flat portion 31 of the graph 33 of Fig. 2. 60 The initial modulation band width is determined by the initial space between the fixed plates 19 and 21 and the movable plate 29 and the current initially applied to the motor 25. Thereafter, the received waves are converted into a D. C. bias 65 which overcomes the normal bias of the A. C. amplifier 9 and thus diminishes the amplifier gain. The diminished gain reduces the amount of current applied to the motor 25 of the frequency modulator 13 and hence the modulation 70 band is diminished as indicated by the curved portion 35 of the graph 33. The resultant effect on the heterodyne frequency is indicated by the graph 37. The altitude is measured in terms of the A. C. 'voltage (or current) applied to the 75 motor 25.of the frequency modulator. It is preferable to calibrate the A. C. voltmeter 17 to indicate directly the altitude in feet or other desired units. It should be understood that the voltage or current corresponding to the altitude may be measured at other points in the system.

For example, the controlling bias or voltage may be measured to indicate the altitude.

Thus the invention has been described as a radio altimeter of the frequency modulation type in which the modulation band width is varied in the conventional manner to obtain the desired accuracy at low altitude. The modulation band width is then made smaller and smaller as the altitude is increased. The control of the band width is brought about by applying a direct current bias which is derived from the received waves. The diminution of the modulation band permits increased amplification in the receiver because the signal-to-noise ratio is proportional to the square root of the frequency band amplified in the receiver. The receiver, transmitter, amplifiers, oscillators and modulators used in the practical application of the invention may be any of the several well known types.

We claim as our invention: 1. A radio altimeter including means for transmitting radio waves of varying frequency to be reflected from the earth, a receiver for receiving said signals and for producing a signal corresponding in frequency to the difference in frequency of the transmitted and reflected waves, means connected to said receiver for deriving from said receiver currents having an amplitude proportional to the difference frequency of the received signals, a low frequency oscillator, a frequency modulator including connections to said transmission means, an amplifier connecting said oscillator and modulator, and means connecting said current deriving means and said amplifier for applying said derived currents to said amplifier to control its gain.

2. A radio altimeter including means for transmitting radio waves of varying frequency toward the earth to be reflected therefrom, a receiver for receiving said transmitted waves directly and after reflection, means connected to said receiver for deriving from said receiver currents having an amplitude proportional to the difference in frequency of the directly received waves and the received reflected waves, a low frequency oscillator, a frequency modulator connected to said means for transmitting, an amplifier connecting said oscillator and modulator for applying currents from the former to the latter, means connecting said current deriving means and said amplifier for applying said derived currents to said amplifier to control its gain, and means for indicating altitude as a function of the current applied by said amplifier to said frequency modulator.

3. A radio altimeter including a transmitter for radiating radio waves of varying frequency toward the earth to be reflected therefrom, a receiver for receiving said transmitted waves directly and after reflection, means connected to said receiver for deriving from said receiver currents having an amplitude proportional to the difference in frequency of the directly received waves and the received reflected waves, a low frequency oscillator, a frequency modulator, an amplifier connecting said oscillator and modulator to apply currents from the oscillator to the modulator, means connected to said current deriving means and to said amplifier for applying said aerivec currents to said amplifier to control its gain, and means connecting said frequency modulator to said transmitter whereby the modulation frequency is determined by said low frequency oscillator and the modulation band width is controlled as a function of \the derived currents.

4. A radio altimeter including a transmitter for radiating altitude determining radio waves of varying frequency toward the earth to be reflected therefrom, a receiver directly responsive to said transmitted waves and to said reflected waves, means connected to said receiver for deriving from said receiver currents having an amplitude proportional to the frequency differ- I ence of the waves received directly and after reflection, a low frequency oscillator, a frequency modulator, an amplifier connecting said oscillator and modulator for applying currents from the oscillator to the modulator, means connected 2 to said current deriving means and said amplifier for applying said derived currents to said amplifier to control its gain, means connecting said frequency modulator to said transmitter to vary its frequency whereby the modulation frequency 2 is a function of said low frequency oscillator frequency and the modulation band width is a function of the amplitude of said derived currents, and a meter for indicating altitude as a function of the currents applied by said amplifier to said 3 frequency modulator.

5. A radio distance-measuring device of the frequency modulator type including means for transmitting waves to an object to be reflected) therefrom, means for receiving said waves di- 3 rectly from said transmitting means and for receiving said waves after reflection, means connected to said transmitting means for varying the frequency of the transmitted waves, means connected to said receiving means for deriving 4 currents having an amplitude proportional to the difference in frequency of the waves directly received and the waves received after reflection, and means connecting said current deriving means and said frequency varying means for applying currents corresponding to said derived currents to control the frequency limits of the frequency varying means.

6. A radio distance-measuring device of the frequency modulator type including means for transmitting radio waves toward an object to be reflected therefrom, means for receiving said transmitted waves directly and after reflection, means connected to said transmitting means for varying the frequency of the transmitted waves, means connected to said receiving means for de- 6 riving currents having an amplitude proportional to the difference in frequency of the directly received waves and the waves received after reflection, means connecting said current deriving means and said frequency varying means for ap- 6 plying currents corresponding to said derived currents to control the band width of the transmitted waves, and means for indicating distance of said obiect as a function of said band width.

7. A radio distance-measuring device of the 61 frequency modulator type including means for transmitting frequency varying radio waves toward an object to be reflected therefrom, means for receiving directly said transmitted waves and for receiving said waves after reflection, means 7 connected to said transmitting means for varying the frequency of the transmitted waves, means connected to said receiving means for deriving currents having an amplitude proportional to the beats of the directly received trans- 74 mitted waves and the received reflected waves, and means connecting said current deriving means and said frequency varying means for applying currents corresponding to said derived currents to control the band width of the transmitted waves.

8. A radio distance-measuring device of the frequency modulator type including means for transmitting frequency varying radio waves toward an object to be reflected therefrom, means for receiving directly said transmitted waves and for receiving said waves after reflection, means connected to said transmitting means for varying the frequency of the transmitted waves.

cludes transmitting a wave, varying the frequency of said wave, receiving said wave directly and after reflection, heterodyning the direct and reflected waves, deriving a voltage corresponding to the difference of frequency of said direct and reflected waves, applying said voltage to control the frequency limits of said frequency variation as a function of said distance, and measuring the voltage applied to the frequency control to indicate distance.

13. The method of measuring distance which includes transmitting a wave, varying the frequency of said wave, receiving said wave directly and after reflection, heterodyning the direct and reflected waves, deriving a voltage corresponding to the difference in frequency of said direct and reflected waves, generating a low frequency current, amplifying said low frequency current, controlling said amplification by said derived voltage, and applying said controlled amplified voltage to determine the frequency sweep of said frequency variation.

14. The method of measuring distance which includes transmitting a wave, varying the frequency of said wave, receiving said wave directly and after reflection, heterodyning the direct and reflected waves, deriving a voltage corresponding to the difference in frequency of said direct and reflected waves, generating a low frequency current, amplifying said low frequency current, controlling said amplification by said derived voltage, applying said controlled amplified voltage to determine the frequency sweep of said frequency variation, and measuring the controlled amplified voltage to indicate distance.

IRVING WOLPF.

ROYDEN C. SANDERS, Ja.