Low altitude f. m. altimeter

Abstract

Claims

3. IN A FREQUENCY MODULATED RADAR ALTIMETER, A RANDOM NOISE SOURCE, A KLYSTRON OSCILLATOR, MEANS FOR FREQUENCY MODULATING SAID KLYSTRON OSCILLATOR, MEANS CONNECTING SAID RANDOM NOISE SOURCE TO SAID MODULATING MEANS FOR RANDOMLY VARYING THE PERIOD OF THE FREQUENCY MODULATION, A TRANSMITTING ANTENNA CONNECTED TO SAID KLYSTRON OSCILLATOR, A RECEIVING ANTENNA, A BALANCE MICROWAVE MIXER CONNECTED TO SAID RECEIVING ANTENNA, ATTENUATION MEANS COUPLED BETWEEN SAID KLYSTRON OSCILLATOR AND SAID MIXER TO APPLY A PORTION OF THE TRANSMITTED SIGNAL DIRECTLY TO SAID MIXER, SO THAT SAID MIXER PRODUCES A DIFFERENCE FREQUENCY BETWEEN SAID TRANSMITTED SIGNAL AND A RECEIVED SIGNAL REFLECTED FROM THE GROUND WHICH IS PROPORTIONAL TO ALTITUDE, WIDE-BAND AMPLIFYING MEANS CONNECTED TO SAID MIXER, A BALANCED MODULATOR HAVING TWO INPUTS, SAID AMPLIFYING MEANS CONNECTED TO ONE OF SAID MODULATOR INPUTS, A VARIABLE FREQUENCY LOCAL OSCILLATOR CONNECTED TO THE OTHER OF SAID MODULATOR INPUTS, A NARROW BANDWIDTH I.-F. AMPLIFIER COUPLED TO THE OUTPUT OF SAID MODULATOR, A BALANCE F.M. DISCRIMINATOR CONNECTED TO SAID I.-F. AMPLIFIER, MEANS FOR GENERATING A SAWTOOTH VOLTAGE WHEN THE OUTPUT SIGNAL OF SAID DISCRIMINATOR IS ZERO AND FOR AMPLIFYING THE OUTPUTS SIGNAL OF SAID DISCRIMATOR WHEN IT EXCEEDS A PREDETERMINED NEGATIVE MAGNITUDE, MEANS CONNECTED TO SAID LAST-MENTIONED MEANS FOR VARYING THE FREQUENCY OF SAID LOCAL OSCILLATOR IN RESPONSE TO SAID SAWTOOTH VOLTAGE AND FOR LOCKING THE FREQUENCY OF SAID LOCAL OSCILLATOR WHEN THE OUTPUT SIGNAL OF SAID DISCRIMINATOR EXCEEDS SAID PREDETERMINED NEGATIVE MAGNITUDE, ALTITUDE INDICATING MEANS CONNECTED TO SAID DISCRIMINATOR OUTPUT SIGNAL AMPLIFYING MEANS, AND RANGE SELECTING MEANS CONNECTED BETWEEN SAND SAWTOOTH VOLTAGE GENERATING MEANS AND SAID FREQUENCY MODULATING MEANS FOR INCREMENTALLY CHANGING THE AVERAGE MODULATING FREQUENCY OF SAID MODULATING MEANS TO MEASURE ALTITUDES IN A PLURALITY OF DISCRETE RANGES.
Oct. 29, 1963 K. E. HARDINGER .ETAL 3,109,172 LOW ALTITUDE F.M. ALTIMETER Filed Aug. 10. 1960 10 Sheets-Sheet 1 T y E R TIME musnmzn i SIGNAL 3 I m u. l fi/ 7 3 A TIME DIFFERENCE g o FREQUENCY a J V V V m ll- I z q. d B AM I \lllHll mrmuauce V I SPECTRUM FL; Fly. 16 INVENTORS PHILIP W. BOESCH KERMIT E. HARDINGER BY 1 FpZZZ Q Cid/a 4 Oct. 29, 1963 K. E. HARDINGER ETAL 3,109,172 LOW ALTITUDE F'.M. ALTIMETER Filed Aug. 10, 1960 1Q Sheets-Sheet 2 e F F1 c A mnsm'nso 5 SIGNAL 5 a TIM: g b d u- 0 T Flg.2A sf g TIME DIFFERENCE g FREQUENCY g G IL II 1| DIFFERENCE SPECTRUM o 8 Fl gdlc INVENTORS PHILIP W. BOESCH KERMIT E. HARDINGER Oct. 29, 1963 K. E. HARDINGER ETAL 3,109,172 LOW ALTITUDE F.M. ALTIMETER Filed Aug. 10, 1960 10 Sheets-Sheet 6 TRANSNHTTED SIGNAL 5 D S E- TlME DIFFERENCE E f FREQUENCY 2 l l DIFFERENCE T SPECTRUM o J] F- SF INVENTORS PHILIP W. 'BOESCH KERMIT E. HARDINGER J flaw a4. Q BY 44%;; aw Oct. 29, 1963 K. E. HARDINGER ETAL 3,109,172 LOW ALTITUDE F.M. ALTIMETER Filed Aug. 10, 1960 1o Sheets-Sheet 7 use E TO LF. II AMPLIFIER BhLANCED MODULATOR L-O. INPUT VIDEO AMPLIFIER INVENTORS PHILIP W BOESCH KERMIT E. HARDINGER BY MMW WWW FROM MIXER Oct. 29, 1963 K. E. HARDINGER ETAL 3,109,172 LOW ALTITUDE F.M. ALTIMETER 1O Sheets-Sheet 10 Filed Aug. 10, 1960 I FIXED AGC FREQUENCY MC United States Patent Ofi ice 3,109,172 Patented Oct. 29, 1963 Army Filed Aug. 10, 1960, Ser. No. 48,786 3 Claims. (Cl. 343-14) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon. This invention relates to continuous wave radar distance measuring systems and more particularly to frequency modulated radar altimeters. A primary requirement for safe instrument navigation in aircraft, as well as for safe instrument landings, is an altimeter which can produce very accurate altitude information within a short response time and over a wide range of altitudes. Instrument flying becomes increasingly important as aircraft speeds are raised, as the airways become more densely populated, and as flying is extended to low Visibility weather conditions. Even under visual flight regulations, the altimeter is necessary to facilitate traflic control and to place all aircraft in an immediate vicinity on a common basis. For low altitude flight, such as takeoff and landing, a highly accurate and dependable terrain-clearance altimeter is indispensable, whether or not ground-control equipment is employed. The error of a fl-areout altimeter for blind landings must be less than five percent, even at low altitudes. Such an altimeter, in conjunction with a high-resolution surveillance radar, could furnish the pilot, or a computer in an automatic landing system, suflicient information to make a safe zero-visibility landing. An object of the present invention is to provide a radar altimeter of greatly improved accuracy. Another object of the invention is to furnish a radar altimeter in which 'both the range ambiguities and the critical-distance errors of known frequency modulated altimeters are eliminated. A further object is to provide a frequency modulated radar altimeter which measures altitude to within five percent at all altitudes in a range from five feet to several thousand feet. Another object of the present invention is to furnish a radar altimeter having improved sensitivity and signalto-noise ratio. A further object is to extend the range of a frequency modulated altimeter over which altitudes may be measured with a high degree of accuracy by maintaining a constant transmitter frequency deviation and varying the modulating frequency under control of a constant dilference frequency servo system. A feature of the invention involves the application of a random-noise signal to the frequency modulator of an altimeter transmitter to obtain a jittered trianguilar modulation signal. This type of modulation substantially eliminates the critical-distance errors and range ambiguities which would otherwise exist in the receiver, as explained in detail below. Another feature relates to the production of a constant difference frequency signal in the altimeter by a servo mechanism control which functions so that maximum frequency deviation of the transmitter oscillator is maintained at all times, over several operating ranges of the altimeter. A typical embodiment of the RM. altimeter system in accordance with the present invention comprises a transmitter having a thyratron circuit operating as a diode random-noise generator, a free running phantastron triggered by the random-noise generator to produce a jittered triangular modulating voltage, a reflex klystron frequency modulated by the phantastron modulator, and a transmitting antenna. The altimeter further comprises a receiver including a receiving antenna, a balanced microwave mixer delivering an output signal which is the difference frequency between a portion of the transmitted signal and the received reflected signal, and a wide band video amplifier. The receiver further contains a balanced modulator which mixes the difference signal from the video amplifier with a heterodynin-g signal generated by a variable frequency local oscillator to produce a predetermined constant difference frequency signal, a narrow bandwidth I.-F. amplifier tuned to the aforesaid constant difference frequency, a balanced F .M. discriminator which generates a DC. output voltage proportional to the frequency variation of the heterodyned output signal of the balanced modulator from the predetermined constant difference frequency, and a phantastron acting as a combined sawtooth voltage generator and D0. amplifier connected between the discriminator and the local oscillator, completing a servo loop which searches for the difference frequency signal from the microwave mixer containing the altitude information and locks 1011 to it when the local oscillator is set to heterodyne the said difference frequency to the constant difference frequency of the I.-F. amplifier. An altitude calibrated voltmeter connected to the sawtooth generator-DC. amplifier (output provides altitude readout for the altimeter, after the servo locks on to the incoming signal. Further, a range selector responsive to the number of oscillations of the sawtooth generator alters circuit parameters in the modulator phantastron to change the modulating frequency without altering the frequency deviation of the transmitter klystron, if the reflected signal is not found in a given altitude range within a predetermined time. The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which: FIGS. 1A, 1B and 1C are, respectively, graphs of the frequency variations of the transmitted and received signals, the frequency variation of the beat or difference frequency signal produced by mixing the transmitted and received signals, and the frequency spectrum of the difference frequency signal of a sinusoidally modulated F.M. altimeter of known type. FIGS. 2A, 2B and 2C are similar graphs for a triangularly modulated F.M. altimeter of the prior art. FIGS. 3A, 3B and 3C show similar graphs for the jittered triangular modulation employed in the altimeter of the present invention. FIG. 4 is a schematic block diagram :of a system illustrating one form of this invention. FIG. 5 is a Specific wiring diagram of the random-noise generator and transmitter modulator of FIG. 4. FIGS. 6A, '6B and 6C are graphs of certain waveforms employed in the modulator of FIG. 5 "and the system of FIG. 4. FIG. 7 is a schematic diagram of the video amplifier and the balanced modulator of FIG. 4-. FIG. 8 is a wiring diagram of the I.-F. amplifier and the balanced discriminator of FIG. 4. FIG. 9' is a schematic wiring diagram of the sweep generator-DC. amplifier and the local or search oscillator of the system of FIG. 4. FIG. 10 illustrates the voltage response curves for the A I.-F. amplifier and the balanced discriminator of FIG. 8. One conventional form of radar altimeter consists basically of a frequency modulated radio transmitter, a receiver containing a mixer-detector, amplifying circuits, and a frequency counter. In such systems, the RM. transmitter is sinusoidally modulated with a high modulation index, as shown in FIG. 1A, and is employed both as the signal transmitter and as the local oscillator. The transmitted signal, having a frequency variation curve as in FIG. 1A, is reflected from the ground and re turned to the altimeter system. The received signal, whose frequency variation appears in FIG. 1A, is the transmitted signal delayed by a time where h is the altitude and c is the velocity of light. The received signal is mixed with the local-oscillator signal, producing an instantaneous difference frequency signal f which is sinusoidal, as illustrated in FIG. 1B. The average magnitude of the difference frequency depends upon the time delay 1' which is, in turn, proportional to altitude. The diiference frequency spectrum of FIG. 1C is composed of lines spaced equally by an amount equal to the modulation frequency, starting at zero frequency. The frequency counter is arranged to count each zero crossover of the difierence frequency signal voltage and average these counts over a time period long compared with the frequency of crossing, so that the average number of counts per unit time is proportional to the altitude being measured. The frequency counter used in these altimeters registers only discrete steps, i.e., the zero crossings, and cannot count fractional zero crossings. This inability of a frequency counter, or any standard frequency discriminator, to measure the difference frequency continuously, rather than in discrete steps, gives rise to a critical-distance error or a fixed error in such altimeters. This error appears as changes in measured altitude in increments equal to the critical-distance. Further, since the transmitted signal is modulated by a periodic function such as f of FIG. 1A, it exactly repeats itself every period of the modulating signal f The receiver frequency counter cannot distinguish between a signal reflected from the ground at one delay time 1- corresponding to a first altitude and a delay time equal to 1 plus any given number of modulation periods T which corresponds to some other altitude. This periodicity of the modulating signal thus gives rise to range ambiguities. Another type of modulation employed in F.M. radar altimeters known to the art is the linear sweep in the form of triangular or sawtooth waveforms, such as shown in FIG. 2A, the sawtooth merely being a special case of the more general triangular waveform illustrated. The instantaneous frequency difference f between transmitted and received signals, shown in FIG. 2B, is essentially constant with the exception of the regions near the turnaround points. The constant frequency difference of curve f of FIG. 2B contains the altitude information which can be expressed as follows. In FIG. 2A, it is seen that triangle abc is similar to triangle ade. Therefore, we have where F is the frequency deviation of the transmitted signal and T is the period of the modulation signal As stated above, Also, From the Equation 2 it is clear that for a given modulating signal the constant frequency difference 6f, present at times other than those corresponding to turnaround points of f and f is directly proportional to altitude h. It is to be noted that Equation 2 applies equally well in the case of the novel modulation techniques employed in the present invention and illustrated in FIGS. 3A and 6A. With triangular F.M. modulation, the energy of the difference frequency signal h; of FIG. 2B is concentrated in a nalrow band of frequencies. The center frequency of the band or the average diiference frequency is proportional to the distance to the ground. The spectrum of the difference frequency signal is depicted in FIG. 2C. This spectrum is easier to detect than that of the sinusoidally modulated types of FIG. 1C and does not require as much receiver bandwidth, resulting in better sensitivity. However, altimeters utilizing the periodic triangular modulation as discussed above still contain range ambiguities and critical-distance errors which are inherent in periodic systems which determine the difference frequency by measuring zero crossovers. In the altimeter system in accordance with the present invention, a random-noise signal is applied to a triangul-ar modulation signal to eliminate these sources of error. The modulating waveform becomes a triangular wave having a random period and amplitude, as seen in FIG. 3A, but having constant slopes for any given range of altitudes. To measure altitudes in several ranges, the altimeter of the invention incrementally changes the aforementioned slopes in successive time intervals by changing the modulating signals period while maintaining the frequency deviation constant, as explained more fully below. The resulting instantaneous difference frequency ,f of FIG. 3B is still constant, except in the turnaround regions. The spectrum, shown in FIG. 3C, is concentrated at a frequency proportional to the distance from the ground, with randomly spaced lines as shown in FIG. 3C instead of the discrete lines of FIG. 2C. The periodicity of the modulation signal which is obtained by the application of random noise to the RM. modulator eliminates all range ambiguities, as the latter are based on periodic repetition of the modulating waveform. Also, since the period of the modulation signal of FIG. 3A is random, the number of whole cycles of the difference frequency 3; of FIG. 3B in each modulation cycle is also random. Therefore, the average number of whole cycles of f does not change in discrete steps as the altitude being detected varies. Consequently, there are no critical-distance errors in the altimeter of this invention. Referring to FIG. 4 which shows the altimeter system of this invention, block 10 represents a random-noise generator whose output voltage is coupled to "an input of a frequency modulator 1:1. The modulator 11 generates an output voltage whose waveform corresponds to the curve of FIG. 6A. The signal from modulator 11 is then applied to the repeller of a conventional reflex klystron oscillator 12, having a linear frequency-voltage characteristic, which generates a frequency modulated transmitting signal. This signal is radiated by transmitting antenna 13 connected to the output of klystron 12. Noise generator 10 and modulator 11 are shown in greater detail in FIG. 5. Therein it is seen that a thyratron 30 is arranged to function as a diode random-noise generator. The modulator '11 consists of a screen-coupled phantastron oscillator 32 which is free running. The voltage at plate 43 of phantastron 32 falls quite linearly after a triggering voltage is applied to its suppressor grid 42 and then rises exponentially as soon as the plate voltage at 43 bottoms, as is well known in the art. The output from thyratron 30 on lead 41 is R-C coupled to suppressor grid 42 to supply the required triggering voltage. By using only the initial part of the exponential rise of the plate voltage at 43 in phantastron 32, the magnitudes of the slopes of the rise and fall may be made equal. The noise voltage on lead 41 randomly varies the amplitude at which phantastron 32 reverses the direction of its sweep, thus generating the desired modulating waveform at 43, as illustrated in FIG. 6A by curve f Since the slopes remain constant and the amplitude at the turnaround is random, the period is random. The waveform of FIG. 6A generated by moduulator 11 is similar to that of FIG. 3A except that the randomness of the amplitude is substantially reduced due to the characteristics of the phantastron 32. The negative peaks are practically constant due to the bottoming of the plate 43 while the positive peaks vary slightly as determined by the out-put of noise generator 30. However, these variations are chosen to be relatively small and result in an average frequency deviation F in FIG. 6A which is constant, as previously stated. The positive peak variations are still sufficient to give the jittered modulation effect which eliminates critical distance errors as has been fully explained. The output of'phantastron oscillator 32 from plate 46 is R-C coupled to the control grid of a cathode follower amplifier 34 whose output is connected through switch 46 to the input of a triode amplifier stage 36. Amplifier 36 has a wide band response due to the plate circuit inductor which improves its high frequency response. A final dual triode cathode follower 38 provides a low output impedance for modulator 11 and its output is coupled to the repeller of klystron 12. Further, a switch 45 in the feedback loop of phantastron 32, which is ganged with switch 46, selects one of three separate sets of resistor-condenser components to control the time constant of the feedback loop. This allows controlled variation of the nominal time period T of modulation signal of FIG. 6A and thus selection of three different constant slopes of the modulation function corresponding to three altitude ranges in which the altimeter can operate. The setting of switches 45 and '46 is controlled by a range selector 23 which is described in more detail subsequently. Referring again to FIG. 4, the received signal reflected from the ground is detected by a receiving antenna 15 and applied as one input to a conventional balanced microwave mixer 16. A directional coupler attenuator 14- is connected to the output of klystron 12 and feeds a part of the transmitted signal directly to mixer 16 as an R.-F. local oscillator signal. The mixer 16 produces an output signal whose frequency is the mean magnitude of the instantaneous difierence frequency between the transmitted and received signals, as illustrated in FIG. 6B. The balanced irnixer should have an amplitude modulation rejection of approximately 20 db. The difference frequency signal from mixer 16, is amplified by a wideband video amplifier 17 which has a voltage gain of 600 and a bandwidth between half-power points extending from 50! kc. to 12 me. The amplifier 17 is of known construction, the details of which are fully illustrated in FIG. 7. The receiver section of the altimeter shown in FIG. 4 further contains a servomechanism consisting of a-modulator 18, an amplifier 20, a frequency discriminator 21, combined sawtooth voltage generator and DC. amplifier 22 and a variable frequency search oscillator 19 connected in a closed loop as shown. The difference frequency signal f containing the altitude information from video amplifier 17 is applied as one input to the balanced modulator 18. The modulator 18 is preferably of the carrier suppressed type with pushpull output, which is well known to the art. It is shown in detail in the FIG. 7 schematic. The other input to balanced modulator 18 is supplied by a searchoscillator 19 which acts as a local heterodyne oscillator to apply a heterodyning signal to modulator 18. The local oscillator or search oscillator 19 is of the variable frequency type, and its operating frequency is :controlled by circuit 22 in a manner to be explained below. The output signal from balanced modulator 18 is coupled to an L-F. amplifier 20 of very narrow bandwidth. The frequency response of amplifier 20 is centered at 30 rnc. and has a. bandwidth of 800 kc. between its half-power points, as shown in curve e of FIG. 10. The LP. amplifier 20' is of conventional design, preferably incorporating automatic volume control in order to maintain linear operation over a wide range of signal amplitudes. One particular example of a suitable I.-F. amplifier is illustrated in FIG. 8. The frequency of local oscillator 19 is varied or swept from 42 me. to 30.5 me. under [control of a 30 c.p.s. sawtooth voltage from circuit 2 2. The difference frequency signal from amplifier 17 is thus mixed or heterodyned with the swept output of search oscillator 19" and translated to a higher frequency region. The local oscillator -19 is connected to the carrier input of the balanced modulator 18, as seen in FIG. 7, and the video amplifier signal is applied to the modulating voltage input of '18 so that the search oscillator signal is suppressed after mixing. Assuming that the altimeter picks up a signal reflected from the ground at a distance proportional to a particular difference frequency 8 then the output of bal anced modulator 1-8 is a signal whose frequency spectrum, such as shown in FIG. 6C, sweeps from (42 mo. 8f) down to 30 rnc., the center frequency of the L-F. amplifier. It will be seen subsequently that when this modulator output signal onits downward sweep is amplified at the upper end of the L-F. amplifier response, the servo loop functions to lock the frequency of the local oscillator 1 9 at the frequency required to heterodyne the of signal to 30 mc., the I.-F. frequency. Thus the need for very wideb-and response in the receiver of the altimeter to accommodate wide variations of 87 corresponding to a wide altitude range is eliminated. This is an important advantage, since the attainment of wide range for the altimeter while employing narrow bandwidth amplifier circuits results in improved receiver sensitivity. The receivers sensitivity is inversely related to its bandwidth. Also, the conversion to a high frequency region by the superheterodyne circuits 18, 19 and 2d largely removes microphonic interferences and Doppler type error signals which occur in a low frequency range and will not be passed by I.-F. amplifier 20. The constant difference frequency output of I.-F. amplifier 20, which exists when local oscillator 19 is set to a frequency f such that f =3O meld-5f, is applied as an input to a balanced frequency discriminator 21. The particular circuit configuration of discriminator 21 is shown in FIG. 8. The discriminator 21 is arranged to weight the spectral energy of the signal from -I.-F. amplifier 20 on each side of the 30 me. center frequency, and produces an output signal with an amplitude and polarity which is proportional to the imbalance of the energies on the two sides of the center frequency. The curve c of FIG. 10 illustrates the required discriminator characteristic. The discriminator 21 functions to generate an error voltage which is proportional to the frequency displacement of the signal output of I.-F. amplifier 20 from the predetermined constant difference signal which is the L-F. center frequency. This error voltage output from discriminator 21 is connected to the control grid 65 of a circuit 22 which operates as a combined sawtooth voltage generator and D10. amplifier as will now be explained with reference to FIG. 9. The circuit 22 of FIG. 4 is arranged to function as a sawtooth voltage generator during the time intervals when the voltage at control grid 65 of phantastron 50 of FIG. 9 is greater or more positive than a predetermined negative magnitude and further to function as a D.C. amplifier at other times when control grid 65 is more negative than the said predetermined magnitude. The voltage at grid 65 at any given time is determined by the output of balanced discriminator 21, since it is connected to the input of circuit 22. The circuit 22 as shown in FIG. 9 comprises a phantastron circuit 50 coupled to a cathode follower output stage 52. When the voltage at 65 is positive or of a small negative magnitude, stage 50 is effectively a screen-coupled phantastron relaxation oscillator generating a 30 cycle sawtooth voltage at plate 63 which is coupled by potentiometer 67 to control grid 68 of cathode follower 52. The cathode follower output appears at terminal 70. The sawtooth oscillations of phantastron 50 at terminal 70 are applied to capacitor 72 in the circuit of triode oscillator 54 of FIG. 9, the search oscillator or local oscillator 19 of FIG. 4. Tn'ode 54 and its associated circuit components form a conventional Hartley oscillator. Capacitor 7-2 is a voltage sensitive capacitor or Varicap, whose capacitance is determined by the voltage applied to it. When a varying voltage is applied to capacitor 72 to vary its capacitance, the frequency of oscillation of oscillator 54 will be controllably varied, as will be apparent to those skilled in the art. Thus the sawtooth voltage at terminal 70 of circuit 22 varies the bias voltage across capacitor 72 applied by bias source 69 to drive the search oscillator 19 over its 40 mc.30.5 mc. sweep. This completes the search and Ice servo loop. The sweeping local oscillator 19 performs a search function in the altimeter, such that at some point during its sweep the difference between its frequency and the signal from amplifier 17 mixed with it in modulator 18 is equal to the tuned frequency of L-F. amplifier 29. Then, f -f=P, the operating frequency of the L-F. amplifier, here 30 me. As local oscillator 19, decreasing in frequency during its sweep, approaches f the output signal of modulator 18 also decreases in frequency towards frequency P. The spectrum of this output signal is amplified by the upper end of I.-F. amplifier 20, and a negative swing of the output voltage of discriminator 21 occurs upon detecting the amplified signal. The discriminator output voltage becomes sufiiciently negative to cause searc oscillator 19 to cease sweeping and remain at frequency f This locking action occurs because when the discriminator voltage applied to control grid 65 of phantastron 50 becomes sufficiently negative as just mentioned, the gain of the phantastron tube is reduced to a level below that required to sustain the relaxation oscillations producing the sawtooth voltage. The loop gain of the stage becomes so low that feedback through capacitor 61 has no effect. At this point, however, the signal at grid (65 is not at cutotf so stage 50 acts as a D.C. amplifier. Its constant output voltage at 70 maintains the frequency of local oscillator 19 constant at f The servo loop is locked onto the signal from video amplifier 17 and subsequently tracks it. The frequency of search oscillator 19 is controlled by the voltage of the sawtooth generator 22. When a reflected signal has been picked up by the receiver and the servo loop has locked onto this signal, the voltage at terminal 70 of circuit 22 is a measure of the altitude. This D.C. voltage is applied to an altitude readout voltmeter 24 connected to terminal 70 and calibrated to indicate altitude. However, when no signal has been acquired by the altimeter and consequently the voltage at 70 is a 30 :c.p.s. sawtooth, a neon light also connected to terminal 70 (not shown) is energized to indicate that the system is not locked on a target, but is in a searching mode. These latter functions could also be employed in suitable form in conjunction with any type of computer or rate-of-descent meter desired in a particular application. Also connected to terminal 70 of sawtooth generator 22 is a range selector device 23 whose output is coupled back to modulator 11 of the transmitter circuits. The range selector can be of any suitable form adapted to count a predetermined number of cycles of the sawtooth voltage generated by phantastron 50 during the searching mode of the altimeter and then produce an output signal suitable to rotate the movable members of switches 45 and 46 in modulator 11 to a new position. For example, range selector 23 could take the form of a predetermined counter aotuated by individual pulses, or cycles, of the sawtooth voltage at terminal 70 and a stepping relay connected to the above-mentioned counters output to rotate the movable arms of switches 45 and 46 to their next successive positions. As described above with regard to modulator 11, such actuation of switches 45 and 46 will select a different resistor-condenser combination in the feedback loop of phantastron oscillator 32 to alter the nominal or average time period T of the modulation signal of FIG. 6A while still maintaining the frequency deviation 1? of the klystron 12 substantially constant. The above arrangement permits selection of three distinct modulation waveform frequencies corresponding to three separate altitude ranges. This can be seen from Equation 2, i.e., the difference frequency from mixer 16 varies in direct proportion to altitude h for a given modulation waveform which has a constant F and a constant f The range selector 23 causes successive incremental changes in the f of the modulation waveform of FIG. 6A without altering F which means that in the above Equation 2, any signal 6 of a given frequency now corresponds to a new value of h, i.e., the altitude range being measured is changed. When the altimeter is first turned on, modulator 11 generates a triangular wave of random period but nevertheless having an average frequency (see FIG. 6A), and f corresponds to one particular range of altitudes, for examples, 5 to feet. The servo loop under control of the sawtooth generator 22 searches for a received signal indicating the altitude is within this range. Should the altitude be much higher than 100 feet, so that no return signal is obtained within the proper frequency range to pass L-F. amplifier 20 after the superheterodyne action of components 18 and 19, the sawtooth voltage at terminal 70 actuates the predetermined counter of the range selector 23. Modulator 11 is thereby switched to a different average modulating frequency f that is, a lower average modulating frequency. The sawtooth generator 22 again causes search oscillator 19 to make the predetermined number of searches for a signal return within the new altitude range before actuating switches 45 and 46 of modulator 11. This process of sweeping within the three altitude ranges repeats in sequence until a target is acquired, whereupon the sawtooth voltage is no longer generated because of the locking action of the servo loop and no further switching of the modulator altitude range occurs. The servomechanism tracks the received target signal and the correct altitude is indicated by the altitude readout 24. It will be apparent that the embodiment shown is only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims. We claim as our invention: 1. An improved radar altimeter comprising in combination: a transmitting signal source, means for generating a randomly jittered triangular modulating signal, said generating means including control means for maintaining the maximum and minimum values of said modulating signal Within predetermined mutually exclusive ranges, means for frequency modulating said transmitting source with said triangular modulating signal to produce an FM. transmitted signal, receiver means for mixing said transmitted signal with a received signal reflected from the ground, and a servomechanism connected to said receiver mixer means, said servomeohanism including a balanced modulator, a variable frequency local oscillator, an 'L-F. amplifier, an discriminator, and a combined sawtooth generator-DC. amplifier connected together in a closed loop, altitude readout means connected to an input of said local oscillator, and range selecting means connected to said sawtooth generator for varying in discrete steps the average frequency of said triangular modulating signal. 2. An improved altimeter having no range ambiguities or critical-distance errors comprising an oscillator, a modulator for generating a triangular frequency modulating signal, a random noise generator connected to said modulator to randomly vary a predetermined parameter of said modulating signal, means connecting said modulator to said oscillator to frequency modulate said oscillator in accordance with said modulating signal, a transmitting antenna connected to the output of said oscillator, and receiving means comprising a mixer for producing a difference frequency from the output of said frequency modulated oscillator and a received signal which has been radiated by said transmitting antenna and reflected from the ground, frequency responsive means responsive to the output of said mixer for producing a voltage proportional to said difference frequency, and altitude indicating means. responsive to said voltage. 3. In a frequency modulated radar altimeter, a random noise source, a klystron oscillator, means for frequency modulating said klystron oscillator, means connecting said random noise source to said modulating means for randomly varying the period of the frequency modulation, a transmitting antenna connected to said klystron oscillator, a receiving antenna, a balanced microwave mixer connected to said receiving antenna, attenuation means coupled between said klystron oscillator and said mixer to apply a portion of the transmitted signal directly to said mixer, so that said mixer produces a difference frequency between said transmitted signal and a received signal reflected from the ground which is proportional to altitude, wide-band amplifying means connected to said mixer, a balanced modulator having two inputs, said amplifying means connected to one of said modulator inputs, a variable frequency local oscillator connected to the other of said modulator inputs, a narrow bandwidth I.-F. amplifier coupled to the output of said modulator, a balanced F.M. discriminator connected to said L-F. amplifier, means for generating a sawtooth voltage when the output signal of said discriminator is zero and for amplifying the output signal of said discriminator when it exceeds a predetermined negative magnitude, means connected to said last-mentioned means for varying the fre quency of said local oscillator in response to said sawtooth voltage and for locking the frequency of said local oscillator when the output signal of said discriminator exceeds said predetermined negative magnitude, altitude indicat-ing means connected to said discriminator output signal amplifying means, and range selecting means connected between said sawtooth voltage generating means and said frequency modulating means for incrementally changing the average modulating frequency of said modulating means to measure altitudes in a plurality of discrete ranges. References Cited in the file of this patent UNITED STATES PATENTS 2,537,593 Landon et al. Ian. 9, 1951 2,958,862 Rey Nov. 1, 1960 2,969,458 Parkinson Jan. 24, 1961

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