Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
1. Field of the Invention
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This invention relates to improvements in distance ~easuring
apparatus, and9 more particularly, to improvements in CWFM radio
altimeter circuits, and still more particularly to a circuit for
providing an autocalibration timing signal against which a returned
signal mixed with a currently transmitted signal can be compared to
minimize the effects of nonlinearities -in the frequency of the trans-
mitted signal.
2. Description of the Prior Art
Typical radio altimeters transmit a signal to be reflected
from the underlying terrain. The signal carries a known modulation
signal, usually a frequency sweep varying in accordance with a tri-
angular or saw-toothed waveform configuration. The frequency sweep
in the prior art devices must be critically controlled to be linearly
time varying or have a known average ~f/~t.
The reflections from the terrain are detected by a receiver
portion of the altimeter, and the frequency offset between the modula-
tion of the received signal and the currently transmitted signal is
determined. The altitude or distance between the aircraft and the
terrain is then calculated from the frequency offset, which is directly
proportional to the two-way time of travel of the transmitted signal.
The larger the difference in frequency between the received and cur-
rently transmitted signals, the higher the altitude.
As mentioned, the slope and linearity of the frequency sweep
transmitted in the prior art radio altimeter devices have to be pre
cisely controlled for an accurate altitude measurement. If the average
slope of the transmitted saw toothed ramp were not constant, for example,
then the ~f/Qt measurement would not accurately reveal the proper fre-
quency difference, resulting in erroneous measurements.
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lypically, the saw-toothed linearity and slope control is
achieved by using a special (and expensive) filter in the modulator
stage of the transmitter, and by using extremely high quality trans-
mitter circuits and components. This results in a larger weight and
volume in the altimeter, requiring valuable space on board the
particular aircraft with which it is associated.
Another provisîon typically incorporated into prior art
altimeter devices is a calibration feedback circuit which controls the
overall slope or rate of change of the ~Frequency sweep modulated onto
the transmitted signal. Thus, in addition to the weight and circuitry
problems necessitated by the use of the linearity maintainin~ circuits
mentioned, the circuitry used to control the slope of the modulation
frequency must be taken into consideration, as well. Furthermore,
because of the numerous variations in the transmitter components, which
may result in frequency drift and the like, the prior art radio altimeters
had to be necessarily frequently calibrated to ensure their continued
accuracy.
Summary of the Invention
In light of the above, it is, therefore, an object of the
invention to provide a radio altimeter which automatically corrects
for transmitter and transmission line nonlinearities to enable less
critical transmi'ter control.
It is another object of the invention to provide an altitude
indicating apparatus which relaxes the transmitter carrier and modula-
tion fre~uency control requirements.
It is another object oF the invention to provide a CWFM radioaltimeter which has improved accuracy from radio altimeters heretofore
used.
It is another object of the invention to provide a radio
altimeter utili~ing digital data processing techniques incorporating
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a clock frequency ~Ihich varies with nonlinearities in transmitted
signal ~requencies and frequency rate of cnange.
It is another object of the invention to provide a ~ethod ~or
automatically calibratiny a CWFM radio altimeter to correct for or
cancel the effect of the nonlinearities in the frequency sweep of the
transmitted signal.
It is another object of the invention to provide a radio
altimeter which can be used to analyze segments of the transmitted
and received data curves, rather than the total curve as was hereto-
fore necessary.
These and other objects, features, and advantages will becomeapparent to those skilled in the art from the following detailed
description when read in conjunction with the accompanying drawing
and appended claims.
In its broad aspect, the invention provides an autocalibration
circuit for use in a CWFM radio altimeter. The circuit includes means
for transmitting a signal which has been frequency modulated with a
frequency sweep signal. Reflection detecting means receives the delayed
transmitted signal, and a first mixing means produces an altitude
determining signal of frequency equal to the difference of the trans-
mitted signal and the detected reflections. Means for sampling the
transmitted signal produces a sample signal, and means for delaying
the sample signal a predetermined time produces a delayed signal.
Second mixing means produces a calibration signal of frequency equal
to the difference of the transmitted and delayed signals, and means
for comparing the ratio of the altitude determining signal and ~he
calibration signal determines the period of the altitude determining
signal.
In another aspect of the invention, a method of automatically
calibrating a CWFM radio altimeter inc!udes the steps of developing a
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correction signal from a current and a delayecl transmitte~ signal, and
comparing the correction signal to a signal of frequency equal to the
difference of a current and a reflected transmitted signal to produce an
altitude indication.
Brief Description of the Drawin~s
The invention is illustrated in the accompanying drawing, wherein:
Fig. 1 is a box diagram of a radio altimeter usiny the automatic
calibration circuit and method in accorclance with the present invention.
Fig. 2 is a graph of t~e frequency versus time showing ideal
transmitted and received frequency sweeps for use in altitude determi-
nation.
Fig. 3 is a graph of a more realistically encountered trans-
mitted and received frequency sweeps, showing the inaccuracies resultant
therefrom in an altitude determination.
Fig. 4 is a graph of the calibration frequency derived in
accordance with the invention versus time.
Fig. ~ is a graph of the amplitude of the mixed and limited
ground return signal, with respect to time.
Fig. 6 is a graph of an ideal clock signal, with respect to
time, in relative scale to the ground return and calibration frequencies
of Figs. ~ and 5.
And Fig. 7 is a detailed schematic drawing of the automatic
calibration circuit in accordance with the invention, used in conjunc-
tion with a data processing system employing a direct memory access
control for utilization of the data derived, which has been calibrated
by the method and apparatus in accordance with the invention.
In the drawings of Figs. 2-6, the relative scales and propor-
tions of the curves and the number of pulses shown have been exaggerated
or distorted for ease of descripti~n and clarity of illustration.
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Detailed Description of the Preferred En~bod;ments
The autocalibration circuit 10 of the invention is shown in
Fig. 1 in operative relationship with a transmitter section 11 and
receiver section 12 of a CWFM radio altimeter 13. Briefly, the trans-
mitter sect;on 11 includes a transmitter 14 modulated by a modulator
15 to deliver a signal to an antenna 16 upon a transmission line 18.
Typically, the transmitter is operated at a frequency of about 4,300
MHz, and is modulated with a triangular wave of frequency of about
100 Hz sweeping over a frequency of about 100 MHz.
The signal radiated from the antenna 16 is directed to the
underlying terrain 17 or other object from which distance is desired
to be measured, and reflections from the underlying terrain 17 are
detected upon a receiving antenna 19. A coupling element 20 adjacent
; the transmission line 18 samples the transmitted signal, and the sample
is mixed in a mixer 22 with the ground reflected signal received upon
the antenna 19. The mixed ground return signal is amplified by a pre-
amplifier 23, and mixed with an intermediate frequency and limited in
an IF/limiter stage 24. The output from the IF/limiter stage 24, which
is in the form of a square wave (see Fig. 5, below described), is then
applied to a period converter 26, which determines the period of the
returned signal, from which determination the altitude can be directly
derived.
If desired, a number of adjacent periods of the returned signal
. can be produced and listed in a memory means, such as a random access
~ memory (RAM) 27. The data in the RAM 27 can be further processed by a
computer means such as a microprocessor 28 or the like to produce
digital outputs which, if desired, can be converted to analog form by a
; digital-to~analog (D/A~ converter 3Q. The output of the D/A converter
30 can drive a conYentional altitude display indicator 31. If desired,
the digital data can be converted to a serial format in a parallel-to-
serial (P/S~ converter 32 to drive an indicator 34.
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With the system thus constituted, the operation to produce an
altitude indication is as follows, with reference particularly to
Figs. 2-6. The transmitter modulation frequency, indicated by the saw-
tooth waveform f~ (Fig. 2) is transmitted from the antenna l6. The
received signal, indicated by the sawtooth waveform fr is received
on the antenna 19, but displaced in time from the transmitted signal
an amount corresponding to the two-~!ay travel time of the signal. At
any ~iven time, the difference in the frequencies of the transmitted
and received signals can be observed, shown by the portion ~f9 which
is directly relatable to the altitude or distance over which the radio
waves have traversed. In the prior art, as above indicated, in order
to produce an accurate altitude indication, steps have been taken to
ensure that the transmitted frequency is as linear as possible in its
variations with respect to time. ~lowever, even with such linearity
assuring measures, the transmitted frequency assumes a waveform ~hich
has nonlinear segments, such as shown in exaggerated form in Fig. 3.
; Thus, the transmitted curve ft ma~ be rounded in a concave or convex
direction, as shown. The received waveform, of course, follo~s a
similar path, but displaced in time. However, depending upon the time
in ~lhich the change in frequency bet~een the transmitted and received
waveforms is taken, different frequenc~ measurements may be obtained,
as indicated by the distances Qfl and ~f2 shown. Thusg an uncertainty
is introduced as to the precise altitude of the aircraft.
In order to eliminate this uncertainty, regardless of the wave~
form of the transmitted frequency, the autocalibration circuit lO of
the invention is presented. With reference again to Fiy. 1, the auto-
calibration circuit lO inc1udes a coupler 38 to sample the transmitted
frequency. A dela~ line 39 is used to delay the sampled frequency a
predetermined time~ corresponding to any convenient time (or altitude)
such s lO0 feet. The sa pled nd delayed signals are mixed in a
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mixer ~0, then amplified and limited in an amplifier and IF/limiter
stage 42. The a~plified and limited signal is then applied to a phase-
locked loop 43 and applied as clock pulses to the period converter 26.
The operation of the autocalibration circuit 10 of the inven-
tion can be seen from the graphs of Figs. 4-6. Speci~ically, Fig. 4
shows the change in frequency with respect to time of the calibration
signal produced by the m;xed calibration delay s;gnal and the sampled
transmitted signal. Any nonlinearities in the signal are reflected in
a change in frequency of this calibration signal. Thus, as sho~m, the
calibration signal may increase in frequency with respect to time. (It
should be noted that although the QfcAL is shown having a time increase
in frequency, other nonlinear variations may be seen depending upon the
particular nonlinear variations of the transmitted frequency.)
The returned signal difference, as shown in Fig. 5, will also
exhibit a nonlinear time varying function, since it is derived from the
transmitted signal having the precise nonlinearities detected by the
autocalibration circuit 10. Thus, for instance, at a particular alti-
tude, the frequency of the output of the phase-locked loop ~3 may be
four times the frequency of the signal produced at the output of the
limiter stage ?~. This can be seen in a comparison of Figs. 4 and 5.
Thus, as noted, each comparison of the period of the returned frequency
and the calibration frequency has the same ratio, i.e., 4, even though
the actual time of the return frequency period may be increasing or
decreasin3~ If the returned frequency were to be compared, for instance,
to a standard nonvarying clock frequency, illustrated in Fig. 6, the
ratio would change from, for example~ a period of 4 to a period of .5
(using $he arbitrary curves of the dra~ing~. I
It can therefore ~e seen that by utilizing the sampled and
delayed actual transmitted signal as a calibrating signal, the effects
of the nonlinearities of the transmitted frequencies are cancelled.
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The autocalibration circuit of the invention is shown in detail
in Fig. 7, used in conjunction with a microprocessing and direct memory
accesss data acquisition and controllin~ system. The mixed return
signal is amplified in the amplifier 23, mixed with an intermediate
frequency and limited in the IF/limiter stage 24 to be applied to
clock a D-type flip-flop 50 to a set state. The output Q of the D
flip-flop 50 is connected to one input of a NAND gate 51, to enable
the calibration pulses derived as below described to pass through the
NAND gate 51 to the clock terminal of a counter 52.
10The output from the IF/limiter 24 is additionally applied to
the ciock input terminal of a second counter 54 to produce an output
after a predetermined number of mixed return signal periods has been
received. Such number of periods may conveniently be one, ten, sixteen,
a hundred, or any conveniently handleable number.
15- The mixed calibration signal, after being amplified and limited
in the ampli~ier and IF/limiter state 42 is applied to a "signal in"
terminal of a phase-locked loop circuit 43. The particular phase-locked
loop circuit illustrated is of tne RCA type CD4046, although it will be
apparent that any equivalent type circuit can be equally advantageously
employed. The VC0 output of the phase-locked loop 43 is connected to a
clock terminal of the counter G2, and an output derived, for instance
upon the Q4 output terminal,is connected to the QII input of the
phase-locked loop. With the phase-locked loop 43 and counter fi2
thus configured, the output frequency upon the VC0 output terminal
can be adjusted to be 16 times the frequency of the mixed calibration
- signal derived from the amplifier and IF/limiter 42. The VC0 output
is applied to another terminal of the NAND gate 51 to be gated thereby
to the clock input of the counter 52.
Thus, in operation, when a pulse is detected of the mixed
return signal, the flip-flop 50 changes to a "set" state, to enable the
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NAND gate 51 to pass the output from the phase-locked loop ~3. The
mixed calibration signal, multiplied by the output frequency control
provided by the counter 62 is then counted by the counter 52 until the
counter 54 reaches the preselected count. At that time, the QN output
terminal of the counter 54 changes state, signaling the termination o~
the desired period to a direct memory access (DMA) control 64. The DMA
control 64 then latches the count presented at the various output
terminals Ql-QN of the counter 52 into a microprocessor peripheral
interface circuit 65. Additionally, the DMA control 64 resets the
flip-flop 50, and the counters 52 and 54 so that an additional
subsequent period count can be made. It should be noted that at this
point, the data produced by the counter 52 latched into the interface
circuit 65 is representat~ve of a predetermined number of periods (for
instance, one, ten, a hundred, etc.) which can be directly used for
producing an output altitude signal to the digital-to~analog (D/A)
converter 30 or the parallel to serial (P/S) converter 32 to provide an
altitude indication upon indicator 31 or 34, respectively.
It may, nevertheless, be desirable to accumulate a number of
such period indicating data. Con;equently, the random access memory
(RAM) circuit 27 is supplied, into which the data produced at the
interface circuit 65 can be successively written by the DMA control 64.
The microprocessor system 28 together with an assoclated preprogrammed
read only memory (ROM) 66 can then access the RAM 27 to perform the
~ predefined processing procedures to the data therein. Such processingi~ 25 procedures may, for example, include detection and rejection of
contaminated data, averaging a number of period indlcating data, and so
forth. The mlcroprocessor system can then produce outputs to the
digital-to-analog and parallel-to-serial converters 30 and 32 via an
input output device 67.
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Although the invention has been described and illustrated with
a certain degree of particularity, it is understood that the present
disclosure has been made by way of example only and that numerous
changes in the arrangement and combination of parts can be resorted to
without departing from the spirit and scope of the invention as
hereinafter claimed.
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