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BACKGROUND OF THE INVENTION
1. Field of the Invention:
The invention relates generally to apparatus and a method for using
low-power homodyne continuous wave (CW) Doppler radar to mesure the size
distribution of hydrometeors, e.g., falling raindrops and hailstones.
2. Background of the Invention:
The knowledge of hydrometeor dynamics, including data on the size, size
distribution and fall speed of hydrometeors constituting observed
precipitation is essential to an understanding and interpretation of
atmospheric processes. This kind of detailed data of actual precipitation
serves different purposes. Radar meterologists in both the experimental
and applications areas rely on the determination of a drop size
distribution curve to calibrate their radars. Without knowledge of the
drop size distribution within the radar sample area the accuracies of
radar derived rainfall rates are in serious doubt. The distrometer is a
necessary tool used in the discipline of cloud physics. Hydrometeor size,
number, and occurrence rate provide information essential to the
evaluation of cloud physics experiments.
Millions of dollars are spent annually in the field of hail research. A
basic requirement of the researcher is knowledge of the size of the hail
and the relative number of hailstones falling per event. A third parameter
that the research would have to have recorded is hail intensity and size
as functions of time, to the nearest minute. Current instrumentation
techniques do not provide this fine grain time data.
The relevant prior art, therefore, is that which involves apparatus and
methods to provide a user with detailed data on the velocity and size
distributions of liquid or solid hydrometeors, even from relatively
inaccessible regions, in real time or near-real time.
Numerous instruments, of varying capabilities, have been developed to study
the size distribution of falling hailstones. See Towery, N. C., S. A.
Changnon, Jr. and G. M. Morgan, Jr., 1976: A Review of Hail Measuring
Instrument, Bull. Amer. Metero. Sco., 57 1132-1140; and Nicholas, T.R.,
1977: A Review of Surface Hail Measurement. Hail: A Review of Hail Science
and Hail Suppression, Meteor. Monogr., 16, No. 38, 257, 267.
Among the available instruments is the Hailpad. Typical forms of this
device are the Styrofoam Pads Manufactured by Dow Chemical Company and cut
to size by the user. The hailpad is in its simplest form is a piece of
flat inexpensive styrofoam material which is used to infer the size of
falling hailstones by the depth and diameter of the impact craters left in
a hailpad surface exposed to the hailstone precipitation. Hailstone sizes
as small as 5 mm can be so measured and the rate of precipitation can be
computed by also noting the hailpad exposure time.
Although the hailpad itself is a low-cost item, there are many problems in
using it. No generally accepted calibration standards exist, human reading
of each hailpad's indentations is tedious and time-consuming, only solid
hyrometeors can be studied, wind may cause angled impacts requiring more
detailed measurements of elongated craters, craters within or on earlier
craters may cause errors, and styrofoam exposed to sunlight will
deteriorate.
Another popular device, usuable with liquid hydrometeors, is the
Joss-Waldvogel Distrometer. This device provides a record of the number
and sizes of raindrops striking a diaphragm surface as functions of time.
Typical of this device is the Distromet Ltd. system, comprised of model
RD-69 sampler, Model AD-69 analyzer, Model Qt-30 timer and Interface unit
FAD-3. Raindrop diameters in the 0.5 mm to 5.0 mm range are inferred from
the diaphragm's displacement due to actual impacts.
The Joss-Waldvogel distrometer is sensitive to the effect of wind on the
shape and alignment of droplets as they impact the sampler surface; is
sensitive to noise and vibration, e.g., high sound levels, and is limited
to a maximum droplet diameter of 5 mm for accurate measurements.
An instrument suitable for use with both solid and liquid hydrometeors is
the Ground-Based Optical Array Precipitation Spectrometer manufactured by
Particle Measurement Systems, Boulder, Colo. This is a relatively new
instrument that will accurately measure the sizes of rain droplets with
diameters in the 0.2 mm to 12.4 mm diameter range by means of an optical
system to project droplet images on a 64 element photodiode array.
Although expensive and fragile, the optical array spectrometer can be used
for field measurements. It has a typical weight of 50 lbs. and a power
requirement of 100 watts, which tends to limit its operational flexibility
for long-term use in unattended remote locations.
A recent design for a digital distrometer employs a sampling head comprised
of equally spaced wires. See Donnelly, D. P. and Bulson, T. M., 1980:
Digital Distrometer. 19th Conference on Radar Meteorology, American
Meterological Society, 486-489. The droplet diameter is assumed
proportional to the number of wires encountered during each droplet's
passage through the plane array of parallel wires. This technique cannot
readily cope with a mixture of rain and hail or a precipitation of
hailstones alone.
A paper titled: "A New Precipitation Occurrence Sensor System," by
Sheppard, B. E., et al. was published in the Preceedings of "the Fifth
Symposium on Meterological Observations and Instrumentation" sponsored by
the Amercian Meterological Society, Toronto Canada, Apr. 15, 1983 pp
38-41. This paper discusses both apparatus and a method having some
features similar to those of the present invention. The Sheppard, et al.,
reference discloses the use upwardly directed CW Doppler radar with 10.525
GHz Gunn diode, FFT processing of the Doppler spectrum, and a
microprocessor to reduce the data further. The focus of this reference,
however, appears to have been to note the start and stop time of the
precipitation event, identification of the type of precipitation (as snow,
rain or drizzle, or hail), and the precipitation amount, rather than the
size distribution of the hydrometeors. There are, furthermore, numerous
differences in the operation features of the circuitry between the
sheppard et al. system and the present invention.
Scientific studies dating back to 1904 provide both carefully measured data
on the terminal velocities of falling droplets and various curve-fits
relating terminal velocities to hydrometeor diameters in both the liquid
and frozen (or solid) state. Studies of particular interest include the
following:
(i) Lenard, P., 1904: Uber Regen. Meteor. Z., 21, 248-262. Determined the
terminal velocity of droplets in the weight range from 1,000 to 130,000
micrograms.
(ii) Gunn, R. and Kinzer, G., 1949: The Terminal Velocity of Fall for Water
Drop in Stagnant Air. Journal of Meteorology 6, 243-248. One of the
classical references, relating terminal velocity to drop diameter for
droplets whose weights ranged from 0.2 to 100,000 micrograms.
(iii) Laws, J. O., and Parsons, D. A., 1943: the Relation of Raindrop Size
to Intensity. Transactions of the American Geophysical Union, Papers on
Hydrology, 452-459. Reviews the milestone measurements made before 1943,
and includes their own measurements of drop size distribution as a
function of several rainfall rates.
(iv) Marshall, J. S., and Plamer, W. McK., 2948: The Distribution of
Raindrops With Size, Journal of Meteorology, 5, 165-166. Confirmed the
earlier data of Laws and Parsons (1943) and the curve fits therefrom. Also
improved coefficient values for the radar rainfall equation.
(v) Best, A. C., 1950: Empirical Formulae for the Terminal Velocity of
Water Drops Falling Through the Atmosphere. Quarterly Journal of the Royal
Metorological Society, 76, 302-311.
(vi) Atals, D., Srivastava, R. C., and Sekhon, R. S., 1973: Doppler Radar
Characteristics of Precipitation at Vertical Incidence. Previews of
Geophysics and Space Physics, Vol. 11, No. 1, 1-35. A comprehensive review
and extension of the theoretical bases for measurement of the
characteristics of rain and snow with vertically pointing Doppler radar.
(vii) Martner, B. E. and Battan, L. J., 1976: Calculations of Doppler Radar
Velocity Spectrum Parameters for a Mixture of Rain and Hail. Journal of
Applied Meteorology, 15, 491-498.
These studies, together with experimental data obtained from the hailpad
and the Joss-Waldvogel distrometer, serve as sources of calibration data
for the present invention, which provides a single, lowcost, relatively
simple apparatus and method for the measurement of hydrometeor size
distributions, in both the liquid and solid states, over a much wider
range than was possible before.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is the provision of novel
apparatus for measurement of hydrometeor size distributions.
Yet another object of this invention is the provision of novel apparatus
for determination and recordation of the type, accumulation rate, timing
and duration of hydrometeor precipitations.
A related object of this invention is the provision of novel apparatus
which is fully automatic in operation and is capable of providing detailed
hydrometeor data from remote locations.
Another related object of this invention is the provision of novel
apparatus which records measured hydrometeor data only when the
user-specified threshold value of a selected physical parameter is
observed.
Another object of this invention is the provision of a novel method for
measurement of hydrometeor size distributions.
Yet another object of this invention is the provision of a novel method for
determination and recordation of the type, accumulation rate, timing and
duration of hydrometeor precipitations.
A related object of this invention is the provision of a novel method for
automatic measurement and provision of detailed hydrometeor data from
remote locations.
Another related object of this invention is the provision of a method in
which a user specifies the threshold value whose observance, for a
selected physical parameters, causes recordation of measured hydrometeor
data.
Briefly, these and other objects of the invention are achieved by
transmitting a low power homodyne CW radar signal in a vertically upward
direction at hydrometeors falling at terminal speed towards the radar
module. The same radar module receives the reflected signal from numerous
hydrometeors of varying sized falling at correspondingly different
vertical velocities. The received signal constitutes a Doppler velocity
spectrum which is subjected to a Fast Fourier Transform (FFT) and further
processed with calibration data to generate the hydrometeor size
distribution for the observed precipitation. The Doppler power spectrum
from the signals relfected back from the falling hydrometeors may be
processed to yeild information on the type and amount of precipitation,
which can be readily combined with chronological data to provide the
duration, time history and rate of precipitation at the measurement
location. Provision is made to cause recordation of data only when a
user-specified threshold value of a relevant parameter, e.g., of
hydrometeor velocity, is observed. The apparatus of the invention is
convenient for disposition in remote locations for prolonged unattended
service, and may be provided with its own power source. Data from the
apparatus of the invention may automatically be recorded or transmitted to
a remote user.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same become better
understood by reference to the following detailed description when
considered in connection with the accompanying drawings, wherein:
FIG. 1 shows the principal components of one embodiment of the invention;
FIG. 2 is a block diagram of the Doppler distrometer, wherein each block
and branch of the electrical circuitry connected thereto are labelled for
convenient reference.
FIG. 3 is a block diagram of a data reduction method;
FIG. 4 is a block diagram showing the system of the invention; and
FIG. 5 is a visual representation of raindrop velocity data recorded during
a light rainfall using the disclosed invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Data Acquisition and Recordation
An understanding of the functioning of the preferred embodiment of the
invention is facilitated by reference to the drawings, wherein like
reference numerals designate identical or corresponding parts wherever
they are shown. Referring now particularly to FIG. 1 thereof, the
principal components of the invention apparatus include a conventional
Doppler radar unit 1 such as the the Model MA86656 Series Microwave X-band
transceiver, produced by Microwave Associates, Burlington, Mass., a signal
processing and control unit (SPCU) 2, including a panel bearing selector
switches for user adjustment of assorted controls, described more fully
hereafter; a bulk data recorder 3, which in the preferred embodiment is a
conventional magnetic tape cassette recorder produced by Radio Shack, and
a visual strip chart recorder 4 produced by the Easterline-Angus Company,
from which the user can determine at a glance when events occurred and the
maximum velocity of the hydrometeors observed. The apparatus, in typical
operation, samples observable precipitation phenomena as described more
fully below.
FIG. 2 shows in more detail the components together with the
interconnections among the component contained within the Doppler radar
unit 1 and the signal processing and control unit 2 (which under operating
conditions may be located hundreds of feet apart). A conventional Doppler
homodyne radar module 5, such as the Model 86656 X-Band Transceiver
produced by Microwave Associates Inc. operates preferably on a frequency
of 10.525 GHz from a Gunn device transmitter to provide a continuous wave
signal at power outputs that range from 5 mW to 100 mW. In its normal
operating posture, the antenna system 6 would direct the output signal
vertically upward at the falling hydrometeors. Each moving hydrometeor
reflects back a small portion of the radar signal incident on it at a
Doppler shifted frequency determined by its downward velocity, according
to the following relationship:
F.sub..alpha. =2VF.sub.o /C
Where
F.sub..alpha. =Doppler Frequency in Hertz
V=terminal velocity in m/sec
C=speed of light in m/sec
f.sub.o =transmitter frequency in Hertz
These Doppler-modulated relfected signals from numerous hydrometeors are
received by a conventional crystal detector 7, located in the Microwave
Module 5, which produces an output of a composite analogue signal that can
range in amplitude from 30 mV to 200 mV depending on the target cross
section and range. This signal appears on line 8 and is supplied to a high
gain amplifier 9 designed to provide approximately 85 dB gain and a
frequency response flat to within +/-3 dB from 60 Hz to 6000 Hz. The
output of high gain amplifier 9 appears on line 10.
A conventional automatic gain control (AGC) circuit 11, capable of
providing over 40 dB of signal compression without introducing waveform
distortion, compresses the signal it receives on line 10 only when the
signal level exceeds a peak-to-peak level of 2 volts. The output of the
AGC circuit 11 is applied via line 12 to a band-pass filter 13 designed to
provide a roll-off of over 20 dB per octave at out-of-band frequencies and
to have a +/-3dB pass-band region extending from 100 Hz to 4500 Hz.
The output of band-pass filter 13 appears on line 14, and, via a remote
cable if desired, feeds a conventional Dopopler signal processor 15 and
also provides a direct signal to the bulk data recorder 3. In the
preferred embodiment this takes the form of an audio input to a cassette
tape recorder 3. The Doppler signal processor is a standard
frequency-to-voltage converter which converts Doppler frequency to a
proportional d.c. voltage as is well-known to those skilled in the art.
Typically, it is the frequency of the signal with the largest amplitude or
the sum of equal amplitude signals that is converted to a proportional
d.c. voltage which appears on line 16. The d.c. voltage in line 16 is used
to drive the pen of a strip chart recorder 4 and a Doppler threshold
detector 17, formed of a conventional comparator, which controls the data
sampling process when hydrometeors are falling at velocities exceeding a
user selectable threshold velocity.
The velocity threshold is set by means of a selector switch 18, on the
front panel of the SPCU 2, which causes a reference voltage to appear on
line 19. The Doppler threshold detector 17 compares the voltage on line 16
to the voltage on line 19. When the voltage on line 16 equals or exceeds
the voltage on line 19 a time-out module 31 is activated as discussed
below.
A selector switch 21 is used to set the sampler interval for sample
interval timer 20 connected thereto by line 22. By adjusting the setting
of switch 21 the user determines the time intervals at which the apparatus
is activated to sample the environment for data on observable
precipitation. Each time sample interval timer 20 is activated, line 23 is
activated and the voltage on it activates both a sample interval
controller 24 and a tone burst generator 25.
The sample interval controller 24 is activated each time line 23 is
activated, and the ensuing sample time is preselected by the user by
selecting a setting on sample interval control selector switch 26. During
the selected interval the apparatus will take data even if there are no
hydrometeors that exceed the selectable velocity threshold, and line 27
will remain activated for the entire period selected by the smaple
interval control selector 26.
A sample interval priority switch 28 determines how the data sampling is
carried out. During the time that line 27 is activated, sample interval
priority switch 28 activates line 29, causing the apparatus to sample the
environment to measure the size distributions of any hydrometeors falling
within the field of its antenna system 6. If hydrometeors are present, and
their velocities exceed the pre-set velocity threshold set by way of line
19, then line 30 is activated, whereby control priority is turned over the
Doppler threshold detector 17 and the apparatus actively samples data
until the hydrometeor velocity drops below the threshold velocity value
selected by the user-selected setting on selector switch 18. A bulk data
recorder controller 36 is activated via line 35 to record the observed
date.
A signal on line 34A activates the time-out module 31 at the start of each
cycle. The time-out module 31 via line 32 will cause a re-set to occur if
the maximum recording time per event is exceeded on a continuous basis due
to system malfunction and will turn off the system for a pre-set time if a
selectable maximum event time is exceeded. Activation of line 32 inhibits
any control functions on line 30 from controlling the system until the
time-out module 31 resets itself after a preselectable period. The normal
action of the sample interval timer circuit 20 is not affected and time
marks, as explained below, continue to be recorded at fixed intervals, as
does a brief sampling of hydrometeor data during the interval that line 27
is activated.
Power circuit switch 33 is a d.c. switch that activates line 34 to provide
power to the Doppler radar module 5 for the duration that line 29 is
active and starts the time-out Module via line 34A. The bulk data recorder
controller module 36 activates the data recordation process via line 37
when line 34 is activated so that the bulk recorder is recording radar
data at all times when the apparatus is actively sampling data.
The tone-burst 4500 Hz time-mark generator 25 provides a one-second burst
of marker tone each time line 23 is active, which is each time that sample
interval timer 20 times out. The 4500 Hz tone from time-marker generator
25 is fed to the Doppler signal processor 15 via line 38, and an
attenuated tone burst signal is fed to the bulk data recorder, e.g., audio
input to a magnetic tape cassette player, via line 14. Each time the tone
burst appears on line 38, maximum voltage output from the Doppler signal
processor appears on line 16 which provides input to the strip chart
recorder 4 and drives its pen to maximum deflection. This maximum pen
deflection ensures that the time marks recorded on the strip chart will
not be confused with the high velocity hydrometeor data that also appear
on the strip chart record. Line 34 is also active at this time and hence
the data recorder 3 is on at any time that a marker tone is generated. The
marker tone provides information regarding the time at which an event
occurs. The one-second burst of the marker tone is triggered each time
sample interval timer 20 times out, hence the event time of an occurence
can be easily computed by multiplying the number of time bursts between
the event of interest (and a chronological reference point) by the period
between activations of sample interval timer 20 as selected by setting
selector switch 21. Since the tone burst time mark also appears on the
strip chart record, the time that events of interest occur can be rapidly
computed from the strip chart record itself.
The apparatus of FIG. 2 shows an optional dual power supply 39 whereby
electrical power may be provided to the system either from a 12 volts d.c.
(12 VCD) source such as a battery or a 115 volts a.c. (115 VAC)
conventional commercially available electrical power source. The system
may therefore be very conveniently set up to operate on 115 VAC standard
line voltage whereever this is available, on 12 VCD battery power if
operation is in a remote location or, optionally, with 115 VAC backed up
by emergency switchover to 12 VDC reserve batteries. The power supply 39
can be electronically regulated in any conventional manner to supply the
proper operating voltage for the Gunn device transmitter and associated
circuitry in radar unit 1.
The apparatus shown in FIG. 2 includes, as a test feature, an optional
manual override and test module 40 allowing the system to be activated
manually, via a front panel manual test switch 41, regardless of the time
that has elapsed in the timing interval. Such manual activation via line
42 turns on the transmitter of the Doppler radar module unit 5 and bulk
data recorder 3, places a tone burst on the data record of recorder 3, and
causes the pen of strip chart recorder 4 to deflect full scale.
Various modifications of the apparatus depicted in block diagram form in
FIG. 2 are possible and will be apparent to persons familiar with
electrical circuitry and data collection techniques. Thus, for example,
any bulk data recorder such as a disk or magnetic card recorder could
easily be used in place of the magnetic tape cassette recorder 3 chosen in
the preferred embodiment. The Doppler radar unit may easily be made
physically integral with the other principal components of the apparatus
of the invention, without affecting their respective roles or
relationships to each other. On the other hand, through remote cables or
by means of any of various conventional long-range communication links,
certain components may be located in geographically remote locations
without adversely affecting the capacity of the apparatus to perform its
useful functions.
The preferred embodiment utilizes analogue circuitry to achieve the
functions of the modules shown in FIG. 2. However, this basic data
sampling system can also be implemented using micro-processor technology
in a relatively straightforward manner. Knowledgeable practitioners in the
art of data processing will recognize that with currently available
hardware the control fucntions could be programmed in "read only memory"
(ROM) and executed by micro-processor circuitry driving the Doppler
distrometer through a peripheral interface adapter (PIA).
Within the scope of this invention, an enhanced embodiment of the same
could have all important system functions controlled by the same
micro-computer that would also handle all the signal processing shown in
FIG. 4. The radar module 5 would transfer the analogue velocity spectrum
data to the bulk data recorder 3, as described, under micro-processor
control, and after the event of interest ends the micro-processor would
read the analogue data recorded during the event. If the bulk data
recorder 3 is a conventional tape recorder this would involve rewinding
the tape in start-stop fashion. The analogue-to-digital converter, 69,
would read the analogue data until the digitization of 4096 points was
completed. The tape would stop and the FFT would be performed on those
4096 points. These points would be stored in memory, the tape recorder
restarted and 4096 more points would be digitized, FFted, and stored,
until all the analogure data was exhausted. The system would then record
the stored processed data in memory on the cassette behind the analogue
record of the event. Time data from a clock would also be part of the
digital velocity spectrum data record. This data would be added during
digitization. Thus, the user would have the original unprocessed analogue
data and the processed velocity spectrum data on the tape as it came from
the field. The processing of data in the field during idle time would save
time during the final processing of laboratory quality data.
System Operating Cycles
FIG. 3 is a diagram showing the Doppler distrometer cycles that are
possible and the conditions that lead to their occurence. It will be
recognized by persons familiar with current data processing and
computer-controlled equipment that these control cycles are
straightforward and will operate with equal efficiency in a hardware or a
software control system.
Referring to FIG. 2 and FIG. 3, the user initiates the data collection
process taking action as follows:
a. Selecting the minimum velocity threshold of interest; accomplished by
setting selector switch 19. See FIG. 2.
b. Selecting the interval at which data sampling will occur; accomplished
by setting selector switch 21. See FIG. 2.
c. Selecting the sample period duration; accomplished by setting selector
switch 26. See FIG. 2.
The system continues to wait for the sample interval timer 20 to time-out;
when this time-out occurs, 43 in FIG. 3, the system proceeds as
illustrated in FIG. 3 and more fully described below with references
thereto, as follows:
44. The sample interval timer 20 is activated.
45. This action cuases the Doppler radar signal transmitter in module 5 to
be switched on.
46. Simultaneously, the bulk data recorder 3 is switched on.
47. A burst of marker tone is sent to the bulk data recorder 3 and to the
pen of the strip chart recorder 4, causing the pen to deflect full scale.
48. The output voltage of the Doppler signal processor 15 is compared to
the threshold value set via selector switch 18 to determine if a target
Doppler velocity that exceeds this threshold is present.
49. If the answer to the preceding check is negative, a test 50 is
performed to determine if the sample interval increment set via selector
switch 21 has been exceeded. If so,
54, the system should re-set for the next interval trigger. If the sample
interval has not been exceeded, 52, the loop is repeated, 53, and the
tests for velocity threshold, 48, and system timeout, 50, are performed
again.
54. If the system is reset, "time-out" occurs, the "YES" branch, 54, is
taken, and the cycle begins again, 66.
55. When the pre-set Doppler velocity threshold is exceeded, 51, the bulk
data recorder is turned on for the pre-set period, which had been
determined via selector switch 26, and the full spectrum hydrometeor data
are recorded. The transmitter sample cycle is not controlled by the sample
interval timer, 20, after the system determines that an event of interest,
wherein observed hydrometeor velocities exceeding the threshold value are
present, has occurred.
56. When the pre-set Doppler velocity threshold is exceeded, the sample
time is controlled by the Doppler threshold detector 17 and system
"time-out" module 31. Now the system, will actively record all hydrometer
data as long as the velocity threshold is exceeded, 48, or until a default
time-out from the time-out module 31 occurs.
57. While the system is under the control of the Doppler threshold module
the one-second long 4500 Hz tone is placed on the bulk data recorded by
bulk data recorder 3 each time the sample interval timer, 20, times out. A
test, 58, is also performed by the loop to determine if the time-out
function has occurred.
58. When the velocity loop recycles, 61, and the velocity threshold is no
longer exceeded in the data under observation, 49, and the sample interval
increment has been exceeded, 54, the transmitter turns off and the system
is returend to the sample mode 67 controlled by the sample interval timer
20.
In the preferred embodiment the recorded data will reside on magnetic tape
in cassettes which can be physcially removed for non-real time data
reduction at a suitable location. Alternative methods of making the
recorded data available to the data reduction facility include
transmission of the data over remote lines and by radio. Numerous
techniques suitable for storing the recorded data at various stages exist
and are comtemplated as alternatives within the scope of this invention,
e.g., magnetic cards, magnetic disks, laser recorded disks and the like.
Data Reduction
It is hightly probable that meteorological monitoring over an extended area
would require an array of Doppler distrometers and that, consequently, a
considerable amount of data would have to be processed for the full value
to be extracted therefrom. One practical solution would be to periodically
collect and transfer all the data records from the various bulk data
recorders 3, each containing the raw velocity and Doppler spectrum data
provided by its respective Doppler signal processor 15, to a central data
processing facility for non-real time data reduction FIG. 4 shows one data
reduction scheme suitable for such a situation. Persons skilled in data
reduction techniques will readily visualize comparable alternatives to
perform the same function.
FIG. 4 shows a cassette playback unit 68 which uses the distrometer data
record in analogue form to drive a conventional analogue-to-digital
(A-to-D) converter 69, via line 70. Line 71 provides the same analogue
form of the data to a 4500 Hz bandpass filter 72. The recorded waveforms
representing the velocity spectrum and the power amplitude of the Doppler
signal are digitized by the A-to-D converter 69 for transmittal as 16-bit
words to a conventional data format controller 73, via line 74.
Simultaneously, the 4500 Hz bandpass filter 72 allows only the 4500 Hz
time marker signal to pass via line 75 to a conventional tone decoder 76,
which produces a simple pulse of fixed duration each time the 4500 Hz tone
is passed to it. The output pulse from tone decoder 76 is used to perform
two functions. First, via line 77A it supplies a time mark interrupt
signal to the data format controller and, second, via line 77, it advances
the count of a conventional 16-bit counter module 78.
Each detection of a one-second burst of 4500 Hz timer marker tone inhibits
the digitized data stream from the A-to-D converter 69 for the tone's
duration and the count from a conventional 16-bit time word generator 79,
driven by counter 78 via line 80, is written via lines 81 and 82 to the
data format controller 73, and thus to a host device via data bus 83.
Data format controller 73, through bus 83, formats the data to | | |
