Cloud altitude measuring apparatus

A cloud altitude measuring means for detecting the existence of and the distance to a cloud comprising a light emitting means, a light receiving means, a plurality of integrators activated at separate time intervals to receive the output signal from the light receiving means, a signal summation device to receive and determine the difference between the signals in the said integrators, and a signal processing circuit to receive the output signal from the signal summation device, which circuit comprises a memory circuit to store a first series of measurement value pulses from said summation device and a comparison device for comparing the first series of measurement value pulses stored in the memory with a second time delayed series of measurement value pulses to detect a cloud echo when the comparative difference between the measurement value pulses exceeds a specified value.

BACKGROUND OF THE INVENTION 
The invention is related to a cloud altitude measuring means for detecting 
clouds and measuring their altitude by evaluating the light reflected from 
the said clouds in response to emitted light pulses and is more 
particularly concerned with means to provide a better signal-noise 
relationship to increase the accuracy of the measurements. 
PRIOR ART 
U.S. patent application Ser. No. 771,261 filed Feb. 23, 1977 and U.S. Pat. 
No. 3,741,655 describe devices for measuring cloud altitude. These known 
devices include measuring equipment of an optical radar type, whereby an 
emitter emits short light pulses directed towards the object. In the cited 
prior art the object is a cloud and when the light pulses hit the cloud, a 
portion is reflected and part of the reflected light is intercepted by a 
receiver located adjacent to the emitter. The time required for the light 
to travel the distance between the emitter and cloud and the cloud and 
receiver is measured and the altitude of the cloud is determined from the 
known velocity of light. The known devices further comprise two 
integrating devices in the receiver unit, which are alternately caused to 
receive signals intercepted by the receiver. One of the integrating 
devices is designed to receive noise signals as well as echo signals 
expected to be emitted from clouds, and the other integrating device is 
designed to receive noise signals only and to act as a reference. After a 
number of light pulses have been emitted and echo signals have been 
received, the contents of the integrating devices are compared. The result 
of the comparison is placed in proportion to a predetermined signal level, 
and if this level is exceeded the existence of clouds is indicated. 
The receiver scans for the existence of clouds at intervals, for example in 
steps of an extension of 5 meters within the measuring range. In this 
manner one or more cloud layers lying one above the other may be 
continuously registered up to the height at which the reflected light no 
longer returns to the receiver. To achieve this stepwise measurement, the 
receiving intervals for the two integrating devices are moved in parallel 
in time so that the whole measuring range is scanned. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a better 
signal-noise relationship for the signals received from a plurality of 
integrators, which signals are derived from cloud echoes. 
Briefly, the invention in its broadest aspect comprises the use of a memory 
device to store measurement value pulses when a first integrator receives 
cloud echo signals and a comparison device for comparing measurement value 
pulses which are received when a second integrator receives cloud echo 
signals with the stored measurement value pulses coming from the memory. 
The first integrator pulses are delayed such that the measurement value 
pulses compared have substantially the same position in the two series of 
measurement value pulses emitted by a summation device. This measuring 
method allows an improved analysis of the relationship between echo signal 
and noise by the comparison device and a more accurate indication of the 
existence of and the distance to a cloud. 
Further objects, advantages, and features of the invention will be apparent 
in the arrangement and construction of the constituent elements in detail 
as set forth in the following specification taken together with the 
accompanying drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a light emitter 11, preferably a GaAs laser diode with 
the necessary drivers for emitting a series of light pulses 14. A light 
receiver, consisting of a light detector 24, which preferably consists of 
an avalanche photo-diode, and a signal amplifier 26, is arranged to 
receive signals 25 representing reflections of the emitted light pulses 14 
from a cloud and noise, the noise being caused by amplifier noise and 
sunlight diffusely reflected from the cloud. The signals 25 detected by 
the detector 24 are amplified in a signal amplifier 26 and are then 
supplied to two integrators 12 and 13 by way of AND gates 16 and 17. Upon 
the emission of each light pulse 14 in a series of light pulses, emitter 
11 transmits a control pulse 15 to gates 16 and 17, respectively, through 
time delay circuits 19 and 20, respectively. Time delay circuit 19 delays 
pulse 15 for a time T.sub.1 and time delay circuit 20 delays pulse 15 for 
a time T.sub.2. Control pulses 15 thus determine, through the time delay 
circuits, the time intervals during which the detected signals 25 are 
supplied to integrators 12 and 13, respectively. For each control pulse 15 
an additional signal is received in each of the respective integrators, 
and the magnitude of this signal depends on the signal level at the time, 
that is, echo signals in addition to a noise signal. The output voltage of 
each integrator increases with the number of control pulses 15, which is 
equal to the number of light pulses in a light pulse series. 
After the emitter 11 has delivered the last pulse 14 in a series of light 
pulses in one and the same receiving interval for integrators 12 and 13, 
it emits a control pulse 21 to integrators 12 and 13 through a time delay 
circuit 22, which delays pulse 21 for the time T.sub.3. This pulse effects 
a resetting of the integrators, whereby a new series of light pulses 14 
can be emitted towards the target and the corresponding echoes be 
integrated in the integrators. 
Control pulse 21 is also delivered through a time delay circuit 23 with the 
delay time T.sub.4 to memory circuit 36 in signal processing circuit 28. 
The function of memory circuit 36 shall be later described. The control 
pulse 21 to the integrators, coming from circuit 22, effects a resetting 
of these so that a new series of light pulses 14 can be emitted towards 
the target and the echoes caused by these can be received and integrated 
in the integrators. Before resetting takes place, the measurement value 
pulse occurring on the output of a summation device 27 connected to the 
outputs of the integrators has been forwarded to the signal processing 
circuit 28. 
For the delay times mentioned above, T.sub.1 &gt; T.sub.2 &gt; and T.sub.3 &gt; 
T.sub.4. Therefore, integrator 12, which operates with the longest delay 
time, receives echoes from a greater distance than integrator 13, that is, 
it registers clouds at a greater distance than the integrator 13. Due to 
the stepwise movement of each light pulse series, the two integrators 
sense the existence of clouds at an increasingly greater distances. The 
integrator 12 first receives echo signals whereas integrator 13 registers 
only noise. Since the output of integrator 12 is connected to the positive 
input of summation device 27 and since echo signal and noise give a 
greater amplitude than only noise, summation device 27 will emit a 
positive output signal which is fed into the signal processing circuit. 
Such signals are shown in FIG. 3 as pointed peaks 29 and are referred to 
herein as saw-tooth shaped measurement value pulses. 
In FIG. 3 the numeral 30 indicates the time when integrator 12 starts 
receiving cloud echo signals. Since the edge of the cloud is not 
absolutely sharp, the first echo will be relatively weak and the 
corresponding measurement value pulse small. As the forward stepping 
continues, successively stronger echoes are received up to time 31, 
thereafter the integrator 12 starts registering echoes from the reflection 
inside the cloud and successively increasing absorption of the light 
pulses decreases the strength of the echo signals and thereby the 
amplitude of the measurement value pulses which amplitude approaches zero 
at 32. Then the whole light pulse is absorbed by the cloud and integrator 
12 registers only noise. 
Delay times T.sub.1 and T.sub.2 increase in time with the forward stepping 
for each light pulse series, and after some time delay time T.sub.2 will 
equal the value of T.sub.1 at point 30 in FIG. 3. Then integrator 13 
starts receiving cloud echo signals and since the output of this 
integrator is connected to the negative input of summation device 27, the 
summation device starts emitting negative measurement value pulses 33. In 
the same way as for the positive pulses, the amplitude of the negative 
pulses starts increasing to a maximum and thereafter decreases to 
approximately zero at 34 in FIG. 3. 
In the present case delay times T.sub.1 and T.sub.2 have been selected so 
that the cloud echo signals coming to integrator 12 have been attenuated 
before integrator 13 starts receiving cloud echo signals. Therefore, the 
output of summation device 27 will register a number of positive 
measurement value pulses with amplitudes which start at zero, grow to a 
maximum and thereafter decrease to zero. Thereafter, the same thing is 
repeated but with an amplitude of inverted sign. Each measurement value 
pulse has an amplitude that corresponds to the signal level within the 
corresponding height interval. The number of saw-tooth pulses is 
determined mainly by the time between the two times T.sub.1 and T.sub.2 
and by the length of step by which the stepwise movement across the 
measuring range takes place. 
The output signal of summation device 27 is supplied to a signal processing 
circuit 28. According to FIG. 1, the circuit includes an analogue-digital 
convertor 35 which converts the analogue measurement value pulses coming 
from the summation device 27 into digital signals which are fed into a 
memory 36 with a specific number of memory positions in which they are 
stored. The digital signals are also fed into a comparison device 37 which 
is also connected to the output of memory 36. A level sensing device 38 is 
connected to the output of the comparison device, said device 38 emitting 
an output signal 39 when the level of the incoming signal exceeds a 
certain value. 
The memory 36 may be designed to operate in various ways, but in accordance 
with the preferred embodiment, the number of memory positions is 
determined by the difference between T.sub.1 and T.sub.2. With reference 
to FIG. 3 this means that the positive measurement value pulses are 
supplied to the memory as well as to one inout of comparison device 37. 
When the measurement process has proceeded so far that point 32 has been 
reached and negative measurement value pulses start appearing on the 
output of the summation device, these pulses will be supplied to the 
comparison device, but at the same time the memory starts emitting 
positive measurement value pulses to the other input of the comparison 
device. The comparison device 37 then transmits an output signal 
representing the sum of the amounts of the negative and positive 
measurement value pulses. The best degree of efficiency is obtained if the 
capacity of the memory is selected such that the maximum positive 
amplitude is respectively compared with the maximum negative amplitude of 
the measurement value pulses. Both the memory 36 and the comparison device 
37 are suitably controlled by the control pulse 21 received from time 
delay circuit 23. 
According to a second embodiment of the signal processing current 28 
illustrated in FIG. 2, the signal processing circuit includes two 
level-sensing devices, one of which, designed 40, receives positive 
measurement value pulses from summation device 27 and transmits an output 
signal when the amplitude of the input signal exceeds a specified value. 
These output signals from level-sensing device 40 are fed into memory 36, 
the output of memory 36 is connected to one input of an AND gate 42. 
Negative measurement value pulses are received by the other level-sensing 
device 41 which delivers an output signal directly to a second input of 
the AND gate. The memory is set or controlled with control pulses 21 such 
that when the negative pulses start arriving from the summation device, 
the memory starts delivering corresponding positive pulses to AND gate 42. 
Since the signal processing circuit requires measurements with a positive 
and a negative series of measurement value pulses, the risk of false 
echoes is reduced, and the accuracy of the measurement is increased. 
While there has been shown and described what is considered to be preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the invention as defined in the appended claims.