GB1571221A

GB1571221A - Cloud altitude measuring means

Info

Publication number: GB1571221A
Application number: GB9452/77A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1976-03-08
Filing date: 1977-03-07
Publication date: 1980-07-09
Also published as: DE2707739A1; FR2344029B1; JPS52110058A; FR2344029A1; SE7603069L

Description

(54) CLOUD ALTITUDE MEASURING MEANS (71) We, ASEA AKTIEBOLAG, a Swedish Company of Västeras, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a cloud altitude measuring means.

A measuring means which is particularly suitable for measuring the cloud altitude is known from U.K. Patent Specification No.

1,362,880. This known measuring means includes measuring equipment of the optical radar type, comprising an emitter which emits short light pulses directed towards a cloud. When these light pulses hit the cloud, reflections occur, and part of the reflected light is intercepted by a receiver located adjacent to the emitter. The time required for the light to travel from the emitter to the cloud and back to the receiver is measured, and the altitude of the cloud can then be determined from the known velocity of light. This known measuring means further comprises two integrating devices in the receiver, which are alternately caused to receive signals intercepted by the receiver.

One of the integrating devices is designed to receive echo signals expected to be emitted from clouds, and the other integrating device is designed to receive noise signals only. After a number of light pulses have been emitted and echo signals have been received, the contents of the integrating devices are compared, and 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.

With this kind of cloud altitude measuring means a well-adjusted sensitivity control of the receiver is necessary in order that precipitation and haze shall not be detected as a cloud and give rise to incorrect information as to the height of the cloud base. It is normal in this connection to let the sensitivity of the receiver, in each period following the emission of a pulse, increase with time so that the sensitivity of the receiver becomes greater as the echo signals become weaker with increasing distance of the cloud from the receiver.

When the emitter and the receiver, as in the above example, are placed beside each other, the measuring means has been found to have low sensitivity in the region closest to the receiver, and echo signals from clouds at a very low level are thus detected with difficulty.

The low sensitivity for low altitude clouds is illustrated diagrammatically in Figures 1 and 2 of the accompanying drawing, Figure 2 being a section on the line B-B in Figure 1. In Figure 1, the letter S designates the emitter, and the lines Sl and S2 are the effective limits of the emitter beam. The letter M designates the receiver, and the lines Ml and M2 are the effective limits of the field of view of the receiver. The limit lines S1, S2 and Ml and M2 show only an ideal condition. Within the emitter beam and within the field of view of the receiver there is in practice a distribution of the intensity, which diminishes from the centre towards the limit lines.From the crosshatched area A (a low altitude region) the receiver M only receives signals of a very low signal level, due to area A being to the right of line S2 and to the left of line M1. It is possible, therefore, that echo signals from clouds located at a very low height will not be indicated. The cross-hatched area D shown in Figure 2 indicates the part of the emitter beam which lies within the effective field of view of the receiver at the altitude corresponding to the section line B-B.

The present invention aims to provide a cloud altitude measuring means better adopted for detection of clouds, but not haze, etc., within the insensitive area mentioned above.

According to the invention a cloud altitude measuring means comprising an emitter arranged to periodically emit a light pulse towards a cloud, and a receiver located adjacent the emitter and arranged to receive the echo pulse reflected from the cloud, is characterised in that in period following the emission of a pulse the sensitivity of the receiver is made first to decrease to a minimum corresponding to a predetermined cloud height between minimum and maximum ranges of the measuring means and thereafter increase to the time required for echo pulse reception from a cloud at maximum range.

In order to be able to reliably detect clouds at the low end of the measuring range, the less sensitive area has been taken into consideration in the functional relationship which controls the sensitivity of the receiver. The function can then in principle be expressed as

where K is the sensitivity of the receiver at any particular instant and Kmax its maximum sensitivity, C is a constant, Th is the transmission factor of the atmosphere, h is the height above the ground from which echo pulses are being received at the particular instant, hmax is the maximum measuring range and Ah is a complex function of the height h and the intensity distribution of the emitter beam, the directional characteristics of the receiver, the angular distribution of the reflected light, and the spacing between the emitter and the receiver.Th can futher be expressed as equal to e-h, where 2 < o < 4.

In diagrammatic form, the above function has the appearance shown in Figure 3 of the drawing, which is a graph showing the variation in the sensitivity of the receiver (expressed as the ratio K/Kmax as a function of the height h above the ground. At the low end of the measuring range the influence of said less sensitive area is predominant.

The influence of the less sensitive area decreases with increasing height and results in a natural increase in sensitivity up to the height h, where the damping in the atmosphere and the sensitivity, which decreases quadratically with the distance, become predominant and thus the sensitivity decreases in a known manner to the maximum range hmax. Both h, and hmax are dependant on quantities which are characteristic of each type of measuring equipment, the quantities being determined by geometrical and electrical properties in a manner similar to that discussed with reference to Ah above.

WHAT WE CLAIM IS: 1. A cloud altitude measuring means comprising an emitter arranged to periodically emit a light pulse towards a cloud, and a receiver located adjacent the emitter and arranged to receive the echo pulse reflected from the cloud, characterised in that in the period following the emission of a pulse the sensitivity of the receiver is made first to decrease to a minimum corresponding to a predetermined cloud height between minimum and maximum ranges of the measuring means and thereafter increase to the time required for echo pulse reception from a cloud at maximum range.

2. Means according to claim 1, in which its sensitivity is determined from the function:

where K=the sensitivity of the receiver Kmax=maximum sensitivity of the receiver C=a constant Ah=a complex function of height Th=height above ground hrn8x=maximum measuring height.

3. A cloud altitude measuring means as claimed in claim 1 in which the sensitivity of the receiver periodically varies with time substantially as shown in Figure 3 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. sensitivity of the receiver is made first to decrease to a minimum corresponding to a predetermined cloud height between minimum and maximum ranges of the measuring means and thereafter increase to the time required for echo pulse reception from a cloud at maximum range. In order to be able to reliably detect clouds at the low end of the measuring range, the less sensitive area has been taken into consideration in the functional relationship which controls the sensitivity of the receiver. The function can then in principle be expressed as where K is the sensitivity of the receiver at any particular instant and Kmax its maximum sensitivity, C is a constant, Th is the transmission factor of the atmosphere, h is the height above the ground from which echo pulses are being received at the particular instant, hmax is the maximum measuring range and Ah is a complex function of the height h and the intensity distribution of the emitter beam, the directional characteristics of the receiver, the angular distribution of the reflected light, and the spacing between the emitter and the receiver.Th can futher be expressed as equal to e-h, where 2 < o < 4. In diagrammatic form, the above function has the appearance shown in Figure 3 of the drawing, which is a graph showing the variation in the sensitivity of the receiver (expressed as the ratio K/Kmax as a function of the height h above the ground. At the low end of the measuring range the influence of said less sensitive area is predominant. The influence of the less sensitive area decreases with increasing height and results in a natural increase in sensitivity up to the height h, where the damping in the atmosphere and the sensitivity, which decreases quadratically with the distance, become predominant and thus the sensitivity decreases in a known manner to the maximum range hmax. Both h, and hmax are dependant on quantities which are characteristic of each type of measuring equipment, the quantities being determined by geometrical and electrical properties in a manner similar to that discussed with reference to Ah above. WHAT WE CLAIM IS:

1. A cloud altitude measuring means comprising an emitter arranged to periodically emit a light pulse towards a cloud, and a receiver located adjacent the emitter and arranged to receive the echo pulse reflected from the cloud, characterised in that in the period following the emission of a pulse the sensitivity of the receiver is made first to decrease to a minimum corresponding to a predetermined cloud height between minimum and maximum ranges of the measuring means and thereafter increase to the time required for echo pulse reception from a cloud at maximum range.