AU2009201367A8 - Apparatus and method for measuring atmospheric transmission and determining meteorological visual range - Google Patents

Description

Pool Section 29 Regulation 3.2(2) AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: Apparatus and method for measuring atmospheric transmission and determining meteorological visual range The following statement is a full description of this invention, including the best method of performing it known to us: 1 APPARATUS AND METHOD FOR MEASURING ATMOSPHERIC TRANSMISSION AND DETERMINING METEOROLOGICAL VISUAL RANGE FIELD OF THE INVENTION 5 The invention relates to an apparatus and method for measuring atmospheric transmission and determining meteorological visual range. BACKGROUND OF THE INVENTION Transmissometers are used to determine visual range in particular in the 10 aviation sector, eg runway visual range determination, but also in other fields of application, and include a light transmitter unit and light receiver unit in a fixed mutual spacing, the so-called measuring baseline. The visual range measurement range required for air traffic control results in typical standard measuring baselines of 50 m and more in order to be able to convert results of 15 the transmission measurement into corresponding visual ranges in an error tolerant fashion. Also known are embodiments which incorporate a combined optical transceiver unit with a mirror unit on one baseline. With such devices, the transmitter light traverses the path twice. 20 In any event, for airfield applications, both of the devices above mentioned are built on suitable stand structures in order to implement a rated height of measurement at 2.5 m above the runway surface. For reasons of stability for the optical alignment, these stand structures are usually fixed on massive concrete foundations. 25 In order to obtain the total required visual range measurement range for the highest category of instrument flight operation (CAT Ilb), it is normal to combine two different measuring baselines with one another. An additional so called short base (measuring baseline 10 m to 15 m) supplies the measured values for the range of very low visual ranges (< 100 m) which can no longer be 30 generated in an error-tolerant fashion for the standard measuring baseline (50 m to 100 m). It is usual to combine one opto-transmitter and two opto-receivers; arrangements with an optical transceiver and two mirror systems are also known.

2 Also known are so-called long-base transmissometers with measuring baselines of up to 300 m which are used, in particular, for the visual range measuring range of up to 10 km. In known transmissiometers, in order to exclude influences from the 5 ambient light, the transmitter light is intensity-modulated, and the opto-receiver preferably reacts to incident light of the known modulation. This modulation can be executed periodically or in pulsed form. The light sources known are preferably mechanically modulated (by a chopper mechanism) or very low frequency-modulated halogen sources, low-frequency-pulsed xenon flashlamps, 10 or else infrared light-emitting diodes and laser light sources. The meteorological visual range MOR (Meteorological Optical Range) is calculated as follows from the measurement result for atmospheric transmission on the basis of a contrast threshold: 15 MOR(m) = (In K * B)/In T where K = 0.05 (5% contrast threshold), B = the measuring baseline in metres, and T = the normalized atmospheric transmission. 20 Since setting up, commissioning and operation of a transmissometer is attended by numerous problematical individual aspects which are difficult to master, and also since the measuring accuracy of transmissometers is undesirably impaired by various environmental influences, various measures are 25 known for simplifying the setting up and/or commissioning and for reducing the undesired environmental influences on the transmissometer. Given that opto-transmitter(s) and opto-receiver(s) are set up on separate concrete foundations, initial precise optical alignment is imperative. The individual discrepancies in set-up and performance are compensated during the initial 30 alignment. However, even the most massive concrete foundations do not ensure that their positions relative to one another always remain exactly unchanged. Any drifting of the concrete foundations, however, causes the optical alignment to drift off, and this is certainly frequently observed in practice and thereby necessarily 3 leads to visual range measuring errors which appear identical to those arising from fouling of the optical outer surfaces. The alignment must be checked regularly for this reason and corrected as appropriate. Use is made for this purpose of optical aids which are introduced into the beam path in order to check 5 the quality of alignment. Measurement usually has to be interrupted for this purpose. Interventions at the optotransmitter and optoreceiver at the site of installation are required in any case, are time-consuming and may disturb flight operations. A further source of error is fouling of the optical outer surfaces, something 10 which must be detected and then be compensated or removed by appropriate measures. These optical outer surfaces are subject to continuous fouling which, even when it occurs seldom, impairs the transparency of the equipment plates and leads to substantial visual range errors, particularly at the upper end of the measuring range. In addition to the regularly required cleaning, usually to be 15 executed frequently, of the equipment plates, various structural measures are known for reducing this fouling and the considerable attendant maintenance outlay. For example, use is made of flaps which expose the optical outer surfaces only from time to time while the system carries out a measurement. The disadvantages of this method are permanently moving parts in the outer region, 20 the risk of complete loss of function in the case of a flap defect, and the short measuring cycle if the technology is to achieve a noticeable reduction in fouling. Such embodiments are no longer found appreciably in practical use. Patent Specification US 4432649 describes such a mechanism for elements which can be swivelled into the beam path. Weather protection hoods 25 are standard and are to be found in virtually every embodiment. The protective effects of these hoods depends essentially on their extent in front of the optical outer surfaces. However, the length of the weather protection hoods is limited by the required field of view for the optical systems, and by the increasing wind attack area. 30 It is only the extensive fouling effects associated with precipitation which can be reduced by weather protection hoods. The continuous rise in fouling by dust and very fine particles cannot be significantly influenced. A few embodiments of such equipment also make use of fans which produce an air flow onto or in 4 front of the equipment plates. The fans have the advantage of also being able in part to keep away dust and very fine particles from the optical outer surfaces. However, a continuous rise in fouling can also not be avoided here. Because of eddies in the air flow, a certain proportion of dirt particles always reach the 5 equipment plates and impair the measuring performance. Patent Specification EP 1300671 discloses an apparatus in which it is possible, upon requirement to introduce a clean segment of a circular equipment plate into the optical beam path of an optotransmitter and optoreceiver, respectively, by rotating this plate. This measure is suitable for lengthening the 10 period between cleanings in accordance with the number of segments available. This is so even although there are some problems in protecting the clean segments against fouling and the presence of parts in the outer region which have to be frequently moved. In any of the embodiments described above, cleaning of the equipment 15 plates is sooner or later the only reliable means of eliminating the fouling effect. It is not possible for fouling to be completely prevented. In addition to the options listed for preventing or reducing fouling, various methods and apparatuses are known which determine the degree of fouling of the optical outer surfaces of optical measuring systems in outside use. 20 Patent Specification US 4,432649 discloses a method and an apparatus where the change in total reflection of the equipment plate, owing to dirt particles, is evaluated. At the angle of total reflection, light is coupled in at a plate edge with the aid of a separate opto-transmitter. The luminous flux penetrates the entire plate in the typical zigzag profile between the two inner plate boundary surfaces. 25 If dirt particles are located on the surface, a portion of the light is scattered out of the plate. Located at the plate edge opposite where the light is coupled in is the associated opto-receiver, which detects the remaining luminous flux. The degree of fouling of the plate can be deduced from the fading of the light signal after traversing the plate. 30 Patent Specification EP 1300671 discloses a method and an apparatus in which a clean segment of a circular equipment plate is inserted into the optical beam path of an opto-transmitter and opto-receiver, when required, by rotating this plate. A statement on the degree of fouling present can be made by 5 comparing the measured values for a fouled and a temporarily introduced clean plate segment. A disadvantage of this method is that it requires disturbance of the transmission measurement for the purpose of determining the degree of fouling, and the presence of frequently moving mechanical elements of the apparatus. 5 Patent Specification EP 0745838 discloses a method and an apparatus which equips a transmission measuring arrangement with equipment plates that are mounted at an angle in conjunction with two optotransmitters/receivers, the transparency of these equipment plates being determined by means of separate plate transmitter units and plate receiver units and being related to two 10 measurement results on hand for atmospheric transmission. The method described necessarily requires two equipment units, the two of which necessitate both an opto-transmitter and an opto-receiver for measuring the atmospheric transmission on two separate paths through different equipment plates. 15 After being set up and aligned, transmissometers require the measured value determined by them for atmospheric transmission and the visual range value resulting therefrom to be adapted to the real visual range conditions at the installation site. This adaptation operation is usually termed calibration. By taking particular account of the fact that, given an infinitely good visual range, in the 20 perfectly calibrated operating state a transmissometer is to achieve a transmission value of 100%, the calibration is usually carried out in very good visual range conditions of > 10 km in order at least approximately to achieve the required calibration condition, since situations with virtually infinitely good visual range are usually seldom to be met. 25 Thus, trained observing staff also estimate the presently existing visual range, and the measured value for the transmission is set for the relevant transmissometer in accordance with the measuring baseline. This setting-up is often performed purely manually as an electronic "sensitivity setting" at the receiver, or by adjustment of the optotransmitter 30 intensity. Purely computational methods for calibration have also been rendered possible in the course of electronic data processing. An additional calibration factor is applied to the measured value supplied by the transmissometer, which is 6 in line with the visual range determined by the observer, and is calculated automatically by the data processing unit once the observer's visual range has been input via the keyboard. 5 SUMMARY OF THE INVENTION It would be advantageous for the present invention to provide an apparatus and a method with the aid of which it is possible to ameliorate some of the disadvantages of the prior art mentioned above. In a first aspect of the invention, there is provided an apparatus for 10 measuring atmospheric transmission and determining meteorological visual range, having a transmitting unit and receiving unit which are respectively supported above ground level by a stand structure that includes an inner bearing tube and an outer tube which is completely decoupled mechanically from and protects the inner tube. All units necessary for carrying out measurements and 15 effecting optical alignment of the transmitting and receiving units, are fitted to the inner tube. All other structural elements whose position could change as a consequence of their dead weight, wind load stressing or one-sided thermal loads (such as resulting from sun light) are advantageously fitted to the outer tube such that the optical alignment remains uninfluenced by these effects. 20 The apparatus will advantageously further incorporate a scattered light measuring arrangement as an integral constituent of the transmission measuring arrangement and will be directly connected to the outer tube. Equipment plates are also provided, positioned at 90* to one another in V-shaped fashion in front of the transmitting unit and receiving unit, respectively, to protect the optical and 25 electronic components against fouling. Each V-shaped equipment plate arrangement is assigned a dedicated transmission measuring device for measuring transparency, which determines the degree of fouling of the equipment plates. Both the optical system of the transmitter and that of the receiver are advantageously mounted cardanically in an adjustable fashion. 30 It proves to be advantageous when both the transmitting device and the receiving device (also referred to simply as receiver and transmitter units) are equipped with a scavenging air system in such a way that the flight path of precipitation particles directed towards the equipment plates is deflected towards 7 the ground, so that these precipitation particles, such as raindrops or snowflakes, cannot reach the optical outer surfaces of the unit. Since the existing scattered light measuring arrangement detects the presence of precipitation, this information can be used for switching the scavenging air system on and off. 5 Continuous operation of the air system can thus be dispensed with whilst ensuring that the equipment plates remain clean in the event of precipitation. The scattered light measuring arrangement is preferably designed as a forward-scattering measuring arrangement. It is particularly advantageous to use a white light-emitting diode as transmitter light source. 10 It further proves to be advantageous when the receiver is designed for synchronous demodulation and is permanently synchronized with the modulation frequency of the transmitter. In a second aspect of the invention there is provided a method for measuring atmospheric transmission and determining meteorological visual range 15 using a transmitter and a receiver unit located in spaced-apart relationship. The method includes the steps of: determining a calibration factor in automatically selected situations by dividing a visual range value supplied by a scattering light measuring arrangement (and which was converted into an equivalent transmission value) by the value for the atmospheric transmission measured with 20 the transmitter and receiver units; determining a fouling-dependent correction factor in-between the automatically selected situations by continuously measuring the transparency of equipment plates which are located in front of the transmitter and receiver units, respectively; determining an alignment factor from the correction factor and the calibration factor, the alignment factor being indicative of 25 a change having taken place in the optical alignment between the transmitter and receiver units; applying the calibration factor and the correction factor to the measured atmospheric transmission value which was determined by the transmission measuring arrangement; and using the alignment factor to restore an initial adjustment between receiver and transmitter units, as required. 30 The present invention enables decoupling of loads and potentially negative environmental factors that may adversely affect the optical system of the transmission measuring apparatus and, on the other hand, provides means for 8 some of these to be detected so that, if appropriate, compensation measures can be taken. The result of this is a virtually maintenance-free transmission measuring arrangement for determining meteorological visual range at airfields (and 5 elsewhere). Other advantageous features of the invention shall become apparent from the following detailed description of preferred embodiments thereof, which is provided with reference to the accompanying drawings. 10 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a graph representing the relative visual range measuring error for an assumed degree of plate fouling of 1%; Figure 2 shows the basic design of an apparatus for measuring atmospheric transmission and determining meteorological visual range in 15 accordance with a preferred embodiment of the invention; Figure 3 shows traditionally determined relative visual range measuring errors induced by reading errors in comparison with results obtained with the apparatus according to the invention; Figure 4 shows the basic design of the double-tube stand structure illustrated 20 in figure 2, with its essential assemblies fastened thereon; Figure 5 shows the basic design of the transmitting unit of the apparatus of figure 2; Figure 6 shows the basic design of the scavenging air system employed in the apparatus of figure 2; and 25 Figure 7 shows the basic layout of the equipment plates for the transmission measuring device for figure 2. DESCRIPTION OF PREFERRED EMBODIMENT Turning first to Figure 2, in accordance with basic principles of 30 transmissometers, a transmitting unit 3 and a receiving unit 4 are set up at a distance opposite one another on stand structures 1, and are shielded / protected by weather protection hoods 2, also carried by the stands. In the present case, the distance, the so-called measuring baseline, between the two units 3, 4, is 9 30 m, it also being possible for other standard measuring baselines of 50 m and 75 m to be implemented. The transmitter 3 and receiver 4 are mounted on the respective stand structure 1 to be located above ground at a required measuring height of 2.5 m. 5 In accordance with the invention, the stand structures 1 are devised to provide particularly high level of stability, in particular with regard to possible instances of deformation owing to wind loading and differential thermal loading (which will be present, for example, as consequence of heating by sun light of only one side of the units 3,4). 10 To this end, a double tube stand structure 1 is provided that includes an inner and an outer tube 5, 6 (see Figure 4) which extend co-axially in the vertical, whereby these tubes are supported on a base plate common to both. The novel design permits complete mechanical separation of the optoelectronic units necessary to effect measurement, from other components of 15 the apparatus. The optoelectronic units 10 are supported by and mounted on the mechanically decoupled inner tube 5. The outer tube 6 serves the purpose of protecting the inner tube 5, and supports all components which are heavy or are particularly exposed to the environment, including the supporting structures 7 with mounting brackets, fan 8 and weather protection hood 2 (see Figure 4). 20 Because of this novel arrangement, the weather protection hood 2 can be of particularly long and thus effective design since, owing to the vertical double tube structure 1, wind and thermal loadings will be reacted at the outer tube 6 and thus exert no influence on the optical alignment of the optoelectronic unit carried by inner tube 5. Because of the downwardly open embodiment of the weather 25 protection hood 2, the equipment's optical panels (or plates) 9 nevertheless remain easy to access for maintenance staff to effect cleaning thereof. The optoelectronic unit on the inner tube 5, and the supporting structure 7 on the outer tube 6, can be rotated vertically about the tube axes for the purpose of coarse alignment of the transmitter and receiver units 2, 3 and are equipped 30 with locking screws for fixing in a final position. A sighting device on the supporting structure 7 is provided to aid the coarse alignment. An acoustic alignment device can also be provided to aid in the coarse alignment process. Powerful signal transmitters in the optoelectronic 10 units of the transmitter and receiver units 3,4 enable, through detection of an increased signal strength to determine when an optical signal of sufficient strength reaches the optotransmitter optics to effect subsequent fine alignment of the transmitter and receiver units. 5 Automatic fine alignment of transmitter and receiver is rendered possible by the design of the optical systems 10 inside the optoelectronic unit of the transmitter and of the receiver. The optical systems 10 are mounted in the region of the lens above a cardanic suspension 17, while geared motors 11 with eccentric elements 12 in the region of the focal length of the optical systems 10 10 ensure an extraordinarily precise and backlash-free electromechanical possibility of adjusting the optical axes. The geared motors 11 can be controlled through a microprocessor with the assistance of suitable control elements. The position of the eccentric elements 12, and thus of the optical axes, is determined separately, with the aid of potentiometers 13, for the horizontal and vertical setting, and 15 detected by the microprocessor on the control unit after analogue-digital conversion (Figure 5). The transmitter and the receiver optics are adjusted both vertically and horizontally one after another during the automatic fine alignment process. During the adjusting operation, both the mechanical position of the optical systems and 20 the optical measurement signal associated with those positions are recorded permanently and simultaneously. Systematically conducting the adjustment, enables to determine both the intensity profile of the transmitter and the sensitivity distribution of the receiver. After recording of the individual intensity and sensitivity profiles, the 25 resulting optimum horizontal and vertical middle positions of the optical axes are set automatically both for the transmitter and for the receiver. The associated positions of the eccentric elements 12 for this optimum alignment of the optical system are stored in non-volatile fashion in the control unit, and are thus available again at any time when required. 30 Transparent equipment plates which do not restrict the optical beam path are provided both for the transmitter and for the receiver units in order to protect the optoelectronic elements of the transmission measuring arrangement. In a preferred embodiment, a novel scavenging air system prevents wetting of the 11 optical outer surfaces, in particular by wind-driven precipitation particles which have not been screened by the weather protection hood 2. The fan stream of the scavenging air system is channelled so as to produce an air stream directed towards the ground in a region in front of the 5 equipment plates. Precipitation particles are deflected reliably downwards before reaching the equipment plates, the air flow aiding and accelerating the movement of the particles in the direction of the ground. The air duct 14 of the scavenging air system is designed as a constituent of the covering structure of the optoelectronic unit. It is completely decoupled 10 mechanically from the fan section. Vibrations occurring from the fan 8 can thereby exert no influence on the measuring arrangement nor, in particular, on the alignment of the optical axes (Figure 6). In order to counteract the permanent recirculation of dust and very fine dirt particles by the scavenging air system and the risk of depositions of such 15 particles on the equipment plates, the fan of the scavenging air system is activated solely when precipitation is present. Precipitation detection is effected using a scattered light measuring arrangement which is fitted on the supporting stand structure 1 of the transmitter unit 3 and has the required ability to detect the current weather. 20 The scattered light measuring arrangement, with the aid of which it is possible for the quality of the calibration of the transmission measurement to be monitored continuously, is an integral constituent of the apparatus according to the invention. The transmitting and receiving units 3, 4 have a mounting bracket which is 25 a constituent of the supporting structure 7, mounted on the outer protective tube, for the fan 8 and weather protection hood 2. The scattered light measuring arrangement 15 is mounted on this bracket and can therefore carry out the comparative measurement, required for detection of rain, in the direct spatial vicinity of the transmission measuring path. Since weather phenomena restricting 30 visual range typically exhibit an inhomogeneous spatial distribution, this immediate vicinity of the measuring volumes of transmissometer and scattered light measuring arrangement is to be preferred to other arrangements. The mode of operation and the basic measurement methods employed in the scattered light 12 measuring arrangement are known in the prior art. Because of the more reliable measuring performance, a measuring arrangement using optical forward scattering measuring methods is favoured over optical backscatter measuring methods. Moreover, a preferred forward-scattering measuring arrangement 5 permits detection of the current weather and, in this context, generates information relating to precipitation events both for the control of the scavenging air system, and in the case of its determination of the calibration factor described below. Forward-scattering measuring arrangements are far less susceptible in 10 principle to measuring errors induced by fouling and are, moreover, typically capable of reliably determining very high visual ranges of 10 km and more, something which is possible with transmission measuring arrangements only given very long measuring baselines (with the disadvantage of the lack of a visibility range below 200 m, which is, however, absolutely necessary) 15 nevertheless with an ever higher susceptibility to measuring errors induced by fouling. The sources of error with scattered light measuring arrangements are chiefly associated with the relatively small and therefore not always representative air volume of typically < 1 litre which is used for determining visual range, and also with a non-representative measurement of instances of clouding 20 of the visual range present during various precipitation phenomena, which favours the use of representative transmissometers for the visual range measuring range below approximately 3 km, which is relevant to safety at airfields. In the present embodiment, the visual range measured values of the 25 scattered light measuring arrangement are also preferably used for comparison with the results obtained from the transmission measurement wherever: - the visual range measured value of the scattered light measuring arrangement exceeds 10 km, - the variation of the visual range measured value of the scattered 30 light measuring arrangement about the mean value in no case exceeds +/-10% in the period under consideration, - no precipitation is detected by the scattered light measuring arrangement, 13 - there is no interruption in the operation of the scattered light measuring arrangement, - the variation of the transmission measured value about the mean value in no case exceeded +/-1% in the period under consideration, and 5 - there is no interruption in the operation of the transmission measuring arrangement. Based on the knowledge of the installed measuring baseline for the transmission measurement, in these selected situations the measured value for the visual range of the scattered light measuring arrangement is converted into an 10 equivalent transmission value and the latter is compared with the measured value of the transmission measuring arrangement and the quotient of the two is calculated. In this case, use is preferably made as measured value of a mean value of all individual scannings, dependent on modulation frequency, in the respective measurement volume of the transmission measuring arrangement and 15 the scattered light measuring arrangement within the previous minute, and the information, converted therefrom, relating to atmospheric transmission and/or visual range. The calibration factor KF for the transmission measured value is then derived from the calculated quotient. The calibration factor is now applied during the following measurements 20 and, in particular, during instances of visual range clouding below 10 km. It retains its validity until a new calibration factor has been determined in the way described above. The described arithmetic operations are carried out by the microprocessor in the control unit of the transmissometer, and the variation in the calibration 25 factor is subject to a limitation to a maximum step width which counteracts an erroneous development based on temporary disturbances. The respective calibration factor is stored in a nonvolatile fashion in the control unit. An optimum measuring accuracy is always achieved for the visual range measuring range used by the transmissometer owing to the use of the measured 30 value of the scattered light measuring arrangement for the purpose of determining a calibration factor exclusively in the range above the upper boundary of the measuring range of the transmissometer, and to the fact that the visual range 14 measuring error of transmissometers which arises because of the environmental influences described increases with lower visual range. The method just described follows the mode of procedure of transmissometer calibration by a skilled observer, with the difference that use is 5 made of every possible calibration situation at any time of day and night for the purpose of optimizing the measuring performance of the transmissometer. This results in the use of a large number of calibration events, something which certainly cannot be achieved by the known calibration methods based on observers. The automatic determination and application of the calibration factor 10 during the transmission measurement at once enables a permanent and complete compensation of the influences which limit the measuring performance of the transmissometer. Turning then to Figure 7, reference number 9 identifies two equipment plates 9 positioned at 900 relative to one another in a V-shaped fashion. This 15 arrangement enables one and the same plate to be transilluminated along two axes. The main axis represents the direction of the beam path for atmospheric transmission measurement, while the secondary axis, offset by 90*, describes the beam path for a separate transparency measurement of the equipment plates. Both optical axes transluminate one equipment plate at an angle of 450 relative to 20 the plate surface and in the same area of the plate, the other plate being transilluminated only by a beam path following the secondary axis. This arrangement enables continuous measurement of the actual plate transparency and permits immediate accurate compensation of the effects of any instances of fouling which limit measuring performance. The determination of 25 plate fouling neither requires measurement to be interrupted in order to permit comparison with a clean reference plate, nor is use made of empirical conversion quantities derived from the scattering behaviour of the plate. It is possible to assume uniform fouling of the two plates because of the long weather protection hood 2 which is used. Consequently, the correction of 30 atmospheric transmission measurement is permissible on the basis of the transparency measurement described even if the atmospheric transmission measurement is influenced only by one equipment plate 9 in each case.

15 Furthermore, the apparatus according to the invention does not use a separate plate measuring receiver unit 16, the latter being an integral constituent of the control electronics. After traversing the equipment plates in the direction of the optical receiving device of the control unit, the light bundle is deflected via an 5 appropriately shaped part of the housing of the electronic unit (see Figure 7). The control unit microprocessor uses the measurement result relating to plate transparency to determine the correction factor, induced by fouling for the transmission measurement. This correction factor is determined separately for transmitting and receiving units 3, 4. 10 It holds in this case that: VS = 1/(TPS)^0.5 VE = 1/(TPE)^ 0.5 where: 15 VS is the transmitter correction factor induced by fouling, VE is the receiver correction factor induced by fouling, TPS is the normalized measurement result of plate transparency measurement at the transmitter optoelectronic unit, and TPE is the normalized measurement result of plate transparency 20 measurement at the receiver optoelectronic unit. Two correction factors induced by fouling can be combined to form a total fouling factor VG: VG = VS*VE. 25 VG becomes 1 in case of clean equipment plates. The same mechanism is used to calculate a factor VGtemp which, by contrast with VG, is renormalized to 1 with each determination of a new calibration factor. This temporary correction factor induced by fouling is then 30 directly applied alongside the calibration factor to a transmission measurement result. TMcorr = TMmess*VGtemp*KF 16 where: TMcorr is the corrected measurement result of atmospheric transmission, TMmess is the non-corrected measurement result of atmospheric 5 transmission, VGtemp is the temporary correction factor induced by fouling, and KF is the calibration factor. Influences of transmission measurement which are induced by plate 10 fouling are compensated in this way, with the aid of transparency measurements, between situations in which a new calibration factor is determined. Each newly determined calibration factor automatically also compensates the influence induced by plate fouling, which is represented by VG. Together with the knowledge of the above-described calibration factor, the 15 explicit knowledge of the degree of fouling of the optical outer surfaces on the basis of the already described transparency measurement of the equipment plates arranged in V-shaped fashion now for the first time enables the separation of transmission measuring errors, and thus also of visual range measuring errors, induced by alignment and by fouling. 20 Since the calibration factor is composed of a correction factor induced by alignment and the correction factor induced by fouling for the transmission measured value, whereas the correction factor induced by fouling is known individually, the fraction of the calibration factor induced by alignment can be calculated directly. 25 KA = KFNG where: KA is the fraction of the calibration factor induced by alignment, 30 KF is the calibration factor, and VG is the total fouling factor.

17 Knowledge of KA and VG enables the invention presented to make detailed statements on the quality of alignment and on the degree of fouling of the equipment plates. The plate fouling occurring can also be compensated computationally by means of VGtemp for the time periods between situations in 5 which a new calibration factor KF can be determined on the basis of the fulfilled conditions for the use of a measured value of the scattered light measuring arrangement. The alignment quality is also reassessed with each renewed calculation of a calibration factor. The user can thereby inform himself both of the degree of fouling of equipment plates and of the quality of alignment. 10 The introduction of suitable limiting values for KA and VG which are specific to an embodiment clearly defines when the equipment plates have to be cleaned, and where there is a need to realign the optical axes of the transmitter and/or receiver. Realignment can then either be initiated by the user or be performed in the fully automatic fashion. Fully automatic realignment is preferably 15 carried out when - the visual range measured value of the scattered light measuring arrangement exceeds 10 km, - the variation in the visual range measured value of the scattered light measuring arrangement about the mean value has in no case exceeded 20 +/-10% in the period under consideration, - no precipitation was detected by the scattered light measuring arrangement, - there is no interruption in the operation of the scattered light measuring arrangement, 25 - the variation in the transmission value about the mean value has in no case exceeded +/-1% in the period under consideration, and - there is no interruption in the operation of the transmission measuring arrangement. The inventive determination of the calibration factor KF and of the 30 temporary correction factor VGtemp induced by fouling in each case ensures optimum transmission measuring performance and, in the final analysis, an as yet unattained measuring accuracy for the meteorological visual range in conjunction with virtual freedom from maintenance.

18 Permanent synchronization of the optoreceiver with the modulation frequency of the optotransmitter enables the synchronous modulation, known from the literature, of the received intensity-modulated light signal with the known improvements in the measurement properties for small noisy signals. The 5 optoreceiver signal is fed digitally to the microprocessor of the control electronics for further processing after analogue/digital conversion with the aid of over a million increments corresponding to a resolution of better than 0.0001% (see Fig. 3). It is preferred that the transmitter light source is a white light-emitting diode 10 which can attain a service life of over 50 000 hours because of the operating current being reduced far below the admissible maximum current. The intensity of the light-emitting diode is modulated periodically with the so-called modulation frequency. In order to generate a large number of scans, something which benefits measurement stability, the modulation frequency is typically above 15 1000 Hz. The light intensity is modulated with a path duty factor of 50% between zero and the set operating current. The mean value of the operating current is only a few milliamperes. The intensity of the light source is kept highly stable by means of an electronic precision control loop. By comparison with monochromatic light sources such as coloured or 20 infrared light-emitting diodes or else laser light sources, the spectrum of the white light-emitting diode used in the embodiment has the advantage of fully representing the wavelength region recommended by the International Civil Aviation Organization ICAO for light sources in the case of visual range transmissometers. By comparison with mechanically modulated halogen light 25 sources or else low-frequency-pulsed xenon flashlamps, which usually have the recommended spectral region, the advantage consists in the implementation of substantially higher modulation frequencies, and in the associated more frequent contributions to the measurement results during averaging.

19 List of reference numerals used 1 Double tube stand structure 2 Weather protection hood 3 Transmitting unit (Transmitter) 5 4 Receiving unit (Receiver) 5 Inner tube 6 Outer tube 7 Supporting structure 8 Fan 10 9 Equipment plates 10 Optical system 11 Geared motor 12 Eccentric element 13 Potentiometer 15 14 Air channel 15 Scattered light measuring arrangement 16 Plate measuring receiver unit 17 Cardanic suspension

Claims (14)

1. Apparatus for measuring atmospheric transmission and determining meteorological visual range, including a transmitter unit and receiver unit which are respectively secured above ground by means of respective stand structures, 5 wherein each stand structure includes an inner bearing tube and an outer tube which is decoupled mechanically from and protects the inner tube, and wherein components of the transmitter and receiver units for effecting measurement and optical alignment of the transmitter and receiver units are fitted to the inner tube, whereas other structural elements of the transmitter and receiver units which can 10 vary their position owing to their dead weight, wind load stressing or one-sided thermal loading are mounted to the outer tube in such manner that optical alignment remains uninfluenced by these effects.

2. Apparatus according to claim 1, further including: * a scattered light measuring arrangement devised as an integrated 15 constituent of the transmission measuring arrangement but being supported by the outer tube; e equipment plates positioned at 900 to one another in V-shaped fashion arranged in front of the transmitter unit and receiver unit, respectively, such as to protect optical and electronic components of the apparatus against 20 fouling, wherein each V-shaped equipment plate arrangement is assigned a dedicated transmission measuring device for measuring transparency and which determines the degree of fouling of the equipment plates; and e respective cardanic mounts for the optical system of the transmitter unit and that of the receiver unit arranged to enable adjustment of the optical 25 system.

3. Apparatus according to Claim 1 or 2,wherein the transmitter and receiver units each have a scavenging air system arranged to deflect precipitation particles in the direction of the ground before they strike and reach outer optical surfaces of the optical components for effecting transmission measurement. 21

4. Apparatus according to Claim 1 2 or 3, wherein the scattered light measuring arrangement is a forward-scattering measuring arrangement.

5. Apparatus according to any one of Claims 1 to 4, wherein the receiver unit has synchronous demodulation capability and is permanently synchronized with a 5 modulation frequency of the transmitter unit.

6. Apparatus according to any one of Claims 1 to 5, wherein the transmitter unit has a white light-emitting diode as a light source.

7. Apparatus according to any one of Claims 1 to 6, further including a signal finder connected to the receiver unit. 10

8. Apparatus according to Claim 7, wherein the signal finder is an acoustic signal transmitter.

9. Method for measuring atmospheric transmission and determining meteorological visual range using transmitter and receiver units supported above ground at a predetermined distance from one another, including the steps of: 15 e determining a calibration factor in automatically selected situations, the calibration factor being formed by dividing a visual range value supplied by a scattering light measuring arrangement and which was converted into an equivalent transmission value by a measured value for atmospheric transmission; * determining a correction factor which is a function of fouling taking 20 place between the automatically selected situations, the correction factor being determined by continuous measurement of the transparency of equipment plates which are located in front of the transmitter and receiver units, respectively; e using the determined correction factor and the calibration factor to determine an alignment factor which is equivalent to a change having taken place 25 in the optical alignment between transmitter and receiver units; * applying the calibration factor and the correction factor to the measured atmospheric transmission value which was determined by the transmission measuring arrangement; and 22 e using the determined alignment factor to restore an initial adjustment between receiver and transmitter units.

10. Method according to Claim 9, wherein the automatically selected situation is determined to be present when an evaluation of the measured values supplied 5 by the scattered light measuring arrangement indicates that no precipitation is present and there is a visual range of greater than 10 km.

11. Method according to claim 9 or 10, further including the steps of: * initially carrying out a coarse alignment of the transmitter and receiver units subsequent to them being set-up; 10 e carrying out an automated fine alignment first of the transmitter unit and then of the receiver unit both vertically and horizontally; e during the fine alignment process, determining and storing data representative of positions which the transmitter and receiver units assume in the process as well as a corresponding measurement signal at the receiver unit, 15 thereby generating a transmission intensity profile of the transmitter unit and a reception sensitivity profile of the receiver unit; and e using the intensity profile of the transmitter unit and the sensitivity profile of the receiver unit in determining an optimal spatial adjusted position of transmitter and the receiver units. 20

12. Method according to Claim 11, wherein the intensity profile and the sensitivity profile are stored in a non-volatile memory.

13. Method according to Claim 9, wherein the measured values supplied by the scattered light measuring arrangement are used to turn-on or turn-off a scavenging air system of the receiver and transmitter units. 25

14. Method according to Claim 9, wherein the correction factor is subjected to threshold value checking and an equipment panel cleaning signal is generated when overshooting of the threshold value takes place. 23 VAISALA GMBH WATERMARK PATENT & TRADE MARK ATTORNEYS P25173AU01