[ 0049 ] fig1 a and 1b illustrate the omni - directional platform configuration of the new scanning ceilometer 1 . fig1 a is a front elevation ; fig1 b is a side elevation . a ceilometer 15 is mounted on support arms 5 for universal movement . the ceilometer 15 may be a cluster of parallel or slightly angularly offset ceilometers , for example a group of four ceilometers . a servo motor 9 drives a 100 : 1 reduction gear 7 that rotates the ceilometer about its horizontal axis upon load bearings 3 . load bearings 3 are installed on both sides of the support arms 5 , and a bayside 100 : 1 gear reducer 7 is attached in - line along the shaft , which is connected to a parker compumotor ® servo motor 9 that provides power to the shaft of the gear reducer 7 . the bottom of the support arm 5 is secured to a bayside 100 : 1 right angle gear reducer 11 , with a turntable configuration that is driven by a parker compumotor ® servo motor 9 . the lower servo motor and the gear reducer / turntable 11 are mounted on a platform base 13 . the server motors 9 are equipped with shaft encoders , enabling precise position measurement . a pc - based controller board interfaces with the motor amplifiers and shaft encoders to direct motion , to control the servo motors 9 , and to determine shaft positions . national instruments ® labview ® software , running on the pc , is used to drive the controller board and to process position information as well as to perform data acquisition , analysis and communication . features for the motion - control hardware are given in the table 1 , where t r is rated output torque , t cs is the rated continuous stall torque , t p is the rated peak torque , δθ e is the encoder angular resolution , and δθ c is the controller position . [ 0050 ] fig1 c shows the ceilometer 61 in the scanning mirror configuration . a platform 63 with a gimballed - mirror assembly 65 is mounted at the end of the ceilometer 61 . the transmitted laser 67 beam reflects off of the mirror surface 69 toward the target area 71 . backscattered light is in turn transmitted back to the mirror 69 from the cloud 71 and reflected to the ceilometer 61 for sensing . the mirror 69 is controlled using servo motors . [ 0051 ] fig2 shows the front panel 22 of the data acquisition system 20 that controls both the motion of the ceilometer and acquire data from the unit . preliminary data - acquisition software was developed using labview ®, a product of national instruments ®. labview is an object oriented graphical programming language based on sub - codes written in c . running from a pc , motion control and data acquisition routines command motion - control boards and data acquisition boards connected to the pc motherboard . the motion - control boards and the pc motherboard control servo motors 9 . additional embodiments for client or server internet communication , remote access of data , communication with other systems , printed reports , or integration of additional systems are also included . up - down numerical input buttons 18 in the upper left panel 19 allow user inputs of angular increments and ranges for both azimuth and elevation . the status box 21 in the lower - left corner displays any errors associated with the lidar ceilometer unit and the motion control hardware or software . data is displayed in the upper middle portion 17 of the screen 22 in numerical format , giving elevation , azimuth and range . a 3 - d visualization 23 of the data and a landing approach cross - section 25 are presented in the right upper and middle portions of the screen 22 . [ 0052 ] fig3 a and 3b show a measurement grid display for a particular elevation plane 31 . for a single - elevation cloud formation , shown in fig3 a , the scanning ceilometer 1 returns three - dimensional data corresponding to a level plane . the “ x ” marks 29 indicate where the lidar beam 27 , scanning with constant angular increments , would intersect the single - elevation cloud formation . individual measurements represent sub - quadrant sectors 33 on a horizontal plane 31 , shown in fig3 b . [ 0053 ] fig4 a illustrates the divergence of the lidar beam 27 with distance from the origin 37 . this divergence causes the spatial measurement resolution to decrease with altitude . with this in mind , as shown in fig4 b , representation of the elemental data is made using sub - quadrant - shaped polygons 33 whose size is a function of the cloud ceiling height . the polygon object &# 39 ; s dimensions were selected so that its size represented the spatial resolution of the measurement . the two diverging sides 47 are defined by the azimuthal measurement resolution . the inner arc 41 and outer arc 39 are defined as the midpoint radius between successive in - plane measurements 43 at a particular azimuth angle θ . providing a nominal vertical thickness 35 , the sector can be represented as a solid object in 3 - d . [ 0054 ] fig5 a and 5b illustrate the wireframe version 49 and the shaded version 51 of the cloud - hit element for 3 - d visualization . [ 0055 ] fig6 shows a display of the data in the context of the measurement by means of a 3 - d model of the terrain 53 , created using a united states geological survey ( usgs ) digital elevation model ( dem ) grid and surface map of the area . the surface map is “ stretched ” over the surface grid rendering , creating a 3 - d image of the terrain with surface features such as roads 55 . this display helps relate the data to actual position in the measurement volume . the 3 - d wedge section 51 is then placed in the proper position and orientation to indicate the location of the cloud ceiling relative to the terrain 53 . [ 0056 ] fig7 graphically depicts cloud heights measured by the apparatus at times 12 : 45 , 12 : 56 , and 13 : 07 . this representation shows two distinct cloud layers centered at 4500 feet and 8000 feet above ground level ( agl ). the reduced number of upper layer measurements is indicative of laser beam attenuation by intervening clouds . [ 0057 ] fig8 graphically depicts cloud heights measured by the invention at times 12 : 45 , 12 : 56 , 13 : 07 , and 14 : 35 . comparisons with fig7 reveal movement at a later time ( 14 : 35 ) of a lower cloud deck into the area at 2000 feet agl . [ 0058 ] fig9 graphically depicts the same cloud data from fig8 as “ cloud hits ” for heights versus azimuth . this figure more clearly shows that cloud layers extended over all the probed azimuths ( from 45 degrees northeast , to 225 degrees southwest ) in a fairly uniform pattern . the cloud layer near 4000 feet was detected in all directions during earlier measurement sequences . later , at 14 : 35 , the lower cloud deck ( near 2000 feet ) was also detected in all directions . although the azimuthal pattern ( or lack thereof ) is discernable here , there is no indication of the horizontal extent of the clouds in a radial direction for any particular azimuth . [ 0059 ] fig1 shows two data sets for intermediate times 14 : 10 and 14 : 22 . the earlier data set indicates only a single cloud layer at 4000 to 5000 feet , while the later one shows both an upper layer between 4000 and 5000 feet and a lower layer with a base near 1000 feet in all probed directions . these two data sets demonstrate that a change in cloud levels at a low altitude ( in this case , 1000 feet ) may occur in less than the 10 minutes it took to complete one data . this is explained by the fact that a 30 degree elevation angle was used , thereby limiting the range of detection to 2000 feet horizontal range from the lidar . however , this horizontal range need not be a problem for an airport environment where there are no adjacent occupied structures that would preclude low angle scanning . reducing the minimum elevation angle to 5 degrees would extend the horizontal range to about 11 , 000 feet for a layer at 1000 feet altitude . [ 0060 ] fig1 and 12 show plan view plots for times 12 : 45 and 15 : 22 . these plots show the detected clouds overlaying a simple map of the actual area and centered upon the point where the data was collected . the view is looking down on the clouds and the terrain below from an overhead position . all clouds detected are shown as shaded crosses surrounding the lidar location , which is indicated by the thick vertical bar near the bottom center . in order to differentiate the cloud heights measured by the lidar , each cloud segment is shaded , or colored , in accordance with the altitude scale shown on the figure ( note that in subsequent figures , the cloud altitudes are given in feet above sea level ). this type of view has the advantage of a relatively clear representation of the cloud coverage over the xy horizontal plane . colors , or shading in the data plots indicate the cloud height . [ 0061 ] fig1 through 18 illustrate perspective plots of cloud height data . these views present a more realistic look at the clouds as they would be seen by an observer near the lidar location but at an elevated position . to enhance the realism of this view , the cloud height data is overlaid on a contour map of the terrain . a comparison of these figures reveals some of the details observed with the passage of time . it is apparent that the coarseness of the spatial grid attained during this experiment precludes resolution of individual cloud edges and shapes , but it allows for observation of the general motion and progression of the clouds over time . thus , a 3 - d perspective view shows the radial extent , the azimuthal distribution , and the temporal evolution of the cloud field . [ 0062 ] fig1 graphically depicts a vertical slice through the 3 - d data in a plane from an observer looking towards one specific azimuth . in this example , only three cloud hits were measured and are represented by shading and text label . an advantage of this display is that the cloud heights along a particular path are seen from the side , clearly showing the relationship between the clouds and the ground . such a display is an aid if one wants to project an aircraft &# 39 ; s flight path against the measured clouds in a specified direction as a function of distance . while the invention has been described with reference to specific embodiments , modifications and variations of the invention may be constructed without departing from the scope of the invention , which is defined in the following claims .