vio} = 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
TEA CO, LASER 
  
E 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Timing Unit 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Figure 1: Scheme of the DIAL facility at the University of Calabria 
IORI8 (10.260 pm) lines, respectively, in order to take 
advantage of the highest differential absorption coefficient value 
(da = 7.7 107* /cm/atm) within the CO, infrared range, and to 
minimize interference effects by other gases. The energy 
transmitted is monitored on a pulse by pulse basis, and it is 
turned out to obtain about a 3 J/pulse. 
To obtain the density profiles of nitrogen, oxygen and other 
atmosphere components, the relevant lines, within the CO, 
infrared range, can be used. 
The distance between the station and the cell is obtained by 
measuring the time delay between transmitted and received 
pulse. 
For the alignment of the laser beams with the telescope axis, 
some mirrors with micrometric regulation are used. Due to the 
CO, laser wavelenghts, it is impossible to control it in the 
visible range. To obtain the alignment for the first applications, 
IR targets have been used. 
To direct the pulses to a selected external target, a mirror, 
manually driven, has been used. 
THE TEST TO DETERMINE THE GEOMETRIC 
ACCURACY 
Range Measurements 
The accuracy of the distance measurements depends on the 
sampling rate and on the trigger offset. In the figure 2 the typical 
curves of a DIAL signal due to two CO, laser pulses are 
reported. The curves represent the intensity of the signal 
reflected by a solid target. The dotted curve refers to the 10R20 
line; the bold curve refers to the 10R 18 line; the sampling rate is 
equal to 20 nsec. In the X axis the distances from the laser 
source are reported in meters, taking into account the 
synchronization between pulse emission and clock activation. 
It is possible to observe two double peaks, corresponding to the 
reflections of the pointing mirror and of the target. The typical 
CO, laser double peak is due to the de-energization of the 
nitrogen contained in the laser mixture (the delay is equal to 
about 2 usec). To avoid the sistematic error due to the unknown 
trigger offset, the ranges can be measured from the mirror, using 
the delay between two peaks. 
To determine the accuracy of this range measurement, two 
targets has been used, at a distance of about 300 and 1000 m 
respectively. 30 measurements have been made using both the 
10R18 and 10R20 lines, with a sampling rate of 10 nsec. In this 
way, the distance is obtained as a multiple of 1.5 m. 
The same distances have been measured using a ZEISS Elta 3 
total station, allowing an accuracy of 3 mm/km, to obtain the 
reference values. 
The average value and the standard deviation of the DIAL 
measurements have been obtained, and compared with the total 
station results. 
In the figure 3 the curves for 3 pulses are drawn. It is possible to 
observe the different trigger offsets. Three peaks are present in 
every curve, due to the mirror and the two targets. 
In the figure 4 the enlargement of a peak is drawn. It shows a flat 
shape, due to the lenght of the laser pulse (80 nsec) 
corresponding to 8 samples. 
To obtain the time used to compute the distances, the first 
sample of every peak has been chosen. Due to the shape of the 
peaks, in some cases the selection of the relevant time is not 
immediate, so, for every sample, the sum of 8 consecutive 
values has been computed, and the sample with the maximum 
4 International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 
  
À Ella an Salz 
  
Arbitrary Units