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Sensing, 
ul 2004 
  
METHOD OF THE HIGH ACCURACY RESOLVING 
RANGE/VELOCITY UNCERTAINTY FOR THE LASER SCANNER 
WITH LINEAR FM 
V. M. Lisitsyn, S. V. Tikhonova 
7, Victorenko Street, 125319, Moscow, Russian Federation - lvm@gosniias.ru 
———ÁRÀ————— 
Commission I, WG I/3 
KEY WORDS: Laser scanning, Infrared, Method, Measurement, Real-time, Algorithms, Precision, Modelling 
ABSTRACT 
The laser scanning system on basis of CO, laser with continuous radiation with 10.6 uum wavelength situated onboard of aircraft is 
considered. The system is intended for remote sensing of surface ahead of aircraft. Simultaneously distance image of observed scene 
has to be formed and vehicle own velocity has to be measured. The system operates in a mode of azimuthal one-line scanning across 
a trajectory of flight. For distance measurement linear-frequency modulation with pulse compression is used. Pulse compression is 
produced with dispersive acoustic delay line. Mathematical expressions are known that permit to resolve distance/speed uncertainty 
in this case. These expressions need two sensing of each element of a surface. However in reality scanning is carried out 
continuously, so two neighbor sensings is made on different elements of surface. 
Suggested method permit to increase noticeably the accuracy of vehicle own velocity measurement while permitting to measure 
distance to each element of surface in case of continuous scanning. For this purpose the fact is used, that value of the carrier own 
velocity is slowly varying function. So, averaging and extrapolation of the information is made line by line to increase accuracy of 
vehicle own velocity measurement. For each sensing, distance is calculated based on the extrapolated velocity value. 
INTRODUCTION 
The laser scanning system on basis of CO, laser with 
continuous radiation with 10.6 um wavelength situated onboard 
of aircraft is considered. The system is intended for remote 
sensing of surface ahead of aircraft. Simultaneously distance 
image of observed scene has to be formed and vehicle own 
velocity has to be measured. The system operates in a mode of 
azimuthal one-line scanning across a trajectory of flight. For 
distance measurement linear-frequency modulation with pulse 
compression is used. Pulse compression is produced with 
dispersive acoustic delay line [Stephan, B., 1085- Dansac, 1, 
Meyzonnette, J. L., 1985]. 
In the system realizing such kind of modulations (fig. 1), almost 
all laser output power goes to acoustic-optical modulator 
(AOM) where radiation gets linear FM with the triangular law 
of change of frequency. The modulated laser beam goes through 
optics and the scanner and is radiated in space, and the part of 
not modulated radiation branches off to the photodetector where 
it mixes up with the received reflected signal. As a result of this 
mixing optical heterodyning of received laser signal is carried 
out, and linear FM is transferred to intermediate frequency. At 
presence of vehicle own velocity frequency of the reflected 
signal is increased by the Doppler shift value. As vehicle own 
velocity can vary considerably, radio oscillator is used (unlike 
to [Stephan, B., 1985]) to maintain a necessary dynamic range 
on frequency. 
Further the signal goes to a dispersive acoustic delay line 
(DADL) where it is compressed in the number of times equal to 
product of frequency deviation AF and linear FM half-cycle to. 
An output of DADL is series of short pulses going with 
frequency that is equivalent of double modulation frequency of 
laser. The generated pulses are received by the counter which 
measures a time delay /44 between the compressed pulses and 
clock pulses marking the beginning of sensing. 
Measured delay times goes to the processor of preliminary 
processing of the information. A task of the processor is 
range/velocity uncertainty resolution, radio oscillator 
management in real time and transformation of distance 
measurements to a kind convenient for the further processing. 
We shall accept periodicity of sensing equal to tg, = 20 ps, that 
corresponds to one linear section of triangular linear FM with 
deviation of frequency of radiation AF = 40 MHz. One sensing 
gives one element of distance image. 
By results of double sensing slant-distance up to probed surface 
that is perpendicular to axis of beam (R) and a radial component 
of vehicle velocity (V) can be accordingly determined as [2, 3]: 
CT. 
R= 4 " + Eu 
pull dl in (1) 
dr del del 
dev 
where c is the velocity of light; À is the radiation wave-length; 
+ : : > - ^ S ; 
(4, is delay time of the compressed pulse for increasing 
section of linear FM of the reflected signal; 7 4, is delay time of 
the compressed pulse for decreasing section of linear FM of the 
reflected signal, AF is the deviation of radiation frequency, /a« 
is the linear FM half-cycle. 
Delay time is expressed as: 
2. U dev 
d Cea =D; A e te +, (2) 
where D is slant-distance;