Velocimeter utilizing carbon dioxide laser measures true airspeed of aircraft. Results of flight tests indicate that clear-weather airspeeds can be measured with accuracy better than 0.1% at altitudes up to 3000 meters; measurements can be made at much greater altitudes in cloudy or turbid air

Koehler, L. E.

Mocker, H. W.

Munoz, R.

English

NASA Technical Reports Server

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Scattering 
Volume
December 1973
	 B73-10506 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
True Airspeed Measured by Airborne
Laser Doppler Velocimeter 
A Doppler detection velocimeter utilizing a carbon 
dioxide laser has been developed to measure the true 
airspeed of an aircraft. Results of flight tests indicate 
that clear-weather airspeeds can be measured with
measures the apparent shift in frequency (Doppler 
effect), which is proportional to the plane's airspeed. 
The laser optical system is folded for compactness 
and rigidity, and includes a true internal mirror 
V
	 Germanium Aircraft Window 	 Primary Mirror 
Secondary Mirror	 Laser 
Frequency
Tracking
Loop
Variable Attenuator 
Field Lens	 p..- Laser Radiation 
Detector	 )- Backscatter Radiation 
an accuracy better than 0.1% at altitudes up to 8000 
meters; measurements can be made at much greater 
altitudes in cloudy or turbid air. 
The optical system of the velocimeter consists of a 
colinear transmitting and receiving telescope, beam-
combining optics for homodyne operation, and a 
large-aperture infrared window which replaces a 
glass window in the aircraft. The laser beam is fo-
cused at a point about 20 m in front of the aircraft, 
where the air is undisturbed by the bow wave. The 
system detects backscatter from aerosol particles and
system in order to eliminate frequency fluctuations 
which can arise from variations in cabin pressure; it 
operates in a sealed-off configuration and has an ex-
tremely high passive short-term stability of one part 
in 1010. The cavity structure consists of a yoke as-
sembly and two end pieces made of low-expansion 
quartz (thermal expansion coefficient of 2 x 108/°C); 
one end piece is roof-shaped to fold the laser beam. 
An angular alignment transducer is attached on one 
side of the laser and a GaAs output coupler at the 
other. Two water-cooled, gold-coated silicon mirrors 
(continued overleaf( 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000506 2020-03-17T07:33:28+00:00Z
fold the laser beam by 1800. Two plasma tubes con-
nected in series to form the laser are sealed by the 
two end pieces to provide an internal mirror system 
requiring no Brewster windows. Output charac-
teristics of the laser feature a stable single-mode 
(TEM OO) operation at a single frequency correspond-
ing to the carbon dioxide rotational line (P20). The 
beam spread of the laser approaches the diffraction 
limit and is of the order of 2.5 rnrad. 
Referring to the diagram, the (Hg, Cd, Te) detector 
output is fed to a frequency tracking loop which 
consists of a mixer, voltage-controlled local oscillator, 
and a specially-developed zero-IF system that per-
mits selection of the predetection bandwidth to pro-
vide optimum tracking sensitivity for a broadband 
signal. A triangular dither-waveform sweeps the 
voltage-controlled oscillator through a small range 
about its center frequency. When the difference be-
tween the local oscillator and signal frequencies is 
within the bandpass of the video amplifier, a voltage 
is developed at the output of the envelope detector. 
The phase of this voltage with respect to the dither 
signal depends on the relationship of the signal 
frequency and the VCO center frequency. Thus, a 
phase-sensitive detector is used to develop a voltage 
which swings positive or negative, depending upon 
whether the VCO center frequency is above or below 
the incoming signal frequency; the voltage is averaged 
and summed by the dither signal to lock the VCO to
the signal frequency. The preamplifier has a fre-
quency response from 2 to 32 MHz with a noise 
figure of 1.5 db. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94085 
Reference: TSP 73-10506 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94085 
-	 Source: Robert Munoz 
Ames Research Center 
with Hans W. Mocker and Lyle E. Koehler of 
Honeywell, Inc./Systems & Research Center 
under contract to 
Ames Research Center 
(ARC-10763)
S 
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B73-10506	 Category 02
	 E1


True airspeed measured by airborne laser Doppler velocimeter

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