>a constellation of NASA satellites intended to monitor clouds, aerosols, oceans, landmasses
and their effects on climate change over the next 10 to 20 years. The flagship, Terra, was
launched in December 1999 (
http://terra.nasa.gov/
). Aura was launched in 2002, the first of a
series of six A-Train satellites, the last of which is scheduled to be deployed in 2006
(
http://radio.weblogs.com/0105910/2004/05/24.html
). However, satellite measurements of the size,
shape and concentration of cloud and aerosol particles are determined by mathematical
inversion of (passive and active) radiative signatures from distances of 700 km. Thus, in situ
validation of retrieval algorithms used to determine cloud and aerosol properties is essential.

Research aircraft equipped with sensors to probe the microphysical properties of aerosols
and clouds have been in use for over fifty years, but the duration of conventional aircraft is
limited so that long-term measurements are not practical, and long-term, high-coverage
measurements are needed to provide a statistical basis for comparing with satellite
measurements. Small uninhabited aerial vehicles (UAVs) and tethered balloons, however, are
now capable of making sustained, long-term (30 hr) measurements, so that data sets can be
collected that provide much better statistical comparisons with results from satellite retrieval
algorithms. In addition, several small UAVs and tethered balloons can be deployed
simultaneously to improve spatial coverage of the measurements.

Lightweight, low-power cloud and aerosol sensors are needed for applications
on small, relatively low-cost, long-duration UAVs and tethered balloons in
order to validate satellite measurements.

2
. Results of the Phase I Research


2.1 Overview

In Phase I, we investigated the feasibility of adapting three existing sensors commonly used
on piloted research aircraft for use on small UAVs. The three technologies are


a hot-wire liquid water content probe (e.g., SkyTech Nevzorov probe, Korolev et al.
(1998)).


a particle scattering spectrometer probe (e.g., Particle Measuring Systems (PMS)
forward scattering spectrometer probe (FSSP), Knollenberg 1981; Droplet Measuring
Technologies (DMT) Cloud and Aerosol Spectrometer (CAS), Baumgardner et al.
(2002)), and


a cloud particle imager (CPI), Lawson et al. (2001).

We evaluated each of these technologies, which in their current form factor are far too heavy
and consume too much power, to determine those that could best be adapted for use on small
UAVs.


3 The Aerosonde was chosen as the primary target UAV (Figure 1) because it is the
most commonly used UAV for weather research (see

http://paos.colorado.edu
,
www.aerosonde.com

and Holland et al. 2001), it is relatively low-cost, has long (30
hr) duration, and in addition, Aerosonde Pty Ltd of Melbourne, Australia (hereafter
called Aerosonde Corporation) has agreed to be a Phase II and Phase III partner
with SPEC. Aerosonde Corporation will contribute significant technical support in
Phase II and will provide production and marketing in Phase III. See Supporting
Documents Section for more information on Aerosonde Corporations
participation in Phases II and III.


Figure 1. Photograph of the Aerosonde UAV selected as a target for feasibility studies in
Phase I.


While it was deemed (marginally) technically feasible to generate sufficient electrical power
to de-ice and heat a hot-wire sensor, it was considered to be too much of an investment for only
one measurement, which is the mass concentration of liquid water, commonly called the liquid
water content (LWC). As a result, investigations into development of a suitable hot-wire sensor
were dismissed and emphasis was placed on the other two candidate technologies.





4 The other two technologies, an aerosol and cloud particle scattering probe and a cloud
particle imager, are much more attractive in that they give considerably more information,
including:



cloud drop size distribution,


aerosol size distribution,


ice particle size distribution,


high-resolution images of ice particles and water drops.



Derived parameters from the above measurements include (but are not limited to):


cloud drop concentration


mean Cloud drop size


liquid water content


ice particle concentration


ice water content (IWC)


ice particle shape and crystal habit


cloud extinction


cloud effective particle size


cloud equivalent radar reflectivity


aerosol size distribution



We determined that the scattering and imaging technologies could be combined
into a single instrument that contributes minimally to aerodynamic drag,
consumes little power, does not need to be deiced and weighs about 1.2 Kg.
Figure 2 shows a Solidworks model of the new instrument, called a Micro-CPI, as
it would be appear when installed on the Aerosonde.

5
Figure 2. Solidworks model of the Micro-CPI as it will appear when installed on the Aerosonde.

SPEC worked closely with Aerosonde during Phase I to assure that the Micro-CPI will fit
seamlessly into the UAV. Aerodynamics, aircraft center of gravity, power loading and sensitivity
to vibration were all considered in the design of the Micro-CPI. Details of the design process
and parameters are described in Sections 2.2 2.11. Aerosonde expects Phase III
commercial sales of 10 to 20 Micro-CPIs per year initially and will make a significant contribution
to the Phase II effort. Aerosonde has agreed to subcontract 640 hours in Phase II; 320 of these
hours will be contributed by Aerosonde (see Budget and Aerosonde Letter of Cooperation).
Aerosondes home office is in Australia. However the company also has a facility at the NASA
Wallops Island Research Center, and has agreed to make available and support a new
Aerosonde Piccolo for flight tests of the Micro-CPI in Phase II.

The commercially available CPI, used on piloted aircraft, has a particle detection
system (PDS) that identifies candidate cloud particles and triggers the imaging system.
The Micro-CPI will use an innovative particle detection system, that in addition to
detecting candidate particles, will simultaneously count and size particles, including
aerosols, cloud drops and ice particles. The new PDS is called a forward scattering
system (FSS) because it uses focused optics with forward scattered light and qualifying
detectors, similar to the FSSP and CAS, to define particle depth of field and pulse height to
determine particle size. Like the FSSP and CAS, the FSS particle sizing is based on the

6 assumption that particles have a spherical shape. The following subsections in Section 2
describe in detail the design of the Micro-CPI.


2.2 Micro-CPI Imaging Optical System

Figure 3 shows a ray-trace of the imaging optical system for the Micro-CPI. The system is
based on a Keplerian telescope operating at a finite conjugate ratio. Particles passing through
the sample volume are imaged onto the CMOS camera array. A pulsed laser (not shown) is
used to illuminate the particle as it passes through the sample volume. The laser pulse width is
much shorter than the exposure time of the CMOS camera chip, thereby freezing the motion of
a particle in flight.

The working distance for a lens system is the distance from the front of the first lens to the
object plane. For this system the working distance is also equivalent to the focal length of Lens
1. The magnification for the optical system is determined by the ratio of the focal lengths of
Lens 2 to Lens 1 and the total path length is twice the sum of the focal lengths of Lens 1 and
Lens 2. The focal length of Lens 1 must be chosen based on the instrument layout in the
Aerosonde. Lens 2 can then be selected to provide the necessary system magnification and
balance the total path length limited by the instrument layout. In this case, Lens 1 is chosen to
have a focal length of 35 mm and Lens 2 is chosen to have a focal length of 85 mm. The
working distance for this system is then 35 mm. The system magnification is 85/35~-2.4X.
The negative sign implies the image is inverted. The total path length is 2 x (85 +35) or 240
mm. The CMOS array chip selected for the system is a Symagery VCA 1281 with 1024 x 1280
pixels that are 7 um x 7 um each. The system resolution is 7 um / 2.4X or approximately 2.92
um pixel
-1
in the sample volume. The sample volume dimensions are 2.92 um pixel
-1
x (1024 x
1280) pixels or 2.99 mm x 3.74 mm.

Figure 3. Zemax ray trace of Micro-CPI imaging optical system.

7


The imaging system design shown in Figure 3 was prototyped in the laboratory. Figure 4 is
a photograph of the prototype imaging system. A Scorpion camera manufactured by Point Grey
Research was purchased. The Scorpion camera uses the Symagery VCA 1281 CMOS chip.
For the Micro-CPI instrument proposed in Phase II, the Scorpion camera will not be used, but
the CMOS chip will be integrated into a custom designed circuit board. The imaging laser used
in the prototype system is a CW diode laser operating at 680 nm. The laboratory system was
used to capture images of glass beads and reticles on a slide. Since the objects are stationary,
a pulsed laser is not required to freeze the motion of the objects.


Figure 4. Photograph of prototype Micro-CPI imaging system using a Scor