Remote Sensing Using an Unmanned Aerial Vehicle

Aircraft Systems
Mission
The mission of the UC Davis CalSpace UAV project is to develop and implement a modular
remote sensing platform able to be deployed on a variety of vehicles. Sensors to be incorporated
include a multi-spectral RGB/NIR camera, CO2-detector, temperature, humidity, and a
spectrometer. The sensors are controlled by a small computer running Linux and linked to a
ground-based computer via an 802.11 network infrastructure. The imagery obtained may be
automatically georeferenced by means of a GPS receiver and three-axis compass and tilt-meter
module. The test-bed for the platform is an off-the-shelf radio control (RC) aircraft, the Kadet
Senior, powered by an electric motor and lithium-polymer batteries with an endurance of 20minutes. A new custom-designed aircraft is currently being built by the UC Davis Mechanical and
Aeronautical Engineering Department to carry the CalSpace sensor platform on longer missions
of 2-3 hours. The applications of the UAV-based remote sensing platform range from atmospheric
research to monitoring plant vigor.
Remote Sensing Using an Unmanned Aerial Vehicle
Matthias Falk, Quinn J. Hart, Keith S. Bowen, Justin R. Merz and Susan L. Ustin,
CALSPACE, University of California, Davis; [email protected]
Aircraft Performance Parameters
The Kadet Senior RC Aircraft was selected for its stability and large wing area. It was modified
with stronger landing gear and larger wheels to allow for operation in rough terrain as well as
relocating the RC equipment to allow for the sensors and computer. The fully loaded weight of
the modified Kadet is approximately 14-lbs, approximately twice that of the standard model. An
exhaustive aerodynamic performance review was undertaken to predict the flight characteristics
of the modified Kadet and aid in selecting a suitable powerplant. Flight-testing proved to
validate the ability of the Kadet to lift the 4-pound payload safely. The fully loaded Kadet is able
to take off from pavement or gravel in just over 50-feet with zero headwind. Once airborne, the
plane is manually flown over the area of interest taking pictures and relaying position reports to
the ground station. Each time an image is captured, the position and attitude of the plane are
used to calculate the coverage of the image, and a corresponding polygon is painted over a map
on the ground station laptop. This allows the operator to see when full coverage has been
acquired, as well as staying within the limits of the area of interest.
Initially, a 1.6 hp, .91-in3 four-stroke, glow-plug-type internal engine was selected for the higher
energy density possible with gasoline. However, vibrations and low efficiencies led to the
selection of a geared, brushless electric motor. When powered by a 6-cell series, 2-cell parallel
(3S2P) lithium-polymer battery in series, the MaxCim N32-13Y provides approximately 600 W
to the propeller shaft. A 3.53:1 gear-reduction ratio allows the motor to operate near the most
efficient rpm, spinning the propeller at about 6-7000 rpm. A 15X10 propeller was selected as a
compromise between maximizing efficiency at take-off and at the projected cruise speed of 30
mph (13.4 m/s). Flight stabilization is accomplished by an FMA Copilot device that uses infrared
transducers to reduce pitch and roll by minimizing the temperature differences seen on all four
sides of the aircraft. When the plane rolls to the left, the right-facing transducer begins to look
skyward where IR temperatures are relatively lower. Likewise, the left-facing transducer begins
to see more ground than sky, effectively raising its temperature indication. This flight
stabilization effectively counters the effects of turbulence, keeping the camera pointing straight
down. The Copilot receiver also includes a failsafe feature that allows the user to program
failsafe settings for all of the servos. In the event of a loss of signal, the Copilot directs all of the
servos to their preset location allowing for a predicable controlled descent, such as a low-power,
downward spiral.
A new, state-of-the-art UAV is currently under construction by the UC Davis
Mechanical and Aeronautical Engineering department. The composite aircraft
will have a 4-meter wing span, and will be capable of carrying a 4-5 kg payload
on missions up to 2-3 hours using an autopilot.
Computer System
The onboard computer system consist of a single board communications computer (WRAP2C, PCEngines, Switzerland) equipped with wireless radio Mini-PCI card (CM9, Wistron
Neweb, Taiwan) and a 4-port serial card (Aeon, Taiwan). All onboard sensors are connected
to this computer and a wireless link is kept with a base station at all times during the flight.
Total cost of the integrated flight computer system is less than $200
Other Tests
Very high resolution imagery coupled with ease of repeat schedules, allows for many
applications of precision agriculture, with monitoring possible for individual plants. The
right-hand image from the UC Davis Campbell Field tract, shows rows of a tomato crop on
2006/07/26. The flight path in Google Earth is shown in the left hand picture.
Increased repeatability
The fact that the UAV can be deployed rapidly and with a minimum amount of effort means that
many different temporal studies can be carried out. Changes within or between seasons can be
identified. Study sites can be flown with monthly or even weekly repeat cycles.
Storer Garden
In addition, the mosaic can be used to update Arboretum plant collection maps,
with detailed plant type and location information. For example, the Ruth Risdon
Storer Garden includes perennials and shrubs well suited central valley gardens
and designed for year round color with low maintenance and water use.
The insets above show the Shields Oak Grove in late winter, providing an interesting contrast to the
large scene taking the previous summer. Summer drought limits grass growth while the oak trees
remain green throughout the summer. In winter the grass is green while the deciduous trees are
leafless.
Sensors
Telemetry
Aircraft flight telemetry is relayed to the ground over an 900 MHz
wireless telemetry system. This telemetry includes GPS, airspeed,
barometric altitude, motor rpm, motor battery pack voltage and
amperage, servo positions, receiver battery voltage, temperature, and
g-forces.
Additional telemetry including GPS and UAV velocity and
orientation is relayed on the 802.11G wireless network.
When combined with a tool like Google Earth (shown above), these
systems provide heads up display for piloting the UAV and controlling
the instrumentation.
Image data were collected using two compact digital cameras. One camera (NIKON Coolpix
4300, 4.3 Mpixel, focal length 8 mm, FOV = 49o) collected RGB imagery. The other camera
(NIKON Coolpix 990, 3.3 Mpixel, focal length 8 mm, FOV = 49o) was modified to be
sensitive in the NIR by replacing the original internal hot mirror with a clear glass (Edwards
Optics, USA). Thus both cameras flown simultaneously give a four channel image with red
,green ,blue and NIR channels and the ability to calculate simple vegetation indices such as the
NDVI while keeping the cost very low. The latest images were taken with a Canon Rebel XT
DSLR camera (8.1 Mpixel, focal length 50.0mm (35mm equivalent: 81mm), FOV =46o). All
cameras were controlled by the onboard flight computer.
Flight coordinates, plane orientation and movement were recorded with a serial GPS unit
(Trimble Inc., USA) and an electronic compass with built in x-y accelerometer (TCM-3, PNI,
Sonoma, USA). All data were recorded by the flight computer. PNI generously donated the
TCM-3 module for this research project.
Alternative instrumentation available includes an atmospheric measurement subsystem
recording pressure, temperature, relative humidity and CO2 concentrations using a lightweight
InfraRed Gas Analyzer (SBA-1, PP-Systems, USA).
SHIELDS OAK GROVE
The Shields Oak Grove, the largest Oak grove in the southwest
United States, is part of the UC Davis Arboretum, with more
then 346 trees in 89 species. This image from May 2006
covers approximately 9.5 hectares, at 5 cm resolution, resulting
in an image of about 6200x5200 pixels. 18 images were
combined to form this mosaic.
NIR Imagery
This sub-scene shows a processed false-color infrared image taken by the
modified NIKON Coolpix 990.
Rookery
In the last 5 years, a rookery including 4 species of Herons and Egrets, totaling
more than 1000 nesting and roosting birds has developed in the Grove. This
mosaic helps assess any damage to the Oak grove with high resolution RGB and
CIR imagery. To support image processing applications. Preliminary results
indicate some foliage loss in the most dense areas of the Rookery. In this image,
individual birds can easily be identified.
Working with the Sonoma Ecology Center, a survey mission was flown in August, 2006
over the Bouverie Preserve near Glen Ellen, California. The stewards of the preserve
were interested in documenting invasive plant species. The goal of the survey was to
provide a high-resolution, geo-referenced mosaic of the region of interest in order to
obtain a baseline datum and to map out some existing field test plots, (shown as the
rectangular vegetative areas in the right-hand image above).
Future missions will characterize the effectiveness of the various treatments applied to
specific areas, such as selectively grazing cattle, and manual weeding.
The projected (black) and actual (red) flight path shown on the left in Google Earth over
the Bouverie Preserve in Sonoma, CA.