AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-1845-2016The Pilatus unmanned aircraft system for lower atmospheric researchde BoerGijsgijs.deboer@colorado.eduPaloScottArgrowBrianLoDolceGabrielMackJamesGaoRu-ShanTelgHagenTrusselCameronFrommJoshuaLongCharles N.BlandGeoffMaslanikJamesSchmidBeatHockTerryUniversity of Colorado, Boulder, Colorado, USANational Oceanographic and Atmospheric Administration, Earth System Research Laboratory, Boulder, Colorado, USANational Aeronautics and Space Administration, Wallops Flight Facility, Wallops Island, Virginia, USAPacific Northwest National Laboratory, Richland, Washington, USANational Center for Atmospheric Research, Boulder, Colorado, USAGijs de Boer (gijs.deboer@colorado.edu)28April2016941845185713October201518November20159February201627March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/1845/2016/amt-9-1845-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/1845/2016/amt-9-1845-2016.pdf
This paper presents details of the University of Colorado (CU) “Pilatus”
unmanned research aircraft, assembled to provide measurements of aerosols,
radiation and thermodynamics in the lower troposphere. This aircraft has a
wingspan of 3.2 m and a maximum take-off weight of 25 kg, and it is powered
by an electric motor to reduce engine exhaust and concerns about carburetor
icing. It carries instrumentation to make measurements of broadband up- and
downwelling shortwave and longwave radiation, aerosol particle size
distribution, atmospheric temperature, relative humidity and pressure and to
collect video of flights for subsequent analysis of atmospheric conditions
during flight. In order to make the shortwave radiation measurements, care
was taken to carefully position a high-quality compact inertial measurement
unit (IMU) and characterize the attitude of the aircraft and its orientation
to the upward-looking radiation sensor. Using measurements from both of these
sensors, a correction is applied to the raw radiometer measurements to
correct for aircraft attitude and sensor tilt relative to the sun. The data
acquisition system was designed from scratch based on a set of key driving
requirements to accommodate the variety of sensors deployed. Initial test
flights completed in Colorado provide promising results with measurements
from the radiation sensors agreeing with those from a nearby surface site.
Additionally, estimates of surface albedo from onboard sensors were
consistent with local surface conditions, including melting snow and bright
runway surface. Aerosol size distributions collected are internally
consistent and have previously been shown to agree well with larger,
surface-based instrumentation. Finally the atmospheric state measurements
evolve as expected, with the near-surface atmosphere warming over time as the
day goes on, and the atmospheric relative humidity decreasing with increased
temperature. No directional bias on measured temperature, as might be
expected due to uneven heating of the sensor housing over the course of a
racetrack pattern, was detected. The results from these flights indicate that
the CU Pilatus platform is capable of performing research-grade lower
tropospheric measurement missions.
Introduction
The use of unmanned aircraft systems (UAS) for Earth science
missions has become increasingly popular over the last two decades. Interest
in such deployments stems from the ability of these platforms to collect
information on spatial variability of key atmospheric properties and the
underlying surface, and provide profiles of atmospheric quantities related to
aerosols e.g.,, clouds
e.g.,, thermodynamics e.g.,,
turbulence e.g.,, and radiation
e.g.,. Additionally, their use has been
buoyed by the potential to deploy these aircraft to areas difficult to sample
with manned platforms e.g.,, including the
near surface environment at high latitudes
e.g.,, and by the potential for
significant cost-savings relative to routine deployment of manned aircraft
with continued miniaturization of instrumentation and platforms alike.
Programmatic interest in the deployment of UAS developed approximately two
decades ago, with the National Aeronautics and Space Administration (NASA),
Office of Naval Research (ONR) and US Department of Energy (DOE) establishing
UAS-based research programs . These programs generally
focused on larger, expensive platforms such as the General Atomics Altus and
General Atomics Gnat-750. At present, while successful deployments of larger
High-Altitude, Long Endurance (HALE) UAS continue
e.g.,, there has been expanded focus
on the development and deployment of smaller, low-cost systems. This focus
has been fueled in part due to the attainability of such systems for the
university research community, as well as by the continued development of
regulations by the US Federal Aviation Administration (FAA) and regulating
agencies of other countries for small UAS (generally 55 lbs and below).
Some examples of such efforts include research flights to investigate lower
atmospheric structure in the vicinity of supercell thunderstorms
, and campaigns to understand lower
tropospheric thermodynamics and turbulence
.
One area of particular interest for UAS-based research is measurement of
atmospheric aerosol particles. At high latitudes, where substantial
atmospheric stratification is routinely observed , and
long-range transport of particles is central in establishing the local Arctic
aerosol population e.g.,, measurement of
aerosols at the Earth's surface is a critical but insufficient endeavor. In
such situations, there is no guaranteed relationship between aerosols
observed at the surface and those in the atmosphere above relevant for
regulating atmospheric radiative transfer and development of cloud particles.
Recent years have seen limited campaigns with manned aircraft (e.g., ISDAC,
; ARCPAC, ; ARCTAS,
) to better understand the vertical and horizontal
variability of aerosol particles. While such campaigns can provide
substantial insight and have the unique ability to deploy a variety of
instruments to the same location, the cost of such efforts is unsustainable
for routine observing. UAS can play a central role in decreasing the cost
associated with making aerosol measurements at altitude in the high latitude
atmosphere, and to date there have been limited UAS-based measurement
campaigns e.g.,. Of
additional interest is the impact of the aerosol and associated cloud
particles on the transfer of energy through the Earth's atmosphere. While
measurements of irradiance are commonly made at the Earth's surface, such
measurements generally only provide the integrated point of view representing
the entire column, and do not provide information on specific layers of
aerosol or cloud particles and their local radiative impact. Such information
can provide critical insight necessary to reduce uncertainty associated with
the radiative forcing of aerosol particles and clouds
. Again, while such measurements have been made using
manned aircraft platforms, measurements of atmospheric radiation from UAS
have been very limited e.g.,, and
to date have generally focused on downward-looking multispectral cameras to
evaluate surface properties.
In this paper, we describe the development and initial testing of the
University of Colorado (CU) Research and Engineering Center for Unmanned
Vehicles (RECUV) “Pilatus” aircraft. Development of this platform was funded as part of
the ERASMUS (Evaluation of Routine Atmospheric Sounding Measurements using
Unmanned Systems) campaign, supported by the US Department of Energy (DOE)
Atmospheric System Research (ASR) and Atmospheric Radiation Measurement (ARM)
programs. Instrumentation for this platform was selected in order to provide
critically needed information to understand the vertical stratification of
aerosol particles and their radiative impact at high latitudes. At the same
time, care was taken to make the best quality measurements possible in order
to support ARM's history in providing high-quality data sets. We first provide
an overview of the platform, including background on the airframe and flight
control systems, followed by details on the instrumentation payload.
Subsequent sections provide details on initial results from testing and
characterization flights carried out in Colorado, as well as a glimpse into
the future deployment of this aircraft to the Arctic environment.
The Pilatus UASAirframe and avionics
The RECUV Pilatus was developed from the airframe of the 3.2 m Pilatus
Turbo Porter almost-ready-to-fly (ARF) kit distributed by Topmodel S.A.S.
The Pilatus was chosen for this project due to its established structural
integrity, low-speed handling characteristics, and ability to carry
significant payloads while still maintaining a total weight of under the 55 lb weight limit established by the Federal Aviation Administration (FAA) for
small UAS. This aircraft is also known for its short-takeoff-and-landing
(STOL) performance, making it a good candidate for atmospheric research
activities where extended runway surfaces are not always available. From a
size and performance perspective, this aircraft is in a similar class as the
ALADINA aircraft developed at the Technische Universität Braunschweig
, although the measurement targets of the two
systems are somewhat different and the Pilatus is capable of carrying a
slightly heavier payload.
Because the primary mission for the ERASMUS campaign involves operation in
the Arctic environment, a decision was made to replace the original 8.5
horsepower air cooled, aspirated 2-stroke gasoline engine with an electric
propulsion system. This was primarily done due to fears of carburetor icing
in the cold Arctic environment. The electric system that was chosen includes
a brushless electric motor, powered by a set of four six-cell (22.2 V)
10 000 milliAmp hour (mAh) lithium polymer (LiPo) batteries. Unfortunately, this
change in the propulsion system does have substantial impact on aircraft
endurance, with flight times in the new configuration limited to 25–40 min, depending on payload. With continued development in battery
technologies, it seems likely that this endurance will climb steadily in the
coming years.
The aircraft is guided by the Piccolo SL autopilot and ground station from
Cloud Cap Technology, which is widely used by UAS operators. The ground
station's graphical interface communicates with the aircraft via a 900 MHz
spread-spectrum data link. This interface allows an operator to control
flight parameters of the Pilatus remotely, including setting of speed,
altitude and ground track. Waypoints are used to establish the aircraft's
course and flight plans can be set ahead of time and can also be modified
in-flight. The aircraft is also set up to be flown manually by an operator
using a hand-held controller, and flight operations have generally called for
manual take-offs and landings while allowing for the Piccolo autopilot to
handle the remainder of the established flight pattern.
Additional modifications made to the aircraft include replacement of the
landing gear springs in order to handle the impact resulting from increased
landing weight. The interior structure of the original aircraft was modified
to include a plywood subfloor and the original tires were replaced with
larger “tundra tires” for ease of operation from a variety of runway types.
In its current configuration, the Pilatus generally cruises at approximately
92 km h-1 (50 knots), has a stall speed of approximately 52 km h-1 (28 knots), and has a dash speed of approximately 148 km h-1
(80 knots). When carrying payload it has a maximum climb rate of
approximately 2.5 m s-1 and a turn rate of ∼ 30∘ s-1,
resulting in a 91 m minimum turn radius.
Scientific payload
To align with the ERASMUS campaign as well as DOE ASR and ARM programmatic scientific and
measurement objectives , the Pilatus was outfitted with
instrumentation that can provide information on atmospheric thermodynamic
state (temperature, humidity, pressure), broadband radiation (both shortwave
and longwave) and aerosol concentration and size. The following paragraphs
provide descriptions of the sensors used on this platform.
The RECUV Pilatus UAS shown with PTH module (white pod on starboard
wing), POPS aerosol spectrometer (gold box in windshield), and three SPN1
pyranometers (two sensors on roof and one sensor on belly). The inset shows
the aircraft with the upward-looking CGR4 mounted.
Atmospheric state
To measure atmospheric thermodynamic state, a specially designed pressure,
temperature and humidity (PTH) sensor suite, mounted to the underside of the
aircraft wing (Fig. ) was employed. This PTH sensor
module (Vaisala RSS904) is based on the sensor portion of the National Center
for Atmospheric Research (NCAR) miniature dropsondes. This module is nearly
identical to those used in the Vaisala RS-92 radiosondes used widely in the
global radiosonde network in order to derive regular balloon-based
thermodynamic profiles, with the exception of the temperature sensor which is
larger and more mechanically robust than the RSS904 version. It features a
capacitive wire temperature sensor with a 0.1 C resolution, a thin-film
capacitor humidity sensor with a resolution of 1 %, and a silicon pressure
sensor with a measurement resolution of 0.1 hPa.
Broadband radiation
To measure broadband shortwave (400–2700 nm) irradiance, the Pilatus was
configured to carry three Delta-T Devices Ltd. SPN1 sunshine pyranometers
(Fig. , top and bottom of aircraft). Of these, both a
standard and modified version of this sensor look up towards the sky, and a
single modified version looks down towards the ground. The standard SPN1 is
unique in that it uses a shading pattern in combination with seven thermopile
sensors. This shading pattern ensures that one of the seven sensors is always
shaded, meaning that it is only subject to diffuse irradiance from the sun,
and that another of the seven sensors is fully exposed to any direct solar
radiation. This allows the device to separate the contributions of the
incoming shortwave irradiance into cosine-corrected direct and diffuse
components, which is critical for correction of the measurement for aircraft
motion (see following paragraph). The modified SPN1s flown remove the shading
pattern and have the internal programming changed to output the central
detector as one output, and the average of the remaining six surrounding
detectors as the other output. In this way, two separate measurements of
total shortwave irradiance are obtained. Also important is the fast response
time of this sensor (100 m s-1). Because the aircraft will potentially be flying
around broken clouds, being able to quickly resolve transitions in the
measured irradiance is important. The SPN1 is equipped with a heater to
prevent condensate formation on the dome. However, in order to reduce power
consumption and because we are not planning to operate the aircraft in
high-humidity environments, we decided to forgo use of the heater in the
Pilatus installation. Without the heaters, the SPN1 requires a power supply
of 2 mA at 5–15 V. Each of these sensors has a 126 mm diameter and weighs
786 g.
Downwelling shortwave measurements, such as those provided by the SPN1, are
very sensitive to aircraft attitude (pitch, roll) due to changes in the
orientation of the sensor relative to the sun. provide a
technique for correcting for this potential source of error for a combined
angular offset from level of up to 10∘. In order to follow their
approach, it is necessary to be able to distinguish between direct and
diffuse contributions to the irradiance, which the SPN1 allows, as discussed
above. Additionally, it is necessary to have high-precision information on
sensor attitude relative to level. This information was obtained using a
VectorNav VN-200 high precision inertial navigation system (INS). The
VectorNav combines a three-axis accelerometer, a three-axis gyroscope, a
three-axis magnetometer, a barometric pressure sensor and a high-sensitivity
GPS receiver in a small and lightweight housing to provide detailed
information on aircraft attitude. In order to ensure minimal offset between
the position of the VectorNav and the upward-facing SPN1s, the VectorNav was
mounted to the bottom of the plate used to mount the SPN1s to the fuselage.
Although the standard (with shading pattern) SPN1 allows us to partition
between direct and diffuse downwelling total solar irradiance as necessary
for correcting for aircraft attitude, the shading pattern used to block half
of the sky view increases overall measurement uncertainty. To reduce this
uncertainty, we additionally employ a modified (no shading pattern)
upward-looking SPN-1, providing hemispheric total solar measurements. While a
similar SPN1 configuration has previously been installed and flown on manned
research aircraft , to our knowledge, this is the first
application of this sensor to an unmanned research aircraft of any size.
For measuring broadband longwave (4500–42 000 nm) irradiance, up- and downward-facing Kipp and Zonen CGR4 pyrgeometers were integrated into the aircraft
system. The CGR4 is among the best pyrgeometers available commercially and is
among those used in the World Meteorological Organization (WMO)'s Baseline
Surface Radiation Network (BSRN). The CGR4 uses a silicon meniscus dome which
provides a 180∘ field of view. Additionally, the design of the CGR4
reduces dome heating due to absorption of solar radiation to a negligible
level when ventilated eliminating the need for dome temperature measurements
or dome shading. Because the CGR4 has a very low output signal (-1.5 to 0 mV), the instrument is paired with a Kipp and Zonen AMPBOX amplifier in order
to convert this into a more reliably readable 4–20 mA current loop signal.
Each CGR4, with the shading dome, has an exterior diameter of 150 mm, and a
weight of 600 g.
Aerosol size distribution
To characterize aerosol size distribution, the Printed Optical Particle
Spectrometer POPS, designed, engineered and constructed
at the National Oceanographic and Atmospheric Administration (NOAA) Earth
System Research Laboratory (ESRL) Chemical Sciences Division (CSD), was
integrated into the aircraft. This lightweight, low-cost sensor is
constructed using 3-D printing technology and provides aerosol concentrations
and particle size distributions for particles between 140 and 3000 nm. Particles
are sized on an individual basis to provide a continuous size distribution. A
compact data system features a custom electronic design including a single
board computer. The sensor and electronics consume 7 W of power at 9–15 V,
allowing for extended operation on a relatively small battery system. POPS
components combine for a total weight of approximately 800 g. The inlet for
POPS is located on the wing in an isoaxial configuration, but the flow is not
isokinetic, as POPS draws air at a rate of 3 cm3 s-1 using a small
pump. The tubing between the inlet and the sensor is constructed mainly of
stainless steel tubing with some smaller section of conductive silicone. The
tubing has an inner diameter of 0.00159 mm (1/16 inch), with an overall
length of 1.65 m. The inlet tubing does have four bends, three of which are
approximately 90∘, and one is approximately 180∘. The large
difference between the aircraft cruise speed and inlet airspeed results in
some oversampling of larger particles as shown by Fig. ,
based on .
Data acquisition and video camera
To command and collect information from the various payload components, a
custom command and data handling/signal conditioning (C&DH) board was
designed. This board consists of various components, including a main board
borrowed from the autopilot of the DataHawk UAS
consisting of a microcontroller with support components and a Micro-SD card
for data storage. Additionally, the C&DH board includes an IMU with a three-axis
gyroscope, a three-axis accelerometer, a three-axis magnetometer and a barometric
pressure sensor, an XBee 900 MHz radio for real-time telemetry, and signal
conditioning circuitry for the analog components. With this configuration,
there were some key design requirements, including achieving the highest
precision and accuracy possible from the CGR4s and SPN1s, integration of a
high-quality attitude measurement from the VectorNav VN-200 IMU, and
efficient routing of power from the three-cell lithium polymer payload battery,
which is separate from the larger propulsion and avionics batteries. Software
was designed to provide two forms of payload telemetry. The primary mode of
telemetry is a log file generated and stored on the onboard Micro SD card.
Main payload packets are generated at 25 Hz, though individual instruments
may not report at this frequency. In addition to the main data packet, a 5 Hz
GPS packet is also generated. In addition to the SD card logging, a 1 Hz
real-time telemetry stream is generated via the XBee radio, containing only
PTH data in ASCII text format.
A central design requirement for the C&DH board was minimization of
electronic noise on the analog sensor (SPN1 and CGR4) outputs. As discussed
above, we integrated a Kipp and Zonen AMPBOX with the CGR4s. In order to
minimize the potential for noise pickup and generation, this AMPBOX was
mounted directly on the bottom of the CGR4 housing to minimize the length of
the cable carrying the low-voltage signal from the sensor to the amplifier.
In general, all voltages were amplified as early as possible in order to
match the range of the analog-to-digital converter. Additionally, the data
system is powered by a dedicated battery in order to separate the electronics
from those associated with the avionics and motor, and we used linear power
supply regulators and decoupling capacitors on all circuit power lines. The
circuit board and cables were designed using best practices, separating
analog and digital circuits to minimize noise coupling. Finally, all cables
used were shielded and extra care was taken to avoid ground loops. With this,
the estimated electronic noise levels for the analog sensors used are 0.15∘ for the CGR4 temperature reading, 3 W m-2 for CGR4 irradiance,
and 1.5 W m-2 for SPN1 irradiance.
Finally, in order to document the flight environment a FatShark PilotHD V2
video camera capable of recording 720 p video at 30 frames per second (fps) to
an integrated SD card logger. This camera is equipped with a 1/2.5 inch 5
megapixel imager and features a metal-cased shell for protection and
minimization of radio frequency interference with aircraft controls. The
weight of the camera system is approximately 33 g.
An overview of the three different Pilatus payload
configurations.
NameInstrumentsPayloadweightCorePTH, Camera, POPS2 kgShortwaveCore, plus SPN1s, VectorNav4.3 kgLongwaveCore, plus CGR-4s3.3 kgPayload configurations
Unfortunately, due to the weight of the radiation instrumentation, not all of
the instruments listed above can fly simultaneously. Therefore, we have
configured our data logging and electronics systems and the distribution of
sensors on the aircraft in order to allow for easy swapping of three payload
configurations (Table ). The first configuration includes
POPS and the PTH module only and allows for the use of two extra 10000 mAh
propulsion system batteries, extending flight duration to approximately 40 min with a combined instrument payload mass of approximately 2 kg. This
configuration is ideal if an extended range of operation is desired, or if
aerosol profiles to higher altitudes (> 750 m) are desired. The second
configuration carries the PTH module and POPS, as well as upward and downward
looking CGR-4s with a combined instrument payload mass of approximately 3.3 kg. Using this configuration, flight time is restricted
to approximately 25 min, depending on the mission flown. The third, and heaviest
configuration includes the PTH module and POPS in combination with the three
SPN1s and the VectorNav INS, resulting in a combined instrument payload mass
of 4.3 kg. In order to make the instrument swaps as easy as possible, the
CGR4 and SPN1 instruments were fixed to separate mounting plates which had
uniform mounting points for attachments to the airframe. To ensure that the
upward-looking radiometric instrumentation is as level as possible during
flight, the roof-mounted plate is placed upon a shim which angles those
instruments at approximately eight degrees relative to the roofline (see
Fig. inset) in order to set them as close to level as
possible during flight.
The SPN1 configuration mounted on top of a team member's vehicle for
obtaining measurements required to calculate the relative offset between the
VectorNav and upward-looking SPN1s (top). The elevation variability of the
route driven for offset characterization purposes (black line), compared with
pitch (magenta) and roll (red) measured during one transit of this route. The
instrumentation was turned 90∘ (yaw) between laps in order to ensure
that the hill structure provided adequate variation in both pitch and roll
for offset characterization.
Characterization of IMU offset
Options for operation of unmanned aircraft in US airspace for government
operators, including the university research community, are limited. Testing
and evaluation of Pilatus equipment was completed under COA 2013-WSA-26,
allowing for operation of the Pilatus by University of Colorado operators at
the Arvada (Colorado, USA) Field at or below 122 m (400 ft) above ground
level. In order to correct the measured SPN1 values for orientation as
outlined in , it is necessary to determine the angular
offsets between what is deemed to be level by the VectorNav IMU and the
actual level state of SPN1 detectors. While the two are mounted to a common
plate, small differences in the mounting or manufacture of the sensors can
result in an offset in level states. To characterize this, it is necessary to
collect a data set containing various permutations of pitch, roll, and heading
plus changes in latitude and longitude. Because the VectorNav has to be
moving to get accurate measurements of heading, it is necessary to complete
the characterization of it's orientation offset relative to the SPN-1s using
a moving platform. However, the limited spatial domain available for flight
under the COA is not supportive of the execution of extended legs with nearly
“level” (limited pitch and roll) flight, as would be preferable for radiation
measurements and offset characterization. Therefore, we instead implemented a
car-based solution for characterization of the VectorNav-SPN1 offset with a
roof-mounted system (Fig. , top).
In order to obtain the measurements required to characterize the offset
between the VectorNav and the SPN1 sensors, it is required to vary the
orientation of this platform over a range of pitch and roll angles under a
variety of solar zenith angles. Using this ground-based approach, this
requires the execution of a series of rectangular patterns driven on a
cloud-free day over terrain with rolling hills from sunrise until around
solar noon (Fig. , middle, bottom). To ensure variability
in both pitch and roll, the sensor plate was turned 90∘ in orientation
(yaw) between each executed run. In total, 14 circuits were completed on
public roads in northwestern Boulder, Colorado, between 07:20 and 12:20 local
time, with approximately 20 min in between the start of each circuit.
Using the results from these patterns, we applied the technique outlined in
to characterize the offset between the VectorNav and the
SPN1s and to allow for the correction of SPN1 measurements for deviations
from level of up to 10∘. These offsets were found to be very small
(0.4, 0.4, and 2.4∘, for pitch, roll, and yaw, respectively), which is
not surprising considering the VectorNav and SPN1s are co-mounted on a single
plate.
Downwelling broadband shortwave irradiance (W m-2) obtained
using the car-top mounted SPN1s during offset characterization runs. The red
line represents the uncorrected values, while the green line represents the
measurement after attitude correction has been applied. The black line
represents the theoretical clear-sky value for the time and location of the
measurements. The inset shows the corrected and uncorrected values for one
loop around the circuit (shown by black box).
Figure shows the raw (red) and corrected (green) downwelling
broadband shortwave irradiance measurements from the car-top SPN1 runs. The
raw measurements show the effect of small pitch and roll variations on the
measured irradiance, with spikes in the data of up to nearly 100 W m-2
for tilt from horizontal of only up to 7∘. After the correction is
applied, these are efficiently corrected, providing an accurate
representation of the clear-sky irradiance at this time. The reductions in
the measured irradiance visible in both the raw and corrected data are the
result of shadows from trees and structures along the route of travel. The
gradual increase of the measured irradiance with time is the result of the
increasing elevation of the sun in the sky moving from early morning to the
middle of the day.
Flight testing and airborne data
In-flight testing of the aircraft and integrated instrumentation was
completed at Arvada Field under the COA mentioned above during
February–April 2015. Here, we provide an overview of results from a series
of instrumented preparation flights completed on 3 April 2015. These flights
were completed between 11:30 and 15:00 local time, under partly cloudy
conditions and relatively light winds. Synoptically, a weak area of low
pressure moved through the Front Range of Colorado on the evening of April
2nd, resulting in a few inches of snowfall. This snow was covering much of
the ground surface early on, but by the time the first flight began, the
runway was already clear of snow, and the snow cover on the surrounding fields
was spotty. The snow would continue to melt throughout the day, as broken
clouds and sun resulted in fairly rapid heating of the lower atmosphere. In
total, four flights were carried out at an altitude of 100 m, with the
aircraft executing a counterclockwise racetrack pattern under autopilot
guidance. These flights ranged from 20 to nearly 24 min in duration. The
first and fourth flights carried the “shortwave” payload (see Table ), while the second and third flights carried the
“longwave” payload.
Thermodynamic profiles from the four flights are presented in Fig. . It is important to note that these flight patterns were not set
up specifically for profiling, and therefore the ascent and descent rates
were not uniform through the depth of the column. The profiles presented
represent binned distributions covering both the ascending and descending
portions of flight. Figure (top) shows potential temperature
profiles, which depict a lower-atmospheric column that is slowly warming up
as a result of solar heating of the surface. These profiles represent binned
distributions at 5 m resolution, with the mean at each height illustrated by
the filled circles. The thin lines represent the interquartile range at each
altitude, providing insight into the variability at a height. It should be
noted that very little time was spent at intermediate altitudes, and the
majority of each flight was conducted at the cruise altitude (near 90–100 m
in flights 1 and 3 and 80–90 m in flights 2 and 4). This, in combination with
some lag inherent to the sensor response time results in values over the
lowest portion of the atmosphere that appear superadiabatic. The rapid
transit through the area closest to the surface is also the primary driver
for the apparent increase in variability (larger IQR spread) at lower
altitudes. Beginning at around 40 m, the profiles represent a well-mixed
atmosphere, as may be expected on a relatively sunny and warm day with some
wind. Since relative humidity is a function of temperature, over a short
amount of time and without significant advection of water vapor, increasing
temperatures resulting from solar radiation will tend to decrease relative
humidity values. Profiles of relative humidity from the PTH module (Fig. , middle) appear to illustrate this phenomenon, with relative
humidity values decreasing throughout the day as boundary-layer temperatures
increase. There does appear to be a thin layer of elevated moisture levels
near the surface, potentially the result of melting snow and the associated
evaporation of surface water into the relatively dry atmosphere. Atmospheric
pressure drops slightly during the third and fourth flights, with pressures
from the first two flights being nearly identical (Fig. ,
bottom).
Sampling efficiency of POPS on the Pilatus.
One question that we attempt to answer with the test data collected is
whether sensor orientation influences the temperature observed with the PTH module
mounted on the wing. Figure provides distributions of
the difference between the measured temperature above 60 m in altitude
(GPS) and the mean temperature at this elevation, binned by aircraft heading.
The distributions include a mean value (symbol), the interquartile range
(thick line) and the 10th–90th percentiles (thin lines), with positive values
indicating that measurements from that heading were warmer than the mean. The
yellow bars represent the range in solar azimuth angles covered during that
specific flight to provide information on how the sensor is oriented with
respect to the sun. With the sensor mounted on the starboard wing, this
results in the shading of the nose of the sensor housing when the aircraft is
flying away from the sun and across the sun towards the west. While both
flights 3 and 4 appear to demonstrate warming when the starboard wing is
oriented toward the sun (directions less than the solar azimuth angle), such
warming is less apparent in the first two flights. There does not appear to
be a systematic bias based on heading relative to the sun from these flights,
with the caveat that the aircraft is only maintaining any given heading for a
maximum of 20–30 s at a time. This seems to support design of the PTH
sensor housing to both reduce direct airflow (and thereby convective cooling)
as well as direct heating through absorption of solar radiation.
Potential temperature (top), relative humidity (middle) and
atmospheric pressure (bottom) from the four 3 April flights. The circles
represent the mean value at each altitude, while the lines represent the
inter-quartile range.
Distributions of temperature anomalies from the mean of all points
above 60 m for each flight as a function of aircraft heading. The mean of the
distribution is presented as a closed circle, the interquartile range is
presented as the thick line, and the 10th–90th percentile range is presented
as the thin line. The yellow bars represent the range of solar azimuth angles
for each flight.
In addition to the PTH data, we also present measurements from flights
completed with the Kipp and Zonen CGR4 (broadband longwave) and Delta-T SPN1
(broadband shortwave) sensors. As mentioned above, the flights 1 and 4 were
completed with the SPN1s, while flights 2 and 3 were completed with the CGR4s
installed. In general, the scene for these flights is rather complex from a
radiation perspective. The surface included both dark (earth) and bright
(snow) covered areas, and the sky featured broken fair-weather cumulus
clouds. To add to the complexity, there is some terrain around the flight
test site that can enter the field of view of the sensors, and substantial
terrain (front range of the Rocky Mountains) approximately 10–20 miles away.
Given the limited amount of area available for flight, we were also required
to fly a relatively tight racetrack pattern, resulting in a substantial
fraction of non-level (i.e., high pitch/roll angles) flight.
As visible in Figs. and , the factors
discussed in the previous paragraph result in substantial variability in both
the short- and longwave radiation measured during these flights. Both
data sets show regular periodic variability as a result of aircraft motion.
While the downwelling shortwave signal is corrected for tilt effects, the
complex cloud cover scene and surrounding terrain result in real variability
with tilt that is not directly connected to the solar position relative to
the sensor.
Broadband longwave radiation measured during the time of testing on
3 April 2015. Included are upwelling (red), downwelling (blue) and net
(black) radiation from the aircraft (thin, shorter line segments with high
variability) and 1 min averages from the NREL site at Table Mountain
(thicker lines). An inset is included to provide additional detail on the
downwelling radiation measured from the aircraft and at Table Mountain for
the third Pilatus flight of the day.
Looking first at the longwave irradiance (Fig. ), variability
in the signal is largely the result of the wide field of view of the sensor.
While this wide field of view is desirable for surface-based operations in
order to ensure that contributions from the entire hemispheric atmosphere are
represented, unfortunately for aircraft-based operations, each change in
flight heading results in an instrument reading from a non-level
configuration, which results in measurements that represent a combination of
sky and surface radiation. The limited flight area, in combination with the
relatively slow response time of the CGR-4 (18 s at 95 % response, 6 s at 63 % response), results in a periodic oscillation in both the
down- and upwelling longwave radiation measured during these flights. In
order to gain insight into the accuracy of the aircraft-based measurement, we
compare the Pilatus measurements to 1 min averaged irradiances obtained
at the National Renewable Energy Laboratory (NREL) “South Table Mountain”
radiometer facility in Golden, Colorado. This site is approximately 12 km from the Arvada airfield where the flights took place, and
therefore the values are not expected to compare exactly. From a general
comparison, however, the aircraft-based measurements appear to agree
reasonably with the surface-based measurements. The largest difference
appears to be that the surface at Table Mountain appears to be warming faster
than at Arvada, resulting in a larger difference between the two measurements
during the second flight. Interestingly, the Pilatus-measured downwelling
measurement appears to agree very well with the Table Mountain measurement,
once the aircraft is at altitude.
Broadband shortwave radiation measured during the time of testing on
3 April 2015. Included are upwelling (red), downwelling (blue) and net
(black) radiation from the first and fourth aircraft flights (thin, shorter
line segments with high variability from approximately 10:45 to 11:02 and
13:20 to 13:44 MDT) and 1 min averages from the NREL site at Table Mountain (thicker lines).
The tilt-corrected broadband shortwave irradiance measured by the SPN1s are
illustrated in Fig. . The corrections applied for this
specific set of flights are illustrated in Fig. as a
function of aircraft heading and total (two-dimensional) tilt angle. The top
panel of this figure illustrates the general tendency of positive tilt angles
(towards the sun) to have higher irradiance values, and negative tilt angles
to have lower irradiance values. The calculated corrections are illustrated
in the central panel, with corrections limited to a maximum tilt magnitude of
10∘. Finally, the tilt-corrected irradiance is shown in the bottom
panel of the figure.
Looking at the corrected values (Fig. ), the blue color
represents downwelling irradiance from the aircraft (thin line) and the NREL
Table Mountain Kipp and Zonen CMP22 pyranometer (thick line). The large drops
visible in the NREL data set during this time represents the passage of clouds
over the sensor. The Pilatus SPN1 measurements agree very well with the CMP22
measurement for both flights, although data from flight 1 happened to
coincide with the overpass of one of these clouds over the Table Mountain
site. The red lines indicate the upwelling values measured using the downward
looking SPN1 on the Pilatus (thin line) and a downward looking Kipp and Zonen
CM3 at the Table Mountain site (thick line). At the time of flight 1, the
upwelling shortwave measured by the CM3 is substantially higher than that
measured by the aircraft. Because this time period featured a rapidly melting
snow layer on the surface, this difference is likely the result of a more
uniform or thicker snow surface at the Table Mountain site. By the time that
flight 4 occurred, the difference between the two sensors and locations has
decreased dramatically, with the Pilatus SPN1 measurement actually being
slightly higher than the Table Mountain CM3. Again, this is likely due to
differences in the surface state and type at the location of the measurement.
The black lines represent the net broadband shortwave irradiance, calculated
as the difference between the measured downwelling and upwelling irradiance.
Downwelling shortwave irradiance (W m-1) from the two flights
completed with the SPN1s. The top figure shows the mean, uncorrected
irradiance detected across a variety of aircraft headings and tilt angles.
The center figure illustrates the amount of the adjustment as dictated by the
correction algorithm, and the bottom figure shows the final corrected values.
Note that tilt angles greater than 10∘ in magnitude are not
corrected.
One of the unique aspects of the SPN1 instrument is that it provides a direct
measurement to distinguish between direct and diffuse contributions to the
measured irradiance. The values for flight 1 and flight 4 are shown in Fig. , with the light blue line representing the measured
(uncorrected) total irradiance, the dark blue line representing the corrected
total irradiance, the black line representing the direct component of the
measured signal and the grey line representing the diffuse component of the
signal. From this, we can see some subtle differences between these two
flights. For example, flight 1 has substantially greater variability in the
net irradiance, with several instances where the direct component drops to
zero. This is the result of a substantial coverage of broken cumulus clouds,
which, at times completely shielded the SPN1 from direct sunlight. These
clouds had mostly dissipated later in the day, resulting in a more consistent
total irradiance and ratio of direct to diffuse irradiance. We also note that
the diffuse contribution has decreased between flight 1 and flight 4, which
results from a combination of lower sun angles during flight 4, less cloud
cover, and a generally drying atmosphere (see Fig. ).
In addition to the computed irradiances, we can use these measurements to
measure the surface albedo. As discussed previously, 3 April initially
featured a patchy snow-covered ground surface, but warm temperatures helped
to rapidly melt the snow. This is very apparent in the albedo measurements
(Fig. ), with the flight 1 albedo values generally varying
between 0.25 and 0.35, with some higher and lower values. In contrast, the
albedo measurements from the later flight 4, which occurred after the snow had
completely melted, are all generally in the 0.2 range, with the exception of
the portion of the flight over the light-colored concrete runway, where
values were closer to 0.27. Gaps in the albedo measurement are found in
places where the autopilot was transitioning between waypoints with bank
angles in excess of 10∘ (generally at the corners of the racetrack
pattern).
Time series of the SPN1 measured downwelling broadband irradiance for
the two flights on which these sensors were flown. Included are the raw
(uncorrected) total values in light blue, the tilt-corrected total values in
darker blue, as well as the direct (black) and diffuse (grey) contributions
to the measured signal.
Finally, POPS was operated on three of the four flights completed on 3 April
2015, with the instrument disabled during flight 3. Particle size
distributions indicating number (top), surface area (middle) and volume
(bottom) are shown in Fig. for the three remaining flights.
As may be expected, there was very little change to the total size
distributions obtained over the 5 h period between the surface and 100 m
altitude. There does appear to be a slight redistribution of particles on the
small end of the size spectrum (between 200 and 300 nm) from the first two
flights to the last flight. Note that sharp features in the size distribution
like the dip at ∼ 300 nm or the peak at 350 nm are caused by a mismatch of the
index of refraction of the environmental aerosol particles and the particles
used to calibrate POPS .
Surface albedo, as derived from the onboard SPN1 instruments. Note
that the first flight was completed when the surface still had a significant
amount of snow present, while the second flight was completed when most of
the snow had melted. Points with tilt angles exceeding 10∘ were
excluded from this figure.
Summary and outlook
In this paper, the RECUV Pilatus unmanned research aircraft is presented.
This system was developed specifically for the measurement of atmospheric
radiation, atmospheric aerosol particle size distribution, and atmospheric
thermodynamic state. To do so, the aircraft is equipped with up- and downward
looking Delta-T SPN1 broadband pyranometers, up- and downward looking Kipp
and Zonen CGR4 pyrgeometers, the NOAA-designed Printed Optical Particle
Spectrometer (POPS) and a housing to carry an NCAR PTH module. The PTH module
and POPS instrument are flown at all times, and the radiation payload is
configurable to measure up- and downwelling shortwave or longwave, but not
both together due to size and weight restrictions. In order to correct the
measured downwelling shortwave irradiance for variability resulting from
aircraft pitch and roll, the Pilatus is also equipped with a VectorNav high
grade INS. In order to characterize any angular offset between the VectorNav
and the SPN1s, the two sensors were co-mounted on a single plate. This
configuration was calibrated by mounting the system to the top of a car and
driving a predefined path on a cloudless, dry day.
Particle size distributions represented as the number of particles
(top), the surface area of particles (middle) and the volume of particles
(bottom) for the three test flights during which POPS was operating on 3
April 2015.
Measurements from a series of test flights flown on 3 April 2015 are
presented. The first and last flights of that day featured the SPN1
(shortwave) payload, while the second and third flights featured the CGR-4
(longwave) payload. All four flights also included the PTH module and POPS,
although POPS data was not collected during the third flight. These initial
flights clearly illustrated the sensitivity of both the short- and longwave
measurements to aircraft orientation, with a combination of partial cloud
cover and rolling terrain resulting in regular oscillations in both
components. Such oscillations would likely be largely avoided when flying
extended level legs, as planned for future deployments in regions where
airspace is less restricted than at our test site.
The first extended field deployment of this system is planned for early
April 2016. At this time, the aircraft and its crew are scheduled to deploy
to Oliktok Point, Alaska to measure aerosol and radiation properties
associated with the end of the Arctic haze season. Oliktok Point provides a
unique operating environment due to the presence of US DOE-controlled
restricted airspace (area R-2204). This area of restricted airspace is made
up of two cylinders with a diameter of 4 nautical miles, the first of which
extends from the surface to 457 m (1500 ft) above sea level and the second of
which extends between 457 m (1500 ft) to 2134 m (7000 ft). Access to this
airspace allows for the execution of the extended level legs desired for
radiation measurements, as well as increased vertical space to acquire
profiles of aerosols, radiation and thermodynamics over the lowest kilometer
of the Arctic atmosphere.
The general concept of making radiation, aerosol and thermodynamic
measurements from platforms such as the Pilatus holds a lot of promise.
Favorable comparison of Pilatus measurements with those obtained from other
sources gives confidence in the ability of this aircraft to obtain high-quality observations. Future development of a new airframe with similar
instrumentation and payload capabilities would likely result in a more
efficient system, and allow flight duration to increase. Extended flight time
will allow aircraft such as the Pilatus to explore higher altitudes and
greater spatial scales. For the time being, the upcoming Alaska deployment
represents an opportunity to evaluate Arctic Haze in a new manner, with
emphasis on the lowest kilometer of the atmosphere. Using this platform, we
hope to be able to capture information on the vertical variability of aerosol
size distribution, as well as the radiative impact of this polluted layer. We
expect to time the campaign in a way where we will be able to measure the
transition from polluted to cleaner conditions along the North Slope of
Alaska during this deployment. In the near future, we will work to further
improve the quality of the measurements being made by attempting to further
minimize noise, particularly in the radiation measurements. Over a longer
time frame, we hope to deploy the aircraft for future missions in the Arctic
as well as at lower latitudes to observe processes related to aerosols,
radiation and thermodynamics.
Acknowledgements
Funding for the development and upcoming deployment of the aircraft to Alaska
is provided by the United States Department of Energy (DOE) Atmospheric
System Research (ASR) and Atmospheric Radiation Measurement (ARM) programs
under grant DE-SC0011459. Instrumentation for operations is on loan from the
Pacific Northwest National Laboratory (CGR4s and SPN1s), the National Center
for Atmospheric Research (PTH module), the National Oceanographic and
Atmospheric Administration (POPS) and University of Colorado Research and
Engineering Center for Unmanned Vehicles (VectorNav). We wish to thank
Douglas Weibel and Tevis Nichols for their contributions to operation of
the aircraft during test flights and Jack Elston for his input into the
initial discussions for this project.
Edited by: M. Kulie
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