Ames Airborne Tracking Sunphotometer, AATS-14

Philip B. Russell, John M. Livingston, Beat Schmid, Jens Redemann, and
James Eilers

NASA Ames Research Center, Moffett Field, CA 94035-1000

<Philip.B.Russell@nasa.gov>

The 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14,
Fig. 1) measures the transmission of the solar beam in 14 spectral
channels. Azimuth and elevation motors controlled by differential sun
sensors rotate a tracking head, locking onto the solar beam and keeping
detectors normal to it. The AATS-14 tracking head mounts outside the
aircraft skin, to minimize blockage by aircraft structures and avoid
data contamination by aircraft-window effects.

![](media/image1.wmf){width="2.4444444444444446in"
height="4.333333333333333in"}Each channel consists of a baffled entrance
path, interference filter, photodiode detector, and preamplifier.
Filters are at wavelengths 354 - 2139 nm (Fig. 2), chosen to allow
separation of aerosol, water vapor, and ozone transmission. The
filter/detector sets are temperature-controlled to avoid
thermally-induced calibration changes. Detectors in the two
longest-wavelength channels incorporate thermoelectric coolers. The
other 12 channels are maintained at an elevated temperature by foil
heaters.

Sun tracking is achieved continuously, independent of aircraft pitch,
roll, and yaw, provided rates do not exceed \~8° s^-1^ and the sun is
above aircraft horizon and unblocked by clouds or aircraft structures
(e.g., tail, antennas). AATS-14 uses a quad-cell photodiode to derive
azimuth and elevation tracking-error signals.

Data are digitized and recorded by an onboard data acquisition and
control system. Real-time data processing and color display are
routinely provided. The real-time science data set includes the detector
signals, derived optical depths and water vapor column content, detector
temperature, sun tracker azimuth and elevation angles, tracking errors,
and time. Radiometric calibration is determined via Langley plots,
either at high-mountain observatories or on specially designed flights.
Repeated calibrations show that the instrument maintains its calibration
(including window and filter transmittance, detector responsivity and
electronic gain) to within 1% in most spectral channels for periods of
several months to a year.

AATS-14 was developed under the NASA Environmental Research Aircraft and
Sensor Technology (ERAST) Program. AATS-14 is designed to operate on a
variety of aircraft, some of which may be remotely piloted or
autonomous. Hence it can locate and track the sun without input from an
operator and record data in a self contained data system. In addition,
it can interface to an aircraft-provided telemetry system, so as to
receive and execute commands from a remote operator station, and
transmit science and instrument-status data to that station.

AATS-14 made its first science flights on the Pelican (modified Cessna)
aircraft of the Center for Interdisciplinary Remotely Piloted Aircraft
Studies (CIRPAS) during the Tropospheric Aerosol Radiative Forcing
Observational Experiment (TARFOX) in July 1996 \[Russell et al., 1999\].
Other missions in which AATS-14 has participated include the second
Aerosol Characterization Experiment (ACE-2) \[Schmid et al., 2000\],
South African Regional Science Initiative (SAFARI) 2000 \[Schmid et al.,
2002\], Asian Pacific Regional Aerosol Characterization Experiment
(ACE-Asia) \[Russell et al., 2002\], and Chesapeake Lighthouse and
Aircraft Measurements for Satellites (CLAMS) \[Redemann et al., 2002\].

**Figure 1.** Fourteen-channel Ames Airborne Tracking Sunphotometer
(AATS-14). Dimensions are in inches.

![](media/image2.wmf){width="5.090972222222222in"
height="4.055555555555555in"}

**Figure 2.** AATS-14 channel wavelengths (vertical lines with arrows)
in relation to atmospheric spectra. The spectra of transmittance T of
the direct solar beam at sea level were calculated using MODTRAN-4.3
with a Midlatitude Summer atmosphere, a rural spring-summer tropospheric
aerosol model (Vis = 23 km), and the sun at the zenith. Current center
wavelengths of channel filters are 354, 380, 453, 499, 519, 604, 675,
778, 865, 941, 1019, 1241, 1558, 2139 nm. Filter full widths at
half-maximum (FWHM) are 5 nm, except for the 353 and 2139 nm channels,
which have FWHM 2 and 17 nm, respectively.

**References**

Russell, P. B., J. M. Livingston, P. Hignett, S. Kinne, J. Wong, and P.
V. Hobbs, Aerosol-induced radiative flux changes off the United States
Mid-Atlantic coast: Comparison of values calculated from sunphotometer
and in situ data with those measured by airborne pyranometer, *J.
Geophys. Res.,* *104*, 2289-2307, 1999.

Schmid, B., Livingston, J. M., Russell, P. B., Durkee, P. A., Collins,
D. R., Flagan, R. C., Seinfeld, J. H., Gassó, S., Hegg, D. A., Öström,
E., Noone, K. J., Welton, E. J., Voss, K., Gordon, H. R., Formenti, P.,
and Andreae, M. O., Clear sky closure studies of lower tropospheric
aerosol and water vapor during ACE-2 using airborne sunphotometer,
airborne in-situ, space-borne, and ground-based measurements. *Tellus B
52,* 568-593, 2000.

Redemann, J., B. Schmid, J. M. Livingston, P. B. Russell, J. A. Eilers,
P. V. Hobbs, R. Kahn, W. L. Smith, Jr., B. N. Holben, C. K. Rutledge, M.
C. Pitts, M. I. Mishchenko, B. Cairns, J. V. Martins, and T. P.
Charlock, Airborne Measurements of Aerosol Optical Depth and Columnar
Water Vapor in support of the Chesapeake Lighthouse and Aircraft
Measurements for Satellites (CLAMS) Experiment, 2001, Abstracts, 11^th^
Conference on Atmospheric Radiation, American Meteorological Society,
Ogden, UT, June 3-7, pp. 20, 2002.

Russell, P. B., Flatau, P. J., Valero, F. P. J., Nakajima, T., Holben,
B., Pilewskie, P., Bergin, M., Schmid, B., Bergstrom, R. W., Vogelmann,
A., Bush, B., Redemann, J., Pope, S., Livingston, J., Leitner, S., Hsu,
N. C., Wang, J., Seinfeld, J., Hegg, D., Quinn, P., and Covert, D.,
Overview of ACE-Asia Spring 2001 investigations on aerosol-radiation
interactions, 11th Conference on Atmospheric Radiation, American
Meteorological Society, Ogden, Utah, 3-7 June 2002, Postprint Volume,
pp. 1-4.

Schmid, B., J. Redemann, P. B. Russell, P. V. Hobbs, D. L. Hlavka, M. J.
McGill, B. N. Holben, E. J. Welton, J. Campbell, O. Torres, R. A. Kahn,
D. J. Diner, M. C. Helmlinger, D. A. Chu, C. Robles Gonzalez, and G. de
Leeuw, Coordinated airborne, spaceborne, and ground-based measurements
of massive, thick aerosol layers during the dry season in Southern
Africa, *J. Geophys. Res*., in press, 2002.

**Weight, Power, Size, and Related Information, AATS-14**

+------------+-----------+----------------+--------+---------+--------+
| **Part**   | *         | **Size**       | *      | **Type  | **Ex   |
|            | *Weight** |                | *Power | of      | ternal |
|            |           | **(19\" panel  | Requ   | Power** | Sensor |
|            |           | or other)**    | ired** |         | Loca   |
|            |           |                |        | **(V,   | tion** |
|            |           |                | **(    | Hz)**   |        |
|            |           |                | watts, |         |        |
|            |           |                | a      |         |        |
|            |           |                | mps)** |         |        |
+------------+-----------+----------------+--------+---------+--------+
| a\.        | 131.6 lb. | [Outside       | 5.5A   | 28 VDC  | Top of |
| Telescope  | (Includes | A/C            | 154 W  |         | cabin, |
| head w     |           | ]{.underline}: | peak   | or      | nose,  |
| electr     | 121-lb    | 8\" OD dome    |        |         | wing,  |
| onics/data | head      | (hemisphere)   | or     | 120VAC, | or     |
| system     | w/elec.,  | atop 5\" H     |        | 50-400  | pod.   |
| cylinder   |           | pedestal.      | 4.2 A  | Hz with |        |
|            | 3.5-lb    | (Total H: 9\"  | @ 500  | add     | 9\" D  |
|            | isolator, | above A/C      | W peak | itional | port   |
|            |           | skin)          |        | 55-lb   | (See   |
|            | 1-lb      |                |        | power   | note.) |
|            | reinforc. | [Inside        |        | supply. |        |
|            | ring,     | A/C            |        |         |        |
|            |           | ]{.underline}: |        |         |        |
|            | 0.4-lb    | 12\" D x 18\"  |        |         |        |
|            | torque    | H cylinder.    |        |         |        |
|            | link,     |                |        |         |        |
|            |           | (+ laptop      |        |         |        |
|            | 0.7-lb    | computer for   |        |         |        |
|            | mount     | checkout and   |        |         |        |
|            | bolts,    | test flights)  |        |         |        |
|            |           |                |        |         |        |
|            | 5-lb      |                |        |         |        |
|            | cable     |                |        |         |        |
|            | bundle.   |                |        |         |        |
+------------+-----------+----------------+--------+---------+--------+
| b\.        | 6-lb      | Laptop         | \~0.8  | 120 V,  | N/A    |
| Operator   | laptop &  | computer.      | A      | 60 Hz   |        |
| station    | cable,    | Optional       |        |         |        |
| (laptop    |           |                | 92     |         |        |
| computer)  | 15-lb     | tray mounts in | Watts  |         |        |
|            | tray      | 19" rack.      |        |         |        |
|            | w/slides. |                |        |         |        |
+------------+-----------+----------------+--------+---------+--------+
| c\. N~2~   | 30 lb     | 7.5" Dia x 21" | N/A    | N/A     | N/A    |
| gas bottle |           | H              |        |         |        |
+------------+-----------+----------------+--------+---------+--------+

Note: Telescope dome needs to be mounted as far as possible from viewing
obstructions such as A/C tail and antennas.