IEEE Geoscience and Remote Sensing Magazine - March 2016 - 27

and distribution are distributed around the United States
and are accessible through a uniform NASA interface [4].

Facts About JPL Earth Science
◗ JPL built the first satellite to track ocean surface winds, sea surface

ReCeNTLY CoMPLeTeD MIssIoNs

height, and natural hazards.
◗ JPL has been instrumental in understanding global sea-level changes

Active cAvity RAdiometeR iRRAdiAnce
monitoR SAtellite
The Active Cavity Radiometer Irradiance Monitor Satellite
(ACRIMSAT) was designed to monitor the total amount of
the sun's energy reaching Earth. The instrument is a pyrheliometer and directs sunlight onto a thermopile. The thermopile converts heat to a voltage that is measured and recorded
[5]. It is this energy, called total solar irradiance, that creates
the winds, heats the surface, and drives ocean currents. Some
scientists theorize that a significant fraction of Earth's global
warming may be solar in origin due to small increases in the
sun's total energy output since the last century.
ACRIMSAT was launched on 22 December 1999 as
a secondary payload into a polar orbit at an altitude of
685 km. The satellite's ACRIM 3 instrument spent 14 years
in orbit monitoring radiation from the sun. It was the third
in a series of satellite experiments that have contributed
to a critical data set for understanding Earth's climate-a
36-year, continuous multisatellite record of variations in
total solar radiation reaching Earth. These are significant
data that climatologists can use to improve predictions of
climate change and global warming.
ACRIMSAT went silent on 14 December 2013, most likely
due to an expected, age-related battery failure. Attempts
since then to reestablish contact have been unsuccessful.
AquARiuS
The joint U.S./Argentinian Aquarius/Satélite de Aplicaciones Científicas (SAC)-D mission was launched on 10 June
2011, and it ended on 8 June 2015 when an essential part
of the power and attitude control system for the spacecraft
stopped operating. The Aquarius instrument comprised an
L-band real-aperture radiometer and scatterometer, and it
successfully achieved its science objectives and completed
its primary three-year mission in November 2014 [6].
Aquarius/SAC-D mapped the salinity (the concentration of dissolved salt) at the ocean surface using the signal strength of thermal emission received at 1.413 GHz,
corrected for the ocean's temperature and for roughness
effects using the scatterometer. The information gathered
has been critical to improving our understanding of two
major components of Earth's climate system: the water
cycle and ocean circulation. By measuring ocean salinity from space, Aquarius provided new insights into how
the massive natural exchange of freshwater between the
ocean, atmosphere, land surface via runoff, and sea ice
influences the ocean circulation, weather, and climate.
Since ocean surface salinity varies from place to place
and over time, scientists can use it to trace the ocean's
role in Earth's water cycle. For example, more than 85%
of global evaporation and more than 75% of global
march 2016

ieee Geoscience and remote sensing magazine

for the past two decades.
◗ JPL has helped improve weather and hurricane forecasting by studying

ocean winds and clouds as well as the characterization of air quality.
◗ JPL plays a central role in understanding ozone depletion and monitor-

ing its recovery through infrared and microwave instruments.
◗ JPL studies Earth's carbon from space, providing valuable information

about Earth's natural and managed ecosystems.
◗ JPL measures Earth's changing gravity field to monitor its ice sheets

and groundwater.
◗ JPL recently launched a mission to measure soil moisture, freeze/thaw

state, and better understand conditions that influence Earth's carbon
and water exchanges.
◗ JPL operates over 25 airborne instruments, many developed as proof
of concepts for future on-orbit missions. Some are being actively
utilized for practical purposes, such as snow-pack measurements and
levee and other infrastructure integrity monitoring.

precipitation occur over the ocean. By measuring changes
in the ocean surface salinity caused by these processes, as
well as by ice melting and river runoff, Aquarius/SAC-D
provided important new information about how Earth's
freshwater moves between the ocean and atmosphere and
around the globe. Figure 1 shows one of the early maps
of salinity produced by the mission within a few weeks of
launch. Since then, Aquarius has generated a wide array of
science results, from ocean salinity [7] to freeze-thaw and
soil moisture on land [8].
quik ScAtteRometeR
The SeaWinds radar instrument on the Quik Scatterometer
(QuikScat) satellite measured near-surface wind speed and
direction over Earth's oceans [9]. SeaWinds was a recovery
mission to fill the gap created by the loss of data from the
NASA Scatterometer in 1997. SeaWinds used a rotating dish
antenna with two spot beams that swept in a circular pattern. The antenna radiated microwave pulses at a frequency
of 13.4 GHz across broad regions on Earth's surface. The
instrument collected data over ocean, land, and ice in a
continuous, 1,800-km-wide band, making approximately
400,000 measurements and covering 90% of Earth's surface
in one day.
NASA's QuikScat was launched in June 1999 and operated nominally until 2009, when the instrument ceased
spinning. Since 2009, the instrument has been used for
cross-calibration with India's Oceansat-2 scatterometer and for the International Space Station Rapid Scatterometer (ISS-RapidScat). QuikScat served both science
(e.g., [10]) and operational weather forecasting and
hurricane tracking.
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2016