To get the best measurements of Earth’s atmosphere, you sometimes have to leave it. This November, the Sentinel-6 Michael Freilich spacecraft will do just that.
When a satellite by the name of Sentinel-6 Michael Freilich
launches this November, its primary focus will be to monitor sea level rise
with extreme precision. But an instrument aboard the spacecraft will also
provide atmospheric data that will improve weather forecasts, track hurricanes,
and bolster climate models.
“Our fundamental goal with Sentinel-6 is to measure the
oceans, but the more value we can add, the better,” said Josh Willis, the mission’s project scientist at NASA’s Jet Propulsion
Laboratory in Southern California. “It’s not every day that we get
to launch a satellite, so collecting more useful data about our oceans and
atmosphere is a bonus.”
A U.S.-European collaboration, Sentinel-6 Michael
Freilich is actually one of two satellites that compose the Copernicus Sentinel-6/Jason-CS
(Continuity of Service) mission. The satellite’s twin, Sentinel-6B, will launch
in 2025 to take over for its predecessor. Together, the spacecraft will join TOPEX/Poseidon
and the Jason series of satellites, which have been gathering precise sea level
measurements for nearly three decades. Once in orbit, each Sentinel-6 satellite
will collect sea level measurements down to the centimeter for 90% of the
Meanwhile, they’ll also peer deep into Earth’s atmosphere with what’s
called Global Navigation Satellite System – Radio Occultation (GNSS-RO) to
collect highly accurate global temperature and humidity information. Developed
by JPL, the spacecraft’s GNSS-RO instrument tracks radio signals from
navigation satellites to measure the physical properties of Earth’s atmosphere.
As a radio signal passes through the atmosphere, it slows, its frequency changes,
and its path bends. Called refraction, this effect can be used by scientists to
measure minute changes in atmospheric physical properties, such as density,
temperature, and moisture content.
The precise global atmospheric measurements made by
Sentinel-6 Michael Freilich will complement atmospheric observations by other GNSS-RO
instruments already in space. Specifically, the National Oceanic and
Atmospheric Administration’s National Weather Service meteorologists will use insights
from Sentinel 6’s GNSS-RO to improve weather forecasts. Also, the GNSS-RO
information will provide long-term data that can be used both to monitor how
our atmosphere is changing and to refine models used for making projections of
future climate. Data from this mission will help track the formation of hurricanes
and support models to predict the direction storms may travel. The more data we
gather about hurricane formation (and where a storm might make landfall), the
better in terms of helping local efforts to mitigate damage and support evacuation
How It Works
Radio occultation was first used by NASA’s Mariner 4 mission in
1965 when the spacecraft flew past Mars. As it passed behind the Red Planet
from our perspective, scientists on Earth detected slight delays in its radio
transmissions as they traveled through atmospheric gases. By measuring these radio
signal delays, they were able to gain the first measurements of the Martian atmosphere
and discover just how thin it was compared to Earth’s.
By the 1980s, scientists had started to measure the slight
delays in radio signals from Earth-orbiting navigation satellites to better
understand our planet’s atmosphere. Since
then, many radio occultation instruments have been launched; Sentinel-6 Michael
Freilich will join the six COSMIC-2 satellites as the most
advanced GNSS-RO instruments among them.
“The Sentinel-6 instrument is essentially the same as
COSMIC-2’s. Compared to other radio occultation instruments, they have higher
measurement precision and greater atmospheric penetration depth,” said Chi
Ao, the instrument scientist for GNSS-RO at JPL.
The GNSS-RO instrument’s receivers track navigation satellite
radio signals as they dip below, or rise from, the horizon. They can detect
these signals through the vertical extent of the atmosphere – through thick
clouds – from the very top and almost all the way to the ground. This is
important, because weather phenomena emerge from all layers of the atmosphere,
not just from near Earth’s surface where we experience their effects.
“Tiny changes in the radio signal can be measured by the
instrument, which relate to the density of the atmosphere,” added Ao. “We
can then precisely determine the temperature, pressure, and humidity through
the layers of the atmosphere, which give us incredible insights to our planet’s
dynamic climate and weather.”
With the help of JPL’s GNSS-RO principal investigator Chi Ao and NOAA’s National Weather Service meteorologist Mark Jackson, this video explains how the GNSS-RO instrument aboard Sentinel-6 Michael Freilich will be used by meteorologists to improve weather forecasting predictions. Credit: NASA/JPL-Caltech
But there’s another reason why probing the entire vertical
profile of the atmosphere from orbit is so important: accuracy. Meteorologists
typically gather information from a variety of sources – from weather balloons
to instruments aboard aircraft. But sometimes scientists need to compensate for
biases in the data. For example, air temperature readings from a thermometer on
an airplane can be skewed by heat radiating from parts of the aircraft.
GNSS-RO data is different. The instrument collects
navigation satellite signals at the top of the atmosphere, in what is close to
a vacuum. Although there are sources of error in every scientific measurement,
at that altitude, there’s no refraction of the signal, which means there’s an
almost bias-free baseline to which atmospheric measurements can be compared in
order to minimize noise in data collection.
And as one of the most advanced GNSS radio occultation
instruments in orbit, said Ao, it will also be one of the most accurate atmospheric
thermometers in space.
More About the
Copernicus Sentinel-6/Jason-CS is being jointly developed by
the European Space Agency (ESA), the European Organisation for the Exploitation
of Meteorological Satellites (EUMETSAT), NASA, and the National Oceanic and
Atmospheric Administration (NOAA), with funding support from the European
Commission and support from France’s National Centre for Space Studies (CNES).
The first Sentinel-6/Jason-CS satellite that will launch was
named after the former director of NASA’s Earth Science Division, Michael
Freilich. It will follow the most recent U.S.-European sea level observation
satellite, Jason-3, which launched in 2016 and is currently providing
NASA’s contributions to the Sentinel-6/Jason-CS mission are
three science instruments for each of the two Sentinel-6 satellites: the
Advanced Microwave Radiometer, the GNSS-RO, and the Laser Retroreflector Array.
NASA is also contributing launch services, ground systems supporting operation
of the NASA science instruments, the science data processors for two of these
instruments, and support for the international Ocean Surface Topography Science
To learn more
about NASA’s study of sea level rise, visit:
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Source: Jet Propulsion Laboratory