My name is Bob Harberts and I am a system
engineer with the Joint Polar Satellite System, or JPSS, at NASA’s Goddard Space Flight Center. We begin with the Earth rotating about its
axis. A gray grided plane projected through the
equator serves as a reference. In this top-down view, we can see the distinction
between day and night sides of the Earth, emphasized here with a yellow noon vector. The Suomi National Polar-orbiting Partnership
Mission, or SNPP, is the first satellite in the JPSS constellation. The orbit plane for JPSS is shown in orange. We can see how that orbit plane intersects
the gray equatorial plane. The upward side of the orbit always crosses
the equator at the same local time. This is called the local time of ascending
node, or LTAN, which is measured from noon. In this case, the LTAN is 21.25 degrees from
noon at 13:25, or 1:25 p.m., local time. This angle is fixed for JPSS orbits so the
satellite will always cross the equator at the same time. The angle between the plane of the equator
and the orbit plane is called the orbit’s inclination, which in this case is about 99
degrees. Together these fixed angles define a sun-synchronous
orbit. A sun-synchronous orbit means the orbit remains
fixed with respect to the direction of the sun. In this split-screen view we speed up the
Earth’s rotation to show one year of the Earth’s orbit around the Sun and how the JPSS orbit
plane always stays oriented with respect to the Sun. Satellites in sun-synchronous orbits always
pass over the same location on Earth at the same local time with the advantage of having
consistent lighting conditions for observations. The second spacecraft in the series, called
JPSS-1, will be launched and inserted in the same orbital plane as SNPP. JPSS-1 will be placed one-half orbit ahead
of SNPP. This means the JPSS-1 will be about 50 minutes
ahead of SNPP in the same orbit, allowing important overlap in observational coverage. Now let’s look at how the satellites observe
the Earth. JPSS satellites have multiple instruments
onboard to observe the Earth environment and atmosphere. An imager collects data about the Earth’s
surface, depicted in green, and sounders that collect data on the atmosphere beneath the
satellite, is depicted in the blue triangular region. The trailing colors behind the satellite represent
the data swath, or region where the data has been collected. As we zoom back out to include SNPP, we can
see how these data swaths overlap, providing better coverage. Even though the satellites are in the same
orbit, they will fly over slightly different regions as the Earth rotates below the satellites. If we look at the entire imager data swath,
we can see how the data builds up over time, covering the entire Earth. It takes about 14 passes for each satellite
in this orbit to cover the entire Earth’s surface. Now that the satellites have collected data,
we need to send observational data back to Earth for processing and use. At this stage, the JPSS constellation uses
two ground stations: Svalbard, Norway, near the North Pole, and McMurdo Station, Antarctica,
near the South Pole. The yellow cone shown here represent the local
field of view of the ground stations, which is approximately five degrees above the local
horizon projected into space. As a satellite passes into the cone, we see
a yellow line that represents radio contact and a downlink opportunity when the satellite
is in view of the ground station antennas. Approximately six-minute contacts are required
to download stored data to the ground per pass. Notice that the grand station cone is not
precisely at the pole so the ground station appears to wobble causing the contact opportunity
times to vary slightly. SNPP only contacts Svalbard, whereas JPSS-1
contacts both polar ground stations. For the next phase of the JPSS constellation,
SNPP is replaced by the JPSS-2 satellite and leads JPSS-1 by half an orbit. At this point JPSS begins using NASA’s Tracking
Data Relay Satellites, or TDRS, in addition to Svalbard to downlink data. The TDRS satellites are in a geosynchronous
orbit, which means they orbit at a rate that matches Earth’s rotation. As a result these satellites remain above
the same location on the Earth at all times. The JPSS satellites can use these as a relay
to send data down to the TDRS ground station at White Sands, New Mexico. In this view, blue lines depict when a TDRS
satellite is in view of a JPSS satellite and yellow lines indicate data downlinks to the
TDRS ground station. While this visualization depicts all possible
times data can be communicated to TDRS, operationally, only six-minute contacts will be scheduled
to downlink and relay data. JPSS missions will be joining an existing
and future constellation of NOAA, NASA, and international satellites, including European,
Japanese, and US military satellites. These many missions vary in the orbits they
fly, time of day observations they make, and earth-observing goals they will achieve. With this even larger fleet of satellites
contributing to data collection, global space-based observational coverage of the Earth will happen
even faster. Additional richness and value of constellation
missions come from the collective sharing and use of data gained from them. Combining data from these satellites greatly
improves global coverage, improves weather forecasting, and continues to increase our
understanding of the Earth as a system.