Introduction to Remote Sensing
Orbital Considerations
Orbital Characteristics (1)
Orbital Considerations (2)
Height vs. Life of Satellite
10,000 km orbit is permanent
Orbital Characteristics (3) Low Earth Orbits (LEO; <2500km)
Equatorial Orbit (Not very useful)
International Space Station Orbit
The station travels from west to east on an orbital inclination of 51.6°. Each orbit takes 90-93 minutes, depending on the altitude of the ISS. During that time, part of the Earth is viewed under darkness and part under daylight. The ISS orbital altitude drops gradually over time due to the Earth's gravitational pull and atmospheric drag. Periodic reboosts adjust the ISS orbit. The orbit of the Space Shuttle is very similar. http://city.jsc.nasa.gov/orbtutor/page1.htm
International Space Station Orbit
The inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit if they do so close enough. The equatorial plane is the plane perpendicular to the axis of rotation of the central body.
an inclination of 0 degrees means the orbiting body orbits the planet in its equatorial plane, in the same direction as the planet rotates; an inclination of 90 degrees indicates a polar orbit, in which the spacecraft passes over the north and south poles of the planet; and
an inclination of 180 degrees indicates a retrograde equatorial orbit.
Orbital Characteristics (4) Low Earth Orbits (LEO; <2500km)
2. Polar Orbits -Orbit in plane of Earth’s rotation axis -Successive orbits cross the equator at different times -Preferred for missions intended to view longitudinal zones under full range of illumination conditions -Nodes: Nodes mark the intersection of the plane of the equator and the plane of the orbit.
N-flying = ascending node S-flying = descending node
Orbital Characteristics (5) Low Earth Orbits (LEO; <2500km)
3. Oblique Orbits (orbital plane intersects equator not at 90°)
-Most satellites in LEO use near-polar oblique -Launched eastward = direction of Earth’s rotation = Prograde orbit -Launched westward = opposite direction of Earth’s rotation = Retrograde orbit
Cape Canaveral Spaceport
Required for safety reasons to launch over water (eastward). All orbits launched from here are prograde.
NASA Space Shuttle
• Altitude 200-600 km • Inclinations: Max 62°, usually ~28° (latitude
of Cape Canaveral) • Always in a Prograde Oblique orbit.
Vandenberg Air Force Base
Required for safety reasons to launch over water (westward). All orbits launched from here are retrograde.
Orbital Characteristics (6): Oblique Orbits
Orbital Characteristics (7): Oblique Orbits
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Landsat 7 Orbital Characteristics
Landsat Paths on Consecutive Orbits
Spacing Between Consecutive Landsat Orbits at Equator
Complete Landsat coverage takes 15 days
251paths/14 orbits per day = ~18 days for repeat coverage
233 paths/ 14.5 orbits per day = ~15 days for repeat coverage
Eccentricity
The eccentricity of a satellite's orbit is defined as the ratio of the satellite orbit's focus length (c) to the orbit's semi-major axis (average orbit radius) (a). It defines how elliptical the orbit is, and defines the orbit height at both the apogee and perigee points. The eccentricity of a satellite's orbit (or any orbit for that matter) is a unitless value that ranges from 0 (perfectly circular) to 1 (parabolic). All of Earth's artificial satellites have orbit eccentricities of between 0 and 1. Within a TLE (Two Line Element) file, the decimal point is not present, but is always assumed to be placed before the first number, even if it is a zero.
Aqua and Terra Satellites
• Aqua (EOS-PM) Orbit
Characteristics • Sun synchronous, nearpolar orbit • Equatorial Crossing Time – 1:30 p.m., ascending node Inclination 98° Altitude: 705 km Period: 99 minutes Semi-major axis 7085 km Eccentricity 0.0015
•Terra (EOS-AM) Orbit Characteristics •Sun synchronous, near-polar orbit •Equatorial Crossing Time -10:30 a.m., ascending node •Inclination 98° •Altitude: 705 km •Period: 99 minutes •Semi-major axis 7085 km •Eccentricity 0.0015
• • • • •
SPOT has better repeat coverage because it can look off-nadir
SPOT Viewing Opportunities
High-level Satellites: Geosynchronous Orbits
Geostationary Satellites
• That orbit is achieved when the spacecraft is "parked" above the Earth
at 35,800 km (22,300 miles) and is moving along a circular path around the planet at approximately 11052 km/hr (6802 mph). A point on the Equator that remains directly underneath is traveling at ~1667 km/hr or 1042 mph. At these speeds there is no relative motion differences, so that the observing satellite is synchronously locked into a geostationary position above the hemisphere it is intended to view and (unless it drifts) will always view the same scene.
• Most imagery shown on TV news weather segments comes from these
satellites. GOES-1 arrived in a geostationary orbit at 135° W, soon after its launch on October 16, 1975. Others launched at two to three year intervals (GOES-10 entered its orbit on April 25, 1997,).
To exemplify GOES imagery, we show the the first test IR image from GOES-9 on June 19, 1995 (left). Such hemispherical images can be subdivided to concentrate on specific areas. The GOES-8 (East) images shows a large continental storm on March 20, 1994.
• Satellite covering the Atlantic Ocean and the eastern U.S. is called
GOES-East (located above the equator at 75°W longitude), and that over the Pacific is GOES-West (at 135°W longitude). Together, they provide coverage of both the Atlantic and Pacific, as shown in this drawing which also illustrates the full disk nature of the view:
To cover the entire Earth, four GOES would be needed. However, other parts of the world are monitored by other systems. As of late 2006, GOES-11 and GOES12 are operating, with GOES-9 and GOES-10 also in orbit serving as back-up.
• The GOES-8 sounder has a visible band and 18 thermal bands, which
are sensitive to temperature variations related to CO2, ozone, and water vapor at different atmospheric levels. We can convert each band into an image, to which we assign colors, to identify thermal differences, as demonstrated in this panel of images taken on May 5, 1997.