Intro to Remote Sensing- Satellite Orbit and Swath

RS Sensors

Instruments used to collect RS data (red)

RS Platforms

Vehicles used to carry sensors (blue)

• Ground-based

• Airborne

• Space-borne

Ground-based Platforms

Use ground vehicles (e.g. trucks designed for RS)

• Advantages

  • Record detailed info (e.g. higher spatial resolutions)

  • Better characterize the target

  • Better understand the info in the imagery

• Disadvantages

  • Only small area project practical

Airborne Platforms

Platforms that are operating within the Earth’s atmosphere (e.g. aircrafts, drones, balloons, etc.)

Space-borne Platforms

Platforms operating at high altitudes (e.g. Landsat 8&9 = 705km, SPOT 6&7 = 694km)

• Satellites- revolve around the Earth

• Particularly important

  • Repetitive coverage over large areas

  • Provide huge amounts of RS data

Several Major Satellite RS Systems Currently in Operation

Need to understand the key parameters describing satellites and sensors they carry

Landsat Series Satellites

• Initiated by NASA (The National Aeronautics and Space Administration) in 1967

• Landsat 1 launched in 1972

• Landsat 8 launched in 2013

• Landsat 9 launched Sept 27, 2021 (lastest)

• NASA and USGS currently share the responsibility of Landsat 9 operations, etc.

Satellite Characteristics

Two characteristics of satellites are particularly important to understanding how satellites work

• Orbits- the path followed by a satellite 

• Swaths- the width of a ground track imaged by a sensor

Key Parameters Used to Describe Orbits

• Altitude

• Period

• Cycle

• Inclination

Altitude

Height of the platform above earth’s surface

• Landsat 9 is in orbit at 705 km above the earth

Period

Time a satellite needs to complete one orbit

• Landsat 9 orbits earth once every 99 min

• How many orbits does Landsat 9 go around the earth every day?

  • # orbits per day = (24 x 60)/99 = 14.545

Cycle

Time needed for the satellite to retrace its path

• = the time needed for a satellite to pass over the same point on the earth’s surface directly for a second time

• Varies with each satellite

• Landsat 9 orbit cycle is 16 days- the satellite comes directly over same point every 16 days

• How many orbits does one complete cycle of Landsat 9 have?

  • # orbits per cycle = (16 x 14.545) = 233

Inclination

Angle a satellite crosses the equator at

• = the angle b/w the equatorial plane and the orbital plane

• Landsat 9 has an inclination close to 90° (98.2°)

Types of Satellite Orbits

Based on the altitude, direction, and speed of satellite movement; 2 most common types:

• Geostationary Orbits

• Near-Polar, Sun Synchronous Orbits

Geostationary Orbits

The position of the satellite relative to the earth’s surface remains the same

• If a satellite follows an orbit parallel to the equator and in the same direction and speed as the earth’s rotation, then the satellite will appear stationary (“static”) with respect to the earth surface

• Satellites in the geostationary orbits are located at very high altitudes (ca. 36,000 km)

• Advantages- collect info over the same portion of the earth’s surface at all times

• Exs. Weather satellites, communications satellites

  • GOES (Geostationary Operational Environmental Satellites) Satellites

    • 36,000 km above the equator

    • GOES can see an entire hemispherical disk of the earth

    • GOES images are distributed in near real time for use in local weather forecasting

Near-Polar Orbit

The orbital plane inclined at a small angle with respect to the earth’s rotation axis

• Basically N-S orbit (constant E-W position) + the earth’s W-E rotation → allows complete coverage of the earth’s surface after one complete cycle of orbits

• Most RS satellite platforms are in this orbit

• Ex. Landsat 9 has an inclination of 98.2 degrees

  • This means it is at an angle of 8.2 degrees from the earth’s rotation axis (N-S poles)

Sun Synchronous Orbit

The orbital plane processes around the earth at the same angular rate at which earth moves around the sun

• The satellite will always pass over a given location at the same local solar time (i.e. constant local time of day)

• Most earth observation satellites usually follow the sun synchronous orbits

• Advantages

  • Ensure consistent illumination conditions

  • Important for monitoring changes b/w images or for mosaicing adjacent images together

Swath

The width of a ground track imaged by a sensor

• Vary b/w tens and hundreds of km wide (for space-borne sensors)

• Landsat 9 swath width = 185 km

  • A full scene is defined as 185 km x 185 km

  • Also note that the distance between ground tracks of consecutive orbits is 2875 km at equator because of the earth’s rotation 

• In near-polar orbits, areas at high latitudes will be imaged more frequently than the equatorial zone due to the increasing overlap in adjacent swaths near the poles