Aerial Photography and Remote Sensing Study Outline
Satellite Remote Sensing
Your should be able to:
1. describe the differences between different satellite orbits - geosynchronous, polar and sun synchronous polar
2. describe the trade offs between image size, resolution and frequency of repeat images for satellites
3. describe some of the changes and improvement in satellite remote sensing in recent years
A. satellites can be placed in many different orbits.
1. All are circles (or ellipses) around the center of the earth.
a. Low altitude orbits are limited by friction with the atmosphere that slows the satellite
2. The farther from earth, the slower the orbit
3. advantages of low orbits
a. better resolution and more detail for a given imaging system
b. more orbits per day for more complete coverage at high resolution
4. advantages of higher orbits
a. less friction with atmosphere for longer lifetime
b. greater area covered per image
5. resolution/ coverage tradeoff
a. higher resolution means a smaller image area and more orbits required to cover the entire globe,
b. since orbits/day is set by the altitude, higher resolution means more days between return visits to the same site.
c. some satellites can aim their cameras to the side of their track, so you dont have to wait for the satellite to pass overhead to get an image of a location
6. Some special orbits are desirable
a. geostationary the satellite is over the equator and moving as fast as the earth is rotating, so it always stays in the same spot relative earths surface (at an altitude of about 36,000 km)
b. polar goes from the north pole to the south pole, allowing the whole earth to be imaged as it rotates beneath the orbit
c. sun synchronous orbits the earth on a path that allows it to cross the equator at the same sun time every orbit so that lighting angles will be similar in each image (about 700 km altitude)
B. imaging process
1. most satellites collect information one line at a time, perpendicular to the satellites motion with
a. a single detector and a rotation mirror - scanning
a. a line of detectors push broom
2. forward motion of the satellite brings successive lines into view
C. most satellites carry multiple instruments
1. using different lens and sensors the instruments can have different resolutions and coverage and sense different parts of the electromagnetic spectrum
2. successful instruments are usually installed on multiple satellites to allow continuous coverage over time as the satellite or instrument wears out.
3. sometimes the label on an image refers to the satellite, but usually it refers to the instrument (e.g. the Thematic Mapper instrument on the Landsat satellite)
D. two notable series of satellites with similar missions have been launched.
1. Landsat
a. The longest running (since 1972) US imaging satellite series
b. covers the earth in 16-18 days with a resolution of about 70 m
c.
early ones had a black and white camera and multispectral scanner (
d.
the
2.
a. a European satellite
b. better resolution (to 10m), but smaller image area so it is hard to image the whole earth
c. fewer bands than the TM.
d. pointable lens allows easier repeat imaging and stereo images
E. examples of successful satellite instruments
1. AVHRR Advanced Very High Resolution Radiometer
a. longest running (since 1978) infra red satellite
b. 1 km resolution
c. 1 visible and 5 IR bands
d. allows the calculation of the Normalized Difference Vegetation Index (NDVI) to indicate how healthy, and abundant plants are
e. measures land, ocean and cloud temperatures
2. MODIS (MODerate resolution Imaging Spectroradiaometer) launched in 2000
a. on two sun synchronous satellites
b. only moderate resolution (250 m max)
c. scan entire earth in 1-2 days
d. 36 bands, ultraviolet through IR
3. ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer)
a. on one of the satellites carrying MODIS
b. specializing in long infrared and microwave measurement.
c. very high resolution (15 m in visible0