Introduction to Remote Sensing
August 30th, 2007 at 9:58 pm (Education, Spacecraft, Technology)
Today was the first day of my Remote Sensing class, which I am attending remotely through USC’s Distance Education Network. In this 2.5-hour class, we covered an introduction to what remote sensing is, what kinds of instruments are used, and some highlights in terms of scientific discoveries that have been obtained through this technology. Here are some tidbits I took away that were surprises:
- Some of the earliest “remote sensing” involved sending cameras up with balloons, then retrieving them and developing their film. This kind of surveillance was used as far back as the Civil War!
- The (spectral) width of atmospheric absorption bands varies with atmospheric pressure, and therefore with altitude; as pressure increases, they spread out to cover adjacent wavelengths.
- Sea-surface height varies with water temperature (this is how they track El Nino) and with water depth (useful for mapping the ocean floor from orbital observations of the sea surface). Separating the two effects (lower sea-surface can mean colder water and/or an oceanic trough) would seem to be a significant challenge.
- Landsat-7, unlike its predecessors, can detect clouds in images on-board and decide to discard cloudy images.
jim said,
August 31, 2007 at 8:16 am
Cool! (Especially #4 :-) Thank you for posting tidbits from your class.
Rex said,
September 7, 2007 at 10:21 am
It seems as though sea-surface height should track a gravitational isocontour. If you have a trough, that should lower the local gravity (since water is less dense than ocean floor basalt), and therefore the contour of equal gravitational potential will be closer to the center of the earth–so yes, the sea surface should be lower. But cold water is denser than warm water (above 4C, anyway), so that should increase gravity and the contour should be higher, and the sea level should rise, not dip. If the effect is not gravitational, did they explain how the temperature effect works?
wkiri said,
September 7, 2007 at 11:16 pm
Rex: This was a quick overview of remote sensing, and we were blasting through various topics too quickly to get any in-depth discussion of the physics behind anything. I’m hoping this will come up again when we get to the lecture that focuses on altimeters specifically. But basically, our prof showed a plot that (as you posit) shows water level dropping over (e.g.) the Puerto Rican Trench, by about 10 m. But then a later slide showed the El Nino plot that shows the water level being used to infer water temperature, and the magnitude of those drops in sea level (explained as “cold water sinks”) are on the order of 10 cm. So now I wonder if they observe over time to come up with a “reference” sea level, dictated by the ocean-floor topography (since that should be constant over our timescale) and then look at short-term differences and interpret those as temperature-induced?
In fact, the Jason data website captions the images as showing “residuals” and notes that “The mean signal, seasonal signal, and the trend have been removed.” So I think this is exactly what they’re doing.