Image Interpretation and Remote Sensing

This section outlines the basics of obtaining information about an object from a distance. Our eyes are the most familiar source of "remote sensing" information. We gather light waves in the "visible" (referring to human eyes!) part of the elctromagnetic (E-M) spectrum to produce the images we see of the world around us. Spacecraft also gather information in the form of electromagnetic radiation, but specialized instruments can be created the are sensitive to E-M radiation at a much broader range of wavelengths than human eyes. We typically see this information displayed as black and white or color images of a planet's surface.

Creating Images of a Planet's Surface
Typically, a small telescope is aimed at the planet's surface to get an image. The image is then focused onto the CCD chip that converts photons of light into electrical charge. The greater the charge, the 'brighter' that pixel will appear in the resulting image. These chips are the same as the ones used in commercially available digital 35mm cameras. In order to create images from this information, we use computers. Computers need to know how much energy is being received (i.e., how 'bright' an area is), and from where in the image scene the energy is coming. The easiest way to do this is to break a scene into a large number of squares, like the tiles in a mosaic. Each one of these picture elements, or 'pixels' for short, has a grid position, corresponding to a location on the CCD chip. For instance, let's say the CCD image is made up of an 800 x 800 grid of pixels. The pixel located in the upper left corner would have the address 1,1. This indicates the pixel located in line 1 (the first column) and row 1 (the top row). The 'brightness' of this pixel is recorded as a number, commonly from 0 to 255 (256 levels are about how many grays the human eye can distinguish Š this would be 8-bit data - but the number could be anything up to the number of levels distinguishable by your detecting instrument CCD chip). Once a set of brightness values and their addresses has been recorded, they can be reconstructed by computer to produce a mosaic showing the surface of a planet in visible light (a black and white image if only 1 wavelength 'channel' is used as described here), ultraviolet, infrared, X-rays, etc.

Laser Altimetry
Another type of remote sensing that gives information about the shapes and elevations of a planet's surface is known as laser altimetry. This type of instrument (a laser altimeter) works similarly to the radar guns used by the police. A pulse of light energy (in this case, at the long-wavelength radar end of the E-M spectrum) is beamed at a car. It bounces off the car, and the time until the reflection arrives is recorded. The total round-trip time, divided by two, can be used along with the speed of light to get the distance to the car. A subsequent pulse is sent out a specific amount of time later. Its return beam is recorded and a second distance calculated. The difference in these distances divided by the time between the pulses gives the velocity of the car relative to the radar gun. Laser altimeters only need to record the distance, since we will consider the surface of the planet to be fixed (not true, of course, with respect to other planets, the sun, etc., but good enough for our spacecraft if itÕs in a circular orbit). So, the laser pulse is sent out, its return time recorded, and a distance to the surface is recorded. If we know the shape of our orbit precisely enough (circular is easiest to calculate distances), the only differences in distance to the surface will be the result of elevation changes on the ground. For instance, it's farther from the spacecraft to the bottom of a canyon than to the top of a high mountain, etc. If we send out and receive these pulses often enough, we can get a 'topographic profile' of the surface beneath our spacecraft orbit track. This is basically a cross-sectional view as if we sliced through the planet's crust along our orbit and looked at the result from the side. This is very valuable to show the shapes of craters, valleys, mountains, the slopes in plains areas that appear flat in visible light images, etc. Global elevation maps of Mars and the Moon produced by the recent Lunar Prospector and Mars Global Surveyor images were produced using data from this type of instrument

Electromagnetic (E-M) Spectrum

Visible Light is electromagnetic radiation at those frequencies that can be sensed by the human eye. But the electromagnetic spectrum has a much broader range of frequencies than the human eye can detect, including, in order of increasing frequency: radio frequency (RF), infrared (IR, meaning "below red"), visible light, ultraviolet (UV, meaning "above violet"), X rays, and gamma rays. These designations describe only different frequencies of the same phenomenon: electromagnetic radiation.

The speed of light in a vacuum, 299,792 km per second, is the rate of propagation of all electromagnetic waves. The wavelength of a single oscillation of electro-magnetic radiation is the distance that the wave will propagate during the time required for one oscillation. There is a simple relationship between the frequency and wavelength of electromagnetic energy. Since electromagnetic energy is propagated at the speed of light, the wavelength equals the speed of light divided by the frequency of oscillation.

Doppler Effect in the E-M Range

Regardless of the frequency of a source of electromagnetic waves, they are subject to the Doppler effect. The Doppler effect causes the observed frequency of a source to differ from the radiated frequency of the source if there is motion that is increasing or decreasing the distance between the source and the observer. The same effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer, or vice-versa. A train's pitch sounds higher as it approaches (higher frequency of sound waves received) and lower when it moves away (lower frequency of sound waves received).

When the distance between the source and receiver of electromagnetic waves remains constant, the frequency of the source and received wave forms is the same. When the distance between the source and receiver of electromagnetic waves is increasing, the frequency of the received wave forms is lower than the frequency of the source wave form. When the distance is decreasing, the frequency of the received wave form will be higher than the source wave form.