Black Butte

SIR-B Radar Image of Mount Shasta

Microwaves are electromagnetic waves used in radar detection. Because rough surfaces appear light and smooth surfaces appear dark, and slopes facing the radar device appear light and the lee slope appears dark, radar works very nicely in recording the topography of a region, as illustrated in the above image.

SIR-A was carried aboard the space shuttle Columbia on the second shuttle mission (STS-2). SIR-A is a side-looking SAR (Synthetic Aperture Radar). The SIR stands for Shuttle (or Spacecraft) Imaging Radar. The A in SIR-A means it was the first mission. Side-looking radar was originally developed so that a plane could fly along the edge of "friendly skies" and gather images from enemy territory. The illustration to the right shows how the space shuttle carries a side-looking radar. The antenna is the aperture of a radar system. A"synthetic" aperture means that the antenna is moved through a series of positions rather than remaining stationary (the shuttle provides the movement).

SIR-B was the second radar mission in the space shuttle program. One and a half years before the fatal accident of the space shuttle Challenger, Mission 41-G flew over Mount Shasta and Black Butte on October 25, 1984 taking this image using Spaceborne Imaging Radar-B (SIR-B). A 2 1/2 minute video of Mount Shasta (product #AVC-115-84C1M) was made in December 1984 using SIR-B data.

The way radar imaging works is like this: The radar antenna sends a microwave signal to a distant object. This signal is scattered but the radar antenna picks up the signal reflected directly back to the antenna. The time it takes and the strength of the returning signal are recorded on digital computer tapes then later processed and analyzed. About 1,500 pulses per second are transmitted to the surface! Check out this picture of Astronaut Kathryn Sullivan checking the SIR-B antenna on this self-same flight. The length of the antenna determines the resolution of the image (the longer the better).

In the above radar image we appear to be looking at Mount Shasta from the southwest (although the radar device is looking at Mount Shasta from the northeast, as is evidenced by the bright slopes). Black Butte is at the base of Mount Shasta and is rather outstanding because of the halo around its northeastern base (the top-side in this image). The halo is caused because that side of Black Butte is facing the radar device and thus sends back a strong signal known as foreslope brightening. The lava flows (near the North arrow) are bright because rough surfaces reflect a lot of the radar signal back to the radar device in a process called diffuse reflection. The gash on the right (southeast) side of Mount Shasta is Mud Creek. The town of McCloud is near the bottom middle of the image in Squaw Valley. Click on the image above to see an annotated version.