Page 164 - Intro to Space Sciences Spacecraft Applications

151
Remote Sensing
Manual of Remote Sensing, Volumes Z and ZZ, 2nd ed. Washington, D.C.:
American Society of Photogrammetry, 1985.
Beal, R., Spaceborne Synthetic Aperture Radar for Oceanography. Balti-
more: The Johns Hopkins University Press, 1981.

1. A sensor is to be placed into geostationary orbit to monitor its field of
view constantly for infrared signals, indicating possible ICBM launch-
es. The sensor is to consist of a 10 x 10 pixel square array of photode-
tectors looking at the frequencies associated with temperatures between
1,500 and 3,000" K (1.29 pm median wavelength). The electronic out-
put of the photodetectors is a sinusoid with a changing amplitude that
represents the detected temperature. This signal is to be digitized using
16 bits per word with a sampling frequency of 1 KHz.
Draw a sketch and determine the geometry of the situation, including
look angle (en), earth angle (y), and elevation angle (v), for the maximum
slant range (R-). (Hint: v = 0 at Rmm.) Determine the size of each pho-
todetector required to give a ground resolution (the JFOV for one detec-
tor) of 10 km when looking straight down. What is the difference between
this resolution and that when the sensor is looking out at Gax? Compute
the system IFOV and GIFOV at nadir and at R-. Also, compute the tem-
perature sensitivity and transmission rate of the system. Finally, discuss
how you would attempt to scan the entire visible earth's surface for com-
plete coverage. Based on your scan method and the given sampling rate
(1 sample = 1 area covered), estimate how long it would take your system
to scan the entire visible surface once (assuming no overlap between areas
scanned each sample).
2. Determine the size aperture a remote sensor would require if it were
looking for possible ICBM temperatures between 1,500 and 3,000 OK
(1.29 pm median wavelength) and wished to see the entire earth's sur-
face possible at all times from its geostationary position while always
looking straight down at nadir. Use Figure 6-1 to work out the geome-
try of the problem. Hint: To do this you need to find the distance per-
pendicular from the nadir line (between the satellite and the center of
the earth) and the point on the horizon of the satellite's field of view.