GPS (multiple overlapping orbits, 29 satellites and growing)
sensor mechanisms
photographic
earliest satellite photographic images from Corona spy satellite captured on mylar negatives and dropped from the satellite.
snagged in the air by planes near Hawaii.
now in the public domain (resolution down to 6 ft) at USGS Earth Explorer.
missions are named KH1 through KH9. The images extend possible landscape change analysis back at least a decade. Operated from 1959 to 1972, when a Soviet sub was found to be submerged near the recovery site.
active digital sensors – emit radiation and record reflection.
most common one is radar (RAdio Detection And Ranging) (JPL description)
and
the new kid on the block is LIDAR (Light Detection and Ranging)