Originally Posted by
tooty
Why is the same principle not applied to other satcomms (making all dishes smaller) ?
GPS receivers don't ever transmit, so the antennas can be light duty. They are semi-omnidirectional, and the active element is a quarter wavelength of 1.6 GHz, or approx. 2.5 cm.
Voice satcomms, while in the same frequency spectrum, require high data rates and lots of power through a mechanically or electrically steered antenna continuously pointed to a geostationary satellite 25,000 miles up.
GPS, Simply spoken:
A bunch of satellites, orbiting thousands of miles above the center of the earth, transmitting a time signal, etc., requiring lots of software.
A receiver somewhere near the surface of the earth. Upon receiving three signals, it triangulates the distance from each satellite, with reference to the center of the earth. Did I say, Software?
Map? Database? Did I say, Software?
Compare that to an ILS, with the antennas anchored in concrete. 90 Hz modulation above and left; 150 Hz modulation below and right. Your position on the beam is determined by the difference in amplitude of 90 vs. 150. No atomic clocks, no software needed.
It takes mountains of data to prove the reliability of GPS and its software for CAT IIIb approaches and landings.
GB