Tiny instabilities in those orbiting clocks contribute at least a few meters of error to single-receiver GPS measurements. Making the clocks smaller (so that more of them can fit on each satellite) and increasing their stability could reduce such errors to fractions of a meter. Pilots landing on narrow airstrips at night would appreciate the improvement. So would surveyors, prospectors, search and rescue teams ... and farmers. 'Precision farmers' already use GPS-guided tractors to dispense custom-doses of water, fertilizer and pesticides over garden-sized plots. Better GPS data could guide those tractors to individual rows or perhaps even to individual plants for special care.
'If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.'