This talk derives fundamental limits to the accuracy with which clocks and signals - e.g. the global positioning system (GPS) - can measure space and time. By combining quantum limits to the accuracy of measurement with the requirement that the local energy density of clocks and signals be less than the black hole energy density, I derive the quantum geometric limit: the total number of ticks of clocks and clicks of detectors that can take place in a volume of spacetime of radius R over time T is no greater than R times T divided by the Planck length times the Planck time.