Updated online: 22 May 2009Older satellites will need to be replaced quickly if GPS is to maintain its precision.NASA Concern over the US Global Positioning System (GPS) stepped up a notch today after a senior official from the US government's congressional watchdog warned that the US Department of Defense faced substantial challenges meeting its space-programme commitments.Scientists use GPS signals for everything from tracking bird migration to making fine-tuned measurements of sea level change. The precision and accuracy of such measurements depends on how many satellites a GPS receiver can detect, and on the satellite's relative positions in the sky.But on...