The atoms are slowed until they are moving at just a few millimetres a second. They can then be unfrozen for precise lengths of time to take exact measurements of how far they moved, therefore allowing precise calculations on how the whole device moved in that time or changes in gravitation. One problem yet to be overcome is that these two possibilities cannot be told apart – a large object to the north of the sensor would give the same reading as acceleration to the south. It is thought that maps showing high-gravitation areas will be able to solve this issue.