GPS receiver are usually equipped with quartz crystal clocks, which are relatively inexpensive and compact. They have low power requirements and long life spans. For these types of clocks, the frequency is generated by the piezoelectric effect in an oven-controlled quartz crystal disk, a device sometimes symbolized by OCXO. Their reliability ranges from a minimum of about 1 part in 10 to a maximum of about I part in 10 , a drift of about 0.1 nanosecond in 1 second. Even at that, quartz clocks are not as stable as the atomic standards in the GPS satellites and are more sensitive to temperature changes, shock, and vibration. Some receiver designs augment their frequency standards by also having the capability to accept external timing from cesium or rubidium oscillators.