GPS occultation is the only known technique that offers full SI-traceability for atmospheric temperature measurements. What is more, it is the only spaceborne technique that offers the required precision for climate monitoring and an essentially zero bias. GPS-RO temperature profiles averaged for climate analysis achieve an absolute accuracy of better than 0.05 deg C between 5 km and 30 km altitude, an order of magnitude better than any other available technique. How does GPS-RO achieve SI-traceability? First, recall that the RO measurement is obtained by precisely counting phase cycles of the received GPS signal to determine the changing path length through the atmosphere (see GPS Earth Sensing). Those cycles come from the atomic frequency standards that keep time onboard the GPS satellites. The onboard clocks are continuously tied with great precision to International Atomic Time (IAT) — an SI standard — by the US National Institute of Standards and Technology (NIST).