摘要：GPS satellites used to carry two cesium and two rubidium atomic standards on board. Subsequently, GPS switched to all rubidium clocks. Galileo plans to use hydrogen masers instead. What are the relative merits of these clocks for use in navigation satellites?
A: It is well recognized that the space-qualified atomic clocks in the GPS satellites are an enabling technology, if not the enabling technology for the system. However, they are also one of the more difficult technologies to acquire.
The first GPS satellites, known as the Block I generation (Navstar Space Vehicles 1–11), initially used three newly developed rubidium clocks. Development of a space-qualified cesium clock subsequently resulted in one of the latter types being added to the satellites’ clock suite, beginning with Navstar 4.
Over time, the GPS development programs for these space-qualified clocks led to the mixture of the different types of clocks used in the satellites. The rationale for their development and use in the GPS system provide answers to the relative merits of the technologies.
We should recognize that space-qualified atomic clocks are unique in the clock technology area. The electronics used in these clocks are one of the biggest differences from their ground-based counterparts. The key cost driver is that they are comprised of space-qualified electronics (Class S or equivalent), which must not only survive the launch environment but also the operational environment (thermal, radiation, etc.) with the objective of remote operation for design lifetimes of 5 to 7.5 years.
