clock: 1 in x 2 in x 1 3/4 in; 2.54 cm x 5.08 cm x 4.445 cm
box: 1 1/2 in x 4 3/4 in x 4 1/4 in; 3.81 cm x 12.065 cm x 10.795 cm
overall-box: 1 1/8 in x 4 3/8 in x 3 7/8 in; 2.8575 cm x 11.1125 cm x 9.8425 cm
overall-clock: 1/2 in x 1 5/8 in x 1 3/8 in; 1.27 cm x 4.1275 cm x 3.4925 cm
Chip-scale atomic clocks (CSAC) have components comparable in size to the integrated circuits (“chips”) used in modern electronic equipment. They were first developed by the Defense Advanced Research Projects Agency (DARPA) and the National Institute of Standards and Technology (NIST) in 2004. The first commercial version is this cesium CSAC with model number SA.45s, produced by Symmetricom, Inc., in Beverley, Mass., and sold beginning in 2011. Its “physics package” (the set of components that generate the cesium frequency) occupies less than 1 cubic centimeter, thousands of times smaller and lighter than the beam tube of a conventional cesium atomic clock and operates on very different principles. Furthermore, the energy consumption is much less, making extended operation on battery power feasible. Developing such a small clock required many innovations in several disciplines, including semiconductor laser technology, silicon processing, vacuum-packing and firmware algorithms. Its portability and low energy consumption have made possible many new applications, such as geophysical sensors, backpack jammers for improvised explosive devices, backpack military radios, unmanned aerial vehicles (drones) and military GPS receivers, but, as of 2013, it has not yet been used in space.