A typical chip-scale Cesium (Cs) Atomic Clock available for service at $1500 a pop in 2011
Deep Space Atomic Clock
The Deep Space Mercury Ion (Hg+) atomic clock is much bigger. To my minds eye, it seems “less simple”.
From NASA Goddard:
“For 20 years, NASA’s Jet Propulsion Laboratory in Pasadena, California, has been perfecting an atomic clock small and lightweight enough for exploration missions. While atomic clocks at ground stations that are part of NASA’s Deep Space Network are about the size of a refrigerator, the Deep Space Atomic Clock is about the size of a four-slice toaster, and could be further miniaturized for future missions.” (One would hope!)
“The Deep Space Atomic Clock can be so small because it uses new, mercury-ion trap technology. Engineers expect that the clock will maintain a deviation less than two nanoseconds (0.000 000 002 seconds) per day.
“That type of accuracy could enable a spacecraft or astronauts to make navigation decisions on their own, rather than waiting for communications from Earth.
The clock will fly aboard the General Atomics Electromagnetic Systems Orbital Test Bed satellite, launching in June 2019. After launch, it will perform a year-long demonstration, confirming its performance against GPS satellite atomic clocks. Once the technology has been proven, small, mercury-ion trap clocks could be integrated into future deep space missions.
“GNSS Space Service Volume and Beyond
“Spacecraft near Earth don’t usually require a timekeeping device like the Deep Space Atomic Clock. In low-Earth orbit, below an altitude of approximately 1,860 miles (about the distance from Maine to Puerto Rico), a spacecraft can rely on one-way signals from any one GNSS constellation to instantaneously determine position and time.
“These spacecraft could simply use GPS to provide them with all the information they need to navigate, just as it does for users on the ground.
“Beyond this altitude, navigation becomes more challenging. It becomes necessary to combine signals from multiple GNSS constellations. This more challenging region stretches up to geosynchronous orbit (some 22,000 miles) and is known as the GNSS Space Service Volume.
“But, NASA engineers want to reach even higher.
…NASA simulations show GNSS signals could even be used for reliable navigation in lunar orbit (!?), far outside the space service volume, over 200,000 miles from Earth.
“Engineers are considering using GNSS signals in the navigation architecture for the Gateway, an outpost in orbit around the Moon that will enable sustained lunar surface exploration.”
Should read, “But this hasn’t stopped NASA Engineers from ignoring General Relativity…”