Setting satellites to network time

Shortly after NASA demonstrated that standard Internet Protocol-based equipment can be used to communicate with a spacecraft, the agency successfully tested another element of standard IP that will help facilitate a mission's scientific research.

On April 14, the UoSat-12 spacecraft successfully used Network Time Protocol to synchronize its onboard clock with the U.S. Naval Observatory's timeserver, located in Washington, D.C. — about a quarter of the way around the world from the satellite's ground station in Surrey, England.

Using NTP, the spacecraft compares its internal clock with a ground reference, such as the Naval Observatory's server. If it detects two samples with an error of 15 milliseconds or more, it automatically resets the spacecraft's clock to match the reference time, said James Rash, project manager for NASA's Operating Missions as Nodes on the Internet (OMNI) program at Goddard Space Flight Center, Greenbelt, Md.

"Having correct time on board spacecraft has been a problem that required complex software," Rash said. Typically, satellite imagery must be time stamped, particularly when the event, such as a gamma ray burst, is very short-lived, he said.

For the clock synchronization tests, an NTP server was ported to the UoSat-12 spacecraft. Two tests were performed, both following the same scenario. The test started out with the onboard NTP server running but disabled so that it could not change the spacecraft's clock. The onboard server periodically negotiated with the Naval Observatory timeserver to factor out network delay. The onboard server calculated the offset it had to apply to the spacecraft's clock, and this value was sent to the ground in a standard telemetry stream, where it was logged for later analysis.

A short time into the test, a command was sent to the spacecraft to enable NTP to actually change the onboard clock. NTP requires two successful offset calculations before it will adjust the clock. Later in the test, a command was sent to the spacecraft to manually set the onboard clock ahead by about 2.5 seconds. After two successful offset calculations, NTP reset the clock to within 19 milliseconds.

Some NTP configurations also include cryptographic authentication to prevent accidental or malicious protocol attacks.