Setting satellites to network time
- By Paula Shaki Trimble
- May 01, 2000
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.