I am Philip Linden, a Research Fellow at Open Lunar Foundation. Outside of Open Lunar, I am a spacecraft systems engineer for Mission Operations at Planet Labs PBC. My fellowship is focused on considering possible architectures for an open, independent time standard for cislunar space. This post is the first in a series about timekeeping and clock synchronization for lunar activities.
In Sync: a musical timekeeping analogy
Imagine a concert where the sound system goes out. A performer on stage can hear the drummer's beat, but the fans at the back of the crowd can't hear anything from the stage.
The drummer is at the back; the audience can only see the performers at the front of the stage. The performers have an idea: to get the crowd to clap in sync with the beat.
- The drummer has the best sense of rhythm at the venue. They maintain a steady beat, at one hit per second. Without amplification, the only people that can hear the beat are the other performers on stage. The drummer sets the true beat, but the audience can only see and hear the performers.
- With an excellent sense of rhythm and years of training, the performers sync their claps almost exactly on beat with the drums.
- The audience is made up of many people with different levels of musical aptitude.
- Audience members close to the stage have a great view of the performance and quickly synchronize. Folks at the back are lucky to see one of the performers when they make their way to a certain part of the stage.
- There is a jumbotron (video wall) broadcasting live video of the performers, installed at a select location at the venue. The jumbotron is installed for those with a poor view of the stage.
How does the crowd synchronize their claps?
Method 1: Sync to a common timekeeping reference
The audience uses the performers as time references. Each crowd member watches one or more performers and tries their best to clap in sync with them.
People with a poor sense of rhythm often need to synchronize their clapping with the performers. Others can clap along in time for a while before getting out of sync.
Method 2: Sync to a nearby authoritative timekeeper
The audience uses each other as time references. Each crowd member watches the person with the best rhythm that they can see.
Audience members close to the stage watch the performers and easily keep time with the beat, regardless of aptitude for rhythm. The sorry sap behind the soundstage can't see the performers directly, but they stay in time just fine by watching the tall band teacher a few meters away who has a clear view of the stage and great musical timing.
Even when their view is obstructed, any audience member nearby with a better sense of rhythm can be used to get closer to the true beat.
How does this analogy relate to spacecraft timekeeping?
Clocks in a timekeeping network are assigned a "stratum", or level of authority, based on how many degrees of separation are between that clock and the "absolute" time reference.
The drummer is the absolute time reference, like the International Atomic Time (TAI) based on measurements from the most accurate clocks on Earth. The performer is a Stratum-0 time source ― a trusted, accurate, and stable timekeeping service that is broadly accessible, like GNSS, which is synchronized with the absolute time reference.
The jumbotron acts like a Stratum-1 time dissemination server, such as a Network Time Protocol (NTP) server connected to the local network. The NTP server is synced to GNSS or another shared time standard, and actors are able to sync to the NTP time even without a connection to any actor inside or outside the network.
Crowd members are other actors with a diverse set of clocks and oscillators, some are better than others. Actors synchronized to the performers are Stratum-1, actors synced to the jumbotron are Stratum-2, actors synced to a peer Stratum-2 actor become Stratum-3, and so on.
In my next post, I’ll explain how to apply this analogy to develop an in-situ time reference.