RADARSAT-2 Orbit Clock

This project was inspired by my coworker Steve who always told me to use orbital units instead of UTC.

RADARSAT-2 is an Earth observation satellite launched in 2007 that has acquired more than 1 million images. With an orbital period of just over 100 minutes, its ground track repeats every 24 days or 343 orbits, actively maintained to within 100 meters of the reference orbit.

By actively following the repeating reference orbit, it makes planning acquisitions easy, since we have a good idea of where the satellite will be months and even years in advance. But a problem arises when we want to figure out the exact timing of the acquisitions. While ground track repeats nicely each cycle, the node crossing time shifts slowly due to effects from the gravity of the Sun and Moon, this can vary up to 100 seconds, and when the satellite is travelling at 8 km/s, an error of that magnitude means you're taking a picture of a very different place.

We solve this by not using time at all and instead defining acquisition timing using orbital units. Orbital units is always 0 at the ascending-node crossing so it gives us a nice fixed reference. The geographical footprint of acquisitions with the same relative orbital units are always going to be the same from cycle to cycle.

So I was inspired to make this clock, where the minute hand represents the orbit (going around once every 100 mins), and the hour hand represents the cycle (going around once every 343 orbits or 24 days). Now we never have to worry about UTC again!

How it Works

The clock converts timestamps to RADARSAT-2 absolute and relative orbit numbers:

• Absolute orbit: Total orbits since reference epoch (January 24, 2008)
• Relative orbit: Position within the current 343-orbit cycle
• One motor hand shows the relative orbit cycle position
• The other motor hand shows fractional orbit progress
• Both hands are homed at startup using the hall effect sensor

Also if you are just interested in clock projects, this guy on youtube has some great videos: https://www.youtube.com/watch?v=K5sB2GNunMY&t=2s