Public satellite data is normally available as Two Line Element Sets (TLEs for short); which get released for satellites on a daily basis (some high-profile targets like, the ISS, get multiple TLEs released per day). You can generate estimated positional data (usually referred to as ephemeris) from the TLE. How long a TLE is good for really depends on the type of satellite and orbit; but you normally want to get a new one at least every 24 hours.
We normally don’t bring up other products, but the company that started Cesium and continues to employ most of the maintainers, AGI (http://www.agi.com), specializes in aerospace software, so this is actually really trivial with the tools we offer.
For example, you can already track Sentinel 1 in the Satellite Viewer demo on cesiumjs.org: http://apps.agi.com/SatelliteViewer/?Status=Operational Just hit the x next to ComSpoc and then type Sentinel in the name, They’ll be 1 lonely green dot orbiting the earth at that point, you can click on it for information.
We also have a free version of our desktop product, (Systems Tool Kit, STK for short) that can generate positions from the TLE for you. Since it also has a scripting engine, it should be possible to write a simple script that generates a daily CZML file for Sentinel 1. (STK can actually retrieve the best TLE for you automatically from the internet as, so you don’t even have to worry about that part).
Obviously I’m biased towards using AGI tools to solve the problem, but hopefully that helps shed some light on the subject for you.