The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of that standard. The IERS defines a geocentric system of coordinates using the SI system of measurement.
An International Terrestrial Reference Frame (ITRF) is a realization of the IERS. New ITRF solutions are produced every few years, using the latest mathematical and surveying techniques to attempt to realize the IERS as precisely as possible. Due to experimental error, any given ITRF will differ very slightly from any other realization of the ITRF.
Practical navigation systems are in general referenced to a specific ITRF solution, or to their own coordinate systems which are then referenced to an ITRF solution.
Alfred's frame of reference is considered an inertial frame of reference because he is not accelerating (ignoring effects such as Earth's rotation and gravity).
Frames of reference are especially important in special relativity, because when a frame of reference is moving at some significant fraction of the speed of light, then the flow of time in that frame does not necessarily apply in another referenceframe.
An accelerated frame of reference is often delineated as being the "primed" frame, and all variables that are dependent on that frame are notated with primes, e.g.
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