There are two questions in this question. I'll answer the second question first because (a) this second question is easier to answer, and (b) the answer to this question leads to the answer to the key question. The second question:
I haven't yet found exact timelines for GOES-S, GOES-T, and GOES-U, but their test phase isn't nearly as long as GOES-R. Why not?
GOES-S, -T, and -U are intended to be exact duplicates of GOES-R. They will have the same sensors and the same operational processes as will GOES-R. Intents are one thing, reality is another. The sensors on the follow-on satellites will inherently differ slightly from those on GOES-R. The validation period for those follow-on satellites will address the slight differences between those satellites and the already validated GOES-R.
The validation period for those follow-on satellites will not address the complex ground processes that transform the raw data transmitted by the satellite to Level 1A, Level 1B, and Level 2 products. Those processes will already have been validated by the time those follow-on satellites are launched. That will take considerably less time than will the validation effort for GOES-R.
Now the key question:
Why will GOES-R take a whole year for validation?
This is primarily because GOES-R essentially is a brand new instrument. The goal is to reach what's called "provisional status" within six months after launch. This means that all of the processes that transform the raw downlink to Level 1A products to Level 1B products to Level 2 products are functioning correcting, and that the underlying science in those transformations is basically correct.
There are several key challenges here:
- Are the scientific instruments measuring what the scientists think they are measuring?
- Is the processing from raw data to Level 1A products correct?
- Is the processing from Level 1A to Level 1B products correct?
- Is the processing from Level 1B to Level 2 products correct?
- Can all of that required processing keep up with the much increased volume of data transmitted by the satellite?
- Do those Level 2 products agree with alternative observations of the same effect?
There are a lot of ways things can go wrong in the above. The instruments themselves might have problems, the underlying science might have problems, and the complex software that performs the transformations from raw data to derived products might have problems. The system has to function properly, end-to-end, and the derived products have to agree with already validated (but perhaps localized) alternative measures of assessing those derived products.
The goal is to have the system at provisional status within six months after launch. That's not quite good enough. Having the system achieve operational status and have the products compare favorably year-round with validation data will take another six to nine months.