"I think eliminating the GPS receiver is a big plus considering how hard it can be to get a good GPS fix. Plus I sell to many people preparing for natural disasters and government collapse where the GPS system may be shut down for civilian use. "
Yeah that's been my thinking exactly. That's not to say that adding a GPS would hard, but in some ways it's just one more thing to go wrong. Plus adding GPS also adds a lot of cost. It has been awhile since I've looked but I remember that adding GPS would add about $80 to the price. Considering that it only takes a few minutes to set the position manually, it just didn't seem worth it.
I would still like to add a GPS option to the code though. There are situations where it would be cool to have it. When traveling for example. If you search for the username Bob101 on this forum, you'll find a guy who set up a small solar panel that was designed to be mounted on top of an RV and automatically track the sun based on GPS and compass orientation.
According to the datasheet, the RTC's accuracy is:
Accuracy ±2ppm from 0°C to +40°C
Accuracy ±3.5ppm from -40°C to +85°C
Here is the datasheet if you are interested. http://www.mouser.com/ds/2/256/DS3231-102175.pdf
I haven't tested super in depth for drift because, but it seems like they will go for a year or more. There is a certain amount of random chance though when it comes to RTC drift, so it's hard to test unless you have a huge number of RTCs to test. I typically say that the time should be checked at least once a year.
Having high accuracy isn't strictly necessary for sun tracking, so it's not a huge deal if the time is off a bit. This is a somewhat rough comparison, but if you figure that the amount of the sun's energy that is intercepted is modeled by the Cos(degrees pointed away from sun) then you'll find that you can be 8 degrees off and still collect about 99% of the sun's energy.
cos(8 degrees) = 0.990268069 --> 99.0268% of energy is intercepted.
Since the sun moves about 0.25 degrees per minute, we find that the time could be 32 minutes off and and still be intercepting 99% of the sun's energy!
"As long as the Time is correct how far out into the future will the Mega version of your tracking code remain dead on accurate as far as the sun's position goes? " I'm actually not sure. I got the calculations from the book Astronomical Algorithms from Meeus and it doesn't come out and directly say how far you can go with the sun calculations. Or at least, it doesn't say it in any obvious location that I can find. I seriously doubt that anyone living today would need to worry about it becoming inaccurate within their lifetime though.