I am very interested in your project.
I've built my own Heliostat with the help of Mark, from http://heliostaat.nl/.
I bought dual axis gearboxes and Stepper as well as DC motors in the beginning (From China).
I struggled really hard to find someone to build the electronics, although I was prepared to pay a lot of money. The electronic engineers I approached, just did not seem interested, although I was going to pay for their time.
I designed my own hardware bits on Autocad in 3D. I had all the aluminium parts cut out on a waterjet cutter. That was quite interesting how accurate those machines are, and their power. Something like 1900 bar of water mixed with grit. Able to cut through 4 inches or more of solid steel.
My heliostat is not 100% finished yet, but its been working for a few years. I still need to put the electronic switches for the horizontal and vertical stops in place, so that the motos wont run past certain points.
Unfortunately most of my fun stopped when I got divorced, and now I am renting my property out in S-Africa where the heliostat is, while I am living in Austria.
I had many trails done, and have sent my electronic parts back to Holland several times, so that Mark could make some changes as I wanted it.
My total costs were big, as I also mounted my heliostat on a long lamp post of steel, so that the sun could go into my bedroom on the second storey. Most of the pole is hidden, as it is inside/between the branches of trees. I've spent 1000's (yes, converted to Dollars)For your project, just my thoughts:
1) GPS is a definite advantage for position AND time, and that was one of my requirements, as Mark only used a DCF time signal, which do not cover South Africa.
2) As I work offshore, and alongside ROV's (Remote operating vehicles) crew, which use fluxgate Compasses: I know that they are not that accurate, like the fibre optic Gyros we sometimes use. I am not sure about the Arduino unit's accuracy.
Point is, you dont need to calibrate the heliostat that often, so that it knows where 0º is. Mark has a simple way of calibrating: You put a cylinder (Tall empty spraypaint/beer can) on top of a piece of paper on the mirror's surface, aim the mirror electronically at the sun, until there is no shadow from the cylinder/can. When there is no shadow, you tell the electronics that "this is the sun's position"
3) I have some heavy winds in my area during Winter, and during my first Winter, I had to put my Heliostat to rest a lot of times, so that it wont blow away.. Rest position as you know is horizontal. The heliostat will restart its normal cycle the next morning at sunrise. I had to send my electronic parts back to Mark again, so that he could build an interface for an anemometer. It doesn't measure windspeed, but rotations, but I cant remember at which RPM it triggers the rest command.
I used to like it to get the sun on my bed at the exact sunrise time, or even to wake up, hearing my DC motors waking up.
Same for when the days get shorter. Every day you hear the motors start up in fast mode, to go to the sleep position.
Another nice feature I have, is my remote control, which I can select 3 targets, as well as a target straight to the sun, if you have solar panels.
I do like the idea of Ben, to be able to track the Moon as well
I am still looking to change my setup, so that I can set my mirror's angles individually, as I lose a lot of sunlight on the wall next to my windows.
Also, just for interest. Hope that all the fanatics wont drool too much. I saw some solar trackers in Austria. Have a look here:https://www.smartflower.com/en