I have some more thoughts about this project:
- I can build minimal version - without display and buttons, just with logging option. Time setting/readout of logged data would be done via serial interface. The minimal option can be done very small and lightweight. There is also option of using larger photodiodes to generate not just measurement input, but also power to the logger. I wonder if it's possible to power logger from harvested sun energy without any other power source. Accumulator of some kind would be probably needed, though - at least to backup RTC.
- To make longer term logs, more precise RTC is needed. RTC drift causes mainly geographical longitude to drift (latitude is less affected), slowly.
- The accuracy of LLG changes over the year, especially for latitude. As shown here, day length for zero declination is nearly equal for all over the world (with exception of polar areas, not shown in graphs), that happens at spring/autumn equinoxes. In such condition, LLG is almost unable to determine its position and accuracy is bad. Fortunately, it's easy to predict when this does happen and filter out wrong results. Accuracy of longitude is more-less constant all over the year.
- I didn't fully explore the blue filters idea. Filters I employed are probably not the best choice and I need something better.
- I wonder if LDR is good light detection device. Photodiode with TIA is better option in terms of accuracy (it can be done linear over many orders of magnitude of photocurrent), not that much in terms of component count and probably current consumption. The advantage of LDR is that it produces roughly exponential output, but I've been unable to properly use data measured by @Ted Yapo as his logger has different "gain" compared to my combination of LDR/47kOhm resistor. Maybe I need LDR with more than one resistor to achieve different "gain levels". Also, currently there is a few manufacturers of integrated devices photodiode+TIA+ADC in single package, this is also viable alternative. LDR with fixed resistor was simplest and easiest way to do it in short timeframe of contest, and provides quite good results, though.
- The "unlocked" XC16 compiler works great.