I finally got some free time at the same time the sun was visible to take the SUNDAR outside. I took it out on my deck and hoping for the best, turned it on. Here is an output from the SUNDAR:
The blue trace are the pulses from the optical interrupter on the radar head. The yellow trace is the output from the photo diode. It obviously shows a significant peak as the sensor points at the sun. In fact it saturates the sensor even with a very low gain. I will need to add a neutral density filter to the device to prevent saturation.
What is not seen here is that if I rotated the device, the relative position of the yellow peak to the blue sync pulses. What is also not seen is that the output of the Arduino provides the relative azimuth.
It did see a few strange software bugs that I need to clean up, but I think the basic premise will work. Now I need to miniaturize the whole system.