What is written in this satellite photograph? (Characters identified: 忠于毛主席)
The characters read:

忠于毛主席
Zhōng yú Máo zhǔxí



忠 means faithful;
于 is a multi-use preposition which, to me, sounds a bit archaic/formal, and here means "to", but can also mean "in" or "on";
毛 is Mao Zedong's surname;
主席 means "chairman".


So this translates to "Faithful to Chairman Mao". By the way, if you give the correct characters to Google, the translation it gives is Loyal to Chairman Mao.