I have a speeding ticket but not sure what to plea.?

I was caught speeding the other day and not sure how to fight the ticket or take "Probation before Judgment". It is my first offense and he cited me going 40 in a 25. But I am confused on to whether I have a case against him to fight it or not. I was in a turn when he clocked me I guess by the time I was through the turn he was already pulling me over. The turn could no way be taken at 40, 30 maybe yes. He was also a fair ways away and didn't see him with radar or anything. He was in an unmarked car with completely tinted windows all rolled up. Also, he didn't provide the radar gun serial number on my ticket so I can't get information on if he had it calibrated or not. I just don't want the points because of insurance, but I don't mind the fines. I am just not sure on to plead not guilty, or just do the other option of Pleading guilty with explanation. Is there even a way to get Probation before Judgmentif I do end up doing the not guilty plea.