Not seeing the sign for 30 mph is no defense. And just because the video has no sign in the area you video taped is likely not even admissible evidence. Indeed, not knowing the speed limit is an admission by you that you probably were not paying correct attention to your driving, or at least some judges I have been before has so stated. If you knew the airport was posted at 40 pmh why were you driving 47 mph? That is another admission that you knew you were above the speed limit and kept on going so you better not mention that at the hearing, either.
I do not see that what you raised is a winning defense. Contact a local lawyer if you want to fight it and maybe he or she can come up with something more for you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in the subject practice discipline and with whom you have an atttorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.