Grandpa Franklin Rodrick SquarePants is SpongeBob's Grandfather, and he hints of being blind. He was first revealed to exist in the episode Rock Bottom where SpongeBob imitates him with the explanation that you should not run for a bus, "especially when it is drivin' up on a 90-degree angle!". He appeared yet again in the episode The Sponge Who Could Fly when he said, in SpongeBob's thoughts, "If we were meant to fly we would have propellers on our heads and jet engines on our backs.". It seems that SpongeBob really likes to learn wisdom from his sayings. Also, in The Sponge Who Could Fly, Patrick thinks Grandpa SquarePants is his grandfather.
Grandpa SquarePants is presumably deceased because SpongeBob always says "Remember what Grandpa SquarePants said..."
"Hey, I'm not your grandfather!"-The Sponge Who Could Fly.