Place a 9 x 13-inch metal baking dish in the freezer to chill for at least 30 minutes.
Combine the sugar and 1 cup water in a small saucepan. Bring to a boil and cook over high heat until the sugar dissolves, about 5 minutes. Allow to cool completely.
Combine the orange juice and champagne in a large bowl. Stir the sugar syrup into the orange mixture, and pour the mixture into the chilled pan. Place the pan in the freezer and chill until ice crystals form around the edges, about 30 minutes. Remove the pan from the freezer and stir the mixture with a fork to incorporate the ice crystals. Continue freezing and stirring every 30 minutes until all the liquid is frozen, about 2 hours.
Granitas may be made up to 48 hours in advance.
Recipe courtesy Donna Leahy's Recipe for a Country Inn (William Morrow, 2002)