The difference between granita and sorbet is in their texture: granita is slushy, while sorbet is smooth.
Squeeze 5 to 6 oranges to yield 3 cups of juice. In a small saucepan, combine 1/4 cup orange juice and 1/2 cup sugar. Boil over high heat, stirring, until sugar dissolves, about 1 minute. Transfer mixture to a shallow 2-quart dish, and add remaining orange juice.
Freeze, scraping around sides and breaking into crystals with a fork every 30 minutes, 2 to 2 1/2 hours total. Cover with plastic wrap; keep in freezer until ready to serve.