Place watermelon in a blender container or food processor bowl.
Cover and blend or process until smooth.
Stir in sugar.
In a small saucepan combine the gelatin and cranberry juice.
Let mixture stand for.
Stir mixture over low heat until gelatin is dissolved.
Stir the gelatin mixture into the melon mixture.
Pour into an 8x8x2-inch baking pan.
Cover and freeze about 2 hours or until firm.
Break up frozen mixture and place in a chilled mixer bowl.
Beat with an electric mixer on medium to high speed until mixture is fluffy.
Return to pan.
Cover and freeze about 6 hours or until firm.