In a medium bowl, combine Sugar and gelatin. Heat 1 cup tangerine juice to boiling. Add to gelatin and stir until gelatin is completely dissolved. Stir in remaining 2 cups juice and grated peel. Cool. Pour into ice cube trays or 9 X 9-inch metal pan; cover and freeze. When almost frozen, scrape into a large mixing bowl. Beat until smooth, but still frozen. Return mixture to pan; cover and freeze until almost frozen. Again scrape into mixing bowl and beat. Spoon into pan, cover and freeze until firm. Serve.