Combine the orange juice, sugar, and lemon juice in a meduim mixing bowl. Whisk. Let the mixture stand for 10 minutes. Whisk again. Pour mixture into the baking pan. Freeze for 30 minutes. Use oven mits to remove the pan from the freezer. Add the yogurt to the frozen juice, stir with a fork until blended, and return the mixture to the freezer. Every 15 minutes, remove the mixture from the freezer and stir with a fork until the mixture becomes slushy. ( Note: This takes about 45 minutes more.) Spoon the slushy mixture into bowls and serve.