In a saucepan, combine sugar, cornstarch, and salt. Gradually stir in orange juice and water until smooth. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil for 1 minute.
Remove from heat. Stir a small amount of hot mixture into egg yolks, then return all to the saucepan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from heat. Stir in butter and orange zest. Pour hot filling into crust.
In a mixing bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff peaks form. Spread evenly over hot filling, sealing edges to crust.
Bake for 12-15 minutes or until meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving.