In a saucepan, combine 1 cup sugar, cornstarch, and salt. Gradually stir in water and orange juice. 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; return all to saucepan. Bring to a gentle boil; cook and stir 2 minutes. Remove from heat. Stir in butter and orange zest. Pour into pie crust.
In a large bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add 6 tablespoons sugar, beating until stiff peaks form. Spread meringue over hot filling, sealing edges to crust.
Bake in preheated oven for 12 to 15 minutes, or until meringue is golden brown. Cool on a wire rack.