Granita is an Italian granular sorbet which specifically is not made in a churn because it is supposed to have discernible chunks of ice in it. The amount of sugar syrup you use will depend on how tart the oranges are. It is usual, anyway, to add some lemon juice to cut the sugar. Beware those oranges like footballs that have a thick, spongy rind and an excessively pithy flesh. When blood oranges are available use them for their stunning colour and depth of flavour.