Preheat oven to 350. Grease and flour a 10-in tube pan with removable bottom. Tap out excess flour.
Place pecan halves on bottom of pan in even layer.
Using colander, drain sweet potatoes well.
In a bowl, mash potatoes until smooth.
In medium bowl, combine flour, baking powder, cinnamon, baking soda, ginger and salt.
In large bowl, with electric mixer on medium speed, beat butter and sugars 2 minutes, until fluffy. Beat in eggs, 1 at a time, beating well after each addition. Beat in buttermilk, vanilla and mashed potatoes.
With mixer on low speed, gradually add flour mixture, beat on medium speed until well blended. Spoon batter into prepared pan; spread evenly. Bake 60 to 65 minutes or until cake tester inserted in center comes out clean.
Cool on rack 15 minutes. Run knife around edges; remove pan sides. Cool completely.
Run long, sharp knife carefully under bottom of cake between pan and pecans. Turn cake out onto cake plate, pecan side up.
Glaze: In small saucepan, over high heat, combine sugar and orange juice. Bring to a boil. Reduce heat and boil until thick and syrupy, 3 to 5 minutes. Remove from heat. Stir in zest.
Brush over pecans, letting some glaze run down sides of cake.