Note that the S900 receiver has an indication on top of the blue ribbon for slant height measurements, as shown below.

Note that in Carlson Surv software, the antenna height can be recorded either as vertical or slant. The vertical ‘Vert’ height is measured vertically up until the ground to the bottom of the receiver, while the ‘Slant’ refers to the height measured from the ground up to the top of the blue ribbon.