Coordinate system is onto the sky for the POSition TARGet Special Requirement. The short lines indicate the blooming directions of the CCDs. Units are seconds of arc. See POSition TARGet <X-value>,<Y-value> for details.
Note that the POS TARG X- and Y-axes are not the same as the U2- and U3-axes, and that POS TARG axes are associated with each aperture selected (i.e., there is a POS TARG coordinate frame centered on each aperture).
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