The survey coordinate system is a spherical coordinate system with poles at , and , . The survey equator is thus a great circle perpendicular to the J2000 celestial equator, intersecting it at and . Lines of constant are great circles perpendicular to the survey equator and lines of constant are small circles parallel to the survey equator. , is located at , , with increasing northward.
The survey area is divided into stripes, where each stripe is centered
along a line of constant , separated from the adjoining
stripe(s) by . Each drift scan tracks a survey stripe,
offset by arcsec perpendicular to the stripe. Two scans (or
``strips''), one offset to the north and one to the south, are
required to fill a stripe.
The survey latitude tracked by stripe
n is given by
in the Northern Galactic Hemisphere, and
for the three stripes in the Southern Galactic hemisphere. These stripes are superimposed on a Galactic extinction map in Figure 2 of york00.
The natural coordinate system to use for processing a given drift scan is the great circle coordinate system for that stripe, , in which the equator of the coordinate system is the great circle tracked by the scan. This great circle is inclined by to the J2000 celestial equator, with an ascending node of . at the ascending node. increases in the scan direction (east) and increases to the north. Each stripe has its own great circle coordinate system.
For reference, the equations to transform among the different coordinate systems are: