GEOBACK is a knob sticking out of the algorithm that selects sources for use in a geodetic segment. For the current source selection algorithm, it is not too useful and should be set to a value higher than the number of scans expected in the segment. The default is 100. For each new source, SCHED looks for which sources of those available would conribute the most to improving the rms of the SecZ parameters in a dummy fit. That dummy fit does not use all the sources in the segment so far, but rather is restricted to the last GEOBACK sources. Without that restriction, once all antennas are giving reasonable SecZ fits, no new source improves things much and the bias to short slews can lead to selection of sources that don't really contribute much to the segment. By restricting the look-back, each new source must contribute to a fit. Since the algorithm was changed to concentrate on the antenna that did worst in after the previous source was added, rather than the RMS improvement, this restricted look-back capability has not been especially useful.
GEOBACK is also used to inhibit repeated use of any given source in a segment. Rather than prohibit repeats entirely, SCHED only prohibits repeats within each GEOBACK scans.