Single Observation -- The PI is requesting that we observe this source once. Swift may observe it on multiple orbits (snapshots) but this is still considered a single observation. Because Swift is in low Earth orbit, observations are split into 20-30 minute pieces called snapshots. A single observation may be composed of multiple snapshots.
Monitoring Observations -- The PI requests that we observe this source more than once on some specified schedule.
With the exception of occasional manual calibration observations, the operation of the Swift is autonomous and is driven by the on-board observation timeline and by newly discovered GRBs. The Swift Mission Operation Center (MOC) loads pre-planned target (PPT) timelines onto the observatory 5 days a week (Monday through Friday; please note that schedules for Sunday and Monday are loaded onto the satellite on the previous Friday, so requests for ToOs on Monday should be submitted by 9 AM Friday to ensure inclusion in the on-board schedule). New GRBs trigger Automated Targets (ATs), which are generated on-board by the BAT instrument. The Figure of Merit process controls the observing timeline and arbitrates between the PPTs and the ATs according to their assigned observing priorities.
The Swift observatory is in low Earth orbit and the XRT and UVOT cannot be pointed closer than 30 degrees to the Earth's limb. Because of this there is no "Continuous Viewing Zone" and the observations of any given target will be broken into segments of 20-30 minutes, called "snapshots".
The figure below provides an illustration of a typical "day in the life" of the Swift observatory. It shows schematically how a newly discovered GRB (an AT) interrupts a previously scheduled series of observations (PPTs). The details in the third row show how the XRT instrument responds to these observations. (Please note that PD mode is no longer active for XRT and has been replaced by WT slew mode.)
Above is a diagram of a typical "day in the life" of Swift observations covering 9 orbits. The middle box (yellow) shows a timeline for a sequence of snapshots of 5 different targets comprised of four Pre-Planned Targets (PPTs) loaded into the on-board flight timeline, and one Automated Target (AT) representing a newly discovered GRB. Targets are color-coded for clarity. The top bar shows an expanded view of the first two orbits. Snapshots are identified by a target number and a snapshot number (target/snapshot). These PPTs would typically represented follow-up observations of GRBs discovered over the previous week.
In Orbit 3 a new GRB is discovered by the BAT, and the FOM task assigns it a target number of 5 and schedules an immediate observation, interrupting the previously scheduled snapshot 34 for target #2. The automated observation of target #5 continues until it is interrupted by an observing constraint, at which time the FOM resumes the interrupted observation of target #2 with snapshot #34. On Orbit 4, target #5 is observed as soon as it becomes visible, and pre-empts time from both target #1 and target #2.
The bottom bar shows the sequence of XRT events for an AT (target #5) and a PPT (target #3). For the AT, the XRT updates its bias row and bias map during the slew and then switches into Windowed Timing slew mode. As soon as the slew ends, the XRT takes an image, centroids the image to determine the source position, and sends this to the ground through TDRSS along with a postage-stamp image of the source. It then switches back to Photodiode mode (PD) until the flux drops low enough for Windowed Timing (WT) mode, at which point it sends a cumulative raw spectrum to the ground through TDRSS. The sequence of events for a PPST is similar except that no Image Mode data are collected and no TDRSS messages are generated.