There are a number of considerations that will affect the revisit (Repeat Cycle) that can be achieved with Change Detection for a given target. These are summarized below with reference to other Capella Support Articles that should be consulted.
A target’s latitude will play a key role in the number of collection opportunities that are possible in a given time period for Change Detection. Review the Current Tasking Request Considerations for a discussion on latitudes that can be imaged.
Competing Tasking Requests
Some regions receive a high number of tasking requests. Capella’s Tasking Tiers and No Bumping Policy provide guidance on how tasking requests are scheduled. Competition from other Capella users may impact the Repeat Cycle that can be achieved in a given region.
Operational Satellites in Each Orbital Plane
When Capella has more than one satellite that is operational in a given Orbital Plane, then higher revisit may be possible because compatible imagery may be acquired from any satellite within that Orbital Plane.
Satellites in the Capella Constellation are not currently in repeat track orbits, meaning that they do not always revisit the same points on Earth on a consistent cycle. Due to the need for consistent acquisition geometry, repeat tasks that are used for Change Detection will have less frequent opportunities for collection than other repeat tasking series that do not need consistent geometry. Tasking with ACD included is only available for Bi-Weekly, Monthly, and Other Repeat Cycles. Although ‘Other’ can be used to request tasks with a Repeat Cycles that are less than two weeks, using this option may result in a higher number of task rejections if the Scheduler is unable to fulfill the requests. The Support Article Repeat task series scheduling should be consulted.
Look Angle Tolerance
The user must select a Look Angle Tolerance between 0° and 10° during Repeat Task setup. As has been described, a larger Look Angle Tolerance (e.g. 10°) will result in more collection opportunities for the repeat series, making tasks more likely to be accepted by the scheduler. The tradeoff for this increase in opportunities is that image artifacts (differential layover, shadow, etc.) are also more likely be detected as changes. A smaller Look Angle Tolerance (e.g. 0°) will help reduce the quantity of changes detected based on image artifacts. However, a smaller tolerance will also result in fewer collection opportunities and tasks that are less likely to be accepted in competitive areas. The revisit charts below are intended to provide an understanding of the relative difference in the number of passes per month that can be achieved over a given latitude with an initial look angle specification of 32° and two different illustrative Look Angle Tolerances: 3° and 7°. These charts describe the projections for just a single satellite in a given Orbital Plane and do not account for any competing tasking requests or the fact that there may be multiple satellites within an Orbital Plane. The charts should not be taken as a guarantee of achievable revisit with the parameters shown, but are quite useful to understand the change in revisit opportunities on a monthly basis that will result from different Look Angle Tolerance choices.
Passes Per Month with 45° Orbital Plane and 3° Look Angle Tolerance:
Passes Per Month with 45° Orbital Plane and 7° Look Angle Tolerance:
Passes Per Month with 53° Orbital Plane and 3° Look Angle Tolerance:
Passes Per Month with 53° Orbital Plane and 7° Look Angle Tolerance: