Alternative Methods of Describing Satellite Revisit Rate

As NOAA/NESDIS moves toward a more disaggregated Low Earth Orbit (LEO) satellite architecture, the definition of “revisit rate” needed to be reconsidered. A typical update rate for a constellation of three polar orbiting satellites may be written as “every four hours,” where the update rate is an average over time and over the globe. The average update rate can mask significant variations in the frequency of observations as a function of global latitude, where areas near the poles are typically observed more frequently than areas near the equator. The average update rate can also mask significant irregularities in the gap times between observations, with some locations experiencing sequences of observations with some long gaps between revisits followed by a sequence of short times between revisits. The update rate, expressed as an average over time and space, can be misleading to users who interpret the metric as implying a regular cadence between observations at any point on the globe.

The authors examined a variety of different ways to define update, or revisit, rate that help set user expectations and help answer the following questions: help set user expectations:

• What can I expect on average?
• What can I expect most of the time?
• What can I expect all the time?

It is clear that no single metric will capture all stakeholder needs. Tradeoffs between different metrics exist and should be proactively considered and characterized during the requirements development process to ultimately determine requirement metrics that are easy to understand and provide insight into the performance that users can expect from the constellation. To develop feasible requirement values, the process must account for user needs and program constraints (such as budget, primary users, and technical feasibility). The authors will review their analysis and present several metrics that may provide the most use to developing a satellite architecture and to clearly address user expectations and needs.