Orbit Simulation Sampling Options Explained

A Scan Ribbon describes a viewing pattern that is traced out on the Earth's surface by a scanning intrument while the spacecraft is flying along its orbital path. Such an instrument collects measurement data while it is sweeping its sensors back and forth from one Earth limb to the other. Typically, this sweeping is perpendicular to the orbital path. This motion provides measurement views that overlap each other in both time and space resulting in near complete coverage of the viewing area. When plotting this scan over a period of time, it gives the appearance of a continuous ribbon along the spacecraft's orbital track.

A Snapshot is a viewing pattern that is traced out on the Earth's surface akin to taking a photograph picture. Instead of seeing a ribbon-like appearance, the data are grouped together in smaller discrete datasets. These discrete data include measurements that are seen only within a specified rectangular viewing grid area, also referred to as a grid box. Typically, a grid box represents nadir views and there are no overlaps from box to box. When plotting this scan over a period of time, it gives the appearance of a row of separated boxes along the spacecraft's orbital track.

A Conical Scan describes a viewing pattern that is traced out on the Earth's surface by a non-scanning intrument while the spacecraft is flying along its orbital path. Here though, an instrument does not perform any sweeping motions. Instead, it makes measurements by looking downward through a field of view shaped like a cone. This cone view may represent an area of many square kilometers. When plotting this scan over a period of time, it appears as a series of overlapping circles along the spacecraft's orbital track.

A Scan Fence describes a viewing pattern that is traced out on the Earth's surface by a non-scanning intrument while the spacecraft is flying along its orbital path. It operates much like a conical scan except that the field of view is reduced into a very narrow beam. Typically, this beam would be generated by laser instruments. Measurement data are produced from sending a downward transmitted pulse and then viewing the echo. Thus, when plotting this scan geometry over a period of time, it appears as if we're looking down on the top of a picket fence along the spacecraft's orbital track.

A Limb Scan describes a variation on a Scan Ribbon viewing pattern. Instead of tracing a pattern across the Earth's surface, it traces a scan pattern way off to the side of a spacecraft's orbit. The sweeping motion allows an instrument to perform measurements between the Earth's limb (troposphere) and space (thermosphere) at discrete altitudes. When plotting this scan geometry over a period of time, it appears like a vertical profile wall off of the spacecraft's orbital track.

An Occultation Scan is a simplification of the Limb Scan geometry much like going from a Scan Ribbon to a Scan Fence. Here the instrument views a distant celestial body (typically the Sun) while scanning through the atmosphere (typically the stratosphere). Using the Sun as a light source, measurements would be performed during sunrise and sunset periods. When plotting this scan geometry over a period of time, it appears like a vertical, jagged profile line off of the spacecraft's orbital track.

Fanning Scan is similar to a Scan Ribbon except that instead of sweeping perpendicular to the orbit track, it sweeps up and down the orbit track. This is sometimes referred to as along-track scanning. Typically, the instrument's sweeping motion is much faster than the spacecraft's motion around the Earth. This motion provides measurement views that overlap each other in both time and space. This overlapping allows a scene on the Earth to be viewed multiple times at multiple angles. Thus, when plotting this scan over a period of time, it gives the appearance of an upside-down hand fan (with the handle at the spacecraft) as it traverses along the spacecraft's orbital track.

A Hemispherical Scan is an extension of the Scan Ribbon and the Fanning Scan. Here an instrument performs a Scan Ribbon-like sweeping motion while at the same time rotating the sensors from perpendicular to the orbital track to along the orbital track. This allows measurements to be collected over a 360-degree viewing range about the spacecraft's nadir point. When plotting this scan over a period of time, it gives an appearance much like a spirograph while traversing the spacecraft's orbital track.

The "Solar Stare" is the simplest geometric pattern we encounter. The instrument simply points in one direction and stares at a celestial body, typically the Sun. This geometry assumes the source is fixed in space. Thus, data sets are generated during those portions of the orbit when the Sun will be within an instrument's staring field of view.