The Walker Tool makes it easy to generate a Walker constellation using the Two Body, J2, J4, or SGP4 orbit propagators. First, define a satellite with the characteristics and orbit you need and then open the Walker tool by highlighting the satellite in the Object Browser and selecting Walker... from the Satellite menu.
The original satellite that you defined is referred to as the seed satellite, while the satellites generated using the Walker tool are referred to as the children.
If the seed satellite has sub-objects such as sensors, the sub-objects are also created for each of the child satellites.
A Walker constellation consists of a group of satellites (t) that are in circular orbits and have the same period and inclination. The pattern of the constellation consists of evenly spaced satellites (s) in each of the orbital planes (p) specified so that t=sp. The ascending nodes of the orbital planes are also evenly spaced over a range of right ascensions (RAAN).
If you want a grouping of sensors on the satellites in the constellation, create them on the seed satellite and they are copied onto members of the constellation during the creation of the constellation.
The way in which spacing between the ascending nodes that define the orbital planes is calculated depends on the Type of Walker constellation you choose. In addition to specifying the number of satellites in each plane, you must also specify the location of the first satellite in each plane relative to the first satellite in adjacent planes. The way to specify the position of the first satellite depends on the type of Walker constellation you choose.
Walker constellations are based on a simple design strategy for distributing the satellites in a constellation. There are two main variants of Walker constellations: Walker Delta constellations and Walker Star constellations. The two variants differ in the distribution of the ascending nodes between the planes of the constellation. For a Walker Delta constellation type, the ascending nodes of the planes are distributed over the full range of 360 degrees, while in the Star configuration, the ascending nodes are distributed over a 180 degree span. Walker constellations are often identified using the following shorthand notation,
where i is the inclination of the orbital planes, t is the total number of satellites in the constellation, p is the number of planes and f is an inter-plane phasing designation. The relative along-track position of two satellites in adjacent planes is determined by the phase parameter (f) where f is an integer from 0 to p-1. The value of f represents the number of slots of angular measure (360 degrees/t) by which the more easterly satellite leads the more westerly satellite.
Walker Constellation Options
The options that are available are dependent on the type of constellation selected.
When a Walker constellation is created, the original (seed) satellite is duplicated as part of the constellation. The new satellites are considered as children of the seed. If you open a Walker window for a child satellite, all fields are grayed out. In addition, the Number of Planes field and Number of Satellites per Plane field shows the plane number and satellite number for the selected child satellite. Each child has the same base name as the seed satellite plus two numbers: the first number identifies the plane in which the satellite resides and the second identifies the satellite's position in the plane.
For instance, if you defined a Walker constellation as having 2 Planes, 2 Satellites per Plane, an Interplane Spacing of 1 and a RAAN Spread of 360 degrees, the 2D Graphics window would look similar to that in the figure.
The next figure more clearly shows the configuration and spacing of the satellites.
Walker constellation clearly illustrating the satellite configuration
The table describes the spacing between satellites in more detail.