Walker Tool

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. Then open the Walker tool by right-clicking on the satellite in the Object Browser, going to the Satellite menu, and selecting Walker... .

The original satellite that you defined is the seed satellite and the satellites generated using the Walker tool are the children. If the seed satellite has sub-objects such as sensors, the Walker Tool creates the same sub-objects for each of the child satellites. So if you want a grouping of sensors on all the satellites in the constellation, create them on the seed satellite and the Walker Tool will copy them into the constellation.

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).

The Walker Tool calculates the spacing between the ascending nodes. This spacing defines the orbital planes, depending 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.

Defining a Walker constellation

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 shorthand notation

i:t/p/f

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 interplane 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.

Creating a satellite collection

You can use the Walker Tool to create a Satellite Collection object in STK or to add a shell to an existing satellite collection. In the Walker Tool window, in the Container Options area, select Create Satellite Collection. This option also has its own option to add to an existing satellite collection rather than create a new one.

The Walker Tool automatically generates a satellite collection subset for each plane and an additional subset that includes all the satellites. Since all the satellites in the constellation have the same period and inclination, the satellite collection considers them to be in one shell. The Walker Tool assigns the seed satellite as the reference object for all entries in the shell.

Walker constellation options

The options that are available depend on the type of constellation selected.

Option	Description
Type	Select one of the following types: Delta. Delta configurations have orbit planes distributed evenly over a span of 360 degrees in right ascension. This configuration requires an integer value of f for interplane phasing. Star. Star configurations have orbit planes distributed over a span of 180 degrees. This configuration requires an integer value of f for interplane phasing. Custom. You can use a Custom configuration to define the span over which ascending nodes should be distributed and to specify interplane phasing in terms of a true anomaly offset.
Number of Sats per Plane	Select the number of satellites for each plane of the constellation. You can specify the total number of satellites in the constellation or the number of satellites per plane.
Total Number of Satellites	This option (t) is an alternative to Number of Sats per Plane.
Number of Planes	Specify the number of orbital planes for the constellation (p).
Inter Plane Phase Increment	Specify the number of "slots" between the first vehicle in adjacent planes (f). Interplane spacing can be, at most, one less than the number of orbital planes in the constellation (p). For example, the true anomaly difference between the first vehicle in adjacent planes is given by the expression true anomaly offset = f*360/t. For a 35/5/3 constellation (35 satellites in 5 planes and interplane spacing of 3), this results in an offset of 3*360/35 = 30.85 degrees from the first satellite in plane 1 to the first satellite in plane 2. This option is replaced by True Anomaly Phasing for Custom configurations. This option is only applicable for Delta and Star configurations.
Inter Plane Phase Angle	Specify the in-track spacing between the first satellites in adjacent planes, entered in degrees. This option is only applicable for Custom configurations.
RAAN Spread	Specify the angle to be divided by the number of planes to yield the difference in right ascension of the ascending node between adjacent planes. The value is fixed at 360 degrees for Delta configurations and 180 degrees for Star configurations. You can specify the value for Custom configurations. Plane 1 will use the RAAN value defined by the reference satellite. The RAAN for subsequent planes is computed by adding (RAAN Spread / Number of Planes) to the RAAN of the previous plane. The RAAN spread is important for small constellations in preventing undesirable plane orientations. For example, if there were two orbit planes inclined at 90 degrees, a RAAN spread of 180 degrees would result in two perpendicular orbit planes, while a RAAN spread of 360 degrees would result in all the satellites being in the same plane and in danger of head-on collisions.
RAAN Spacing	Specify the spacing of RAAN between orbital planes, entered in degrees. This option is only applicable for Custom configurations.
Color by Plane	This option sets the display color of the satellites in the 2D Graphics window by their planes; all of the satellites in the same plane are displayed in the same color, and each plane is assigned a different color from the other planes. If this option is not selected, all child satellites will inherit the display color of the seed satellite.
Create unique names for sub-objects	Select this option to have unique names assigned to all of the sub-objects of the satellites. If this option is not selected, all sub-objects for all of the child satellites will inherit the sub-object names given for the seed satellite when the constellation is created.
Container Options	Select from the following options: Do not create - This enables you to create Walker satellites without creating any STK objects. Create Constellation - STK automatically creates a Walker constellation object as well as objects for all the satellites in the Walker constellation. Enter the Walker constellation's name in the text box. The Walker Tool names the satellites. Create Satellite Collection - STK automatically creates a satellite collection object that includes all the satellites in the Walker constellation; STK does not create any satellite objects. You then choose from two sub-options. One is "Create new Satellite Collection," for which you enter a name in the text box. The other option is "Modify an existing Satellite Collection," for which you must select a satellite collection and then choose whether to add a new shell or to overwrite the existing satellites with the new shell.

When you create a Walker constellation, the tool duplicates the original (seed) satellite as part of the constellation. The new satellites are children of the seed. If you open a Walker Tool window for a child satellite, all fields are unavailable. In addition, the Number of Planes field and Number of Satellites per Plane field show 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 two planes, two satellites per plane, an interplane spacing of 1, and a RAAN Spread of 360 degrees, the 2D Graphics window would look similar to the figure below.

The 2D Graphics window showing a Walker seed satellite and its children (two planes, each with two satellites)

The figure below shows the configuration and spacing of the satellites.

Walker constellation illustrating the satellite configuration

The table below describes the spacing between satellites in more detail.

Satellite spacing for sample Walker satellites

Satellites	RAAN	Argument of Perigee	True Anomaly
Seed Satellite	0 degrees	0 degrees	0 degrees
Satellite11	0 degrees	0 degrees	0 degrees
Satellite12	0 degrees	0 degrees	180 degrees
Satellite21	180 degrees	0 degrees	90 degrees
Satellite22	180 degrees	0 degrees	270 degrees