Formation Flying

What is formation flying?  

The concept of formation flying satellites is frequently confused with that of a satellite constellation. As defined by the NASA Goddard Space Flight Center, a constellation is composed of "two or more spacecraft in similar orbits with no active control by either to maintain a relative position." 

Groups of global positioning system (GPS) satellites or communication satellites are considered constellations. Again from NASA, formation flying instead "...involves the use of an active control scheme to maintain the relative positions of the spacecraft." 

The difference lies in the active control of the relative states of the formation flying spacecraft. A distinction must also be made between formation-keeping and formation changes. Formation-keeping is the act of maintaining a relative position between spacecraft in the presence of disturbances, while formation changing modifies the formation type, changing the relative satellite dynamics.  This page will discuss formation keeping.

Controlling a group of satellites while they are in a fixed formation allows us to take in situ measurements over a large scale. Although each satellite is small, when grouped together in a formation, the total effect of the measurements can be greater than the sum of the parts.  The ION-F science mission of distributed ionospheric impedance measurements is an example of distributed in-situ space science, .

There are three general cases of formation flight:

• Leader-follower;

• Same ground track, and 

• Side by side. 

Of these, the first two are discussed below. It has not yet been determined if side-by-side formation flight is suitable for the ION-F mission.

Leader-Follower

The most basic configuration in which spacecraft can fly in formation is that of leader-follower. In this configuration, the orbital elements of each spacecraft are identical except for the true anomaly, i.e. meaning that each spacecraft follows the same orbital path, but at different times.

Same Ground Track

A same ground track formation, in one in which two or more satellites have identical ground tracks. Under certain conditions, it is possible to define two or more different circular orbits that create identical ground tracks on the Earth's surface, at different times. 

By flying in formation, the ION-F spacecraft will demonstrate technology for distributed-spacecraft science. The Dawgstar team has begun development of a formation flight simulation that will serve as a testbed for controller development. 

This simulation will also accurately predict the performance of the Dawgstar, and also increase the probability of success of the ION-F mission. The effects of drag and Earth oblateness on spacecraft motion are being examined and compared.  Preliminary findings show that effects due to Earth oblateness will be greater than effects due to atmospheric drag on the performance of the ION-F nanosatellites' formation flying.