A formation of small and large satellites orbits the Earth along the same ground track every 98 minutes in a 705-kilometer sun-synchronous orbit, crossing the equator at 1:30 a.m. and p.m. Termed the “A-Train” (short for the Afternoon Train), this carefully planned and coordinated constellation operates as a single, powerful, virtual observatory. This is a spectacular success story — our understanding of Earth system science has significantly improved due to the synergies from this diverse and complementary suite of instruments.
NASA’s National Polar-orbiting Operational Environmental Satellite System Preparatory Project (NPP) spacecraft is holding to its October launch readiness date for insertion into a sun-synchronous 824-kilometer orbit with a 1:30 pm equator crossing time. NPP will be followed by the first Joint Polar Satellite System spacecraft (JPSS-1), though now with a likely delay from the planned mid-2015 launch. The vision being presented here is one of an operational satellite formation, with NPP being the first car in the train.
Gone are the days of instruments mounted on a common optical bench. Since the launches of the Earth Observing System (EOS) Aqua and Aura spacecraft in May 2002 and July 2004, respectively, and the CloudSat and CALIPSO pathfinders in April 2006, NASA has demonstrated not only how to successfully operate a satellite formation but also the tremendous process-centric scientific synergies that result. Beyond scientific discoveries, the use of data from the A-Train research instruments in operational weather prediction centers has improved weather forecast quality and skill, via model initialization and data assimilation, forecast verification and improved process representation. Data originating from these research satellites are now as valuable to the weather modelers as data from the operational satellites. A second generation of U.S., Japanese and perhaps European spacecraft are now planning to join the train.
Formation flying can deliver multiple benefits, not the least of which being the ability to flexibly combine (and maintain over time) multiple, synergistic and multisensor measurement types. Pointing and position knowledge are required, and orbit insertions, maintenance and maneuvers must be coordinated and controlled as a formation. The best practices and lessons learned in station-keeping multiple satellites in their assigned boxes within the train are now operationally demonstrated through NASA’s A-Train operations.
Advances in both station-keeping ability and protocols now make it possible to achieve formation flight with a diverse set of spacecraft, including the large EOS observatories, small satellites, microsats and co-manifested satellites — launched simultaneously or years apart. Whether three years or a decade, satellites have finite lifetimes — the train would not. The JPSS Train, or J-Train, would bring these improved efficiencies:
- Complementary measurements create synergies.
- The degree of simultaneity can be assigned based upon position within the train.
- The train would be permanent, with its composition changing over time.
- International cooperation is facilitated.
- Technology insertion is facilitated, as research and operational technologies can operate side by side.
- Engineering complexities and management difficulties of integration on a common bus are avoided.
- Individual sensor replacement becomes more agile and cost effective.
The A-Train continues to exemplify the best of international scientific cooperation and coordination to this day. Just as a large department store or grocery store can become the anchor tenant of a shopping center, so can a robust satellite platform become the anchor tenant for a satellite train. When NPP is launched, it should be considered not a single spacecraft but rather the beginning of a new joint polar train — an operational satellite formation to come. And this could be followed by complementary trains within the operational Eumetsat and U.S. Department of Defense morning orbit planes.
Philip E. Ardanuy is principal engineering fellow with Raytheon Intelligence and Information Systems, which is under contract with NASA to deploy and operate the JPSS Common Ground System and to complete work on the Visible Infrared Imager Radiometer Suite, which will fly on NPP and JPSS-1.