Scientists have analyzed data recorded
during the lengthy decent of the US Air Force's
Communication/Navigation Outage Forecasting System (C/NOFS) satellite
into Earth's atmosphere. The observations will be extremely useful
for future operations, significantly improving predictive models for
satellite trajectory and re-entry.
When the C/NOFS satellite burned up during a scheduled re-entry on November 28, its demise was carefully recorded by a joint team of scientists from NASA, the US Air Force, and the University of Texas at Dallas (UT-Dallas).
It marked the end of a seven and a half year mission which saw the probe study a layer of charged particles known as the ionosphere, located between 40 and 600 miles (64 and 965 km) above the surface of the Earth. It's a highly changeable region, being affected by solar activity, electrical field effects and strong upper atmospheric winds. Such disruptions can alter satellite orbits, and produces turbulence known as scintillations, which interfere with radio wave navigation and communications.
For 13 months before the satellite's eventual demise, as its orbit steadily decayed, the team made comprehensive observations, looking at regions of the atmosphere that aren't routinely studied, as it's not possible to sustain an orbit at such altitudes without long-term on board propulsion.
The recorded data shows that at the lower altitudes, the upper atmosphere and ionosphere are strongly influenced by small changes.
"The neutral atmosphere responds very dramatically to quite small energy inputs," said UT-Dallas' Rod Heelis. "Even though the energy is put in at high latitudes – closer to the poles – the reaction at lower latitudes, near the equator, is significant."
The observations shed light on one big mystery – why the low latitude ionosphere causes such pronounced issues with communication and navigation radio signals at night. As the satellite descended through the darkness, it detected what is now deemed responsible for the interference – areas of the ionosphere where charged particles flow by each other horizontally in opposite directions.
The event also provided an opportunity to study the point at which the charged particles in the ionosphere and the neutral particles in the upper atmosphere interact. The readings show that neutral winds create build ups of neutral gas against the edge of the ionosphere, creating previously unobserved density variations. Allowing for such variations will improve accuracy when modelling things like radio wave interference and spacecraft drag.