With some satellites, a related phenomenon can be detected at radio wavelengths.

USA 155 is one of the SDS series of satellites. Its orbit is geosynchronous and it is stationed above 10 degrees west longitude. This makes it an easy object to track from here in the the UK.

Like the other SDS satellites, some of which are in inclined and highly eccentric orbits, it carries a transmitter operating near 2242.5 MHz - otherwise known as SGLS channel 9.

The diagram illustrates what happened when USA 155 entered a period of eclipse at the time of the 2009 Autumnal Equinox. It occurred around midnight Local Solar Time at the sub-satellite point.

Every year, a similar situation occurs daily for a few weeks either side of the two equinoxes.

Once USA 155 entered eclipse, the frequency started to rise as power output was lost from the solar array and the storage batteries ceased to be charged. It continued to rise as the storage batteries ran down. After about 75 minutes, solar power was restored as the satellite moved back into full sunlight.

It then took eleven hours for the storage batteries to re-charge fully from the solar array.

There are small upward deflections in the trace at around 06:00 and 10:00 UTC. They may represent periods when internal power demand increased to support an item of equipment being switched on.

One of the things that can be deduced from this event is that when electrical power is at a premium, transmitter frequency increases. This helps when studying other satellites because it sometimes reveals periods of heavy/low power drain. An example is one of the STSS Demonstrator satellites where it looks as though onboard equipment was powered down over the 2009-10 Christmas and New Year period.

The small gaps in the data are due to radio interference from one of the SDS satellites in HEO. The constantly changing Doppler shift meant that the two satellites were received simultaneously at the same frequency for a few minutes.