Generation of electrical power : the solar array

As it would be too dangerous and restrictive to carry either a nuclear generator or a fuel cell which would require a quantity of reagent incompatible with the nominal satellite lifetime of 5 years, a natural choice was to use a renewable, available and ecological source, the sun.

Solar array design takes into account variations in solar flux, electron and proton fluxes, meteor showers and ultraviolet radiation which degrade performance by about 2% per year.

A solar array consists of several panels covered with solar cells. The panels have a rigid, honeycomb structure with a carbon fiber skin; the boom between the panels and the satellite body is made of carbon composite tubes developed by the Aerospatiale company. For a 5 panel array, once the panels have been deployed, the total area is 25 m2. 8640 silicon cells each 24 cm2 in size, manufactured by ASE in Germany, are bonded on one side and provide more than 2200 W of power

Solar array deployment

After the satellite separates from the launcher, the flight software progressively stabilizes it by slowing the rates of rotation about the different axes and by pointing the earthward side vertically down.

The next step is to open the solar panels so that these can start generating electrical current to power the onboard equipment and recharge the batteries for the coming periods of darkness.

The solar array consists of five large solar panels. During the launch phase, these are folded one on top of the other on the satellite's "roof", since otherwise, the satellite would not fit inside the rocket's payload fairing. The panels are secured in the stowed position then released by firing pyrotechnic devices controlled by the flight software.
Springs then deploy the solar array in two stages:

  • primary deployment, which takes place off the coast of Newfoundland, releases the main boom and the first of the panels,
  • secondary deployment, which takes place while the satellite is within range of the Japanese control stations, frees the remaining panels. The next step is to wait until the next time the satellite crosses the ecliptic plane (i.e. the plane defined by the Earth's rotation around the Sun). When this occurs, the array is set in rotation so that from this moment on the array begins tracking so its panels remain perpendicular to the Sun's rays. For a launch on 21 March, the day of the equinox, the ecliptic plane will be intersected when the satellite crosses the equator.

Each step is monitored closely, and with a degree of apprehension, by the project engineers since, without electricity, the satellite will survive no more than three or four orbital revolutions (i.e. 300 to 400 minutes). If anything goes wrong at this stage, there is very little time indeed to remedy the problem. For precisely this reason, the project and operational teams spend a lot of time, prior to the launch, rehearsing various scenarios to ensure that they are ready to respond very quickly in the event of a mishap. Fortunately, problems at this stage are very rare.