Airbus Defence and Space

The Envisat polar platform

Rising to a truly monumental challenge

The Envisat satellite platform (known as the Polar Platform), built by Airbus Defence and Space in the UK, is a derivative of the architecture of the Spot 4 spacecraft bus design, for which Airbus Defence and Space was also responsible. But it is much, much bigger.

Spot 4’s vital statistics are a petite 2 x 2 x 5.6 m, whereas the supersize Envisat’s dimensions are 10 x 4 x 4 m. And that’s in the tightly packed ‘stowed’ configuration for launch; fully deployed for business in orbit it measures a whopping 25 x 10 x 7. Its solar array, unfurled, is the size of a tennis court (70 m2)!

The payload component, too, is on a more ambitious scale than anything that had flown before. The 10 instruments to study a multitude of Earth science variants and parameters across land, sea and air weigh a total of 2.2 tonnes. Some of these instruments, such as the Advanced Synthetic Aperture Radar with its 725-kg, 1.3 m x 10 m antenna, clearly had to be mounted on the exterior of the spacecraft, while others are hosted in the internal equipment bay. And then there are all the other elements, such as communications and data-handling equipment, that are necessary to keep the satellite functioning properly, for which appropriate housing needs to be found.

The main driver for the overall satellite configuration was thus the need to maximise the payload instrument mounting area and meet all the instruments’ viewing requirements while remaining within the physical constraints of the Ariane 5 launcher’s fairing and interfaces.

The design of the satellite was particularly demanding. It had to have the strength to hold in place the extremely sensitive instruments yet still be light enough to be launched, a challenge at the best of times. (And one that almost foxed Earth-bound freight possibilities, solved in this case by dividing the satellite structure – supplied by Airbus Defence and Space Spain – into two parts for transportation to its launch base in Kourou.)

Manipulating a monster

The Envisat platform’s sheer size and weight – coupled with its highly delicate nature – posed quite a logistical headache. Many of the existing satellite-handling structures and equipment simply were not suitable for this ground-breaking undertaking, and new versions had to be designed specially to cope, capable of lifting, lowering, turning, and allowing access to the satellite securely but with infinite gentleness. The handling frame, seen in the photo repositioning the satellite to the horizontal for equipment access, is one example of a mechanism purpose-built for Envisat.

Repositioning Envisat to the horizontal for equipment access © ESA

The problems of handling Envisat were not only those of moving or accessing but also of testing. ESA’s solar simulator at European Space Technology Centre (ESTEC) in the Netherlands is a vast vacuum chamber used to receiving satellites with deployed antennas mounted on them, but, as the photo below shows, Envisat was a tight fit!

The chosen design also had to ensure that the rocket launch forces, particularly the vibrations, would not be amplified by any part of its structure and so shake the mounted instruments to pieces. Demonstrating this required that test facilities at ESTEC were built specifically with the giant Envisat structure in mind: shaker platforms capable of handling 10-tonne loads and acoustic chambers able to house the height of the structure were required.

Envisat in ESA’s solar simulator at European Space Technology Centre (ESTEC) © ESA