Scaling up and beaming down

De-regulation of the telecoms industry, the resulting escalation of demand and the development of relatively low-cost launch vehicles has propelled the industry into a new environment and away from its research-based origins.

At Matra Marconi, orders from the European Space Agency (ESA) for the original Orbital Test Satellite (OTS) has led to a fast filling order book. The latest Matra Marconi built Hot Bird 3 is the first of its series with a steerable antenna.

Commissioned for Eutelsat, Hot Bird is a high-powered direct broadcast satellite transmitting 160 TV channels. With a launch mass of 3tonne it was, until recently, the largest built in Europe. In the wake of this success has come similar orders from Egypt, Singapore, Taiwan, as well as from Astra and Intelsat.

To meet this heavy schedule, existing antenna testing needed radical rethinking. Options included re-jigging the Stevenage facility or, alternatively, transporting antennae to test facilities in either France, Germany or elsewhere in the UK. However, the estimated total of £60,000-£70,000 per test at these alternative sites could be halved if testing was kept in-house.

After initial discussions with Time & Precision Industries, a positioning systems specialist, design proposals were agreed and engineers from both companies were able to begin work on a new generation of antenna test rigs.

While adequate for research and pre-production purposes, the original test rig suffered from a certain lack of rigidity and its welded steel construction made it difficult to modify without time consuming and expensive reconstruction.

Initial discussions centred around a vertical rig consisting of a large aluminium ‘I’ beam carrying a bulky slide and guide-rails. However, after further consideration, a radical solution was proposed. A lightweight modular construction system, already used in any number of industrial applications would, it was thought, be an answer to the design problems met in building a structure some 5m tall that had to be relatively light in weight for mobility, yet mechanically rigid enough to avoid unwanted vibration.

MODULAR EXTRUSIONS

The result was a cross-braced 5m tower built of MiniTec lightweight modular extrusions. A vertically mounted linear slide with a 4.5m, 40mm diameter by 10mm precision rolled leadscrew was fitted with micro-switches to ensure rail linearity within 0.1mm over a 3m travel. A triple-stack 400 half-steps/rev motor was linked to a 5:1 reduction gearbox, to provide a positioning accuracy of 0.05mm/300mm travel and a repeatability of 0.0025mm.

With limited power, typically between 15-135W/channel, a satellite must re-broadcast signals to a precise ‘footprint’ on the Earth’s surface 36,000km below. To achieve such accuracy, the surface of each dish is subtly shaped so that signal strength across the receiving area is within specific limits.

A typical satellite is equipped with solar panels and on-board back-up batteries. So power is limited and great care must be taken to reduce power loss and eliminate undue signal atten-uation. Many current antennae employ two dishes in tandem.

Typically, the front dish gathers the vertical X-axis component and the rear dish the horizontal Y-axis component of the signal. In theory each dish should operate at 100% efficiency. However, in practice there is always some lowering of efficiency as the front dish inevitably traps a small percentage of the Y-axis signal as it passes through. With such small reserves and relatively weak signal strengths, great care is needed to determine exact signal strength and to minimise power loss.

OPERATING IN SPACE

In addition to coping with the need to deliver an adequate signal strength throughout a precisely defined area with very limited power, the telecoms satellite must also be able to withstand the rigours of operating in space for extended periods.

During a service life of around 15 years, each satellite will encounter intense solar radiation and extremes of temperature of 170 C. Simulating these conditions, each antenna is put through a heating and cooling cycle, with its performance measured before and after each cycle.

Detailed pre-test and post-test plotting of antenna performance is carried out in a 5m high anechoic chamber. A large vertical jig, mounted on a rotating base, holds the antenna, which can range from a 100mm diameter feed horn to a 2.5m reflecting antenna.

This allows the RF signal to be plotted as a cylindrical field as the antenna is raised and lowered inside the anechoic chamber and the sensor is rotated about it. The field plotted within the test chamber is the ‘near field’ measurement. This is extrapolated to give the size, shape and power distribution at the Earth’s surface the ‘far field’ measurement.

OUTSOURCING ADVANTAGES

The move away from in-house design and build by Matra Marconi reflects the increasing tendency of industry to outsource specialist engineering expertise, as the cost of maintaining expensive in-house teams becomes ever more prohibitive.

Such a change delivered a number of advantages that might not have been apparent at the outset of this project.

Original thinking was influenced by past experience, resulting in the tendency towards a massively engineered welded structure. By bringing in outside expertise, an entirely new design philosophy was jointly developed that resulted in employing a highly adaptable modular construction system.

Such a radical move required much discussion between Matra Marconi engineers and Time & Precision’s motion control specialists. After extensive calculation and evaluation, MiniTec showed distinct advantages over more conventional solutions. Its lightweight structure ensures that even at a height of 5m, the complete test rig would be relatively easy to manoeuvre, whilst the inclusion of numerous cross bracing elements still allowed a high level of mechanical stability. Benefiting from its patented fastening method, this modular system delivered extremely rigid joints whilst reducing cutting and machining to a minimum. This, and the use of hinged joints, enabled extruded sections to be hinged and rotated, providing a highly versatile test rig capable of quick and easy modification to meet the varying demands of diverse test schedules.