GAIA testbench: monitoring the basic angle with micro-arcsecond accuracy

A test-bench simulating the GAIA metrology system has been developed by TNO-TPD. The lines-of-sight of the two telescopes in the GAIA instrument are separated by an angle (called `basic angle') of about 106 degrees. The basic angle stability should be within 10 microarcsecond rms over the satellite revolution period of 3 hours, or should be at least known with this accuracy. The basic angle is monitored by a high-precision interferometric laser metrology system.