Our goal is to measure the gravitational constant G to within 1-10 ppm using a high-finesse Fabry-Pérot optical cavity. The two high-reflectivity mirrors of the Fabry-Pérot cavity will be suspended separately by two nearly identical simple pendulums of length about 1 m. For the source of attraction, we have built a rotating mass source with 12 lead bricks (each brick is of mass 11.5 kg) on each of two opposite sides of a rotating table. The period of rotation can be adjusted. We normally use a rotation period of 200 s. For this rotating mass, the relative equilibrium displacement of the two mirrors is over 10 nm and we are working for a preliminary measurement of this distance to one part in 10⁴. Auxiliary quantities such as the positions of the test mass and the positions of Fabry-Pérot cavity mirrors can be monitored using real-time laser metrology. The stability and real-time monitoring of the pendulums, building stability, and the compensation or averaging of mass inhomogeneity need special considerations.