The Sounding-Rocket Principle Of Equivalence Measurement (SR-POEM) will test the weak equivalence principle (WEP) to 2 × 10~(-17) g in an experiment launched into free fall by a sounding rocket. The high sensitivity is possible in a short time because: (1) our laser distance gauges measure to 0.1 pm in 1 second; (2) the high measurement speed allows us to keep the temperature of the critical region stable to within a few μK using two cascaded thermal low- pass filters; (3) the spacing between the physics package and the test masses is kept constant by virtue of a servo (but not a drag-free satellite); (4) the test masses (TMs) are unconstrained during drops, avoiding constraint-force imperfections; and (5) the position measurement is to a plate that is almost stationary with respect to the TMs (by virtue of the position servo and the initialization of the TMs).
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