We present a new technique for measuring the stray electric field in precision space inertial sensors by modulating the electric charge of a free-falling test mass and measuring the resulting coherent Coulomb force. The free charge of the test mass is controlled by ultraviolet photoemission using a pulsed light source synchronized with an oscillating potential capacitively induced on the test mass. We can modulate the test mass charge sinusoidally at an arbitrarily chosen frequency by varying the phase of the UV light pulses relative to the induced test mass potential at the appropriate rate. This technique allows us to optimize the precision of the measurement by choosing a modulation frequency that is within the most sensitive band of the sensor. We present an experimental validation of this approach using an inertial sensor integrated with a torsion pendulum, measuring the equivalent stray potential of the sensor with milliVolt precision in ${10}^4\mathrm{s}$. We discuss the applicability of this technique for the upcoming Laser Interferometer Space Antenna (LISA) gravitational-wave observatory.

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• Gravitational wave detection
• Laboratory studies of gravity