We have estimated the observed system response of STS-1 sensors installed at stations of the IRIS/IDA Seismographic Network using cross-spectral analysis of random binary calibration signals. Systematic deviations of up to 1% in amplitude and 1° in phase between the measured and the manufacturers “nominal” responses are observed in the range 0.2 to 100 mHz and up to 12% in amplitude and 5° in phase for frequencies above 5 Hz. A parametric analysis of the STS-1 seismometer is developed that leads to a more complete pole and zero model of the sensor, accounting accurately both for small changes in individual instrument parameters and for the effects of an incomplete compensation of the mechanical response by the feedback circuit. This model is then used to fit the observed responses with a precision of 0.2% for the amplitude and 0.1° for the phase over the entire useful bandwidth of the instrument.

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