The X-ray monitor consisted of 2 collimated argon proportional counters. The instrument bandwidth of 3–10 keV was defined by the detector window absorption (the window was 0.01 inch beryllium) and the upper level discriminator. The active gas volume (P-10 mixture) was 2.54 cm deep, providing good efficiency up to 10 keV. Counts were recorded in 2 energy channels. Slat collimators defined the field-of-view for each detector; the long axes of the FOVs were perpendicular to each other. The long axes were inclined 45 degrees to the scan direction, allowing localization of transient events to about 1 degree. The centers of the FOVs coincided, and were pointed 40 degrees below the scan equator of the wheel in order to avoid scanning across the Sun. The spacecraft wheel rotated once every 6 seconds. This scan rate corresponds to 1 degree every 16 milliseconds; counts were telemetered in 64 or 32 millisecond bins to minimize smearing the collimator response.

The platform spin vector was perpendicular to the plane of the Sun-synchronous orbit. Because the detector FOV was not symmetric about the spin vector, the scan pattern repeated with a 1 year cycle, rather than the 6 month cycle of the HEAO platforms. Some sources, as Vela X-1, spend as much as 2 months within the half response bands.

P78-1 was in a noon-midnight, Sun-synchronous orbit at 600 km altitude. The orbital inclination of 96 degrees implied that a substantial fraction of the orbit was spent at high latitude, where the particle background prevented detector operation. In-flight experience showed that good data were obtained between 35 degrees N and 35 degrees S geomagnetic latitude outside the South Atlantic Anomaly. This yields an instrument duty cycle of 25–30%. Telemetry data were obtained for about 40-50% of the orbits, yielding a net data return of 10–15%. Though this data rate appears low, it means that about 10⁸ seconds of good data reside in the XMON data base.

The instrument parameters and data yield imply a 3 sigma point source sensitivity of 30 UFU in one day’s operation (1 UFU = 2.66 × 10^-12 erg/cm²/s/keV). Each detector was about 0.1 of the area of the Uhuru instrument. The instrument background at low geomagnetic latitudes was ∼16 ct/s. Of this background, ∼6 ct/s comes from the diffuse cosmic X-ray background, with the rest being instrumental. Assuming a conservative 10% data return, the net source duty cycle in scanning mode is 1.4 × 10^-3, implying a source exposure of 120 seconds per day. For a background of 16 ct/s, the 3 sigma error in determining the flux from a given sky bin is then 4.5 ct/s, or about 45 UFU, after 1 day. A limiting sensitivity of 30 UFU can be obtained by combining both detectors. A comparable error will exist in the flux determination for moderately bright galactic sources. Source confusion due to the 5 degrees FOV projected along the scan direction will complicate the observation of sources in the galactic bulge region (approximately |l| < 30 deg, |b| > 10 deg).