  NICER has a comparatively low background rate, but it is highly variable, and
its spectrum must be predicted using measurements unaffected by the science
target. We describe an empirical, three-parameter model based on observations
of seven pointing directions that are void of detectable sources. An
examination of 3556 good time intervals (GTIs), averaging 570 s, yields a
median rate (0.4-12 keV; 50 detectors) of 0.87 c/s, but in 5 percent (1
percent) of cases, the rate exceeds 10 (300) c/s. Model residuals persist at
20-30 percent of the initial rate for the brightest GTIs, implying one or more
missing model parameters. Filtering criteria are given to flag GTIs likely to
have unsatisfactory background predictions. With such filtering, we estimate a
detection limit, 1.20 c/s (3 sigma, single GTI) at 0.4-12 keV, equivalent to
3.6e-12 erg/cm^2/s for a Crab-like spectrum. The corresponding limit for soft
X-ray sources is 0.51 c/s at 0.3-2.0 keV, or 4.3e-13 erg/cm^2/s for a 100 eV
blackbody. Faint-source filtering selects 85 percent of the background GTIs,
and higher rates are expected for targets scheduled more favorably. An
application of the model to 1 s timescale makes it possible to distinguish
source flares from possible surges in the background.
