  A systematic programme of calibration observations was carried out to monitor
the performance of the SPIRE FTS instrument on board the Herschel Space
Observatory. Observations of planets (including the prime point-source
calibrator, Uranus), asteroids, line sources, dark sky, and cross-calibration
sources were made in order to monitor repeatability and sensitivity, and to
improve FTS calibration. We present a complete analysis of the full set of
calibration observations and use them to assess the performance of the FTS.
Particular care is taken to understand and separate out the effect of pointing
uncertainties, including the position of the internal beam steering mirror for
sparse observations in the early part of the mission. The repeatability of
spectral line centre positions is <5km/s, for lines with signal-to-noise ratios
>40, corresponding to <0.5-2.0% of a resolution element. For spectral line
flux, the repeatability is better than 6%, which improves to 1-2% for spectra
corrected for pointing offsets. The continuum repeatability is 4.4% for the SLW
band and 13.6% for the SSW band, which reduces to ~1% once the data have been
corrected for pointing offsets. Observations of dark sky were used to assess
the sensitivity and the systematic offset in the continuum, both of which were
found to be consistent across the FTS detector arrays. The average point-source
calibrated sensitivity for the centre detectors is 0.20 and 0.21 Jy [1 sigma; 1
hour], for SLW and SSW. The average continuum offset is 0.40 Jy for the SLW
band and 0.28 Jy for the SSW band.
