The latitude range covered by the measurements is shown in the BADC catalogue, and a typical range over the course of a year is shown in Figure 3 . A polar projection of the measurement points shows a pattern of measurements which appears to spiral towards the poles before changing direction and spiraling away. Since launch there have been a number of periods during which data collection has ceased. These are tabulated below, with comments. The vertical resolution of the data varies with channel and is not equivalent to the vertical spacing in the level 2 file.
Several factors contribute to the resolution.
Instrument optics, signal measurement bandwidth and data processing procedures are the primary factors. The resolution can be further degraded by the processing. In fact, nearly any attempt at improving the stability of retrievals has the consequence of smoothing results in the vertical. Consequently, resolution at the higher altitudes will be closer to 6 or 7 km, quickly improving as signal increases at lower altitudes. Although variable smoothing can violate conservation of signal rules, the error is a very small fraction of total estimated error.