The attitude mode of ENVISAT for nominal operations is the stellar yaw steering mode. Yaw steering is required by the radar imaging instruments in order to compensate for the rotational velocity of the Earth’s surface in the sub-satellite point. This mode is obtained via small rotations about the roll, pitch and yaw axis, with the transformation around yaw being by far the dominant component. Maximum yaw amplitude of ±3.92° occurs when ENVISAT passes the equator at ascending or descending node. Close to the poles it reaches 0°, i.e. yaw steering imposes a sinusoidal ‘wobble’ of the platform around the flight direction. On the dayside part of the orbit the platform is turned to the right side of the flight direction, in the eclipse part to the left. For measurements with long Line-of-Sights requiring high spatial resolution, particularly when SCIAMACHY is operating in limb mode, the quality of the scientific results, e.g. altitude profiles, is strongly dependent on the precise knowledge of the platform attitude.
ENVISAT On-board Resources
The total ENVISAT payload shares the same on-board resources. Thus instrument operations must be designed such that no interferences occur. For SCIAMACHY this is particularly important in respect of the allocated data rate. Generated data rate depends on an instrument and its measurement mode. The Regional Mission of ASAR (Advanced Synthetic Aperture Radar) and MERIS produces data with high and medium rates whilst the Global Mission, with its lower data rate, is established by all other instruments together with ASAR and MERIS in low rate modes. Global Mission instruments are operated continuously throughout the orbit. Measurement data from the Global Mission instruments are processed on-board via the High Speed Multiplexer (HSM). SCIAMACHY uses HSM resources together with MERIS in Reduced Resolution (RR) mode (fig. 2-3). As long as MERIS runs in RR mode, only the low rate data can be generated by SCIAMACHY. This MERIS mode is operational for solar zenith angles < 80°. Although SCIAMACHY is able to generate data with a data rate of 1.8 Mbit/sec yielding measurements with high spatial resolution, for most parts of the orbit only the low rate of 400 kbit/sec is therefore possible. Only when ENVISAT is close to sunrise during each orbit, the solar zenith angle condition is not fulfilled for MERIS and measurements with the higher rate are possible.