During most of its passage through the dayside zone, SCIAMACHY carries out an alternating sequence of limb mode and nadir mode measurements.

Fig 1.3.2.2.2.1: SCIAMACHY nadir mode scanning.

Image courtesy of :

In nadir mode SCIAMACHY scans a region of about 960 km across track by 450 km along track. This scan lasts approximately one minute and is known as a 'state', since it is a sequence of individual measurements that are performed with identical instrument parameters (such as integration time and, therefore, horizontal resolution). SCIAMACHY then carries out a limb scan of a similar size, and then another nadir scan, and so on. About 20 nadir regions are scanned during the dayside of each orbit, each nadir region being separated by a limb scan region.

Fig 1.3.2.2.2.2: SCIAMACHY limb mode scanning.

Image courtesy of :

The alternation of the two modes is arranged so that the same atmospheric region is measured in both modes within a short interval. The limb scan obtains data from the stratosphere 3290 km ahead of the current nadir position. Since the instrument is travelling at a ground speed of 7.5 km/s, after approximately 435 s (7.25 min, during which the instrument continues to alternate nadir and limb scans) SCIAMACHY arrives over the same atmospheric region that was previously scanned in limb mode and now scans it in nadir mode.

Because it is downward-looking, the nadir mode scan obtains data that relates to the light path through the entire atmosphere as well as interactions with the surface.

SCIAMACHY therefore observes the same atmospheric region in two different ways within a period of about seven minutes. The concentrations of NO₂ in the stratosphere retrieved from the limb mode scan can be subtracted from the 'total column' concentrations measured by the nadir mode scan to produce the concentrations of NO₂ in the troposphere. The way this is done is described in the 'Retrieval procedures' section of this module.

Limb/nadir matching is one of SCIAMACHY's most important and novel features.

Fig 1.3.2.2.2.3: Dual-mode scanning of the same air volume by SCIAMACHY.

Image courtesy of :

The alternating limb/nadir mode scanning of a swath width of 960 km means that global coverage at the equator is achieved after 6 days.

Fig 1.3.2.2.2.4: Earth surface coverage of SCIAMACHY in dual-scan mode.

Image adapted from :