If you have questions about using polar satellite data, please e-mail them to Patrick Dills (dills@ucar.edu) or Sherwood Wang (woody@ucar.edu). We will continue to expand this FAQ as the need arises.
Answer: Progression of orbits and orbital periods are not exactly the same between two satellites. When satellite orbits overlap, it leaves other areas without coverage. Consequently, on some days orbits overlap considerably leaving large gaps, while on other days the smaller overlap results in no data gaps.
For more information see the following pages:
AMSU & SSM/I Scan Strategies
AMSU & SSM/I Footprint Plots
Answer: The SSM/I scans in a conical pattern, taking measurements in 102-degree portions/arcs of each circle. Due to quality control and sometimes individual scan measurements that are simply missing, these data voids translate into this arc pattern.
For more information see the following page:
AMSU - SSM/I Scans and Table
Answer: The strong emissivity contrast between land and water, as well as the rather large FOVs of the measurements, cause the ocean algorithms to suffer from land contamination as the measurements approach the coastline. There are several techniques under development to help account for the fractional amount of land in the FOV and allow for retrievals closer to coastlines.
For more information, see the following page:
Edge Effects in Microwave Products
Answer: The new AMSU contains several channels between 23 and 150 GHz that will allow for the development of products similar to those with SSM/I. For a look at real-time AMSU products already being generated, visit either the CIRA or NESDIS Microwave Sensing Group Team Web sites via links in the module's Resources section.
For more information see the following page:
Microwave Spectral Channel Descriptions
Answer: The passive microwave channels used to retrieve winds detect only the changes in ocean surface roughness (foam, capillary waves) caused by the wind. Therefore, AMSU and SSM/I cannot retrieve wind direction. Wind directions are typically filled in from other sources such as the GOES cloud-track winds, surface wind observations, and pressure or model analyses.
For more information see the following pages:
Ocean Surface Wind Speed
Ocean Surface Wind Speed Application
Answer: Not very reliably. There is simply not enough "unique" information in the satellite measurements to separate snow on the ground from falling rain. Since both snow cover and rain tend to strongly scatter microwave radiation at the higher microwave frequencies used for their retrieval, their signatures also appear very similar. Unfortunately, the same applies to falling snow over snow on the ground.
For more information see the following page:
Rain Rate-Ocean Versus Land