Regional and Mesoscale Meteorology Branch

GOES stands for Geostationary Operational Environmental Satellite. It has a geo-synchronous orbit in that it remains over a fixed point on the earth above the equator. In order to do this, the orbit must be 36,000 km above the earth. This is in contrast to the POES or Polar-orbiting Operational Environmental Satelllites, which does not remain at a fixed point over the earth – it has a sun-synchronous orbit (follows the sun). It operates at a lower altitude of 850 km above the earth. The diagram represents some, but not all of the geostationary satellites positioned around the globe which are operated by the European community (Meteosat), the Russian Federation (GOMS) and Japan (GMS). There are also many polar orbiting satellites that are represented here by only one satellite to demonstrate the 850 km orbit. (Note the image is not drawn to scale).

You should be familiar with the first item listed under Environmental Sensing – obtaining the data, relaying it to the ground and redisseminating it. You may not be aware that the GOES satellite is tasked to do many things which help with forecasting.

This is a pictoral representation of some of the other tasks of GOES. For Environmental sensing other that the imaging and sounding capabilities that we will be focusing on shortly, there is the Space Environment Monitor. It monitors the occurrances of events on the sun that will affect the electromagnetic environment surrounding the earth. These space weather events could impact the functioning of the satellites and communications relying on satellites. In the area of data collection, the satellite relays realtime data from a variety of platforms. The Tsunameter Mooring System is pictured here.

For Data Broadcast, the GOES satellite relays distress dignals from those who are lost or need help (and have the emergency transmitters).

Because there is repeat coverage of the same location on the earth in a matter of 15 – 30 minutes during normal operating conditions, the GOES satellite is ideal for viewing the weather and it’s evolution. There are many scales of weather interaction and the GOES satellite provides the opportunity to view starting at the large scale with the full disk down to the mesoscale which we can look at with higher resolution in the visible imagery and coarser resolution in the infrared. Note the resolution is defined in reference to nadir, as if you were looking directly down to the earth at a 90 deg angle. The footprint that is viewed is larger as we go away from the equator and have larger viewing angles. It will also introduce a parallax effect which will make the top of the cloud appear at a different location than it actually is. We won’t be going into all the details about these effect right now. You will come across these concepts in other modules as the need to discuss them arises.

Ultimately what resolution – spatial and temporal – you are seeing has come about from a number of decisions of which the cost is often the greatest limiting factor. It takes a long time to plan and launch a satellite. We’re currently utilizing technology that is at least 10 years old. As will be shown in coming slides, the imager has better resolution in the horizontal (smaller spatial footprint and extensive full disk coverage) and the sounder has more detail in the vertical (more channels placed in specific wavelength regions to capture vertical detail)

Depiction of the GOES 8-12 spacecraft showing the location of the imager and sounder instruments.

Each GOES satellite has 5 channels: GOES 8-11 have channels 1-5, GOES 12 has channels 1-4 and 6. (GOES 10 and 12 are currently operational.) This was done to avoid some confusion between channel and wavelength. We generally refer to the channels by their central wavelength even though it represents a broader wavelength band. The characteristics of the channels will be reviewed in the GOES channel selection module.

The sounder instrument was designed to provide more information in the vertical. The nominal ground resolution at nadir is 10 km (coarser than the imager) and there are 18 channels to provide measurements to infer moisture and temperature in the atmosphere profile along with a visible channel to provide information on cloud top and surface features. More will be said about the sounder in a later module.

Where are the satellites located? To provide the ideal coverage over the continental US. The most useful imagery for viewing is within a 60 degree circle from the satellite subpoint. The communication range is larger (as long as you can keep a clean line of site to the satellite).

Here are the GOES east and GOES west routine scan sectors. Note the size and time it takes for the satellites to complete a scan. The satellite scans from north to south while performing scans from west to east and back again (from east to west). Due to the different areal coverage, the sectors are different between GOES east and GOES west. Both schedules are set up to give coverage over the continental US every 15 minutes (except when there is a FULL DISK image) and coverage over the N. Hemisphere every half hour. The FULL DISK scan occurs every 3 hours.

This hour of schedule is a good example of what to expect throughout the day. In terms of coverage over the CONUS, essentially there are 4 images per hour except for when there is a FULL DISK image. A FULL DISK image takes 26 minutes to complete and therefore limits the number of CONUS images to 3 per hour. For GOES west, the FULL DISK occurrs every three hours at the top of the hour: 00:00, 03:00, 06:00, 09:00, 12:00, 15:00, 18:00, 21:00. For GOES east, the FULL DISK occurs every three hours at 15 minutes to the hour: 02:45, 05:45, 08:45, 11:45, 14:45, 17:45, 20:45, 23:45. The schedule is also disrupted at the beginning of the day (00:00 hour for GOES east and 03:00 hour for GOES west) when extra time is used for spacecraft checks. See the full schedule for more details. GOES schedules

A rapid scan schedule of the satellite is called in support of environmental and operational events that warrent additional satellite imagery. This is what to expect for satellite coverage. GOES schedules

In order to get 8 images per hour over the CONUS, what is changed in the scan schedule? Differences for GOES West: the southern hemisphere sector is smaller and there is an additional smaller CONUS sector called the subconus sector. If you are along the west coast, you will notice that these subconus sectors are resopnsible for the cropped images over the Pacific during an RSO. Differences for GOES East: there is no Extended Northern Hemisphere sector and there is a much abreviated southern hemisphere sector. When there is a rapid scan, most of the Southern hemisphere/South America only gets imaged every 3 hours (during the Full Disk scan).

SRSO is the acronym for Super-Rapid Scan Operations. SRSO is a special imaging mode of the GOES satellite which is currently used for research purposes. The sector generally takes 1 minute to complete. Only two srso’s have ever been called for GOES WEST – both were for hurricanes and occurred 4-10 years ago. If there ever is an srso again, and you are along the west coast and not in the SRSO sector, like the rapid scan mode, you will notice when there is a Super Rapid Scan because the smaller subCONUS sector results in limited coverage over the Pacific. If you are in the east and you are not in the SRSO region, you may not even notice that a SRSO is occurring because CONUS coverage will be normal. The coverage during FULL DISK scans is a little variable during SRSO. See the full schedule for details. GOES schedules

The most important thing to notice is that Sounder coverage does not cover the full disk. For GOES west, the CONUS sector occurs every hour. The other sectors alternate with the CONUS sector. The most frequently scanned sector is the N. Pacific (off 2 hrs, 4 consecutive scans), then Hawaii (2 consecutive scans, off 4 hrs). The Mexico sector is not done routinely. For GOES east, the CONUS sector occurs every hour. The E. Caribbean, Gulf of Mexico, and N. Atlantic Sectors alternate with the CONUS sector: E. Caribbean (1 every 6 hrs.), N. Atlantic (1 every 6 hrs), then 4x Gulf of Mexico (off 2 hrs, on 4 hrs. except at end of day only 3 hrs.)