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GOES Frequently Asked Questions for Forecasters

VISIT is a joint effort involving NOAA-NESDIS Cooperative Institutes, the National Environmental Satellite Data and Information Service (NESDIS), and the National Weather Service (NWS). The primary mission of VISIT is to accelerate the transfer of research results based on atmospheric remote sensing data into NWS operations using distance education techniques.

1. What is GOES RSO?
GOES "RSO" is Rapid Scan Operations. This is a mode in which the GOES satellite can provide images over the conterminous US region approximately every 7.5 minutes (8 times per hour). This doubles the number of GOES images generated per hour (from 4 to 8) and is requested for a particular satellite.
2. If an NWS forecaster initiates an RSO request, does that impact the instruments, operations or the lifetime of the satellite?
No. There is not a technical reason why RSO would reduce instrument or spacecraft life. The same mechanical parts are used, whether the GOES imager is in RSO or a routine schedule.

The GOES Imager scan mirror moves at constant rate. Rapid scans are really no more rapid than the full-earth scans, they merely repeat a short sequence often. The moving parts on the instruments are in motion all the time anyway because the observing schedule is full. So, rapid scans do NOT impose extra wear-and-tear.

3. How does an NWS forecaster initiate an RSO request?
Call your respective regional point of contact (POC) and ask for an RSO. The POCs are at the following forecast offices:
  • Central Region - Detroit, MI
  • Eastern Region - Pittsburgh, PA
  • Southern Region - Tulsa, OK
  • Western Region - Monterey, CA
  • Pacific Region - Honolulu, HI
  • Alaska Region - Anchorage, AK

If there are no conflicting satellite operations, your RSO request will go into effect at the next available picture time (for GOES-East: 26 minutes after the hour; for GOES-West: 3 or 33 minutes after the hour).

NOTE: Procedures require at least one hour advanced notice to implement an RSO request.

4. How does a non-NWS (eg. Department of Defense) user submit an RSO request?
It is recommended that non-NWS users contact the NWS forecast office in your area and coordinate your RSO request through them. Establishing communications with the NWS will be beneficial not only for satellite requests, but for other issues as well.
5. What is the launch schedule for future GOES and POES satellites?
Latest GOES Flyout schedule
6. The AWIPS "Fog Product" looks different than RAMSDIS - Why?
It is different. The AWIPS/WFO-A product is calculated in a different way. It's also important to note that the AWIPS product is only valid at night when there is a lack of sunlight. There are instructions on how to modify the fog product in AWIPS to make it appear as it does on RAMSDIS online.
7. What are "single chord" operations?
Each GOES satellite utilizes a "dual-chord" scanning Earth sensor that provides spacecraft roll and pitch error estimates to the satellite Attitude Control System (ACS). The sensor employs two scan mirrors that measure the chord length of the Earth disk in the Northern and Southern hemispheres. As a result of the relative satellite and lunar & solar geometry’s, the moon or sun will periodically drift into one of the Earth sensor chords. If either Earth sensor chord actively scans the sun or moon, large erroneous Earth sensor inputs will be fed into the ACS, potentially placing the satellite in an uncontrolled state. Fortunately, it is easy to precisely predict the occurrence of Earth sensor intrusions well in advance. In these cases, the chord containing the intrusion will be inhibited from scanning, thus the name "single-chord" operations. In the past, spacecraft pointing errors due to "single-chord" operations were large and manually corrected via operational procedures. Presently, each GOES satellite autonomously corrects for these pointing errors, dramatically improving image navigation and registration (INR) performance during this necessary operation.

Single Chord is in reference to our Earth Sensor operations, which normally use two scans, one in the North and on in the South, for attitude control. Whenever the Moon or the Sun is about to enter the scan area, we inhibit that scan so that the offending object does not corrupt our attitude control.

8. What is a satellite eclipse period?
Since GOES is in a geosynchronous orbit, the sun will yearly traverse a +/- 23.5 degree angle perpendicular to the Earth’s equator (GOES orbit plane). As a result, near the Vernal and Autumnal Equinoxes the Earth disk will periodically occult the sun, from a GOES perspective. Essentially, there are two eclipse seasons for each GOES spacecraft. Each eclipse season spans a 48-day period, symmetric around the equinox and the sun occultation lasts for a maximum of 72 minutes/day during the equinox. Each GOES spacecraft utilizes a solar array that converts sunlight into electricity in order to power the satellite. Each day during the eclipse season the sun is blocked by the Earth and sunlight is not available to the GOES solar array. Therefore, the energy needed to power the instruments is not available and the instruments are powered off. There is typically a 0-3 hour outage of imagery each day as GOES progresses through eclipse season. The maximum outage of 3 hours will occur at or near the equinox.

GOES-13 has a newer spacecraft bus that allows it to operate through eclipse and keep out zone periods. The GOES-R series spacecraft will be designed to support full operations through the maximum geosynchronous eclipse duration of 72 minutes. All instruments will also be capable of continuous operation through eclipse.

9. What are "keep out zones"?
KOZ is an acronym for Keep-Out-Zone, another term related to eclipse season operations. The GOES imager and sounder instruments have temperature constraints that prevent them from scanning an area too close to the sun. If the GOES imager or sounder is allowed to scan an area near the sun, they could potentially overheat and become permanently damaged. Prior to and following each eclipse season as the sun is "close" to the Earth disk, image regions in the proximity of the sun will be deleted. Historically, the size of these regions, and hence Keep-Out-Zones for the GOES imager and sounder were a six degree radial region. Therefore, if the sun were predicted to be within a six-degree radius of the instrument field of view for a particular image, that image would not be commanded. More recent thermal analysis has been performed leading to a reduction in size of the KOZ, allowing images that were deleted in the past to be restored.

KOZ is the Imager and Sounder Keep Out Zone, it is times of the day centered around Spacecraft midnight when the instruments would be looking at the Sun when scanning. The KOZ starts about 10 days before each eclipse season and ends about 10 days after the last eclipse. Major concern is rapid heating of the instrument mirrors which could compromise the instruments and the mission.

GOES keep out zone / eclipse seasonal charts

10. What is a solar intrusion and why does it affect GOES?
Solar intrusions are only a concern for GOES during the eclipse seasons (See Question 7). During a 48-day eclipse season as the sun moves from North-to-South (Autumnal Equinox) and South-to-North (Vernal Equinox), it will move closer to the Earth sensor chord, eventually entering the chord’s field-of-view. In order to prevent the Earth sensor from scanning the sun, the chord containing the intrusion will be inhibited from scanning. Since solar intrusions only occur during eclipse season, there will be an image outage whenever GOES is performing single-chord operations due to a solar intrusion.
11. What is a lunar intrusion and why does it affect GOES?
A lunar intrusion is when the moon drifts into a GOES Earth sensor chord, requiring the sensor to be operated in “single-chord” mode. Due to improvements made in the flight software, there is no impact to products during this mode of operation. Each GOES satellite will typically experience 4 to 6 lunar intrusions per month, each lasting approximately 90 minutes.
12. What does GVAR mean?
GVAR stands for GOES I-M VARiable data transmission format. The GVAR format was developed to take advantage full advantage of the new GOES instruments and support variable length scan lines, while retaining as much commonality as possible with previous formats. This allows for more rapid scanning and more flexible image sectors.
13. Why aren't the NESDIS hourly skew-t plots available in a "truncated" version when clouds prohibit a complete retrieval? For instance, a lower cloud deck may preclude a skew-t from being generated, but there would be good data down to and including cloud top. Why not send what data is available...rather than nothing at all?
The reason we don't do this is due to the large vertical extent of the weighting functions for each sounding channel. All of the tropospheric sounding channels are affected to some degree by clouds, and this effect is very difficult to account for. The answer to this problem is going to an interferometer, which will provide high vertical resolution, and will enable us to sound down very close to the cloud tops.

Another answer, in addition to the interferometer, is to go to a microwave sounder, which would sense through the clouds.

14. What are the estimated accuracy of the GOES sounder-derived Lifted Index (LI) and Total Precipitable Water (TPW) products?
One way to estimate the accuracy of the GOES sounder products is by comparing them to co-located radiosondes. Of course the GOES sounder does not measure the exact same space (point measurement versus a volumetric one) or time (co-locations are required to be within one hour) as the radiosonde, but the comparisons can still be informative. In general, due to the limited number of channels, the retrieved profile will be much smoother in the vertical than a radiosonde.
  • TPW: An improvement in moisture detection is evident when comparing both sounder retrievals and the first guess to radiosondes. For example, for the period from April 1996 through February 1997 the improvement for the Root-Mean-Square Error (RMSE) of the total water vapor (TPW) is roughly 20% (3.3 to 2.6 mm).

    The RMSE = SquareRoot ((Bias*Bias) + (StandardDeviation *StandardDeviation))

  • For the lifted index (LI), the RMSE between the retrievals and the radiosondes is approximately 2 degrees. The changes in space or time may be more accurate than this value.

    For additional information: http://cimss.ssec.wisc.edu/goes/sndprf/sndprf.htm

15. How often will LI and TPW sounder-derived products be updated during convective season, and how long is the delay between availability and display due to processing time?
The sounder Derived Product Images (DPI) are available every hour (except during satellite eclipse). There is a lag of approximately one hour between the start time of the sounder scaning (e.g. 11:46 UT) and completion of the DPI processing (e.g.12:44 UT). Of course given that there is hourly sounder coverage, one can look at the previous hour if the current hour is not yet processed.
16. What is parallax?
Parallax in the difference in geolocation between the satellite-observed earth surface location and the actual cloud location. Parallax is most significant when GOES observes opaque clouds at high altitudes.
17. What is SRSO?
SRSO is the acronym for Super-Rapid Scan Operations. High temporal resolution satellite imagery is available during SRSO. Typically, this means 1 minute resolution data, although it can be as high as 30 seconds. A RSO request superceeds any SRSO requests. SRSO data is not available for viewing on AWIPS, you may view it on RAMSDIS online.
18. What conditions must be satisfied in order for a satellite to have a geosynchronous orbit?
In an ideal geosynchronous orbit, a satellite moves around the earth in the equatorial plane so that the period of revolution of the satellite is exactly equal to the period of the rotation of the earth about its own axis. Hence, to an observer on the earth, the satellite appears to be stationary. In the real world, the gravitational forces of the sun and moon tend to perturb the orbit, causing the satellite to drift latitudinally and longitudinally relative to the spacecraft’s subsatellite point. If a line is drawn from the spacecraft to the center of the earth, the point where the line intersects the earth’s surface is the subsatellite point. The basic conditions for a satellite to be considered "geosynchronous" are the following:
  1. The satellite orbit must be circular (orbital eccentricity = 0).
  2. The satellite orbit must be coplanar with the earth’s equatorial plane (orbital inclination = 0).
  3. The satellite must be at a constant altitude from the earth’s surface (orbital semi-major axis = 42164 km).

For example the GOES-8 spacecraft’s orbital parameters are controlled to within the following limits via satellite maneuvers: Orbital Eccentricity 0.0005 Orbital Inclination 0.4 degrees 42161 km Orbital Semi-Major Axis 42169 km.

19. If I call RSO, do I impact other offices?
Yes, they get RSO as well. RSO covers the CONUS sector for each satellite. See next question for the areas covered by the CONUS sectors.

20. Is RSO different for GOES-E and GOES-W?
Yes. See the pictures of the GOES RSO domains:

21. How long can RSO run?
There is no time limit (other than periodic interrupts for full disk, winds, and some other functions). If you want to call for more than 6 hours, you should determine the expected threat. In an instance where a threat continues for several hours, the requesting/coordinating regional focal point should determine a suitable time frame.

22. How does RSO help in winter storm cases?
Rapidly-changing onset and explosive development of large scale synoptic systems and the important regional and mesoscale components of these systems can be observed and tracked. Areas of significant instability and intensification can be determined in addition to the tracking of model output/forecasts. There are techniques for both heavy snow and severe weather which can be associated with these winter systems. Recall that RSO is intended as an "alert" mode in AWIPS which means that warnings and short term forecasts are the products that RSO should support.

23. What is a batwing?
Batwing was a feature discovered after we were on orbit. It is caused by stray sunlight getting into the Earth Sensor. The error introduced into the attitude control system resembles the BATWING symbol used in the movie. Satellite Motion Compensation SMC loads are uplinked into the IMC (Image Motion Compensation) logic that counteracts the pointing errors by deflecting the Imager Scan line in an opposite direction of equal proportion to the error. Batwing occurs around the solstices when the Sun declination is greater than 16.4 degrees + and -.
24. What is the bevel effect?
The Bevel is another Earth Sensor stray light phenomenon that causes the same pointing error signature as Batwing, but is an order of magnitude smaller. It is handled by Single Chord operations.
25. Does RSO get activated automatically by any forecast products from the NCEP national centers?
Yes.
If a hurricane watch or warning is in effect for CONUS coastal locations, NHC will activate RSO.
SPC will activate RSO for a Day-1 enhanced risk (or greater).