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GOES Satellite FAQs
Answers provided by Nick Pinkine (NESDIS) and Brian Motta (CIRA)
(Revised 17 February 1999)		 Link to NOAA
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What is a GOES Rapid Scan Operation?
A GOES Rapid Scan Operation (RSO) is a mode in which the satellite can provide images over the conterminous US region approximately every 7.5 minutes. This doubles the number of images generated per hour and is requested for a particular satellite.

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 INR performance during this necessary operation.

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.

What is a lunar intrusion? How 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 (See question 6). 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.

What is a solar intrusion? How does it affect GOES?
Solar intrusions are only a concern for GOES during the eclipse seasons. 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.

What are Keep Out Zones?
Keep-Out-Zone (KOZ) is 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.

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created by Bernard N. Meisner
last update: 15 May 2000