Another nice Metop-Flare here last night (5-MAY-07, 20:01:16 UT) at mag -2 next to Mizar. There now has been a fair number of observations of these flares to confirm my way of calculating them. For the benefit of those who would like to come up with their own programs here is how to do it: Metop can produce Iridium-style flares with its ASCAT (scattering radar) antennae. There are six such antennae, three pointing to the left of the flight path and the other three pointing to the right. Those pointing to the right never receive sunlight and cannot produce flares (and they are coated differently from the left-pointing antennae for passive thermal control). The antenna surfaces point into the following (Yaw, Pitch) directions: (-45, -46.3), (-90, -56.5), (-135, -46.3). In this same system the directions of the Iridium MMA antennae are (-120, -40), (0, -40) and (120, -40). If you have a program to calculate Iridium flares you could substitute the Iridium MMA angles by the corresponding Metop ASCAT angles. But you have to do one further modification to get correct results: Metop operates with a modified local attitude called Yaw Steering Mode. In this mode the satellite will yaw to allign one axis with the ground trace velocity vector (rather than with the inertial velocity vector as do most other satellites). (Also the nadir pointing direction is with regard to the local normal of the Earth reference ellipsoid rather than the center of the Earth.) The difference amounts to 4.6 deg at the equator to 0 deg at the apex near the poles. Once you account for this extra tweak you can proceed as you would with calculating Iridium flares. Due to the smaller surface of the ASCAT antenna compared to the Iridium MMA you should reduce the calculated magnitude by about 1 or 1.5 mag. Due to its special sun-synchronous orbit Metop flares show some peculiarities: They never can be seen during the winter except for extreme high latitudes. During the summer mid- or high-latitudes will not see any (-90, -56.5) flares. Flares from the (-45, -46.3) surface can only be observed from the Northern Hemisphere and (-135, -46.3) surface flares can only be seen on the Southern Hemisphere. A brief increase in brightness (not necessarily a flare) has often been observed each time the Yaw angle of Metop needed to produce a flare at that position is at -45 degrees (and probably -135 deg on the Southern Hemisphere) - independent of the Pitch angle. Some reports mention a distinct reddish color to it. Gerhard HOLTKAMP Darmstadt, Germany 49.8822 N, 8.6558 E ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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