Calculation of Metop Flares

From: Gerhard HOLTKAMP (
Date: Sun May 06 2007 - 15:27:50 EDT

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    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
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