Flares

 

Lacrosse 4 Sunday 23 April  2006 

Based on analysis of a flare of Lacrosse 4 on Christmas Eve 2005 I estimated its proposed radar panel was at an of 25 and that it used XVV flight mode. I also had 3 observations of Lacrosse 5 (see below) which suggested panel angles between 29 and 36 in YVV flight mode. So I tried making predictions for Lacrosse 4 on the basis of 33. The first prediction that tied up with a clear night was on Sunday 23rd April 2006. Amazingly weather conditions were good at the designated time and it was 10C in the observatory at 21:50BST. Details of the prediction are show below:

Maximum altitude 77.2 at 23/04/06 20:59:47UTC
Minimum  range km 705.845543583738
Sun Altitude -12.7
Ingress 23/04/06 21:03:19
Flare Prediction for Flight Mode XVV Panel Angle 33
 MA 0.28 at 23/04/06 20:59:24UTC Alt=71.6

A flare duly occurred at almost exactly the predicted time, as analysis of the finder tape shows. But then the satellite showed strange cyclic variations in brightness, some of which matched the flare itself. I have no idea  what caused this behaviour!

One possible aspect for the flare. The incoming sunlight (yellow) is reflected off the radar panel at rear. The panel is rotated by 33 from the x-y plane about the y-axis. The reflected ray (orange) is nearly coincident with the satellite observer vector (red). The blue blob marks the location of the observer. 

 

Background

During an observation of Lacrosse 5 on March 6th 2006 I noticed a mild flare around 20:19:05. Here is a brightness trace taken from the finder camera video. Shortly after the flare there is a drop in brightness which was probably caused by cloud.

At 20:20:54 the brightness plummets. Again I'm not sure why but it could have been cloud. 

On the graph the blue line indicates the angle between the observer-satellite vector and light reflected from the radar panel. I have calculated the rotation of the panel required to bring this angle to zero at the time of the flare.

A similar calculation is performed for the solar panel. Since the result is nowhere near zero, this panel cannot have caused the flare. 

 

This screen-capture from a VRML simulation of the pass shows the instant of the flare. As the simulated  telescope view (inset) shows the possible radar panel has been positioned to cause it to flare. 

In the orbital view the incoming sunlight (yellow) is reflected from the panel as the orange vector, which hits the ground quite near to my location (red). 

To achieve this I have a program which can calculate the rotations that a panel needs to cause a flare for any location at any time. These rotations are calculated for the x and y-axes of the satellite in the specified flight mode (YVV here). 

In reality I assume that the panel can ONLY rotate about the y-axis and the good news is that at  the observed time of the flare no rotation about x was required whereas a rotation of 36 degrees was required for y. 

Two other flare observations for Lacrosse 5 reported to me suggest a panel angle between 29 and 36 degrees. If I assume say 33 degrees I can predict future flares. Some are predicted for May. Watch this space.  

 

 

 

 

Home Up Next

Send comments & suggestions to the webmaster