The Last Minutes on June 25, 1998 Before the Loss of Telemetry

Matra-Marconi Space UK in Bristol (the subcontractor responsible for the Attitude and Orbit Control System of SOHO) has performed a dynamics simulation of the last ESR (Emergency Sun Reacquisition) which matched the available telemetry from the three Sun Acquisition Sensors (SAS1, SAS2 and SAS3).
From this simulation the simulation section at ESTEC (TOS-EMM) has taken the period 100 - 268 seconds and produced a 4800 frane MPEG animation. This animation covers the period June 25, 1998 04:40:35 -- 04:43:23 UTC: from the start of the deviation of the sun till just before loss of telemetry.
At entry of ESR (t = 0 in the simulation) SOHO was already spinning at a rate of 7.2 deg/sec around its X axis and 5 degrees off pointing from the sunline
The normal observing attitude of SOHO is the +X axis pointing toward the Sun and the +Z axis pointing toward the ecliptic north pole.  A positive roll angle is a roll from Y to Z. A positive pitch angle is a roll from Z to X, and a positive yaw angle is a roll from X to Y. 

Note also the position of the Sun Acquisition Sensors (SAS). SAS1 is located on top of the spacecraft with its line of sight in the +X direction. SAS2 is located at the bottom facing +Z and SAS3 on the other side facing -Z. In the figure only SAS3 is marked SAS. although SAS2 is also visible next to the low gain antenna on the +Z face.


In the figure below you have the attitude history of the first 400 seconds of the simulation. It is represented in the form of Euler angles in the 123 sequence:
The roll angle (phi) shows the spin: angle wrapping around from -180 to +180 degrees. The other angles show the deviation from the sunline. From the simulation results we also computed what the 3 SAS sensors should have been seeing. In the 3 plots below you will find the theoretical and the telemetry values.

The red plus signs represent the processed telemetry values. Note that when the sun is out of the field of view of a SAS it will return a zero value. You see a telemetry point like that in the Beta output from SAS1. Based on the corrspondence between telemetry and simulation results we believe that the simulation and the resulting animation show what happened fairly accurately.
The animation is made looking from the Sun toward SOHO. It is done in an orthographic projection, meaning if a part of the solar array is obscured then it will actually be in the shadow. 
The red, green and blue arrows sticking out of the central body represent the +X, +Y and +Z axes respectively. The sun vector is represented by a yellow arrow. Since we are looking from the sun toward SOHO it shows up as a yellow dot in the center of the image. 
The actual animation is in the file  soho.mpg (8700 kByte). It is recorded with 35 msec between frames. To get a real-time impression you will need to instruct your MPEG player to play the animation at 30 frames per second. 

This page has been written by Ton van Overbeek,

Last modification: June 04, 2002
Feedback and comments about this site, please direct to the
SOHO Webmaster.
Questions about our mission, the spacecraft and science, please check: Dr. SOHO.

Click here to escape from frames