# SWSU-55 satellites – determination of the spin rate

I am currently working on determining the spin rate of the SWSU-55 satellites. You see from most SSTV images that all SWSU-55 satellites spinning very fast.
Now we are lucky and can read the angular velocity (xyz axis) from the beacon. This shows that the satellite is rotating in all 3 axes, i.e. it is tumbling.
If you look at the field strength, you can see a period of 4.5 sec. I assume that this is one rotation. This results in a value of 13.3 rpm. Based on the SSTV image you can also see 8 noise dips (lines) within the 36 seconds.

The angular velocity can be computed as w = sqrt(Vx^2 + Vy^2 + Vz^2) and the rotation axis can be computed as the unit vector (Vx, Vy, Vz)/w. So we see similar rotation values. This is what we get from the gyros, and also from the fades in the signal power and SSTV image. Based on the radiation pattern of a dipole I assume the assignment to the axis as shown in my drawing below. … and we still observe the Dzhanibekov effect, also called the intermediate axis theorem or tennis racket theorem describes the behaviour of a rigid body with three distinct principal moments of inertia.

```13:08:39 RS5S>ALL>UI,C,F0 (1124 baud): SWSU-55 #7 S=290 U1=4.51 U2=4.37 T1=12.72 Mx=9.643372 My=32.069817 Mz=-48.665386 Vx=-38.816795 Vy=56.412212 Vz=45.847328 Time=1080475103 R 13:09:18 RS5S>ALL>UI,C,F0 (1124 baud): SWSU-55 #7 S=291 U1=4.48 U2=4.45 T1=13.04 Mx=-16.819834 My=4.709554 Mz=-3.812496 Vx=-48.839695 Vy=-9.167939 Vz=70.931297 Time=1080510009 R 13:12:40 RS5S>ALL>UI,C,F0 (1127 baud): SWSU-55 #7 S=294 U1=4.43 U2=4.28 T1=14.01 Mx=7.400727 My=-52.253620 Mz=-47.319801 Vx=-48.114502 Vy=6.099236 Vz=-69.580154 Time=1080625225 R 14:42:50 RS5S>ALL>UI,C,F0 (1124 baud): SWSU-55 #7 S=391 U1=5.00 U2=4.98 T1=12.72 Mx=25.117619 My=42.161720 Mz=34.536728 Vx=-52.977100 Vy=51.541985 Vz=-45.351147 Time=1085566194 R 14:46:13 RS5S>ALL>UI,C,F0 (1125 baud): SWSU-55 #7 S=394 U1=4.48 U2=4.46 T1=13.37 Mx=-16.595570 My=4.036760 Mz=11.213223 Vx=-45.190838 Vy=-70.725189 Vz=-12.167939 Time=1085680123 R 14:51:34 RS5S>ALL>UI,C,F0 (1126 baud):```