See the updated graphics for ARISSat height here:

http://www.qsl.net/py4zbz/arissat.htm#r

Often, ARISSat switch from high power to low power mode when

passing over my QTH in Brazil, despite being fully illuminated by the sun.

Coincidentally, this happens when he enters the SAA, South Atlantic Anomaly!

Is there any relationship?

73 de Roland PY4ZBZ