I processed the data. The plot below shows the signal strength for satellite 15 on the successive recording days. The SNR values have been obtained by averaging the raw SNR values from the Garmin over 2 minutes. The signal strength reaches its maximum not at the highest elevation but appr. Big Brother is again the clear winner, Mighty Mouse somewhat lower, and Lowe Active in between.
All plots show some bumps at the same time.
I have no idea about the cause, my best guess would be multipath. Drop me a line. A Beautiful Evening in the Amsterdam Forest First about the weather. I believe that we have the best weather since 1945 or so. Up to 30 deg C during the day, cooling down to 15 deg in the night.