Hi, Jacob,
Thanks, Yes, indeed, the time is meaningful to the data. We can easily find the periodic waveform from the image, In effect, one cycle is just a day, as the records generated every half an hour, so 48 records together make up one cycle.
Thanks for your patient and detailed response, thanks sincerely.
I have learned much from your answers. But I am still in puzzle about the question below, if you will give me a hand, I will appreciate it greatly.