The distance D from the ground follows a sine law, which begins at D=5 where t=0 and reaches a maximum of D=25 where t=30.
This implies that the sine function has an amplitude of 25-5=20, and a period of 30 seconds.
The resulting function is

In fact, this function:
Has a value of 5 where t=0:

Reaches the maximum at t=30:

Resets after 60 seconds:

So, the period is 60 seconds, the midline is 15 (the middle value between 5 and 25), the amplitude is 20, and there is no horizontal phase shift.