(a)    The following are eligible to participate in the DROP: a covered police employee eligible to receive retirement benefits under Section 18-42(a), or a covered firefighter employee eligible to receive retirement benefits under Section 18-67(a).

    (b)    A covered employee eligible under subparagraph (a) may voluntarily make an election to participate in the DROP at any time during the period beginning on the effective date of the DROP and ending on the day prior to the sunset date provided for in Section 18-91(a).