System for providing alert-based communication services for which corresponding alert conditions to be met by mobile stations are defined. The system includes an alert engine capable of firing alerts associated with the alert-based communication services if location data regarding the mobile stations is indicative of the mobile stations meeting the alert conditions corresponding to the alert-based communication services. A requirements engine is provided for determining an expected earliest future time at which at least one alert condition is capable of being met by a particular mobile station and outputting a data element indicative of a requirement to obtain updated location data about the particular mobile station in advance of the expected earliest future time. Also provided is a scheduler for receiving expiry times data indicative of a plurality of expiry times relating to respective location requests, processing the expiry times data for determining an order for servicing the location requests by positioning determining equipment (PDE) at least in part on a basis on the expiry times of the location requests and an output for interfacing with the PDE, allowing it to service the location requests according to the order determined.