from the packet header and send an echo data response command back to the source modem with the echodata the echo data response command 12 is the response to theecho data request command any modem will respond to an echo data request command by sending the echo data response message the base station will send the power mode message command 13 to a daps unit to request that the daps unit change the wake period and sleep period the u201c number of slots u201d command command 16 will be sent by the base stations to request the daps units to set the total number of time slots in one embodiment the base stations will send rtca command 17 to the daps units to convey differential gps information in another embodiment different commands can be used to convey different types of differential gps information the debug command 20 is sent to convey special debugging information to the addressed device that is parsed and handled by an object separate from the standard command hierarchy in one embodiment the system will operate with 51 time slots two slots are reserved for transmit and receive transition 6 time slots are reserved for base stations and 43 time slots are reserved for remote stations the position of each daps unit is determined by using the global positioning system.