In one implementation, a method includes receiving versions of a message from a first satellite-based receiver and a second satellite-based receiver that both received a radio frequency (“RF”) transmission of the message, the message comprising a self-reported position of a transmitter of the message. The method also includes determining a time difference between a first arrival time of the RF transmission of the message at the first satellite-based receiver and a second arrival time of the RF transmission of the message at the second satellite-based receiver. The method further includes determining a measure of the likelihood that the self-reported position of the transmitter is valid based on the time difference between the first and second arrival times. The method still further includes transmitting an indication of the measure of the likelihood that the self-reported position is valid.