Communications to high latitudes using a commercial low Earth orbit satellite system

Low Earth orbit (LEO) satellite communications systems could be deployed providing voice, data, facsimile and paging services to mobile users in many regions of the world. The LEO systems are expected to be used by the Department of Defense (DoD). The DoD will be able to use these systems for other applications that take advantage of the special features resulting from the LEO configuration, such as global connectivity and small mobile terminal size. We consider the ability of a LEO system to support communications to mobile terminals in the high latitudes. The study is based upon the IRIDIUM system, which will provide continuous polar coverage to commercial users. The two key criteria for assessing the ability of the LEO system to support high latitude missions are detectability of the radio frequency (RF) transmissions of the mobile terminal and communications availability. Detectability is critical because there are not expected to be many IRIDIUM users in the high latitudes. Detecting a high latitude user implies some degree of automatic identification, whereas detection at a lower latitude must be followed by some identification process to discriminate between many users. To decrease the detectability of the mobile unit RF emissions, it is assumed that the mobile terminal antenna is modified to reduce low elevation angle RF emissions. Therefore, the impact on detectability, as well as connectivity, when using a modified mobile terminal antenna is examined. After a description of the mobile terminal RF characteristics, the detection and connectivity analyses are presented.