are hand-held and are equipped with calculator-like keyboards and liquid-crystal readouts. Power for such transceivers is from penlight batteries. SYSTEM ARCHITECTURE ESTABLISHED Each transceiver has a unique digital identification code which is built in at the factory, and is included in all of its transmissions. The identifier is similar to that used in the ICAO world-wide Selective Calling (SELCAL) system. By selecting the digital address which heads each outbound message, the ground station can send a message to all transceivers, or to a group, or to a single transceiver. The satellite relays are similar to conventional communications satellites in that they have no intelligence. They simply pass the ground station's transmissions to the user transceivers, and return the replies to the ground station. The Geostar process commences with an interrogation signal (Fig. 2), sent out many times per second from the ground station and relayed through a satellite to all transceivers. The normal state of the transceivers is quiet listening with no transmission. A transceiver can be changed to an active state by any of three events: the expiration of a preset time delay, or the user's command, or the ground station sending a “wake-up” message addressed with the transceiver’s identifier. When any of those three events has brought the transceiver to the active state, it responds to the next interrogation with a short burst reply. The reply contains the unique transceiver identification code, any message which the user wishes to transmit, and also error detection codes and an error correcting code. After that reply, the transceiver turns itself off. The transceiver signal, emitted from a nondirectional antenna, is relayed by at least two Geostar satellites to the ground station. There, the times of arrival are combined with terrain height information, stored in the ground computers, to yield a three-dimensional position for vehicles, boats and hand-held transceivers. In the case of aircraft with minimal equipment, height is obtained by the pilot's entering his altitude as a message while cruising. In aircraft equipped with ICAO standard encoding altimeters or with precision encoding altimeters, barometric altitude is included in the transceiver reply, and is combined at the ground station with current data on local barometer settings for the calculation of a three-dimensional position. When replies through three satellites are available, a low precision calculation of position independent of altimetry is made, as a back-up. Position computed at the ground station is sent to the user as a message through one of the satellite relays, addressed with the identifier code of the transceiver. All messages are completed by the transmission of a positive reply, signifying that the message has been received correctly, satisfying the embedded error code. If that acknowledgement is not received, there is an automatic retry. The Geostar process takes 0.6 sec, and a further 0.7 sec if a retry is required. Since Geostar is primarily intended as a civil system, its data protocols will be published. However, the privacy of messages unrelated to air traffic safety can be assured by government-approved encryption, as is customary in civil telecommunications.
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