MHz, with a bandwidth of 16.5 MHz. Both interrogations and addressed messages are contained in that data stream. Spread-spectrum transmission originally was developed for jam-resistant military systems, such as the U.S. Global Positioning System (GPS). In such transmission, the information is superposed on a carrier which alternates phase in a predetermined pattern at a high rate, producing a wide bandwidth signal (Fig. 4). In Geostar, as in GPS, that rate is about half the bandwidth. The replies from transceivers are also spread-spectrum, with the same alternation rate and bandwidth. Their carrier frequency is 1618 MHz, in the L-band. While both Geostar and GPS use spread-spectrum techniques, their system architectures are completely different. GPS is a one-way, open-loop system providing a single function: user position fixing. Geostar is a two-way, closed-loop system providing many functions, of which user position fixing is only one. But the antennas for Geostar are similar to those of GPS. They are printed circuits on the aeroplane surface, with no projection into the slipstream. Therefore, they do not impose any technical, operational or economic penalties of drag or weight. Table 1 gives the main Geostar System parameters relevant to communications reliability and ranging accuracy. Using spread-spectrum transmission and the Geostar System architecture, two or more RDSS systems can share the same spectrum. Two such independent systems each could provide the full functions of surveillance, navigational guidance, approach guidance, two-way messaging, terrain avoidance and emergency location. Short of that extreme in redundancy, a Geostar-type RDSS can provide a high degree of redundancy economically in each system segment. For the user segment, carrying two transceivers will provide full on-board redundancy for all Geostar Services. For the space segment, the satellite relays will be fully redundant internally, in accordance with normal practice for high reliability long-life space systems. A six- satellite global system will recover instantaneously from loss of transmit capability of any satellite. Receive capability requires very little weight on the satellite, and almost no power. Redundant receive capability can therefore be provided most economically by lightweight packages carried on ordinary communication satellites. With such provision, recovery from total loss of a satellite will also be instantaneous.
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