Note: A total of 55 proposals have been filed with the International Telecommunications Union (ITU).

Note: A total of 55 proposals have been filed with the International Telecommunications Union (ITU).

13.5.3 Antijam Techniques

Because the satellite is usually in view of a large segment of Earth, RF interference from Earth-based transmitters, either unintentional or deliberate, may occur. The frequency allocation procedures discussed previously minimize unintentional interference. Intentional interference, or jamming, is of particular concern in military applications. Jamming consists of transmitting a large modulated carrier to the receive terminal at approximately the same frequency, overwhelming the desired signal and thus disabling the link.

We can reduce the effects of jamming by using spread-spectrum modulation techniques [Dixon, 1984] to spread the transmitted signal in a pseudorandom manner over a bandwidth much larger than the data rate. The receiver takes advantage of the fact that he knows the code used to modulate the transmission while the jammer does not A replica of the pseudorandom waveform is generated at the receiver and correlated with the received signal to extract the data modulation. Using this method, we can reduce the received jamming power relative to the desired signal by the ratio of the spread-spectrum bandwidth to the unspread signal bandwidth. For example, by hopping a BPSK-modulated signal of 100 bps over 1 MHz, the jamming power, on the average, is reduced by a factor of approximately 10,000, or 40 dB.

In a communication relay satellite, onboard processing is highly desirable to despread the received signal before retransmitting it on the downlink. Otherwise the uplink jamming signal will capture most of the satellite transmitter's power, leaving little for the signal. Another technique for countering uplink jamming, employed by the DSCS-ni, is to generate a null in the antenna beam pointed toward the jamming source. This technique can lower the jamming power by 20 to 40 dB relative to the power of the received signal.

The satellite crosslink may also be jammed. The satellite can reject jammers located on Earth by using narrow antenna beams pointed away from the Earth. Operating at 60 GHz takes advantage of the oxygen absorption band, thus shielding the satellite from the Earth. Crosslinks may also use spread-spectrum and antenna-nulling techniques.

13.5.4 Security

A characteristic of space-ground communication is the ease with which the link can be intercepted by an unauthorized user, who may receive the data for his own use, or, even worse, take control of the satellite by transmitting commands to it Data encryption techniques help us avoid these problems by denying access to the data and the satellite command channel unless the user has the correct encryption key. Recent developments have led to complete encryption and decryption devices being placed on single VLSI chips, thus adding little to satellite mass. Tbe main issues are distributing the key and synchronizing time. To make sure a link remains secure over the life of the satellite, the encryption key must change at regular intervals, because others will monitor and eventually uncover it

The receiver decryption device must be accurately synchronized with the transmitter encryption device to recover the original data. With some systems, both the satellite and ground station may need a very accurate atomic clock, especially if the data rate is high and one must acquire the signal within seconds. For command links to stationary satellites, the data rate is low and the acquisition time can long. For this application, crystal oscillators are accurate enough. An alternative to atomic clocks is a GPS receiver, which automatically synchronizes itself to the GPS time standard.

13.5.5 Diversity Techniques

Diversity techniques consist of transmitting or receiving the same signal more than once to increase the probability of receiving the signal correctly. For example, a satellite may transmit a signal simultaneously to two ground stations. If the distance between these ground stations is greater than about 5 km, the probability of high rainfall attenuation existing at both locations at the same time is small. This technique is called spatial diversity and is an effective way to increase the availability of a satellite-ground station link operating at frequencies above 20 GHz, where rain attenuation can be large [Ippolito, 1986].

A second example of diversity is to transmit the same signal two or more times at different frequencies or time intervals. For example, multipath fading may be caused by reflections of the signal from parts of the aircraft structure in a satellite-to-aircraft link. The multiple reflected signals will interfere with the main signal, causing the amplitude of the received signal to vary with frequency and time as the aircraft moves. Use of frequency or time diversity will increase the probability that the message will be received correctly in at least one frequency or time interval. In military applications, frequency hopping over one data symbol provides frequency diversity protection against partial band jamming. Forward error correction coding followed by interleaving (e.g., scrambling the order in which the bits are transmitted) provides a form of time diversity protection against a pulse jammer. Time interleaving improves the decoding performance by randomizing a burst of errors caused by pulse jamming.

A third example of diversity is a technique used by Globalstar, "satellite diversity." In this case a ground station talks to a UT with circuits through two separate satellites. This is not to avoid rain outages as discussed above, but rather outages caused by blockage from buildings or trees for mobile UTs. If the path to one satellite is temporarily interrupted, the power is increased on the other link to maintain the contact Also, when one satellite is "setting," this technique maintains the conversation through the "other" satellite while a new "rising" satellite can begin to carry a circuit. In this way, handover between satellites is transparent This is accomplished by using a RAKE receiver. A RAKE receiver has the property of having several parallel digital processing channels which can correct for Doppler frequency offset and time delay so as to combine several digital signals in time alignment for the maximum signal-to-noise ratio. The technique requires two ground antennas, which are putting many circuits through each satellite simultaneously.

13.5.6 Optical Links

In recent years lasers generating narrow band energy at optical frequencies provide an attractive alternative to the microwave frequencies discussed above. Unfortunately, clouds and rain seriously attenuate optical links. Therefore, optical links have limited application in satellite-Earth communications. However, an optical link is well suited for communication between two satellites. Intersatellite links have been designed using optical links with capacities above 300 Mbps.

Optical crosslinks are superior to microwave crosslinks for high data rates because they can obtain extremely narrow beam widths and high gains with reasonable size (see Fig. 13-19). Also, frequency allocation problems do not exist in the infrared or visual bands. On the other hand, the narrow optical beams are difficult to acquire and point accurately, requiring complex and sometimes heavy pointing mechanisms.

Figure 13-20 compares RF and laser crosslinks, demonstrating that RF links are generally better for data rates less than about 100 Mbps because of their lower mass and power. However, development of more efficient lasers and lighter steerable optics may soon make lower rate optical links attractive.

Fig. 13-19. Optical Systems (that Is, Direct Detection and Heterodyne) Require Smaller Antenna Diameters Compared to RF Crosslinks. [Chan, 1988].

One application of optical links between satellites and Earth is die blue-green laser link being developed by DARPA and the U.S. Navy for submarine communications [Weiner, 1980]. The laser frequency of 6 x 1014 Hz was chosen for its ability to penetrate sea water. Even so, the water loss can range from 5 to 50 dB or more, depending upon the actual depth of the submarine. In addition, loss due to cloud scattering is

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