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Satellite Crosslinks

"No limit In exclusively military band of 7.70-7.75 GHz.

"No limit In exclusively military band of 7.70-7.75 GHz.

A criterion for frequency band allocation is the potential for one link to interfere with another. Extensive analysis is required when applying for a frequency band and orbit to avoid interference with, or by, existing services such as terrestrial microwave links and ground-based radar operations. Especially significant are the antenna side-lobe levels and the dynamic range of powers over which the system must operate. For ground-station antennas operating in the 4-6 GHz and 12-14 GHz bands, the FCC specifies the maximum sidelobe gain as (32 - 25 log 0) dBi for 1 deg<0<48deg; otherwise -10 dBi for 48 deg < 0< 180deg, where dBi is dB relative to an isotropic radiator (0 dB gain) and 0 is the angle in deg off the axis of the main antenna beam. Also shown in Table 13-12 are the maximum allowed power flux densities radiated by the satellite onto the Earth. These limitations, set by the ITU, are necessary to avoid interference to existing terrestrial services, such as microwave relay links.

Two geostationary satellites in approximately the same orbit location servicing the same ground area may share the same frequency band by: (1) separating adjacent satellites by an angle (typically 2 deg), which is larger than the ground station's beam-width, and (2) polarizing transmitting and receiving carriers orthogonally, which allows two carriers to be received at the same frequency without significant mutual interference. Right-hand and left-hand circular polarization are orthogonal, as are horizontal and vertical linear polarization. Commercial systems use these frequency-sharing techniques extensively [Morgan and Gordon, 1989].

13.3.6 Link Budgets

The link budget provides the designer with values of transmitter power and antenna gains for the various links in the system. It is therefore one of the key items in a space system design, revealing many characteristics of the overall system performance. Table 13-13 presents link budgets for FireSat command and telemetry using the equations and figures given in this chapter.

TABLE 13-13. Unk Budgets for FireSat.

Item

Symbol

Units

Source

Command

Telemetry and Data

Frequency

f

GHz

Input parameter

0 0

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