Introduction
1705A Spectrum Monitor 1-9
APPLICATIONSThe principal applicationfor the TEKTRONIX 1705A S pectrumMonitor is to
provide a convenientmethod to locate and identify Ku-Band satellites, find the
correcttransponder and check on its availability, and optimize antennaposition-
ing and polarization. The remainder of this section provides basic satellite
communications application data. Specific operating instructions are located i n
Section 2, Operating Instructions.
The communicationssatellites that the television industry is interested in lie in a
band directlyover the earth’s equator, at a distance of approximately35, 900 km
(or about 3.7 times the earth’sdiameter). Travelingat approximately
11,000 km/h thesatellite completes one full orbit in 23 hours, 56 minutes,
4.9 seconds, which is referredto as a sidereal day. Whena satellite completes
one orbit in a siderealday it is geographically stationary (geostationary) to a
point on the earth’ssurface. Without being geostationary,using a satellite would
be extremely complicated and the calculations requ ired to determine when they
were in theusable window, and how long they would stay there would, in most
cases, requirea computer.
At 35,900 km distance the earthsubtends an angle of 18˚, which provides
coverageof approximately 40% of the earth’s surface. SeeFigure 1 -2. Forty
percent(40%) of the earth’s surface corresponds to an areastretching from 70˚
North latitude to 70˚South latitude. F orrough assumptions, a satellite, in
geostationaryorbit over the equator, could cover latitudes from the Arctic circle
(66˚30’N) to the Antarctic circle(66˚30’S). Howeverit should be noted that
even though the satellite is capableof covering 40% of the earth’s surface the
actual coverage will be less in most cas es because of the antenna design and
availabletransmitter power.
9,675 km
35,900 km
18°
70°N
70°S
Figure1 -2: Relationshipof a com munications satellite to earth
Determining the exactangle from the horizon to a satellite (other than at the
equator)requires a knowledge of trigonometry, because of the relatively clo se