Archive for November, 2009
Satellite Footprints
Wednesday, November 11th, 2009The area of coverage on the earth’s surface that is effectively irradiated by a satellite’s antenna is called its “footprint.” This footprint is also the earth coverage area from which a satellite’s antenna can effectively collect signals transmitted to it. Theoretically, like the beam from a flashlight, a footprint should be circular. However, the earth’s terrain is uneven, the thickness of the atmosphere changes, and different satellite antennas may have differently shaped beam patterns. The footprint, in reality, usually has an irregularly shaped beam pattern with signal intensity strongest in the central parts of the projected ground coverage pattern and tapering off towards the edges of the pattern.
Satellite Coverage Areas
Saturday, November 7th, 2009Coverage refers to that portion of the earth’s surface over which SATCOM services are provided. Global coverage is defined as that coverage of all longitudes and latitudes and geographic regions. There are five primary overlapping geographical regions:
CONUS
ATLANTIC
INDIAN OCEAN
PACIFIC
NORTH POLAR REGIONS
A sixth region, the South Polar Region has had no requirements to date but it may receive increased emphasis as new requirements emerge for that part of the world.
Satellite Antennas
Thursday, November 5th, 2009Antennas are essential components of a satellite system. They are located aboard the satellites and as part of the ground equipment suites. The signal transmitted from a ground terminal antenna up to a satellite is called an uplink, and the transmission from a satellite to a ground terminal antenna is called a downlink. The frequencies of the uplink and downlink are never the same. The uplink frequency is normally higher. The reason for this, it is easier to generate radio frequency power on the ground rather than aboard the satellite where weight and power are limited. For a large dish reflector antenna on the ground, using the higher frequency (which has shorter wavelength) for the uplink will maximize the signal power that is transmitted to the satellite. This is important because the satellite, in comparison, will be more disadvantaged in its ability to strongly receive and transmit signals. Again, the larger ground antenna will have the advantage to more strongly collect the weaker, lower frequency downlink signals transmitted by the satellite.