Project OSCAR (Orbiting Satellite Carrying Amateur Radio), a west coast USA-based group which built and launched the very first amateur radio satellites began in 1961. OSCAR I was launched on December 12th, 1961 barely four years after the launch of Russia's first sputnik. OSCAR I was followed by OSCAR II six months later. These satellites were built, quite literally in people's basements and garages. Originally these were "bleep sats" but now they carry sophisticated repeaters or transponders.

Even today's satellites give out bleeps (beacons) on a particular frequency so that you know the satellite is passing by. The time the satellite is visible (in range) to an observer is called a satellite "pass". During the pass, you are in the "footprint".

In simple terms a communication satellite is usually nothing but an orbiting repeater in the sky. The satellite picks up your signals on a particular frequency (uplink) and re-transmits the same on a different frequency (downlink). Your original signals are hence enhanced and made available to a huge area depending upon the coverage (footprint) of the satellite.

The transmitter-receiver combination on a satellite is called a transponder. Satellites do not have the physical space to separate receive and transmit antennae, so they use different bands ie. most satellites have their uplink frequencies in the V.H.F band and their downlink frequencies in the U.H.F band. You may not be able to hear a satellite on its exact downlink frequency right here on earth. The frequency on which you may get the signal from the satellite may vary due to doppler shifts. A doppler shift is the variation in frequency caused by the motion of the satellite or the earth

 

 

 

Communications satellites are put into orbit by means of rocket-powered launch vehicles, or boosters. Most are placed into geostationary orbit, in which the satellite follows a circular path around the Earth in the plane of the Equator, at an altitude of 35,900 km (22,300 miles). At this height the satellite's period of revolution around the Earth is the same as the Earth's period of rotation, so that the satellite maintains a continuous position above the same spot on the globe.

 

Throughout its service life, a satellite must be maintained in the correct attitude (i.e., pointing in the right direction). Attitude is controlled by one of three methods: "spin-stabilizing" the entire satellite, including the antennas; spinning the body of the satellite while the antenna platform is "despun," or counter-rotated, in order to point stably at some point on Earth; and maintaining "three-axis" stability of the entire satellite by means of an internal gyroscope and small thruster jets.

Mode	Uplink (to satellite)	Downlink (from satellite)
A	145 Mhz	29 Mhz
B	435 Mhz	145 Mhz  -- also known as UV mode
J	145 Mhz	435  Mhz  -- here JA is analog and JD is Digital
JL	1.2 Ghz / 145 Mhz	29 Mhz
K	21 Mhz	29 Mhz
KA	21 Mhz / 145 Mhz	29 Mhz
KT	21 MHz	29 Mhz / 145 MHz
L	1.2 Ghz	435 MHz
S	1.2 Ghz	2.4 GHz
T	21 Ghz	145 Mhz
UV	435 Mhz	145 Mhz -- same as Mode 'B'
V	29 Mhz	145 Mhz -- inverse of Mode 'A'