The present invention pertains to communication systems and, more particularly, to apparatus and methods for transmitting and receiving signals between satellites in communication systems.
International regulations govern the placement and station keeping for geosynchronous satellites. These regulations require the ground path of a geosynchronous satellite to intersect the equator only within a tolerance window, or xe2x80x9corbital slotxe2x80x9d, which is allocated to the satellite. Typically, each orbital slot is centered over a single longitude and is defined about the central position by xc2x10.05 degrees to xc2x10.1 degrees of longitude. Orbital slots currently are centered at every two degrees of longitude (e.g., 180 slots exist around the earth). This separation helps to ensure that signals emitted from satellites located in adjacent orbital slots will not significantly interfere with each other.
The finite availability of orbital slots encourages satellite designers to design geosynchronous satellites having the largest possible data-carrying capacity. The capacity of a geosynchronous satellite is typically proportional to the size of the satellite and is limited by the state of current technology. Large, prior art geosynchronous satellites are expensive to build and place in orbit. Because of the expense, it is not typically feasible to frequently replace geosynchronous satellites which have too little traffic-carrying capacity due to inadequate size and/or outdated technology.
In some prior art systems, multiple geostationary satellites are placed within a single orbital slot in order to increase the traffic carrying capacity of the system within that slot. This is referred to as co-positioning or co-location. In other prior art systems, multiple geostationary satellites are placed in different orbital slots and interconnected using links between the satellites.
In designing better satellite communications systems, the designers must be primarily concerned about maintaining the link. In some cases, the link is between a satellite and a user, and in other cases, the links are between different satellites. This means that a link analysis must be performed. The radio frequency (RF) carrier-to-noise power ratio (C/N) at the receiving end depends on power delivered to the antenna, antenna gains, propagation losses, and effective noise temperatures of the receiving system.
The link equation can be written as
C/N=EIRP(1/L)(G/T)(1/(k*B))
where EIRP=Effective Isotropically Radiated Power;
L=[(4*II*R)/(xcex)]2 where R is the range from the transmitter to the receiver and xcex is the RF wavelength;
G/T=figure of merit;
k=Boltzmann""s constant; and
B=noise bandwidth of the receiver.
In the above equation, L is the ratio of the spreading area to the effective area of an isotropic antenna, and G/T is the ratio of the gain of the receiver""s antenna to the noise temperature of the receiver""s antenna.
Every communication system has a link budget associated with it. The link budget is a necessary prerequisite for the establishment of any communication channel whether that channel is a terrestrial based channel, a terrestrial to space channel or a space-based channel.
The link budget is used to determine if the link will xe2x80x9cclosexe2x80x9d. A primary concern is whether the required carrier-to-noise power ratio (C/N) can be achieved.
In prior art systems, designers have viewed antennas as devices for a single function. This leads to satellite payloads which are heavier and more complex than they need to be.
What are needed are a method and apparatus which overcome these limitations and allow smaller and less costly satellites to be constructed.