Section 5 - Basics of Satellite Communication
Section 5 - Basics of Satellite Communication
5.1 - Satellite Orbits & Relative Positions
Geostationary Satellites are in an orbit that matches the earth’s rotation; one orbit every 24 hours. To maintain this orbit and appear stationary in the sky, they must be orbiting 22,236 miles (35786 km) directly above the equator. This orbital path is also known as the “Clarke Orbit”. This type of orbit is most employed for telecommunications satellites, due to the lowest cost and complexity of the related ground station antenna hardware. Though there are many other types of satellites and orbital paths, this review will focus on geostationary satellites.
There is a telecommunication satellite with similar RF frequency characteristics located approximately every 2 degrees of longitude over the equator. From the ground, this ring of geostationary satellites will appear to be situated as an arc. Although they are not visible with the naked eye, this arc will be higher (and wider) or lower (and narrower) depending on what latitude it is being “viewed” from.
For Example: From the state of Missouri, USA (Approx. 92° West, 39° North) there is a range of geostationary telecommunication satellites above the Southern horizon. From that location the eastern-most viewable satellite is Intelsat 14, parked at 45° W longitude. The western-most viewable satellite is AMC 8 (also occupied by “Spaceway2”), parked at 139° W longitude. Both the East and West extremes of the viewable arc are closer to the horizon and are not preferred, due to their vulnerability to obstructions such as tree lines. Many orbital slots are occupied by multiple satellites on various frequency bands. At the example location there is a total of about 38 commercial telecommunication satellites operating in the Ku-Band (discussed in the next section) available from horizon to horizon.
5.2 - Radio Frequency Application
Commercial satellite signals typically operate in one of 4 bands of the microwave radio frequency (RF) spectrum: L-Band, C-Band, Kµ-Band or Ka-Band. Each band has its own characteristics and advantages. Kµ-Band (10 to 18 Gigahertz) is the frequency spectrum used in ODN satellite communications, due to its combination of information capacity and its endurance in cloud cover. In North America, Ku-Band signals are transmitted in the range of 14 to 14.5 Ghz and are received in the range of 11.7 to 12.2 Ghz.
Active Antenna Components:
From the modem to the antenna components, the data is modulated and carried on the L-Band. The L-Band is a frequency range from 1 to 2 Ghz that is also used for cell phones and some cable TV channels. This allows the data signal to be carried on a standard RG-6 cable television cable between the local components at the satellite ground station. The antenna receives and transmits the signals at much higher frequencies that can not be carried on a standard cable, and so the transmitter circuit (BUC) and receiver circuit (LNB) must be part of the antenna assembly. BUC stands for Block Up Convertor; it converts the L-Band frequency presented at its input to a higher Ku Band frequency and transmits it. LNB stands for Low Noise Block down-convertor. The LNB receives the Ku-Band signals from the satellite, converts them down to an L-Band frequency and then amplifies the signal. The amplified L-Band signal is then sent to the modem via cable.
The exact amount of Ghz that is either stepped up or down is called the Local Oscillator or “L.O.” frequency, This number will be labeled on the BUC or LNB and is a valuable piece of information required for setting up a modem.
Microwave RF signals can be “polarized” vertically or horizontally so that the orientation of the receiver at the antenna can either maximize or nullify a signal.
Polarity allows the same frequencies to be used for two signal sources, without interference.
When the polarity of satellite signal is called “Horizonal” or “Vertical”, the term refers to the perspective of the satellite’s orientation over the equator. So, the ground station antenna polarity must be adjusted to match that satellite’s orientation from the perspective of the ground.
What a telecommunication satellite does:
A typical telecommunications satellite has many transponders that are focused on their areas of service. The focus of these areas or “footprints” can include an entire continent or just a 100-mile-wide area.
A satellite transponder is a microwave RF receiver/transmitter combination that receives a signal, converts the frequency of that signal down by 2.3 gigahertz and re-transmits it in real time. For instance, a signal received by the transponder from a ground station at 14.26 Ghz horizontal will be re-transmitted at 11.96 Ghz. vertical. On both the satellite transponder and the ground stations, signals are received on the opposite polarity than they are transmitted.
The following graphic depicts the path when the remote station transmits, and the gateway hub receives:
The next graphic depicts the path when the gateway hub transmits, and the remote station receives:
5.3 - VSAT Network Topology
VSAT stands for Very-Small-Aperture-Terminal and is a two-way satellite ground station with a dish antenna that is smaller than 3.8 meters. A VSAT station transmits and receives data (modulated in the RF Kµ-band) via satellite. There are several different network configurations for the data path, two if which will be discussed here.
In a star network configuration, the VSAT station accesses the internet by communicating with the hub, which is connected to the terrestrial internet. The data packets are relayed both ways through the satellite. The hub antenna is typically large and well-maintained, as it serves to connect the internet with many remote VSAT stations.
In a mesh network, VSAT stations can communicate directly with each other, relayed both ways via satellite.
Basics of Satellite Communication.pdf
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