You’ll probably want to start with the entry on satellite communication and antennas before this one.
Orbits are put into four levels based on altitude. Low Earth Orbit is about 100 miles to 1240 miles. The short distance allows low-power devices with omni-directional antennas to be used. The most common example is satellite phones, like Iridium. These satellites circle the Earth in 90 minutes. From any spot on the Earth, the satellite will only be visible overhead for 10 minutes.
Being visible isn’t referring to seeing it with your naked eye. Visible means a direct line-of-sight view of a spot so communications can occur. It also assumes a large sky, such as being in Montana or the open ocean. The more clutter blocking the sky reduces the satellite visibility. This includes hills, mountains, trees, buildings and other obstructions, or being in a low spot like a valley. It is nearly impossible to use satellite equipment in downtown New York City at ground level due to all the buildings.
Medium Earth Orbits are 1240 miles to under 22,236 miles. At this altitude, the satellites orbit the earth in four to six hours. That extends the visibility overhead to about two hours. GPS satellite operate in this orbit.
Geosynchronous Orbits are satellites at 22,236 miles. It takes a full day to orbit the Earth so the satellite will appear in the same spot of the sky once a day.
Geostationary Orbits are satellites at 22,236 miles and parallel to the equator. Since the satellite is moving at the same speed the Earth is rotating and in the same plane as the rotation, the satellite is in the same spot of the sky all the time. This is the most popular orbit to park a satellite in.
High Earth Orbits are satellites above 22,236 miles. They are not commonly used for our purposes.
Footprints, beams and look angles
A satellite’s footprint is the circular area on the surface of the Earth that is visible to the satellite. This is the potential area of coverage that the satellite can communicate with. Areas directly under the satellite will receive a stronger signal then those on the fringe areas due to the increased distance and atmospheric interference.
Satellite operators want efficient use of their equipment so they use beams, or specifically focused transceivers, to cover areas within the footprint. Imagine a satellite positioned above the equator roughly centered on the United States. The satellite operator’s intended audience is maritime users. They would focus the beams toward the waters of the Atlantic, Pacific, Gulf and Great Lakes; and away from inland areas knowing that there are few oceangoing freighters in Colorado. No energy is wasted trying to fill a part of the satellite’s footprint where it will never be used. When evaluating a satellite for use, you need to consider the beams and not the total footprint.
The more transceivers a satellite has, the more traffic it can handle at simultaneously. Satellite operators rate their satellites by the total cumulative traffic it can handle simultaneously through the entire satellite. The other factor when evaluating service is how much traffic a specific beam can handle. In normal daily use, it is hard to overload a single spot beam as the resources are geographically dispersed. A catastrophic disaster will bring many of these resources to a single geographical location; all trying to use the same beam. That is when the beam will be overloaded.
Outages and overloads on satellite services are common during major hurricanes such as Katrina, Rita, Gustav and Ike. This is most common on shared satellite services. Consider the satellite user density that occurs with the convergence of local, state and Federal responders; media and observers; utility companies restoring service; private companies COOPing; and NGOs, CBOs and FBOs responding as well. Many of these rely on some form of satellite service. Paying for dedicated satellite airtime is quite costly especially when they only use it occasionally. Now that a disaster has occurred, they all want to use it at the same time. In many ways, a satellite in orbit is similar to a cell tower: they are designed to maximize revenue efficiently for normal use, and extreme circumstances quickly exceed the designed capacities.
Finding a satellite in the sky is done through look angles. These measurements are unique based on the observer’s location. With geostationary satellites, the look angles will remain constant so long as the observer’s location remains constant. A look angle is made up of three parts: the azimuth, elevation and polarization. The azimuth is the compass direction (0-360°). The elevation is how high to look up (0-90°). Polarization is rotating the transmitter to align the radio waves with the satellite.
Here’s an exercise. Imagine that we are using Intelsat’s G-18 satellite located at 123° West. This is a geostationary satellite so we know that it will be above the equator and 22,236 miles up. 123° West is near the California coast. If we are in San Francisco, the azimuth would be 180° and elevation 46°. The higher the elevation, the easier it is to clear tree and other obstacles. Move to St. Thomas, USVI; the azimuth is 258° and elevation is 22°. St. Thomas is an island with hilly peaks in the middle so a satellite shot is unlikely from the NE side of the island due to the low look angle. Change our location again to Boston; the azimuth becomes 242° and elevation drops to 19°. The same situation occurs in Maine where the elevation is very close to the horizon. We were lucky during an operation in Maine and setup headquarters at a military airbase that had a runway near the same angle we needed.
Bands and frequencies
Just as two-way radios have a number of different bands with difference characteristics, so do satellites. Satellites operate at higher frequencies then two-way radios. A number of these frequencies are shared with terrestrial services. For example, the S Band (2-4 GHz) includes both satellite radio (Sirius), and Wifi and Bluetooth signals.
Inmarsat BGANs operate on the L-band (1-2 GHz). These are small, easy to point and decent global coverage. The major issue with BGANs is the low data throughput (32 to 256 kbps) and high cost.
C-Band (3.7-8 GHz) is well known for the “direct to home” TV signals using larger dishes of 2 – 3½ meters in diameter. The downside is that C-band has power restrictions and receives interference from microwave services.
Ku-Band (12-18 GHz) doesn’t have the power restrictions of the C-band and is used by the DirecTV system. The main challenge of Ku-band is the nearness to the resonant frequency of water. This means that water absorbs radio waves reducing the strength of the signal. This is commonly called rain fade. If you have DirecTV, you’ve experienced this when your signal goes out during heavy rain storms. The wavelength absorption peaks at 22.2 GHz. For non-technical purposes, think of it this way: The subscript U representing being under this peak. The subscript A of the Ka-band represents being at or above this peak.
These characteristics are important considerations depending how satellite service will be used. A Ku-band service will not help you for communications during the storm, but it will have the fastest speeds before and after the storm. The C-band could work in the storm, but the size of the dish makes portability unlikely and temporary setups risky in high winds. L-band will get through nearly all the time, but only a relatively slow speeds and high cost.
- Intelsat. Satellite basics. http://www.intelsat.com/resources/satellite-basics/
- Intelsat. Solar weather effects on satellites. http://www.intelsat.com/resources/tech-talk/solar-weather.asp
- Satellite Industry Association. First responder’s guide to satellite communications. http://www.sia.org/frg_files/FirstResponder%27sGuidetoSatelliteCommunications.pdf