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Satellite Communications

Very Small Apeture Terminal (VSAT)

In many remote locations the only practical communications method is through a Very Small Aperture Terminal (VSAT) satellite system. The two satellite Internet providers used by PBO are HughesNet (formerly called Direcway) and in Alaska, several sites use the Starband system. Note that the installation document that is linked to from this page describes a typical HughesNet installation. The only significant difference in installation procedures for a Starband is that the Starband system does not have a pointing interface installed on the modem. For this reason it is necessary to have a separate outdoor pointing interface, typically a "Bird Dog" satellite meter is utilized to tune the dish.

Each VSAT communication installation uses a relatively small ("very small aperture" means less than 3 meters in diameter) radio-reflective paraboloid dish with a transceiver mounted in front of it that is pointed toward a single GEO satellite about 35800 km above the Earth's surface at the equator.

When possible, a satellite with longitude fairly close to that of the station should be selected. This way, the satellite will be to the south at the highest possible elevation. However, the signal footprints of satellite transponders can point at various angles, so that the strongest signals are not always from the closest satellite. Usually, the broadcast company assigns satellites based on usage patterns as well as the particular ground location. At high latitudes, pointing to a satellite near the station longitude becomes more important. At 80° latitude (north or south), even a satellite directly to the south is essentially on the horizon, so using VSAT at more than 70° is difficult at best.

Though, it is certainly not neceessary to understand how the geostationary orbit works. It can be useful to visualize what the orbit looks like from various latitudes. During recon of a potential VSAT site, one can quickly gauge which particular spots will have a clear shot at a satellite. During installation, it is easier to reality-check the dish pointing and polarization values.

At the equator, the geostationary orbit is a "straight arc" from due east to directly overhead to due west. At higher north latitudes, the arc becomes a curve with its high point further and further to the south. Before doing recon for a VSAT site, and certainly before committing to a final dish location, the pointing azimuth and elevation of the available satellite(s) should be noted. In the field a compass and clinometer can be used to verify that the intended satellite is in unobstructed view. A useful web-based application to find pointing parameters, incorporating Google Maps, can be found here.

At most PBO stations that use VSAT, the dish is mounted on the top of the enclosure pole, and the modem (currently called the DW7000) is placed in the top enclosure compartment along with the NetRS GPS receiver. At locations where there is no enclosure post a secondary post is either attached to the side of the enclosure hut or another post is installed in the ground. As noted above, the first siting requirement is that there is a clear line of sight to the intended satellite. This is determined by finding the sky location of the satellite and making sure that no ground obstructions block that spot, and that no trees or buildings are likely to become obstacles during the life of the station. In one case, on an oil platform off the southern California coast, the originally reconned dish location was blocked by an escape pod (enclosed lifeboat) hanging from a higher deck. Be aware that the true beam path is not the apparent direction that the dish is pointing, by about 20°. To avoid having the transceiver block a large part of the signal, it is mounted off-center. With the typical vertical receive, horizontal transmit setup, this means that the transceiver is low and bounces the signal upward off the dish. However, if the transponder is polarized for horizontal receive, the transceiver will be to the side of the dish, and the pointing azimuth will be around 20° from where the dish appears to be facing.

Broadband Global Area Network (BGAN)

BGAN communication uses a satellite constellation, operated by Inmarsat, using three geostationary satellites providing near-global coverage. Satellite communication via BGAN uses a modem-antenna unit about the size of a laptop computer. Since the communication is with one of the three GEO satellites, the unit needs to be roughly oriented, usually in a fixed position, for optimal data transmission.

UNAVCO BGAN installations currently use the Hughes Broadband Satellite IP Terminal. Installations done by UNAVCO typically incorporate a timer switch to limit the BGAN modem on-time to reduce power consumption at the site.

Iridium

The Iridium satellite constellation, developed by Motorola between 1993 - 1998 and now owned and operated by Iridium Communications Inc., consists of 66 active LEO satellites and additional spares at a height of approximately 781 km above the Earth at an orbital inclination of 86.4° in six orbital planes with 11 active satellites in each plane. Satellite communication via Iridium allows users to place voice and data calls from remote locations any place in the world using a handset about the size of a cordless home phone. Calls can be made to land line telephone systems, cell phones, or other Iridium phones. Internet access is also available through Iridium's ISP or other ISPs, using an L-Band Transceiver (LBT) at the remote site in place of the Iridium phone. This is the only communication system with global coverage, which is especially useful for remote sites in polar regions or other sites where no other communications options are practical.

Iridim communication allows up to 2400 bps which is suitable for normal rate GPS/GNSS data downloads of remote sites. At each site a Iridium tranmitter/receiver modem with omni-directional antenna needs to be installed. The power consumption is about 1.0 W when downloading a typical 1 MB data file per day. The Iridium modem is sensitive to antenna cable losses thus requiring overall cable loss between the modem and antenna to be < 3dB. Installations done by UNAVCO typically incorporate a timer switch to periodically reboot the Iridium modem. Proximity of the Iridium antenna to the GPS/GNSS antenna has been known to cause RF interference with the relatively weak GPS/GNSS signals when the Iridium modem is transmitting.

 

For more information about radio modem communication technologies, please see the UNAVCO Knowledge Base on Satellite Communications Summary.

Last modified: Friday, 01-Apr-2016 23:53:16 UTC

 

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