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Modbus Communication with the SAT50 Iridium Satellite Modem


Satellite Communication technology is commonly utilized for remote site communications when there is no other practical means available such as when the site is located in areas not serviced by terrestrial communications such as dedicated line, or cellular. The site may be located in rugged terrain where line of site communication to a radio or cellular tower can obstructed by trees, hills, and mountains. Alternatively the application may be mobile so that an elevated antenna tower cannot be deployed.

GEO vs LEO Satellite Communication Technology

Satellite communications can use either Geostationary (GEO) or Low-Earth Orbit (LEO) satellite technology.

GEO satellite systems (such as VSAT or Inmarsat) utilize Geostationary satellites located at a fixed location approximately 23,199 miles above the equator to relay data from a ground station to satellite data terminals.  The Geostationary remains fixed relative the ground station and remote data terminals; a GEO satellite terminal located at a fixed site will have the same constant view to the Geostationary satellite at all times. Remote sites located at extreme northerly or southerly latitudes will have a extremely low look angle (ie 4 degrees in Fairbanks Alaska) to the GEO satellite and thus can be easily obstructed and be unreliable.

LEO satellite systems such as Iridium or GlobalStar use a network of Low Earth Orbit Satellites (66 satellites for Iridium) orbiting at a lower altitude above the earth (483 miles for Iridium constellation).  The motion of the Iridium satellite network over the site insures a only a short period where there is no view to one of the satellites. The low altitude of LEO satellite systems provide low latency path for voice and data communications but at a higher data costs. The Iridium network includes SBD (short burst data) packet data mode that is designed for M2M, GPS tracking, and messaging applications.

High Availability Low Bandwidth Communications Applications

The SCADALink SAT50 Satellite Modem operating on the Iridium SBD network is designed for SCADA communications to remote M2M applications where cost effective but reliable low bandwidth communication is required.  This can be ideal for some remote monitoring, data retrieval  applications, or remote control applications.

The SAT50 is easy to deploy and cost effective since no large towers are required to clear obstructions.  As long as the SAT50 can see a reasonable degree of the sky, the SAT50 will communicate. Even within heavily treed areas there should be clear sky overhead. This makes the SAT50 ideal for mobile and temporary applications where the site location cannot be widely cleared of trees and foilage. As an example, pipeline applications where equipment is installed on pipeline right of ways may not have clearance from obstruction from foilage.

Modbus Protocol over the Iridium Low-Earth Orbit Satellite Network

Modbus is a REQUEST – RESPONSE Master – Slave protocol commonly used in SCADA Communications over Wireless networks.  Modbus Protocol (Modbus RTU, ASCII,  Modbus TCP) is supported by many commonly used industrial control devices including PLC, RTU, flow meters, data loggers, and smart instruments; and SCADA Host systems.

The SCADALink SAT50 supporting full bi-directional communication can support Modbus communication to  sites with these remote devices . Note the higher network latency is accounted for and configured in the setup of the SCADA Host; and the messaging frequency is low. The  Modbus register map should be packed so a single poll can transfer the required site status in a single contiguous block.  Multiple message polling of site would not be recommended. WRITE to site registers would have to be infrequent. Communications to Modbus TCP devices onsite can be supported in a Modbus Ethernet to Serial Gateway such as the SCADALink IP100 is used to for protocol conversion.

The SAT50’s support for Modbus Protocol can enable use of many common industrial controller in communications challenged remote site applications. without need for implementation of custom messaging protocols.


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