Inmarsat-C

Inmarsat-C

Dear List, I have been monitoring this list for quite some time and have found it to be an excellent source of information concerning the various forms and types of systems being integarted around the world. Since I work for a manufacturer of PLCs and RTUs designed specifically for the SCADA market, I have often felt it would be inappropriate to speak and/or respond to the list in order to avoid turning it into a sales forum. I do not wish to use this list to promote our products unless someone requests specific information that I may be able to help with. In light of this, I could use some of your expertise. I am working on a project in which a Modbus compatible RTU will be used to collect data from remote sites and send it over radio to a central sites. Some sites are so remote that we have to use Inmarsat-C link. Does anyone have information on how to integrate a Modbus RTU system and Inmarsat-C? Any ideas will be much appreciated. Thanks in advance for all your help. I look forward to being able to contribute much more in the future.

Re: Inmarsat-C

As an employee of a company that manufactures and sells radio- based modem equipment compatible with the Modbus protocol among others, I am familiar with some of the facts of Inmarsat satellite communication since several of our clients use our technology for the 'last mile' link to fan out from an earth station to their various remote points. Simply put, Inmarsat-C is treated often like a POTS dial-up circuit, with delay. I also believe that a VSAT style full time link can be rented from the carrier, so these would appear to be full duplex low speed dedicated point- to-point circuits. Most lower speed modems have no trouble with using the satellite as the transportation link and the Modbus (not plus) protocol will have no problem dealing with the encountered delays if you set the message acknowledgement timeout values for the host and PLCs long enough to allow for the half second + communication delays that will be encountered. regards to all

 Bram Frank
 Director of Sales
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      Tel: +1 514 737-0020        Post: Dataradio Inc.
      Fax: +1 514 737-7883              5500 Royalmount Avenue, Suite 200
                                        Town of Mount Royal, Quebec, Canada
   e-mail: bfrank@dataradio.com         H4P 1H7
 Web page: http://www.dataradio.com
      

Re: Inmarsat-C

The Inmarsat C system is a slow speed 600 baud store forward system, the big advantage is the data will always get from the transmitter to the reciever. It might take some time but it wil get there. Expect up to minutes before it has gone through the system. The time depends on the Land Earth Station LES, some of them are not doing too well with small messages. Inmarsat has two systems for data one is for ASCII and the other is for raw 8 bit binary. Some of the LES do not know what raw 8 bit binary is. It is possible to send a global poll command to all the transcievers with one message.

The Modbus protocol at least the original one is based on the master sending a poll command and the data is recieved. There is many different implementations of Modbus. From this you will see the Inmarsat is not good for Modbus. There must be some kind of protocol converter because Inmarsat messages is send in an envelope, the converter must put the modbus data in the envelope. There is also many different routing characters added by Inmarsat, but they are normally stripped from the message by the transciever equipment.

Do not expect to connect you Modbus RTU/PLC to a Inmarsat transciever, it will not work.

The next problem is to get the data from and to the LES. They do not operate on any standard, it is normally a dial up PSTN line or possible X.25. All the LES is build to send Telex and/or Fax data. It is a real problem to figure out how to get the data in/out of the LES. There has been made a lot of improvement here.

The next is the price. An Inmarsat C transciever will cost about 3000 USD, including antenna etc. Then there is a montly charge to the LES, plus a charge for each message send.

Althoug the LES providers like to get traffic, most of them do not understand SCADA, but it is getting better. They normally expect you to change your connection to suit thie system, nothing wrong here expect the system was designed for old 300 baud teletype communication. The Inmarsat C system is perfect for a SCADA system with low data and data send like every 4 hours. The RTU or flow computer will send the data, maybe after being polled. The system is perfect in an area where there is no other way of communication, like in a jungle, in the middle of the ocean, in the desert etc.

It is also possible to use it to send controls to the RTU/Flow computer for shut downs etc, there is a priority transmitting feature which can be used if the data has to get through quick, but it will cost you. The conclusion is. Inmarsat C is a perfect system for slow communication, works perfect for like a natural gas pipeline, enviromental data colletion where the RTU/PLC/Flow Computer is smart enough to initiate the trasnmission based on some parameters.

I have seen several Inmarsat C applications and they work very well as long as the customers understand the limitations or operation of the system. Most do not understand it in the beginning they think in the good old fasion SCADA communication mode, poll - data back, turn around time less than a .5 sec. I hope this hels you. JJ

Ref: c:\scada\031997\msg00010.xml

Re[2]: Inmarsat-C

Have you ever thought to take into account other satellite communication services, other than Inmarsat-C? If you need to collect data on a regional basis using geostationary satellites, there are, for example, Mobilsat by Optus in Australia and ITALSAT F2/EMS by TELECOM Italia in Europe, using NEC or WESTINGHOUSE satphones with Voice, Fax (9600 baud Half Duplex) and Data transmission (2400 baud Full Duplex) features. If your application is designed to handle short messages coming from anywhere in the world with a low duty cycle, I suggest you evaluating the possibility to use new satellite constellations, like ORBCOMM (Web site: http://www.orbcomm.net).

      Ciao.
  Sandro SEVERONI
  Environmental and Industrial 
  Monitoring  Control Services Manager
  PSM Division              Nuova Telespazio Spa
  965, Via Tiburtina        I-00156 ROME (ITALY)
  Phone: +39-6-4079-3641    Fax: +39-6-4079-3894    
  E-mail: Sandro_Severoni@telespazio.it
    

Ref: c:\scada\031997\msg00011.xml

Re[4]: Inmarsat-C

Right! EUTELSAT is also going to resell EMS services all over Europe, excepting Russia.

  Sandro Severoni
  Nuova Telespazio SpA
  Sandro_Severoni@telespazio.it
  

 Oggetto: Re[3]: Inmarsat-C
 Autore:  Steve.Draper@cmgplc.com in SMTPLINK-PO
 Data:    19/03/97 18.22
    

      Stephen Draper
      CMG
      steve.draper@cmgplc.com
    

Ref: c:\scada\031997\msg00012.xml