CONCEPT PAPER -- IRIDIUM SHORT-BURST DATA (SBD):
Remote Control and Data Acquisition for Remote Instrumentation

R. Michael Reynolds
3 Nov 2008

This memo describes a design for a global satellite communication system that makes use of the newly introduced Iridium Short-Burst Data (SBD) capability. A new market in SBD modems opens the way for a globally available remote data telemetry system for transmitting low-bandwidth, high accuracy data in real time. Applications include weather stations, shipboard measurements, and similar geophysical instrumentation systems. Hardware costs are reasonable and typical message costs minimal (hourly messages of 30 bytes come to about US$100 per month). Data telemetry is internet based and bi-directional.

Iridium Satellite LLC is the only provider of truly global satellite voice and data solutions with complete coverage of the earth (including oceans, airways and Polar Regions). Iridium Satellite LLC launched global voice services in March 2001 and added ubiquitous Internet connectivity in June. The company successfully launched seven additional spare satellites in 2002 to ensure the system's long-term performance. Iridium delivers essential communications services to and from remote areas where no other form of communication is available. The service is ideally suited for industries such as maritime, aviation, government/military, emergency/humanitarian services, mining, forestry, oil \& gas and heavy construction. Iridium currently provides service to the U.S. Department of Defense under a multi-year contract. Iridium works with more than 30 seasoned service partners to sell and support the service globally.

The system is being used extensively by the U.S. Department of Defense for its communication purposes through the DoD Gateway in Hawaii. The commercial Gateway in Tempe, Arizona provides voice, data and paging services for commercial customers on a global basis. Typical customers include maritime, aviation, government, the petroleum industry, scientists, and frequent world travelers. Iridium Satellite LLC claims to have approximately 137,500 subscribers as of September 30, 2005, which is a 22% increase from the third quarter 2004. Revenue for the nine months ended September 30, 2005 was up 24% over the nine months ended September 30, 2004.


Iridium Short-Burst Data system

SBD provides a new capability for vertical market applications in the Oil, Gas, Rail, Maritime, Aeronautical, and Utility industries as well as applications in the Government and Military sectors. The SBD service is available to certified Value Added Resellers (VAR) who include it as part of a complete end-to-end solution for specific wireless communications needs.

Remote Applications send Mobile Originated SBD (MO-SBD) data messages via an Iridium SBD modem. The application microcontroller or microprocessor communicates with the modem using Hayes-type "AT" commands over an RS232 serial port. The application loads the data message into the modem buffer and then instructs it to send the data message. The data message is transmitted across the Iridium satellite network utilizing inter-satellite links to reach the Iridium Gateway. From there the data message is transferred via e-mail to the VAR's host computer system. Here the message is stored in a database for further data processing. Mobile Terminated SBD (MT-SBD) messages are sent to the Iridium Gateway via e-mail from the VAR's host computer system. MT-SBD data messages are delivered to the LBT following a MO-SBD or mail-box check initiated by the remote application.

Several SBD providers are available and for our initial development effort we plan to use the SBD modems provided by NAL Research, Manassas Virginia. The SBD system provides the following data packet capability. The maximum length of a MO-SBD message is 1960 bytes. The maximum length of a MT-SBD message is 1890 bytes. Global network transmit latency for delivery of messages ranges from approximately 5 seconds for short messages to approximately 20 seconds for maximum length messages.

SBD Costs

The Control Data Unit (CDU) will provide the interface between the instrument system and the communication modem. The SBD modem accepts up to 205 bytes in a single message. For each message the first 30 bytes cost $.04 per byte. Beyond 30 bytes the cost is $0.0015 per byte. Thus a 50 byte message will cost $1.23 and a 200 byte message will cost $1.45 per message. Messages can be in either direction. Incoming messages, to the remote phone, are free of charge. Thus hourly transmissions of 110 bytes can result in a monthly charge of $115 per month and 200 byte messages would be $225. The primary features of the SMS are

The sketch here shows shows the main components of the hardware. The development system will use off-the-shelf components to the extent possible. The Power UPS will be a standard off the shelf AC to 15 VDC converter with a wide (100--240 VAC, 50--60 Hz) input range suitable for worldwide coverage. The Power System provides DC power at the correct voltages and current demands of the instrument, the translator, and the Iridium modem. The Translator ingests the data strings from the remote instrument and creates the SBD packet. In the development system the trasnslator will be a laptop PC running Linux OS. In later versions the Iridium CDU will be a much more compact and energy efficient package.

The translator receives output from data system (meteorological station, Infrared Seasurface Temperature Autonomous Radiometer (ISAR)) and creates SBD messages. In addition, it will receive SBD commands and can perform a multitude of tasks in support of the remote instrumentation. For this proposal we will make the prerequisite that the software in the ISAR instruments is not changed. The translator can perform the following tasks under control via SBD commands:

The translator can receive the approximately 5-sec raw measurement strings from ISAR as well as the final SSST strings and the intermediate strings. We expect the translator can have considerable intelligence and thus is able to make computations of SSST on it's own.

The SBD Data String

The SBD message can be a maximum size of 205 bytes. ASCII or binary formats are allowed. The Iridium SBD provider provides error checking and packet repeat protocol as part of its operation. However, it is recommended to include a CRC checksum during the first period of operation as an extra safeguard. The remaining 195 bytes will be a pseudo-ascii binary grouping as was developed for the GOES data system. (C source code for both the CRC checksums and pseudo-ascii data compression have been written in prior projects.) In pseudo-ascii compression, the string is comprised of printable ASCII characters and each character has six bits of a binary number. Thus an 18-bit binary number can be represented by three printable characters, and a message using the entire 195 bytes will contain 1170 bits of data. Pseudo-ascii is selected over straight binary because it works well with cell phone SMS and with text email formats. With a little practice the pseudo-ascii characters can be read with using a conversion program. An example of pseudo-Ascii compression for a large sodar data block is below. It is estimated that hourly transmissions of this sodar message would cost about $225 per month.

In the event of an emergency a command can instruct the CDU to send wind profiles at a faster rate, say every 10 minutes.

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The following data packet, for an Infrared Seasurface Temperature Autonomous Radiometer (ISAR), can be incorporated into the SMS message with 271 bits (45 printable characters):
Bits Time Resolution
18 year day 2 min
18 latitude 76 m
18 longitude <76 m
15 Temperature 1 .0015 °C
15 Temperature 2 .0015 °C
12 Signal 1 .15 mV
12 Signal 2 .15 mV
12 Signal U .15 mV
12 Signal D .15 mV
12 emissivity 2x10-5
15 SSST mean .0015 °C
15 SSST last .0015 °C
15 SSST (Translator) .0015 °C
10 Pitch mean .04 °
10 Roll mean .04 °
9 pitch standard deviation <.01°
9 roll standard deviation <0.01°
8 speed over ground .08 mps
8 course over ground 1.4°
12 Window temperature 0.012 °C
1 shutter position yes/no
15 rain rate ~0.04 mm/hr

The above packet would cost approximately 10¢ to send. Hourly transmissions from a single station would cost $876 per year. In an emergency situation, packets could be transmitted every minute and the cost would rise to $144 per day. For emergency situations the stations would be turned on as required.

Software Development

The software for this system would be written in a Unix/Linnux shell (similar to the code currently used for RSMAS Weatherpak operations and download). In this way existing C modules can be intergated with PERL, EXPECT, and shell commands. This format allows development of additional software offline with easy integration into the Translator package. We expect to be able to use existing C code for the packet creation and checksums. Existing PERL modules will be used for parsing the ISAR NMEA strings. Much of the code (TCL, shell, EXPECT) has been written by RSMAS programmers and can be adapted to Translator software. The PERL and C modules were developed by RMR Co. for NOPP and other projects, and can be adapted to this project within the scope of existing NOPP funding. Thus software can be developed in minimal time and with small cost.

Links

Iridium L.L.C.

NAL Research

Infosat Inc.

InfoSat

Global Information Technologies