Friday, 12 July 2013

GRIBs on PredictWind Satellite Communicator

Hi everyone,
An update on progress in obtaining GRIB files from our PredictWind Satellite Communicator.  Another geeky post!

SailDocs GRIB file received on our Satellite Communicator
At present PredictWind doesn't support the supply of GRIB files to the Satellite Communicator.  This is ostensibly because the Iridium Short Burst Data (SBD) service doesn't support email attachments.  However there is a way around this, as the above image shows!

By placing the email attachment (the GRIB file) into the body of the email the GRIB data can be communicated.  This isn't cheating in any way - you still pay for the data you transfer.  The overall email still has to fit within the 2KB Iridium SBD limit after compression.  The actual limit is about 4KB before compression.  This is about the size of GRIB file we normally downloaded via HF, and provides very useful data.

Unfortunately the GRIB files available via email from PredictWind appear to contain a minimum of 5 forecasts.  This means the area covered can only be about 5degrees*5degrees, with only a single variable (one of wind, rain, atmospheric pressure etc) in the file.  Anything bigger creates an oversize email.

SailDocs and zyGrib allow the user to define the GRIB file required with more freedom.  For example SailDocs allows a single forecast 10degree*10degree GRIB with two variables or a dual forecast 5degree*5degree with 4 variables are both well within size limits.

After researching all this we've designed and tested a system which allows us to get GRIBs on our Satellite Communicator.  At present we can get them from PredictWind, SailDocs or zyGrib.  We create request emails aboard in the format required by each service, but send them to one of our shoreside email addresses.  The email is automatically redirected to query(at) or grib(at)  This results in the response email being sent to the ashore email address, where the GRIB attachment is reformatted to lie in the email body.  The modified response email is then sent to the Satellite Communicator.  All automatically.

The extra processing adds a only minute or two to the overall process, and as soon as the response is available it shows up in the Satellite Communicator email inbox.  No connecting twice as is the case with HF email!

Below is an example response email.  The dense block of text is the GRIB file encoded and placed in the body of the email.  It provides two wind data forecasts, each covering a 5degrees by 5degrees area. The block of text is about 700 bytes in size, yielding a GRIB file of about 500 bytes.  This is the GRIB file displayed in the image above.
Example response received aboard
Getting from the text block to the GRIB file involves running a simple script (program) to decode the text into the binary data which forms the GRIB file.

If anyone's interested in accessing our system for their Satellite Communicator let us know!