How to Get Worldwide Satellite Wind Observations (ASCAT) via Email

While sailing offshore, forecasts, NOAA synoptic charts, and other weather information can be accessed through an Iridium GO.

However, on a couple notable occasions, the forecasts—even when just hours ahead—differed significantly from real conditions.

This made me want better access to actual observations. Until now, I did not have a solution for this while offshore outside of the U.S.

What is ASCAT?

ASCAT (“advanced scatterometer“) is a method of observing sea state conditions from a satellite. Roughly speaking, radar is used to observe the sea state, from which the wind conditions can be derived.

NOAA’s ASCAT data comes from the MetOp-B and MetOp-C satellites, operated by EUMETSAT. (Current ASCAT data is no longer available from MetOp-A.)

The satellites are in polar orbit 817 km up, and circle the earth about once every 101 minutes. The earth is rotating underneath the satellites, so when the data is plotted on a map, it appears as a diagonal path. Also, depending on where it is in its orbit, the satellite may be traveling either north-to-south (“descending”) or south-to-north (“ascending”) relative to the earth.

I was unable to find information about the revisit rate (how often it will pass over the same geographic location). However, it appears this is roughly once per satellite per day, or on some cases greater than 24 hours is possible. It also seems that MetOp-C traverses a geographic location roughly fifty minutes (half an orbit) following MetOp-B. If you have more accurate information about these characteristics, please let me know in a comment!

There can be some latency (delay) in the timeliness of the data, because once collected by the satellite, it will need to be downlinked to a ground station before it’s available for processing and use. It’s unclear what the maximum possible latency is. Average latency to ground station is believed to be 47 minutes, with a further 2 hours and 15 minutes needed for processing by EUMETSAT. You can expect the NOAA-disseminated data to lag a bit behind this (by how much I don’t know).

How to Get the Data

The ASCAT data is available as image files that show wind barbs. An example image appears at the top of this article.

Part 1 of the process will be determining the names of the files to get. Part 2 will be retrieving the files from the web via email.

Note that the instructions here are for the 25 km resolution data. 50 km resolution data is also available; if you’d like instructions for this, let me know in a comment.

Also, these instructions are appropriate only for latitudes below 80°, although practically speaking, there’s no useful data above 75°N and 70°S.

Part 1

The globe is broken up into several distinct image files. The image URLs are of the form[sat]/zooms/WMB[dir]s[num].png


  • [sat] is replaced with either B or C to denote the MetOp satellite
  • [dir] is replaced with either a or d to denote ascending or descending
  • [num] is replaced with a number between 0 and 383 based on the geographic region; how to compute this from latitude and longitude is discussed below

Generally speaking, once you figure [num], you’ll want to get all four image files for that region: ascending and descending from both B and C. Figuring which of the four will have the most recent data is complicated enough that just grabbing all four is simplest. Nonetheless, bear in mind that it’s possible that all four most recent images do not contain data for your precise location.

Each image is about 30 KB in size, so downloading four is feasible for a low-bandwidth connection like Iridium GO or SSB.

For the steps below, only the whole number latitude and longitude are needed.

How to figure [num] for latitudes below 60°

  1. Using negative longitude for west and positive for east, add 180
  2. Take the result from step 1 and divide by 15; truncate the result to just the whole number (i.e. ignore the remainder or fraction)
  3. Take the result from step 2 and multiply by 12
  4. Using negative latitude for north and positive for south, add 60
  5. Take the result from step 4 and divide by 10; truncate to a whole number
  6. Add the results from steps 3 and 5; this is [num]

How to figure [num] for latitudes between 60° and 80°

  1. Ignoring north and south, subtract 60 from the latitude
  2. Take the result from step 1 and divide by 10; truncate to a whole number
  3. Take the result from step 2 and: for northern hemisphere, add 12; for southern hemisphere, add 14
  4. Take the result from step 3 and multiply by 24
  5. Using negative longitude for west and positive for east, add 180
  6. Take the result from step 5 and divide by 15; truncate to a whole number
  7. Add the results from steps 4 and 6; this is [num]

Example 1

For a position of 24°N, 110°W:

  1. -110 + 180 = 70
  2. 70 ÷ 15 = 4 (ignoring remainder)
  3. 4 × 12 = 48
  4. -24 + 60 = 36
  5. 36 ÷ 10 = 3 (ignoring remainder)
  6. [num] = 48 + 3 = 51

URLs to get would be


Example 2

For a position of 62°S, 56°W:

  1. 62 – 60 = 2
  2. 2 ÷ 10 = 0 (ignoring remainder)
  3. 0 + 14 = 14
  4. 14 × 24 = 336
  5. -56 + 180 = 124
  6. 124 ÷ 15 = 8 (ignoring remainder)
  7. [num] = 336 + 8 = 344

URLs to get would be


Part 2

Now that we’ve determined the files we want, we’ll use Saildocs to retrieve them via email. Note that before using this service, you need to agree to their very reasonable terms and conditions.

Now compose an email:

You’ll receive four separate emails in response, one for each file. The attached images will look similar to the one above.

Pay close attention to the notes at the bottom of the image, in particular the notations relating to the times of observations.


And there you have it… Real wind observations available to you offshore!

References and Further Reading

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