Monday, July 23, 2012

Predicting ASCAT Satellite Pass Times

This 2012 article is being updated in July 2023, to account for the loss of Metop A wind data, replaced now by Metop B and C.  We will remove this notice when the full update is competed.

ASCAT is the name of the scatterometer that provides true wind speed and direction on the ocean surface about once a day in most places. 

A scatterometer is on board the EUMETSAT ( satellites Metop-B and Metop-C, so it is helpful for us to know when the satellite will pass by to give us this valuable data.  (The original Metop A is still providing research data but not scatterometer wind data.) 

And I should say up front, there are other ways to predict ASCAT pass times (ie mobile apps), but for now we start with the most basic of predicting this pass time from the basic satellite characteristics and some generic online data.

The practical application is this: We find the ASCAT region we care about from the bottom of our ASCAT portal page (or from our book Modern Marine Weather, which has the same info) and then we download all four of those files. Then we look at these to see which are helpful now, and then also to use the content of this present blog post to figure when we should download which of the four again for new and latest data. Without that preparation, we risk waisting satcom air time downloading data we already have.  (Looking ahead to the not too distant future when we will all have affordable high speed internet at sea, we will just download the files again every couple hours and not have to bother with the time prediction. The files are about 40kB each.)

It takes processing time to get the actual data after the satellite goes by, but the valid time is when the satellite actually went by...and you may want to know this ahead of time so you can prepare to make weather map comparisons with the actual winds you can learn from the satellites. 

Graphic ASCAT data are available 2 to 3h after each pass from:, and,

but for now we are discussing the time predictions, not the wind data itself (see for practical links).

We know roughly when the satellite will pass by us because it is in a sun synchronous orbit, meaning it crosses the equator every day headed north (ascending) at the same time relative to the sun. For Metop-B this is 2128 local mean time (LMT). This is not zone time nor standard time nor even GMT. It is a special time concept, which is best interpreted as the GMT of the event observed from the Greenwich meridian. To figure the crossing time at other longitudes, we have to account for the westward motion of the sun at 15º of Lon per hour. Thus on a Lon near Hawaii of exactly 150º W, the ascending node would take place 10h later at 3128, or 0728 LMT.  In the middle of the Mediterranean at 15º E, Metop-B heads north across the equator one hr before it gets to Greenwich, at 2028. Near Australia at 135 E the time is about 1228.

The satellite circles the earth once every 101 minutes (1h 41m), so the date is not important here; we just want to illustrate the LMT concept.

Here we see an ascending B pass crossing the equator at 0728 at 150 W (10 hr after 2128 at Greenwich), and also at 1228 at 135 E, where it goes by before passing over Greenwich.The ASCAT data are the two swaths on either side of this track.  This picture shows a total of about 14 ascending passes. The swaths are the same width on the earth at all latitudes  (500 km) but get wider with Lat due to the map projection, which is similar to a mercator. Hourly swath indexes like this from the manati link above are also found at the KNMI site.

We can also figure the timing of the descending pass since it will take half an orbit (50.5 min) to go over the top and back down to the equator.

Here we see the satellite coming back down on the other side of the earth 50 m later. Note that it does not cross at 180º (dateline), but farther W by (50.5/60)x15=12.6º, at about 168E, because the earth rotated to the east by that amount during the 50.5m travel over the top.  

Knowing this behavior of B, we can figure the times of C as they will be half an orbit (~50m) behind B—or vice versa. This is likely enough information for estimates of passes across the regions we have named that match the OSWT file sizes, namely 10º of Lat by 15º of Lon, which is just over one orbit span. But this is not  very specific prediction information for an arbitrary point on earth for several reasons. 

First, the earth is rotating beneath the orbit of the satellite, so the satellite path  tracks to the west as it proceeds north as the earth rotates toward the east. So we don't immediately know what its longitude was when it crossed the equator to reach us at our more westerly longitude.

Second, though it does move north very fast (360 x 60 nmi in 101 min) it still takes about 10 min to get from the equator up to 45 N, so pinpointing the time it will pass is more involved.

To make things worse, knowing when it crosses the equator every 101m does not tell us if it will indeed cross at a place that provides data because of the wide nadir gap along the track. In other words, if i am going to get data near HI, it will be in the around 0730z or so (2130 HST), but i do not know where in our window the track will be so no way from this along to know how much data we get.  We also get the descending B and both passes for C. As noted earlier, we typically do get data about 4 times a day—and as outlined below we have a prescription for accessing this data that does not require these specifics.

With these complexities in play, if we do want to know more precise satellite pass times,  we should turn to typical astronomer's satellite prediction apps.

(1) The Data Center at the University of Wisconsin-Madison Space Science and Engineering Center ( has plots online of the Metop-B and C positions such as shown below for specific dates. You can get the present date and the next 3 days.

If you are starting a voyage on Aug 10, 2023, for example, which happens to be day 222 on a DOY calendar, then you could request up to four days of data from Saildocs with this email format:


(2) A very neat free PC app called Orbitron not only has a versatile predictor, but you can also use it as a screen saver so Metop-A and OceanSat2 (the OSCAT satellite from India) are always on  your mind!  Get it at  It is postcard software.  The author just asks that you send him a post card if you like it.  We have sent ours.

The ring around the satellite is its visible horizon, so we see that the satellite is over the horizon from our position, about 7º high in the sky, bearing 328. Same vessel position.

Historically, Orbitron called Metop-A by the name SARSAT 11, which seems to imply the author attributes more importance to its search and rescue pay loads than the meteorology instruments. As mariners, we have to be grateful for both.... but you must know this to find it in Orbitron!

As of Aug 11, 2023, however, i cannot find any of the Metop sats in Orbitron. I have written to the author to ask about this.

Another bit of hitory... this neat app below is also gone.

3) Another, maybe most convenient of all, option is the iPhone app called Satellite Tracker by Susan Mackay. She is in Australia, I believe.  Output is shown below so you can compare all of these methods for the same day of July 21 about 2200z. It will do both Metop-A and OceanSat2.

In this radar type view, the local position (20N, 170W) is in the center. Each ring is 30º of altitude. The sun is high and to the East. The moon is at about 095 at about 40º high. If you watch this app live, you see the satellite moving along the track. This example is in a descending mode, headed south but still bending to the west.


This display shows the next several passes, ascending and descending. Click any one of these in the app to get the picture above it.

In the above examples, i put the boat position at 20 N, 170 W so we could compare them. Each of these apps let you watch the satellite come cover the horizon, peak out, and then set.  For the best satellite predictions, your stand alone options (Orbitron and Satellite Tracker) require you to update the latest satellite parameters every few days, called TLE (two line element) files. They are provided by NORAD. The programs do this automatically. It takes just seconds. 

These apps show all the passes, not just the ones that till yield data. If the satellite passes too low in the sky, its scatterometer can not reach out to where you are, so no data. And with ASCAT, if it passes right over head there is also no data since it has a nadir gap of no data right below it.  Its data starts some 360km either side of the track below it and then they extend out 500km from there.  OSCAT does not have a nadir gap. 

It is my feeling that if we are to incorporate this valuable data into our navigation and weather routing, it pays to understand the basics of what is involved.  In other words, so you could answer the question, why don't we have data all the time, whenever we want it?

Once a satellite goes by on a favorable pass, we generally get actual ocean wind speeds and directions along those two swaths in about 2 to 3 hours.  This may seen a long delay, but recall we do not get the surface analysis maps for about this same delay past the synoptic times. So in that sense, this is as fresh as it comes in the middle of the ocean. 

There is another satellite (from India) sharing data with us now called OceanSat 2 (OSCAT), and hopefully more on the horizon. So this important area of meteorology is getting better all the time. These satellites are timed to give optimum coverage, not just redundancy.


Another Aug 11 note, there were several sections here about other scatterometer satellite systems, but most are no longer working, and the one or two others beside ASCAT seem to be undependable... ie they are not used in any way by US or UK weather services.


Anonymous said...
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Manuel GOACOLOU said...

Python is your friend...

just take a look to this project, and in particular in tests for examples:

David Burch said...

RapidSCAT was relatively short-lived. It turned out to be too complex having it attached to the ISS. It provided data from September 2014 and operated until August 2016, but is not planned to come online again.