A scatterometer is on board the EUMETSAT (www.eumetsat.int) 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.)
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.
but for now we are discussing the time predictions, not the wind data itself (see starpath.com/ascat for practical links).
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.
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.
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.
(1) The Data Center at the University of Wisconsin-Madison Space Science and Engineering Center (https://www.ssec.wisc.edu/datacenter/polar_orbit_tracks/) 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.
send https://www.ssec.wisc.edu/datacenter/polar_orbit_tracks/data/METOP-B/2023/2023_08_10_222/GLOBAL.gifsend https://www.ssec.wisc.edu/datacenter/polar_orbit_tracks/data/METOP-C/2023/2023_08_10_222/GLOBAL.gif
(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 www.stoff.pl. 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.
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.
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.