MODIS scenes (Terra / Aqua)

Satellite → Remote sensing → MODIS scenes (Terra/Aqua)…

Which MODIS scenes did Terra or Aqua take, when, and over where — for any date, past or present — and the files NASA keeps for them. The tool does two things:

• Plot scene footprints: draw the ground area of every 5-minute MODIS scene in a time window.
• Find scenes: list the scenes that cover one point over a number of days.

Both give the scenes' files at NASA's Ocean Biology DAAC (OB.DAAC) as clickable download links.

The orbit is iGMT's own SGP4 propagator (the one behind Satellite orbits…); nothing is installed and no external program is run. It is the last example of the RemoteS documentation (Aqua orbits), made interactive.

The dialog

The dialog has two tabs, Scenes and Accounts, and a status line under them. The status line says what was done, how many scenes OB.DAAC does not have (see below) and how far the orbital elements used are from the requested time.

Satellite and orbital elements

• Satellite — AQUA or TERRA.
• Elements — where the orbit comes from. An orbit is computed from a TLE (two-line element set), and a TLE is only good for a few days around its own date (its epoch): used weeks away from it, the satellite's position can be off by tens of kilometres, which is a whole scene. So the right elements depend on the date you ask about:

With the last two, of all the sets available the one whose epoch is nearest the requested time is used. The status line always says which epoch that was, and warns when it is more than 15 days away.

• TLE file and its … button — only for the TLE file source: a text file of two- or three-line element sets. It may hold many satellites; only the chosen one (AQUA = NORAD 27424, TERRA = NORAD 25994) is read.

Scene footprints

• Start (UTC) — the start of the time window. MODIS scenes are 5-minute granules starting on the 5-minute marks, so the start is rounded to the nearest 5 minutes. The dialog opens on now.
• Minutes — the length of the window; each 5 minutes is one scene.
• Plot scene footprints — draws one polygon per scene. On an empty window a map with coastlines is made around the scenes first; on a window that already shows something the footprints are laid over it, in its own frame. They land in Scene Objects as one line layer named after the satellite and the window, e.g. AQUA scenes 2021-09-02 13:30+10m.

Find the scenes that cover a point

• Lon, Lat — the point.
• Days — how many days to search from Start: a negative number searches that many days before it, a positive one that many days after it.
• When — Day passes, Night passes or Any time. A pass that crosses the day/night line is kept as either.
• Product — Chlorophyll-a (L2 OC) or Sea surface temperature (L2 SST). Chlorophyll is only measured in daylight, so a night pass has no OC file.
• Find scenes — a scene covers the point when the satellite's ground track passes within half a MODIS swath of it (1163 km).

The scene files

The list under the two groups gives every scene as a link to its file, https://oceandata.sci.gsfc.nasa.gov/getfile/<file name>. Clicking one opens it in your web browser, which asks for your Earthdata login (see Accounts). Long lines scroll sideways; the list opens scrolled to the end, where the file names are.

How the names are found. A scene's file name carries the time of its first scan: AQUA_MODIS.20210908T125500.L2.OC.nc. The orbit gives the scene's minute exactly, but not its seconds: NASA's files end in 00 or 01 in runs that no orbit predicts (…T125500, …T134001). So every name is asked of OB.DAAC itself (its file search service), all scenes at once, and the name it answers is the one shown. This also settles the version of the file:

• For recent days OB.DAAC only has the near-real-time file, ….L2.SST.NRT.nc.
• Once reprocessed, a date also has the final file, ….L2.SST.nc. When both exist, the final one is given.

A scene OB.DAAC does not have is left out of the list, and counted in the status line ("1 not (yet) at OB.DAAC"): it is either not processed yet — the newest scenes appear with a delay of a few hours — or it has no ocean data. When OB.DAAC cannot be reached at all, the computed names are shown, marked (unchecked); their seconds are then a guess. OB.DAAC's answers are kept for 10 minutes, so pressing the button again is immediate.

A progress window follows the three steps: reading the elements, propagating the orbit, and asking OB.DAAC for the names, one step per scene answered, with the seconds waited counting up.

Region

Every scene over a rectangle in a period — a month, a year — written as a download script, one wget line per file, newest first. Satellite and Elements are the Scenes tab's.

• Region — the W, E, S, N limits (in the same N/W/E/S cross as Rectangle limits). Use the window's limits copies the region the window shows; the tab also opens on it when the window already shows something.
• From, To — the first and the last day, both included (UTC). The tab opens on the last complete month.
• When, Product — as in Find scenes.
• File — the script to write: a .bat file on Windows, an executable shell script elsewhere. Run it in the folder where the files should land; wget -nc skips a file already there, so an interrupted download is resumed by running it again. Left empty, the script opens in a text window instead, where it can be edited, copied, or saved wherever you want.

The scenes are found by NASA's CMR granule search, the catalogue behind OB.DAAC: it selects, on NASA's side, the files whose own footprint covers the rectangle in the period, and the day or night ones by each file's flag. No orbit is computed and no orbital elements are needed, so any past period works, and a year over a region takes a few seconds. OB.DAAC keeps each product in two collections, reprocessed and near-real-time (MODIST_L2_SST and MODIST_L2_SST_NRT, MODISA_… for Aqua, …_OC… for chlorophyll); both are asked and, when a scene is in both, the reprocessed file is the one written.

The Earthdata login of the Accounts tab is written into the script (--user, --password), so the script holds the password in plain text: keep it to yourself. The links use https://oceandata.sci.gsfc.nasa.gov/getfile/; the …/cmr/getfile/ address also works, but it only redirects to that one.

Accounts

Two free accounts, each saved as one entry of the file .netrc in your home folder — the standard place where curl, wget, GDAL and most download tools look for logins:

machine urs.earthdata.nasa.gov
    login <your Earthdata user name>
    password <your Earthdata password>
machine www.space-track.org
    login <your Space-Track login e-mail>
    password <your Space-Track password>

The tab opens filled with what the file already holds. Save login writes or replaces that one entry and keeps every other entry of the file as it is. The file is plain text: the passwords are readable by anyone who can read your home folder, as for every program that uses .netrc. Setting the environment variable NETRC to another path makes the tool use that file instead.

NASA Earthdata

Downloading the scene files from OB.DAAC needs an Earthdata login. Register at urs.earthdata.nasa.gov. Note that a link clicked in the list is downloaded by your web browser, which asks for the login itself; the entry in .netrc is what command-line tools (curl, wget, GDAL) use to fetch the same files.

Space-Track

Space-Track is run by the US Space Force and keeps every element set ever published for every satellite — several a day for Terra and Aqua, back to their launch. Celestrak, by contrast, only serves the current ones. That archive is what the Space-Track history source reads. Register at space-track.org; the account is free.

The local history — each satellite's elements are kept. Space-Track limits automated use and asks that history be downloaded once, not again and again. So the elements are asked by calendar month, and every answer is stored for good in ~/.gmt/iGMT/tle_history/:

• <NORAD>.tle — all element sets downloaded so far for that satellite (27424.tle for Aqua, 25994.tle for Terra), in date order;
• <NORAD>.months — the months already complete on disk.

For a date, the months covering it ±3 days are needed; only those not on disk yet are asked for, in one Space-Track session. A complete month (one that ended more than a day ago) is never asked for again, so working on dates you have used before needs no network and no login. The current month is not complete — new sets are still being published — and is asked for again at most every two hours. The files are plain TLE text: they can also be used as a TLE file anywhere else.

A refused login, or a Space-Track error, is reported in the status line. Space-Track may lock an account after repeated failed logins: if the login is refused, check it on the website first.

Planned

• Download the files from inside iGMT, with the Earthdata login in .netrc, and open them straight into MODIS L2 swath to grid — instead of going through the web browser.
• A manual button on the dialog opening this page, as the GMT tool dialogs have.

Current limits

• MODIS L2 swath to grid has not yet been tried on a real MODIS L2 file.