Enable computation of the expiration date of the BSP file(s)

.gitignore

@@ -2,3 +2,4 @@ dist/
 build/
 *.egg-info/
 __pycache__/
+venv/

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## master (unreleased)
 
 * Added a ``--check-only`` argument to ``download.py`` to display the expiration dates of the files currently on disk.
+* Enable computation of the expiration date of the BSP file(s) on disk (requires to install the local repository using the [dev] option / See README for more information).
 
 ## 0.0.2 (2019-08-23)
 

Makefile

@@ -5,6 +5,8 @@ help:
 	@echo "             (see \`python download.py --help\` for options)"
 	@echo " * clean: remove all skyfield data from data directory."
 	@echo " * package: build python source package."
+	@echo ""
+	@echo " * install-dev: Install the requirements to execute download.py"
 
 download:
 	python3 download.py
@@ -14,3 +16,6 @@ clean:
 
 package:
 	python3 setup.py sdist bdist_wheel
+
+install-dev:
+	pip install -e .[dev]

README.md

@@ -11,16 +11,15 @@ Several issues are raised by these data files:
 * In some countries, or behind some filtering proxies, the USNO is considered as a military website, and thus is **blocked**.
 * These files have **an expiration date** (in a more or less distant future). As a consequence, even if the files are already downloaded in the right path, at each runtime you could possibly have to download one or more files before making any computation using them.
 
-## Currently known expiration dates
+### Currently known expiration dates
 
-|      File       |     Date      |
-|:---------------:|:-------------:|
-|   deltat.data   |  2020-06-01   |
-| Leap_Second.dat |  2020-07-28   |
-|  deltat.preds   |  2021-01-01   |
-|    de421.bsp    | *unknown*(\*) |
+|      File       |    Date    |
+|:---------------:|:----------:|
+|   deltat.data   | 2020-06-01 |
+| Leap_Second.dat | 2020-07-28 |
+|  deltat.preds   | 2021-01-01 |
+|    de421.bsp    | 2053-10-08 |
 
-(\*) Even though its expiration date has not been extracted through a convenient script, we can state that it should expire in the year 2053.
 
 ## Goal for this project
 
@@ -60,8 +59,15 @@ load = Loader(get_skyfield_data_path(), expire=False)
 
 We're providing a ``Makefile`` with basic targets to play around with the toolkit. use ``make help`` to get more details.
 
+In order to be able to run the `download.py` script, we recommend to run it **from a virtualenv** where you'd have installed the "dev" dependencies, using:
+
+```sh
+make install-dev
+```
+
 *Note:* This project is, and should be compatible with Python 2.6/2.7 and Python 3.3+, to be kept the same Python compatiblity that `skyfield` has.
 
+
 ## Copyright
 
 ### Data files

download.py

@@ -5,6 +5,7 @@ from datetime import datetime, timedelta
 from os.path import join, exists
 import shutil
 from urllib.request import urlopen
+from jplephem.spk import DAF, SPK
 
 from skyfield_data import get_skyfield_data_path
 
@@ -13,6 +14,37 @@ USNO = "http://maia.usno.navy.mil/ser7"
 IERS = "https://hpiers.obspm.fr/iers/bul/bulc"
 
 
+def calendar_date(jd_integer):
+    """Convert Julian Day `jd_integer` into a Gregorian (year, month, day)."""
+
+    k = jd_integer + 68569
+    n = 4 * k // 146097
+
+    k = k - (146097 * n + 3) // 4
+    m = 4000 * (k + 1) // 1461001
+    k = k - 1461 * m // 4 + 31
+    month = 80 * k // 2447
+    day = k - 2447 * month // 80
+    k = month // 11
+
+    month = month + 2 - 12 * k
+    year = 100 * (n - 49) + m + k
+
+    return date(int(year), int(month), int(day))
+
+
+def bsp_expiration(fileobj):
+    """
+    Return the expiration date for a .bsp file.
+    """
+    daf_object = DAF(fileobj)
+    spk_object = SPK(daf_object)
+    dates = [segment.end_jd for segment in spk_object.segments]
+    # We take the closest end date, to expire the file as soon as it's obsolete
+    end_jd = min(dates)
+    return calendar_date(end_jd)
+
+
 def deltat_data_expiration(fileobj):
     """Return the expiration date for the USNO ``deltat.data`` file.
 
@@ -100,6 +132,9 @@ def download(url, target):
 
 
 def get_expiration_date(target, params):
+    """
+    Return expiration date for the given ``target``.
+    """
     expiration_date = None
     if exists(target):
         expiration_func = params.get("expiration_func")
@@ -136,7 +171,10 @@ def check_should_i_download(target, params):
 
 def main(args):
     items = {
-        "de421.bsp": {"server": JPL},
+        "de421.bsp": {
+            "server": JPL,
+            "expiration_func": bsp_expiration,
+        },
         "deltat.data": {
             "server": USNO,
             "expiration_func": deltat_data_expiration

setup.cfg

@@ -32,5 +32,10 @@ packages = skyfield_data
 setup_requires =
     setuptools
 
+[options.extras_require]
+dev =
+    jplephem
+    numpy
+
 [bdist_wheel]
 universal = 1