Orbit Corrections in SNAP
Published:
There’s a pretty common issue while performing orbit correction in ESA-SNAP. There’s a simple way to pre-download the required orbit files:
- All the files one at a time (bulk)
- As per the requirement.
The required orbit files need to be placed in appropriate folder .snap/auxdata/Orbits/Sentinel-1/POEORB/S1A/2019/04/19.
NOTE - POEORB stands for precise orbits, RESORB stands for restituted orbits. And depending on the satellite vehical S1A and S1B can be used. The files can be organized in the following structure - year, month and day. Individual files for each day can be placed in their respective folder.
1. Bulk download (Orbit data till date)
NOTE Precise orbit data is delayed by around 2 weeks.
The bulk data can be downloaded from the very well maintained GitHub repo by Alexey Pechnikov (PyGMTSAR maintainer). Simply clone or download the repo and place the orbits folder at .snap/auxdata/Orbits/Sentinel-1/POEORB/.
2. Individual download
Head to the https://step.esa.int/auxdata/orbits/ in order to download the required orbits depending on your required scene.
One can follow the automated approach using Python scripts. A crude script can be found to bulk download, this can be simply modified to download specific files too.
"""This script is to fix the issues with Sentinel-1 Precise Orbit download
issue faced in SNAP.
"""
import requests
from bs4 import BeautifulSoup
import wget
import os
from tqdm import tqdm
# get a list of all the updated
url = "https://step.esa.int/auxdata/orbits/Sentinel-1/POEORB/S1A/"
response = requests.get(url)
soup = BeautifulSoup(response.content, "html.parser")
years_links = [link['href'] for link in soup.find_all('a', href=True)][1:]
print(years_links)
download_root = "../.snap/auxdata/Orbits/Sentinel-1/POEORB/S1A/"
for year in years_links:
yearly_url = url + year
yearly_response = requests.get(yearly_url)
yearly_parser = BeautifulSoup(yearly_response.content, "html.parser")
monthly_links = [link['href'] for link in yearly_parser.find_all('a', href=True)][1:]
print(monthly_links)
for month in tqdm(monthly_links):
monthly_url = yearly_url + month
monthly_response = requests.get(monthly_url)
monthly_parser = BeautifulSoup(monthly_response.content, "html.parser")
orbit_links = [link['href'] for link in monthly_parser.find_all('a', href=True)]
# structure of download folder as per year/month
download_folder = download_root + year
if os.path.exists(download_folder):
pass
else:
os.mkdir(download_folder)
download_folder = download_folder + month
if os.path.exists(download_folder):
pass
else:
os.mkdir(download_folder)
# download the file
os.chdir(download_folder)
for link in tqdm(orbit_links):
if link.endswith('.EOF.zip'):
file_url = monthly_url + link
filename = link.split('/')[-1]
wget.download(file_url)
print(f"Downloaded {filename}")
break
break
References
- STEP forum - https://forum.step.esa.int/
- ChatGPT for web scrapping
- PyGMTSAR - https://github.com/AlexeyPechnikov/pygmtsar