SP3 Downloads¶

LuPNT can download precise GNSS SP3 orbit products from NASA CDDIS and cache them under output/gnss_files/sp3. The Python and C++ loaders use the same cache directory, so a file downloaded from either interface can be reused by the other.

This page describes the setup required before downloading, then shows how to run the included Python and C++ examples.

Before You Download¶

CDDIS access may require NASA Earthdata Login authentication. Before running the examples, do the following:

Create an Earthdata Login account:
```
https://urs.earthdata.nasa.gov
```

Review NASA’s Earthdata Login API and integration page:

https://www.earthdata.nasa.gov/engage/open-data-services-software/earthdata-developer-portal/earthdata-login-api

Configure command-line credentials with ~/.netrc.

The file is named .netrc and lives in your home directory. It is not ./netrc in the repository.
```
touch ~/.netrc
chmod 0600 ~/.netrc
cat >> ~/.netrc <<'EOF'
machine urs.earthdata.nasa.gov
  login YOUR_EARTHDATA_USERNAME
  password YOUR_EARTHDATA_PASSWORD
EOF
```
Keep this file private and never commit it. The 0600 permission matters: many HTTP tools refuse to use a world-readable .netrc file.
If this is your first scripted CDDIS access, sign in through a browser once and approve any Earthdata application-access prompt for CDDIS.

Optional Environment Variables¶

For short local tests, both LuPNT loaders also accept credentials from environment variables:

export EARTHDATA_USERNAME="YOUR_EARTHDATA_USERNAME"
export EARTHDATA_PASSWORD="YOUR_EARTHDATA_PASSWORD"

Prefer ~/.netrc for normal use so credentials are not exposed in shell history, process environments, or terminal logs.

Run The Examples¶

The Python example downloads or reuses SP3 files for several UTC epochs:

pixi run download-sp3-example

The C++ example exercises the same CDDIS filename/URL logic and cache:

pixi run download-sp3-example-cpp

Both examples print the selected SP3 product, local cache path, satellite IDs, and TAI epoch coverage parsed from the file.

Use From Python¶

The Python loader accepts either explicit filenames or target epochs. When target epochs are provided, it automatically computes the CDDIS product name, downloads missing files, and parses the SP3 contents.

from datetime import datetime, timezone

import pylupnt as pnt

loader = pnt.SP3Loader(
    target_dt=datetime(2025, 1, 1, tzinfo=timezone.utc),
    sim_t=0,
    dt_timesys=pnt.UTC,
)

print(loader.filenames)
print(loader.sats[:8])

Use From C++¶

The C++ loader provides static helpers for epoch-based download/cache and then normal parsing:

#include <lupnt/lupnt.h>

using namespace lupnt;

Real epoch_utc = GregorianToTime(2025, 1, 1, 0, 0, 0.0);
std::filesystem::path sp3_path =
    Sp3Loader::DownloadFileForEpoch(epoch_utc, Time::UTC);

Sp3Loader loader(sp3_path);
const auto& sats = loader.GetSatellites();

Troubleshooting¶

If LuPNT reports that CDDIS returned an Earthdata Login HTML page, the request reached the login service but did not receive the SP3 file. Check:

~/.netrc exists in your home directory, not in the repository.
The machine entry is exactly urs.earthdata.nasa.gov.
The file permissions are 0600.
Your Earthdata username and password are correct.
You have approved CDDIS access in a browser if prompted.

You can test the same credential path with curl:

curl -L -n -O "https://cddis.nasa.gov/archive/gnss/products/<week>/<file>.SP3.gz"

The flags matter: -L follows Earthdata redirects and -n uses ~/.netrc.

Cache Location¶

By default, SP3 files are stored in:

output/gnss_files/sp3

When running through pixi, LuPNT environment variables are configured from the workspace. If you run binaries manually, make sure LUPNT_DATA_PATH and any custom output path are set consistently with the rest of your workflow.