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Satellite Laser Ranging Data: Access for Orbit Analysis

Card No. 06·66266 · Updated · Standard formula, shown below

This guide shows you how to access satellite laser ranging (SLR) data and prepare it for orbit analysis. You’ll learn where to find the data, how to download it in the correct format, and how to perform basic orbit determination steps using free tools. No prior SLR experience is needed, but basic familiarity with orbital mechanics and command-line tools will help.

What You’ll Need

  • A computer with internet access
  • A text editor (e.g., Notepad++, VS Code)
  • Optional: Python 3 with numpy and matplotlib for data processing
  • Optional: GMAT or Orekit for advanced orbit analysis

Step-by-Step

  1. Identify the data source. The primary source for SLR data is the International Laser Ranging Service (ILRS) data centers. The two main archives are the CDDIS (Crustal Dynamics Data Information System) at NASA and the EDC (European Data Center) at DGFI-TUM. Both offer free, public access.
  2. Choose the data product. For orbit analysis, you typically need either:
    • Normal points (NP): Range measurements averaged over a short interval (e.g., 5 minutes) for a specific satellite pass. This is the standard product for most analyses.
    • Full-rate data: Raw individual laser shots, used for detailed studies but larger in size and noisier.
  3. Access the CDDIS archive. Go to https://cddis.nasa.gov/archive/slr. You’ll need to create a free account (or use your Earthdata login) to download data. Alternatively, use the anonymous FTP at ftp://cddis.nasa.gov/slr.
  4. Navigate to the data directory. The CDDIS SLR directory is organized by product type. For normal points, go to slr/np. For full-rate data, go to slr/fullrate. You’ll see subdirectories by year and then by day-of-year (DOY).
  5. Download the data for your satellite. Files are named with a convention like np_<satellite>_<date>.gz (e.g., np_lageos1_2024001.gz). Use your browser or wget to download the file for your target satellite and date.
  6. Extract the file. Most files are gzipped. Use gunzip np_lageos1_2024001.gz or a GUI tool like 7-Zip to extract the text file.
  7. Understand the format. SLR normal point files follow the ILRS Consolidated Range Data (CRD) format. The file starts with a header (lines beginning with h1, h2, etc.) and then contains data records. Each normal point is a line starting with 20 (the epoch) followed by the range, time-of-flight, and other parameters. Refer to the ILRS Data Format Specification for full details.
  8. Load the data into your analysis tool. If using Python, write a script to parse the CRD file. A simple approach is to read the file line by line, extract lines starting with 20, and store the epoch (in MJD) and range (in meters) into arrays. Here’s a minimal example: 
    import numpy as np

def read_crd(filename):
    epochs = []
    ranges = []
    with open(filename) as f:
        for line in f:
            if line.startswith('20'):
                # Epoch is in MJD, range in meters
                mjd = float(line[4:18])
                rng = float(line[18:30])
                epochs.append(mjd)
                ranges.append(rng)
    return np.array(epochs), np.array(ranges)
  9. Perform basic orbit analysis. With the range data, you can compute the satellite’s position relative to the station. The range is the distance from the station to the satellite. To reconstruct the orbit, you need to combine observations from multiple stations over time. For a simple pass, you can plot range vs. time to see the pass shape. For full orbit determination, use a tool like GMAT (free) or Orekit (open-source Java library).
  10. Validate your results. Compare your computed orbit with the official ILRS predicted orbits (available from the same archive under slr/pred). The difference should be within a few meters if your processing is correct.

Common Mistakes

  • Using the wrong file type: Confusing normal points with full-rate data. Normal points are pre-processed and easier to use; full-rate data requires significant filtering.
  • Ignoring the station coordinates: The range is measured from a specific ground station. You must include the station’s geocentric coordinates (available in the CRD header) to convert range to a satellite position.
  • Not accounting for atmospheric delays: The laser pulse travels through the atmosphere, which slows it down. The CRD file includes a correction, but if you use raw range, you must apply the tropospheric delay correction yourself.
  • Mixing up units: Range is in meters, but some older formats use microseconds of time-of-flight. Always check the units in the file header.
  • Using the wrong epoch: The epoch in CRD is in MJD (Modified Julian Date), not a standard date. Convert carefully to UTC if needed.

Tips

  • Start with a well-observed satellite like LAGEOS-1 or LAGEOS-2. They have many normal points and are easier to analyze.
  • Use the ILRS Data Handling Software (DHS) or the SLR Resampling and Editing Kernel (SREK) to filter outliers before analysis.
  • Download a week of data for your satellite to get better orbit coverage.
  • If you’re new to orbit analysis, use GMAT’s built-in SLR tracking data import (it supports CRD format).
  • Check the ILRS website for the latest data format changes; the format is updated periodically.

Troubleshooting

  • Cannot log in to CDDIS: Ensure you’ve created an account and are using the correct credentials. Try the anonymous FTP if you’re having issues with the web interface.
  • File not found: Verify the satellite name and date. Check the directory listing for the exact filename. Some satellites have multiple names (e.g., LAGEOS-1 vs LAGEOS1).
  • Data appears noisy: Check if you’re using full-rate data instead of normal points. Also, ensure you’re applying the range corrections listed in the file header.
  • Orbit solution diverges: This often happens when you have insufficient data or bad station coordinates. Try including more passes and verify the station coordinates are correct.
  • Python script errors: Ensure the file is extracted and you’re reading the correct line numbers. Use a debugger or print a few lines to verify the parsing.

Frequently Asked Questions

  1. What is the difference between normal points and full-rate data? Normal points are averaged range measurements over a short interval, reducing noise and data volume. Full-rate data contains every laser shot, which is noisier but retains all information.
  2. Do I need to apply any corrections to the range data? Yes, the CRD file provides corrections for atmospheric delay and relativistic effects. If you use the raw range, you must apply these corrections yourself; otherwise, your orbit will be biased.
  3. Can I use SLR data for any satellite? SLR is only available for satellites equipped with retroreflectors. Most GNSS satellites, geodetic satellites (LAGEOS, Starlette), and some Earth observation satellites have them.
  4. How often is SLR data updated? Normal point data is typically available within a few hours to a day after observation. Full-rate data may take longer.
  5. Is there a way to visualize the data before analysis? Yes, you can plot range vs. time using any plotting tool. For a quick look, use the ILRS Quicklook plots on their website.
  6. What is the accuracy of SLR normal points? For well-observed satellites like LAGEOS, normal point accuracy is typically a few millimeters to a centimeter.

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