Drift Scan

Drift-scan radio astronomy is a simple but powerful observational method used in radio astronomy — especially for mapping the sky or studying large-scale sources — that takes advantage of the Earth’s rotation instead of mechanically moving the telescope to track celestial objects.

Here’s a clear breakdown of how it works and why it’s useful:

🛰️ What It Is

In drift-scan mode (also called transit mode):

  • The telescope is fixed in position, pointed at a constant azimuth and elevation (or fixed declination in equatorial coordinates).
  • As the Earth rotates, the sky “drifts” past the telescope’s field of view.
  • The radio signals from each celestial source are detected as they pass through the beam, producing a time series of signal strength.

🕒 How It Works

  1. Fix the telescope at a desired declination (δ).
    The telescope beam will naturally trace a path across the sky at that declination once every sidereal day (~23h 56m).
  2. Record signal vs. time.
    As the sky drifts through the beam, sources at that declination produce peaks in signal power as they transit.
  3. Calibrate using known sources (like Cas A or Cyg A) to measure beam shape, sensitivity, or system temperature.
  4. Repeat at different declinations if you can adjust the elevation — allowing you to build up a 2D sky map.

📊 Applications

  • Sky surveys: Used to make all-sky or partial-sky maps (e.g., early 21 cm surveys).
  • Beam characterization: Easy way to measure the antenna’s response pattern.
  • Pulsar and transient searches: Sensitive to sources that drift through the beam.
  • Calibration and monitoring: Tracking system gain, RFI, and background temperature over time.

🧮 Example: 21 cm Hydrogen Line Drift Scan

If you point a small dish or horn antenna at the celestial equator and record the 1420 MHz band:

  • You’ll see the Galactic plane as a strong increase in brightness as it drifts through.
  • By measuring Doppler shifts, you can infer the rotation of the Milky Way.

⚙️ Advantages

✅ No need for precision tracking motors or mounts
✅ Simplifies mechanical design and data collection
✅ Ideal for long-term monitoring or mapping large sky regions

⚠️ Disadvantages

❌ Limited to one declination at a time
❌ Integration time per source is limited to beam transit time
❌ Lower signal-to-noise ratio unless you use wide beams or repeated scans

🧰 Typical Setup

  • Antenna: Fixed parabolic dish, horn, Yagi array, or even a simple dipole array
  • Receiver: LNA → bandpass filter → SDR (e.g., RTL-SDR, Airspy, etc.)
  • Software: GNU Radio, SDR#, Radio Eyes, or custom Python code for FFT/spectral averaging
  • Data: Power vs. time and frequency; post-processed into sky maps or spectra