How do radar satellites measure ground movement after an earthquake?

In one line

The rule

Radar satellites send microwave signals to the ground and measure the time and phase (position in the wave cycle) of the signals that bounce back. By comparing the phase of signals from two different times, they can detect ground movement as small as a few millimetres.

No single formula, but the key idea is: Δd = (λ × Δφ) / (4π) where:

• Δd = change in ground distance (displacement)
• λ = wavelength of the radar signal
• Δφ = change in phase between two radar images

Let's understand it simply

Imagine you take a photo of your house before and after a storm. You compare the two photos to see if anything has changed. Radar satellites do something similar, but instead of normal photos, they use microwaves (a type of invisible light). These microwaves bounce off the ground and return to the satellite. By comparing the 'before' and 'after' signals, the satellite can spot even tiny shifts in the ground—like if your house moved just a few millimetres!

Think of it like rolling out a long measuring tape from the sky to the ground. If the ground moves, the tape will show a different length the next time you measure. Radar satellites use the change in the signal's 'phase' (like the position of a wave) as their measuring tape.

How it works

1. The satellite flies over an area and sends a pulse of microwaves to the ground.
2. The ground reflects these microwaves back to the satellite.
3. The satellite records the time and phase of the returning signal, creating a radar image.
4. After an earthquake, the satellite flies over the same area again and repeats the process.
5. Scientists compare the two radar images using a method called interferometry (studying the difference in phase between the two signals).
6. The difference in phase shows how much the ground has moved—up, down, or sideways.

The story behind it

The technique is called Synthetic Aperture Radar Interferometry (InSAR). Scientists started using radar from planes in the 1950s, but it became powerful with satellites in the 1990s. The European Space Agency's ERS-1 satellite (launched in 1991) was one of the first to use this for earthquakes. The problem they wanted to solve: How to measure ground changes in remote or dangerous places, where it is hard to send people.

A memorable detail: In 1992, a big earthquake in California was measured using InSAR, showing the ground had moved several metres—without anyone needing to visit the site.

Where you see it in real life

• After an earthquake: Satellites map how much the ground has shifted, helping rescue teams know which areas are most affected.
• Tracking landslides: In hilly areas, radar satellites spot slow ground movement before a landslide happens.
• Monitoring volcanoes: Scientists watch for swelling or sinking of the ground around volcanoes, which can warn of eruptions.
• City planning: In cities like Mumbai or Delhi, radar can detect if the ground is sinking due to too much groundwater use.
• Flood mapping: After heavy rains, satellites can see which areas have sunk or changed shape.
• Checking dams and bridges: Engineers use radar data to make sure big structures are not moving or cracking.

Why it matters

• Disaster response: Fast, accurate maps help save lives after earthquakes and landslides.
• Safety: Early warning for landslides or sinking ground can prevent accidents.
• Urban planning: Helps cities plan better and avoid building in risky areas.
• Science: Teaches us how the Earth's crust moves and changes over time.

Worked example

Let's say a radar satellite uses a wavelength (λ) of 5.6 cm (0.056 m). The phase difference (Δφ) measured between two images is 1.57 radians (which is about 90°).

Given: λ = 0.056 m, Δφ = 1.57 radians
Step 1: Write the rule        Δd = (λ × Δφ) / (4π)
Step 2: Put in the numbers    Δd = (0.056 × 1.57) / (4 × 3.14)
Step 3: Calculate numerator   0.056 × 1.57 = 0.08792
Step 4: Calculate denominator 4 × 3.14 = 12.56
Step 5: Divide                Δd = 0.08792 / 12.56 = 0.007 m
Step 6: Convert to mm         0.007 m = 7 mm

So, the ground moved about 7 millimetres!

Common mistakes

• Thinking radar takes normal photos: Radar uses microwaves, not visible light, so it can see through clouds and at night.
• Believing it only measures big movements: Radar satellites can detect shifts as small as a few millimetres.
• Mixing up phase and time: The key measurement is the phase difference, not just how long the signal takes.
• Forgetting about buildings and trees: Changes in buildings or plants can confuse the radar, so scientists must be careful when interpreting the data.

Remember it

Radar satellites "see" ground movement by comparing the phase of microwave signals before and after an earthquake—like a super-precise measuring tape from space.