- How Does turfRad Measure Root-Zone Soil Moisture?

turfRad measures root-zone soil moisture using passive L-band microwave radiometry — the same physical principles used by leading Earth-observation satellites such as NASA’s SMAP and ESA’s SMOS missions. By bringing this technology from orbit down to mower and utility-vehicle level, turfRad delivers high-resolution, subsurface moisture data suitable for day-to-day turf management.

What Is L-Band Microwave Sensing?

L-band refers to a portion of the electromagnetic spectrum between 1–2 GHz, corresponding to wavelengths of approximately 21 cm. turfRad operates in the 1.4–1.427 GHz band, which is internationally protected for passive scientific sensing — no transmissions are allowed in this frequency range. This protection is critical because it ensures:

• extremely low background interference,
• stable and repeatable measurements,
• and high sensitivity to soil moisture.

Longer wavelengths like L-band are able to penetrate turf canopy and sense moisture beneath the surface, unlike optical or infrared technologies that only observe surface conditions.

Passive Microwave Remote Sensing: Listening, Not Transmitting

turfRad sensors are completely passive. They do not transmit any signal into the ground.

Instead, they measure the natural microwave energy (Planck radiation) that soil continuously emits. This emission changes in a predictable way depending on how much water is present in the soil. Why this works:

• Dry soil emits more microwave energy.
• Wet soil emits less, because water has a much higher dielectric constant than dry soil.
• By measuring this difference, soil moisture can be quantified accurately.

Because the signal is naturally emitted, measurements work day or night, and are unaffected by clouds, shade, or lighting.

Measurement Depth

Passive L-band sensing is most sensitive to soil moisture in the top ~7–10 cm (3–4 inches) of soil. This depth aligns closely with the active turfgrass root zone and the layer that most directly influences firmness, playability, irrigation decisions, and early stress development.

Field validation has shown that turfRad’s measurements correlate strongly with TDR readings taken at approximately 3 inches depth, making the data highly relevant for day-to-day turf management.

Dual Polarization: Improving Measurement Stability

turfRad measures microwave emissions in two polarizations, which improves the stability and reliability of soil-moisture estimates. This approach helps account for surface and vegetation effects and supports more consistent measurements across varying turf conditions.

While vegetation influence is typically small in turfgrass systems, dual polarization enhances overall signal quality and repeatability.

Resolution: Why Ground-Based Matters

Satellite-based L-band radiometers measure soil moisture at very coarse resolution (often tens of kilometers per pixel), due to physical antenna constraints. By mounting the radiometer on a mower or utility vehicle, turfRad achieves:

• spatial resolution down to ~0.8 m (2.5 ft),
• full-fairway and full-course coverage,
• and repeatable, high-density moisture maps.

How the turfRad Technology Compares to Other Sensing Technologies

TDR

• Measures soil moisture at a single point using electrical pulses.
• Highly accurate locally, but labor-intensive and spatially limited.
• Sensitive to probe placement, soil texture, and salinity.

NDVI & Optical Imaging

• Measure reflected light from the turf canopy.
• Indicate plant stress after moisture problems appear.
• Do not measure soil moisture directly.

Thermal Infrared

• Measures surface temperature only.
• Strongly affected by wind, shade, slope, and seasonality.
• Limited usefulness for consistent moisture measurement.

Ground Penetrating Radar (GPR)

• Detects subsurface layers and material changes.
• Not sensitive to subtle moisture variation within soil volumes.

Why Passive L-Band Is Different

Passive L-band radiometry offers a unique combination of benefits:

• Direct subsurface moisture measurement
• Independence from sunlight and weather
• Sensitivity to moisture changes before visual stress
• Repeatable, scalable coverage
• Reduced sensitivity to salinity compared to lower-frequency systems
  
  

Summary

turfRad applies proven satellite remote-sensing physics in a practical, ground-based system. By passively measuring natural microwave emissions in the protected L-band spectrum, turfRad delivers reliable, root-zone moisture insight at the resolution turf managers need.

It does not replace traditional tools — it expands what’s possible by making subsurface moisture visible everywhere, every day, and without disruption.

Want to go deeper?
For a more detailed, scientific explanation of turfRad’s sensing technology, measurement physics, and validation work, you can download our full technical white paper here: White Paper