How do Radar Altimeters Perform Over Water?

     With the rapid growth of the UAS industry, both commercially and tactically, flight over water has become increasingly common. Many current and emerging UAS applications implement flight over water, including:

• Search and Rescue
• Surveying
• Environmental Monitoring
• Mapping
• Inspection
• Aerial Target Operations
• Medical/Equipment Delivery
• ISR
• Sea-Skimming Tactical Applications
• Takeoff and Landing on Moving Maritime Vehicles

     As these over-water applications continue to expand, the necessity for reliable sensors and systems onboard the UAS becomes essential. One onboard sensor that is crucial for a successful over-water UAS flight is an above-ground-level (AGL) altimeter. Flight over water typically involves changes in atmospheric pressure, BVLOS operations, and limited visual references, resulting in AGL altimetry becoming critical for providing aircraft with real-time, accurate above-ground-level altitudes to ensure aircraft safety and mission success. Other sources of altitude, such as barometers, require calibration to maintain accurate readouts, whereas AGL altimetry, such as radar altimeters, requires zero calibration and can immediately provide accurate altitude data.

     The most common AGL altimeters in the UAS industry are laser altimeters and radar altimeters. Accuracy over water is a specific strength of radar altimeters compared to alternative equipment at lower altitudes. The near-infrared (NIR) or visible light spectrum of laser altimeters causes irregularities in altitude information. In some instances, laser altimeters can penetrate the water below and provide a returning altitude much greater than the aircraft’s true altitude above the surface. In other instances, laser altimeters may provide reasonably accurate, yet inconsistent and noisy altitudes.

     Radar altimeters, on the other hand, perform exceptionally well over water and maintain high accuracy throughout flight. The frequencies at which radar altimeters operate allow for favorable altitude data, producing consistent surface returns with minimal penetration. It is uncommon for radars to lose range fidelity over water, and they often serve as a complementary source at lower altitudes, typically below 200-300m, where GPS, barometric, or Optical sensors may experience reduced reliability. Radar altimeters can seamlessly transition between land and water without compromising data fidelity. Optimal radar altimeter accuracy is achieved in lower sea state levels, such as sea state levels one or two, though reliable performance is maintained across a wide range of conditions.

     The first video (US-D1 vs Laser Altimeter) showcases a comparison in altitude data between Ainstein’s US-D1 and a laser altimeter during a flight over water.