Radar angels

Lore & Background

Early radars suffered from strong ground returns that blanked portions of the display, making angels difficult to distinguish. The development of the COHO concept in the UK eliminated these permanent echoes, allowing angels to be seen clearly for the first time. One early example in 1953 on the Radar, Anti-Aircraft No. 4 Mk. 7 led an ornithologist to purchase a surplus Radar, Anti-Aircraft No. 3 Mk. 7 for bird tracking. Many angels were initially attributed to meteorological effects, but this theory could not explain their behavior. Experiments by the Radar Research and Development Establishment showed the radar cross section of a dead bird was about 0.01 square meters, much smaller than normal detection limits, and certain motion aspects contradicted the bird hypothesis. In one case, the experimental COHO MEW radar at Great Baddow observed ring-shaped angels radiating outward from a point and drifting in the wind, only in the morning. Investigators expected a factory steam plant but found open parkland with a stand of trees. The mystery was solved when huge flocks of starlings were seen leaving the trees in a wave-like pattern: at night the birds clustered in the center, then at dawn tree-hopped outward; on an invisible signal, birds at the outside left simultaneously, radiating outward, and the process repeated. At night, birds arrived in small groups and caused no display. It was not until 1957 that the Royal Society concluded birds were the primary cause.

Reader's Guide

Radar angels significantly impacted radar operations, particularly for the Canadian Mid-Canada Line, which became almost unusable in spring and fall during bird migrations. The problem was worsened by birds landing near warm diesel generators at the stations. While typical pulse radars could use sensitivity time control (STC) to suppress nearby returns, the Mid-Canada Line used continuous wave (CW) radar with no inherent timing, so STC could not be applied. This led to a significant feature on the similar AN/FPS-23 radars used on the DEW Line: Doppler filtering to remove objects slower than 125 mph. For pulse radars, swept gain (STC) was a solution: it lowered receiver sensitivity for nearby targets before reaching maximum gain at longer range, typically around 50 miles. By adjusting gain suppression, bird returns could be eliminated while aircraft remained visible. Beyond birds, any periodic structure in the radar's view can cause similar effects, notably in sea-scanning radars on aircraft and satellites when wave patterns match multiples of the radar wavelength. This effect has been exploited in radars that measure sea state offshore and in wind-measuring radars that create required patterns using acoustic waves from large loudspeakers.

Did You Know?

• Radar angels were first noticed in the 1940s and became a major study topic in the 1950s.
• The radar cross section of a dead bird was measured at about 0.01 square meters, similar to a bag with 1 lb of water.
• The Royal Society concluded in 1957 that birds were the primary cause of radar angels.
• The Mid-Canada Line was almost unusable during bird migrations because its continuous wave radar could not use sensitivity time control.

The Science and Operating Principles

GPR works by sending high-frequency radio waves—typically between 10 MHz and 2.6 GHz—into the ground through a transmitter and antenna. When those electromagnetic pulses strike a buried object or a boundary where the electrical properties of the material shift, part of the energy bounces, refracts, or scatters back toward the surface, where a receiving antenna captures the returning signal. The underlying logic mirrors seismology, except that GPR relies on electromagnetic energy rather than acoustic waves, and reflections occur at boundaries of differing permittivity rather than mechanical impedance. The technique is entirely non-intrusive, meaning no drilling or excavation is required to peer beneath concrete, asphalt, soil, ice, or even fresh water. Practitioners can identify subsurface objects, shifts in material composition, voids, and cracks, making it a versatile surveying tool that leaves the ground undisturbed.

A History of Subsurface Exploration

The roots of ground-penetrating radar stretch back further than most people realize. In 1910, Gotthelf Leimbach and Heinrich Löwy filed the first patent for a continuous-wave radar system aimed at locating buried objects, just six years after the original radar patent. A decade and a half later, in 1926, Dr. Hülsenbeck filed a patent for a pulse-based radar approach, which offered notably better depth resolution. By 1929, W. Stern had already used the technique to measure the depth of a glacier. After a long period of limited progress, the 1970s brought a surge of interest driven largely by military research, and the first affordable consumer-grade equipment hit the market in 1975. One of the most striking early milestones came in 1972, when the Apollo 17 mission placed an instrument called ALSE—the Apollo Lunar Sounder Experiment—into lunar orbit. It recorded depth information reaching 1.3 kilometres beneath the Moon's surface, storing the data on film because suitable digital storage did not yet exist.

A Tool for Many Disciplines

The versatility of GPR is one of its most remarkable qualities. In Earth sciences, researchers use it to map bedrock, study soil composition, trace groundwater, and probe ice sheets. Prospectors have applied it to find natural traps in buried stream beds where gold nuggets or diamonds in alluvial gravel might accumulate. On the Moon, the Chinese rover Yutu carries a GPR unit on its underside to investigate lunar soil and crust. In engineering, the technology supports nondestructive testing of pavements and structures, locates buried utility lines—including non-conductive plastic conduits and concrete sewers that standard electromagnetic induction tools cannot detect—and characterizes soils and bedrock. Environmental teams deploy it to delineate landfills and contaminant plumes, while archaeologists use it to map buried features and cemeteries without disturbing the site. Law enforcement has turned to GPR to find clandestine graves and buried evidence, and military units employ it to detect mines, unexploded ordnance, and hidden tunnels. In 2020, the US military placed a $200.2 million order with Chemring Sensors for a GPR system designed to find improvised explosive devices buried in roadways.

The Limits of Seeing Through the Earth

Despite its power, GPR is not a universal solution, and its effectiveness is tightly bound to the electrical and physical properties of the material being scanned. The conductivity of the ground, the center frequency of the transmitted signal, and the radiated power all constrain how deep a practitioner can see. Higher frequencies deliver sharper resolution but are attenuated more quickly, so they cannot travel as far. Lower frequencies penetrate deeper but sacrifice detail, making frequency selection an inherent trade-off between clarity and reach. In ideal conditions—dry sandy soils, granite, limestone, or concrete—penetration can reach around 15 metres. In ice, the numbers are staggering: low-frequency GPR has reached bedrock in Greenland, a depth of several thousand metres. At the opposite extreme, moist or clay-rich soils with high conductivity can limit penetration to mere centimetres. Antenna placement also matters; contact with the ground generally yields the strongest signal, though air-launched antennas allow scanning from above. Within hydrogeophysics, a specialized cross-borehole variant has emerged as a practical way to assess soil water content.