Atmospheric Sounding
Understanding Atmospheric Sounding
Weather forecasting, climate monitoring, and RF propagation prediction all depend on knowing the atmosphere's vertical structure — temperature, humidity, and wind from the surface to the stratosphere. Atmospheric sounding is the science of making these measurements, and many of the most important sounding technologies are RF-based.
Radiosonde (Active UHF)
The workhorse of atmospheric sounding is the radiosonde — a small instrument package that ascends on a weather balloon, measuring temperature, humidity, and pressure while transmitting the data via a UHF radio link at 400–406 MHz. Radiosondes provide the highest-accuracy profile measurements but are limited to twice-daily launches at fixed stations.
Microwave Radiometry (Passive)
Ground-based microwave radiometers measure the thermal emission of the atmosphere at multiple frequencies around the oxygen absorption complex (50–60 GHz) and water vapor line (22.235 GHz). By measuring brightness temperature at several frequencies with different absorption characteristics, the radiometer can retrieve temperature and humidity profiles continuously, filling the temporal gaps between radiosonde launches.
GPS Radio Occultation (Space-based)
When a GPS signal passes tangentially through the atmosphere as seen by a LEO satellite receiver, the atmosphere bends and delays the signal. By measuring this bending angle as a function of tangent altitude, the receiver retrieves a high-resolution refractivity profile from which temperature and humidity can be derived. This technique provides global, all-weather sounding with no consumable hardware.
Key Equations
TB(f) = ∫W(f,z)T(z)dz
W = weighting function
T(z) = temperature profile
Channel selection:
O2 line complex (50–60 GHz): temperature
H2O line (183 GHz): humidity
Vertical resolution:
Δz ≈ 2–5 km (microwave)
Comparison
| Channel | Center freq | Weighting peak | Observable | Instrument |
|---|---|---|---|---|
| AMSU-A ch6 | 54.4 GHz | Surface | T surface | NOAA/EUMETSAT |
| AMSU-A ch9 | 57.3 GHz | 250 hPa | T mid-tropo | NOAA/EUMETSAT |
| AMSU-A ch14 | 57.6 GHz | 2 hPa | T stratosphere | NOAA/EUMETSAT |
| MHS ch3 | 183±1 GHz | 500 hPa | Humidity | NOAA/EUMETSAT |
| MHS ch5 | 190 GHz | Surface | H2O total | NOAA/EUMETSAT |
Frequently Asked Questions
What frequency does a radiosonde transmit on?
International radiosonde systems operate in the 400.15–406 MHz band, allocated by the ITU Radio Regulations for meteorological aids (MetAids). The radiosonde transmitter typically operates at 10–200 mW, sufficient for a ground receiver with a tracking antenna to receive data from balloon altitudes up to 35 km. Modern radiosondes use GPS for position tracking, eliminating the need for ground-based radio direction finding.
How does wind profiler radar work?
Wind profiler radars transmit vertically or near-vertically and detect backscatter from turbulent fluctuations in the atmosphere's refractive index. By measuring the Doppler shift of the returned signal at multiple beam angles, the radar computes the three-dimensional wind vector as a function of altitude. UHF profilers (449 MHz) measure wind through the entire troposphere, while boundary layer profilers (915 MHz or 1.29 GHz) provide higher-resolution measurements in the lowest 3–5 km.
Why is atmospheric sounding important for RF engineers?
Atmospheric sounding data directly feeds the propagation models that RF engineers use to design radar and communication links. The refractivity profile determines ray bending and ducting conditions. The water vapor profile determines gaseous absorption. The turbulence profile determines scintillation. Without accurate sounding data, propagation predictions — and therefore link budget calculations — are unreliable.