What Is an Upper Air Sounding System? Radiosonde, Pilotsonde and Ozonesonde Explained

What is an upper air sounding system

An upper air sounding system is a measurement chain made up of a balloon borne instrument, a radio telemetry link and a ground receiving station, used to record temperature, humidity, pressure and wind continuously as the instrument rises through the atmosphere.

The instrument is expendable, the ascent cannot be repeated, and every reading is tied to an altitude. That is what separates a sounding from any measurement taken at the surface, and it is why the sounding record remains the reference dataset for atmospheric science.

Three elements have to work together. The balloon borne sonde carries the sensors and the radio transmitter. The telemetry link carries the data down, typically in the 400 to 406 MHz meteorological band. The ground station receives, decodes, validates and archives the message stream, turning a sequence of readings into a vertical profile a forecaster or researcher can use.

Sertel builds this chain around three instrument types: the Radiosonde T-RS-300, the Pilotsonde T-PS-300 and the Ozonesonde T-OS-300. All three share the same telemetry architecture, the same GNSS wind finding approach and the same ground reception workflow, so one observatory can run all three from a single ground setup and choose the instrument launch by launch.

Why vertical atmospheric profiles matter

Surface observations describe one plane of a three dimensional atmosphere. Forecasting, aviation planning, range safety and climate research all depend on knowing how temperature, humidity, pressure and wind change with height.

That vertical structure governs stability, cloud formation, jet stream position and the dispersion of anything released into the air. A sounding gives it directly, as measured values rather than modelled ones, which is why upper air observation networks remain the benchmark against which satellite retrievals and model outputs are validated. The value lies in the fact that a sonde physically occupies every altitude it reports.

Where sounding data is used

• Weather forecasting and numerical model initialisation
• Aviation support, including wind and temperature profiles for flight planning
• Launch range safety and trajectory assessment for aerospace operations
• Disaster management, particularly cyclone and severe weather monitoring
• Climate research and long term atmospheric trend analysis
• Stratospheric ozone monitoring and environmental compliance reporting
• Research and education at universities and atmospheric science institutes

The three instrument classes explained

Sondes differ mainly in what they measure. A radiosonde returns a full atmospheric profile, a pilotsonde is a dedicated wind finding instrument, and an ozonesonde adds chemistry to a meteorological profile.

Choosing between them comes down to how complete a profile the mission needs and what the payload budget allows.

Radiosonde: the full atmospheric profile

A radiosonde measures temperature, humidity, pressure and wind throughout flight and transmits in real time to the ground. It is the standard instrument for routine synoptic soundings because it returns the complete PTU set, pressure, temperature and humidity, alongside GNSS derived wind. The Sertel T-RS-300 weighs less than 30 g, runs 180 minutes on a Li-SOCl2 battery, profiles to 30 to 40 km altitude and transmits over a proven range of up to 300 km.

Pre flight integrity matters as much as flight performance. The T-RS-300 carries onboard functional checks covering temperature verification, humidity sensor reconditioning and humidity calibration, together with configurable sonde parameters, so measurement integrity is confirmed before release rather than discovered later in the data.

Pilotsonde: focused wind profiling

A pilotsonde is a windfinding instrument. Where a complete atmospheric profile is not required, it delivers upper air wind speed and direction with temperature sensing at lower payload cost than a full sensor radiosonde. The Sertel T-PS-300 weighs less than 27 g, profiles to 30 km and transmits up to 250 km using LoRa or GFSK telemetry.

This suits high frequency wind observation programmes, aviation support and range safety work, where the operational question is what the wind is doing at altitude. Because the T-PS-300 keeps the same balloon borne ascent, telemetry architecture and ground reception compatibility as the radiosonde family, switching between the two costs nothing in ground infrastructure.

Ozonesonde: chemistry added to the profile

An ozonesonde adds an electrochemical concentration cell ozone sensor to a meteorological radiosonde, so one flight returns both the ozone profile and the meteorological context needed to interpret it. In the Sertel T-OS-300, ambient air is drawn through a potassium iodide sensing solution where an iodine and iodide redox reaction generates a current proportional to ozone concentration. Combined with the onboard pressure and temperature reference, that current yields the ozone partial pressure profile from the surface through the stratosphere.

The T-OS-300 is designed for 2000 g weather balloons and supports ozone profiling from mean sea level up to 50 km, the band that matters for stratospheric ozone monitoring. It weighs less than 30 g without the ozone sensor and is fully compatible with the IMD provided ECC ozone sensor, which is required for ozone sounding applications.

One family, one ground workflow

The T-RS-300, T-PS-300 and T-OS-300 share their core telemetry, GNSS wind finding and pressure sensing architecture. Operators running mixed sounding programmes can standardise on a single ground station and a single reception workflow across all three instrument types, which simplifies training, spares holding and data handling.

Radiosonde, pilotsonde and ozonesonde compared

The three instruments share telemetry, GNSS and pressure architecture but differ in measured parameters, profiling altitude and telemetry range.

Figure 1: Comparison of the Sertel upper air sounding instrument family across measurement scope, altitude and telemetry performance.

Inside a modern GNSS sounding instrument

A research grade sonde combines four subsystems: a calibrated sensor package, a dual band GNSS receiver for wind finding, signal integrity protection, and a narrow band telemetry transmitter.

How GNSS wind finding replaced theodolite tracking

GNSS wind finding derives wind from the sonde's own position fixes instead of from angles measured on the ground, removing the dependence on visual line of sight, daylight and operator judgement that limited optical theodolite tracking.

For decades, pilot balloon wind observation depended on an operator tracking a rising balloon through a theodolite and reading angles at fixed intervals. The method worked but carried three structural limits: it needed visual line of sight, so cloud cover truncated the profile; it needed daylight, or a lit balloon at night; and it depended on operator judgement, so results varied between observers and between stations.

The T-PS-300 replaces manual theodolite based tracking with a fully automated GNSS solution, delivering consistent, repeatable wind data for every ascent regardless of visibility or time of day, at theodolite grade accuracy with no manual optical tracking.

For an observation network the practical consequence is comparability. Two stations running the same automated instrument produce profiles that can be compared directly, which is the precondition for using the data in numerical models and in long term climatological series.

What a complete sounding station needs

Besides the sonde, a working station needs a ground receiving station, receiving and GNSS antennas, a balloon filling unit, an unwinder, ground check equipment and balloons matched to the target altitude.

• Sonde radiosonde, pilotsonde or ozonesonde, selected per mission
• Ground receiving station receives, decodes and archives the telemetry stream, in standard or MIL grade configuration
• Receiving antenna UHF antenna for the 400 to 406 MHz telemetry link, with a GNSS antenna for the station reference
• Balloon filling unit controlled, repeatable gas filling so ascent rate stays within the planned envelope
• Unwinder releases the suspension line during ascent, keeping the sonde clear of the balloon wake
• Ground check equipment pre flight verification of sensor readings against a known reference
• Balloon and expendables balloon size matched to the target burst altitude and payload mass

Because Sertel supplies the sondes, the ground system, the antennas and the accessories as one product family, the interfaces between these elements are already defined. That removes the integration work that otherwise falls on the observatory when equipment is sourced piecemeal.

A launch to launch sounding workflow

A repeatable launch sequence runs from mission planning and sonde preparation through ground checks, link verification, balloon filling, release and profile validation.

1. 1
   Plan the launch Confirm the observation schedule, select the instrument class, and match balloon size and gas volume to the target altitude and payload mass.
2. 2
   Prepare the sonde Activate the battery, configure the transmission frequency, and set the sonde parameters for the flight.
3. 3
   Run the ground check Verify temperature readings, recondition and calibrate the humidity sensor, and confirm the pressure reference against the station barometer. Record the results.
4. 4
   Confirm the link Verify telemetry reception at the ground station, check GNSS lock and satellite count, and confirm the data message decodes cleanly at 1 Hz.
5. 5
   Fill and rig Fill the balloon to the planned volume, attach the unwinder and suspension, and secure the sonde for release.
6. 6
   Release and track Launch on schedule and monitor the incoming profile in real time, watching ascent rate and signal quality as the balloon rises.
7. 7
   Close out the flight Record burst altitude and termination, validate the profile for gaps or anomalies, and archive the sounding with its pre flight check record.

How to choose a sounding system

Selection turns on eight criteria: measurement scope, altitude requirement, telemetry range, GNSS capability, signal integrity protection, payload weight, ground system compatibility and field practicality.

How Sertel supports upper air sounding programmes

Sertel designs and manufactures a complete upper air sounding family in India, covering sondes, ground receiving stations, antennas and launch accessories.

Building a dependable sounding programme

An upper air sounding system is easy to describe and demanding to run well. The instrument is expendable, the flight cannot be repeated, and the quality of the record depends as much on the discipline of the ground procedure as on the specification of the sensors. Programmes that standardise on one instrument family, one ground workflow and one launch procedure get data that stays comparable across sites and across years, which is ultimately what the data is for.