Vaisala RD94 Technical data

Vaisala Dropsonde RD94

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The Vaisala Dropsonde RD94 is a meteorological measurement device to be used in atmospheric prolings from aircraft ight level to surface. Descending through the atmosphere by a parachute, it measures the proles of pressure (P), temperature (T), relative humidity (U) and wind direction and speed from the point of launch to the ground. It has been used from low, 1500 ft, to high, 66 kft, altitude drops. The RD94 transmits data using a narrow bandwidth FSK (Frequency Shift Key) modulation in the 403 MHz meteorological band to the AVAPS (Airborne Vertical Atmospheric Proling System) onboard the aircraft.

Proven Measurement Performance

The Vaisala Dropsonde RD94 sensors are calibrated using similar methods as with the Vaisala Dropsonde RD93

and the Vaisala RS92 radiosonde family. Calibrations are traceable to the international standards. Accuracies have been validated e.g. in the WMO comparisons.

The winds are measured using a commercial GPS receiver, which has a custom made rmware to ensure proper wind measurement. The wind proles from multiple simultaneous dropsoundings show a good consistency.

Designed for Aircraft Use

The Vaisala Dropsonde RD94 is designed for aircraft use, and is launched through the body of an aircraft. Specially designed launchers are used for both pressurized and un-pressurized cabin aircrafts. The delayed deployment mechanism ensures proper clearance from the aircraft, and allows the parachute to open safely. The specially designed parachute stabilizes the RD94 aligning it properly for measurements, and its small gliding factor ensures proper measurement of the wind speed and direction.

The descent speed of the RD94 is approximately 11 m/s at the sea level, and about 21 m/s at a 12 km altitude. In strong convective situations the descent rate can vary a lot, and even negative (descent) rates are measured. The RD94 measures the PTU twice and wind speed and direction four times a second. This means that e.g. the last measured pressure level is at maximum about 5 - 6 meters above the surface.

The robust design of the RD94 ensures safe use of the legacy launchers and high speed deployment of the parachute.

Dropsonde Receiving System

Onboard the aircraft the AVAPS system receives, displays, stores, and distributes dropsonde data for further

use by the application in question. The AVAPS is not compatible with Vaisala radiosondes and vice versa.

AVAPS can be congured to track up to four dropsondes at the same time. This is an essential ability in operations that are carried out with high-speed, highaltitude aircraft when dense horizontal resolution of data is required.

The AVAPS can be set to provide data automatically for the aircraft data system for further use. The dropsonde technology developer NCAR/EOL provides ASPEN data post descent processing software for dropsonde users.

Intellectual Property Rights and Development

Vaisala Inc., USA, manufactures and markets the Dropsonde RD94 and AVAPS Dropsonde Receiving System under license to University Corporation for Atmospheric Research Foundation (UCARF). The dropsonde and receiving system hardware and software have been developed at the Earth Observing Laboratory (EOL) of the National Center of Atmospheric Research (NCAR), in Boulder, Colorado, USA.

Vaisala Dropsonde References

Since 1996, about 50,000 earlier versions of the Vaisala Dropsonde RD93, have been used in eight countries by various operators. The main uses have been hurricane research, and data acquisition over oceans as input for Numerical Weather Prediction (NWP) models. In addition, a large number of Vaisala Dropsonde RD93s, have been annually used in various eld campaigns to acquire data for meteorological research and for validation of other airborne instrumentation.

Technical data

Vaisala Dropsonde RD94

Weight 350 g Size 7 cm in diameter, 41 cm in length Maximum deployment airspeed 250 kt IAS (= 125 m/s IAS) Shelf life 1 year from delivery

Transmitter - EN302054

Frequency range 400 MHz to 406 MHz Frequency stability ±1 kHz RF power output 100 mW Channel spacing Modulation FSK Harmonic & spurious output >50 dB below the carrier level Total modulation Telemetry range with recommended receiving antenna 325 km

Battery

Type Three lithium CR-2 cells in series Voltage 9 V nominal Current Max. 235 mA, 200 mA average Life 2 hours (operating), 3 years (shelf)

Pressure sensor (P)

Vaisala BAROCAP® silicon sensor Range 1080 hPa to 3 hPa Resolution 0.1 hPa Accuracy

Repeatability* 0.4 hPa

Temperature sensor (T)

Vaisala THERMOCAP® capacitive bead Range –90 °C to +60 °C Resolution 0.1 °C Accuracy Repeatability * 0.2 °C Response time (when used and measured in Vaisala Radiosonde RS80)

6 m/s, 1000 hPa < 2 s

Relative humidity sensors (U)

Vaisala H-HUMICAP® thin ﬁlm capacitor, twin-sensor design Range 0 % to 100 %RH Resolution 1 %RH Accuracy

repeatability * 2 %RH Response time (when used and measured in Vaisala Radiosonde RS92)

6 m/s, 1000 hPa, +20 °C < 0.5 s

6 m/s, 1000 hPa, –40 °C < 20 s

Wind (horizontal)

Commercial GPS receiver

Data rates

PTU update rate 2 Hz Wind update rate 4 Hz

Descent

Descent speed ~11 m/s at sea level Descent time

From 14 km ~15 mins

From 7.5 km ~8 mins

*Standard deviation of differences between two

successive repeated calibrations, k = 2 conﬁdence level

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