THE INSTRUMENT PACKAGE forTROPOSPHERIC EFFECTS(CLOUD ATTENUATION) ON SIGNAL TRANSMISSION
The main component of the radiosonde is a sturdy, lightweight, white cardboard (or plastic) instrument package, approximately the size of a large shoe box. The package is attached to the train containing the balloon and parachute. The following weather sensing instruments are located within or attached to this package:
The resistance thermistor is a white ceramic covered metallic rod that serves as a temperature sensor on most American radiosondes. The diameter of the rod is approximately 0.7 mm and its length is no more than 2 cm. The electrical resistance of this rod changes with a change in the air temperature. To increase contact with the air, the thermistor is located on an outrigger, extended a distance from the outside of the instrument package. The thermistor is white to minimize the heating by sunlight. The temperature range for the thermistor lies between approximately +40° C to 90° C.
The hygristor is a humidity sensor consisting of a glass slide or plastic strip covered with a moisture sensitive film of lithium chloride (LiCl) and a binder; metal strips are located along the edges. The electrical resistance of the chemical changes with a change in the atmospheric humidity. The hygristor is located within the instrument package at a place where the outside air passes the hygristor. The hygristor on most radiosondes is designed to record the ambient relative humidity in the range from 15% to 100%.
The radiosonde measures pressure by means of an aneroid barometer, consisting of a small, partially evacuated metal canister. This temperature compensated instrument is central to the instrument package. The volume of the canister expands as the radiosonde ascends, in response to a reduction in the atmospheric pressure aloft. The aneroid is designed to register pressures from 1040 mb to 10 mb or less. The aneroid also serves another function as described below. A pen arm is attached to the aneroid.
A switching mechanism, called a baroswitch, was used on older radiosondes to switch between the two sensor and the reference elements. This baroswitch involves a contact arm connected to the barometer passes over the commutator bar. The commutator bar is a selector switch with 180 contact points; the switch occurs at approximately 10 mb intervals. The contact arm, responding to the pressure changes, moves across the contact points on the commutator bar.
Newer model radiosondes use a capacitive transducer with an aneroid capsule.
Alternating conducting and insulating strips are used to change the frequency. The conducting strip transmits humidity and reference information while the insulator strip transmits temperature information. Each fifth contact is a reference contact.
OSCILLATOR RADIO TRANSMITTER
A miniature radio transmitter generates the FM radio frequency carrier, operating on a modulated carrier frequency of 1680 MHz. Variable modulation is used to transmit the collected information. That is, the radio frequency is changed by the position of the contact arm on the baroswitch. The radio transmitter is located in the pointed plastic cylinder attached to the base of the instrument package. In flight this conical antenna housing is pointed downward.
A small battery is contained in the radiosonde package to serve as the power supply for the weather sensing instruments and the radio transmitter. The battery is activated and tested prior to launch.
BALLOON & PARACHUTE
The radiosonde package is carried aloft by a spherically shaped balloon. The balloon is made of a film of natural or synthetic rubber (neoprene). Before launch, a neoprene balloon is inflated with lighter-than-air gas, typically helium, to approximately 6 foot (2 meter) diameter. This size provides sufficient lift to carry a radiosonde payload of several pounds. The thickness of the balloon skin ranges from 0.002 to 0.004 inch at the time of inflation, but becomes 0.0001 inch just before the balloon bursts. As the balloon ascends, it expands in size from approximately 6 feet to a diameter between 24 and 32 ft before it bursts. The balloon carries the instrument package to an altitude of approximately 25 mi (27-37 km) where the balloon bursts (at a pressure of approximately 10 mb). An attached parachute returns the instrument package safely to the ground. Return mailing instructions are included in the instrument package. Those radiosondes that are found and returned can be refurbished for subsequent flights, saving a considerable amount of the cost of a new radiosonde.
The following equipment is located at the upper air observing station to track the radiosonde, receive the telemetry data and process these data into a useable form.
The antenna receives the telemetry signal transmitted from the radiosonde. Highly directional radio direction finding antenna is used also to obtain the wind speed and direction at various levels in the atmosphere by tracking the radiosonde and determining the azimuth and elevation angles. The ascent rate of the radiosonde is known and timed between intervals.
(Output to other devices)
Before launch, calibration of the baroswitch is made, with appropriate adjustment. The battery is activated. The balloon is carefully inflated to such a size that it will provide the proper lift. The balloon, parachute and instrument package are attached. The radio equipment is tuned. Just before launch the surface weather conditions are measured. The balloon and instrument train is launched. Care is taken so that the radiosonde does not become entangled with local obstacles.Tracking of the radiosonde is begun immediately upon launch. Visual observations are continued until the radar tracking has locked on to the ascending instrument package. The data are recorded automatically during the flight and then processed for transmission.