Each sonde type contains the appropriate radionavigation signal receiver, a 400MHz transmitter, and a microprocessor which processes and formats thermodynamic data from sensors for transmission to the aircraft. The aircraft portion of the dropwindsonde system consists of a 400MHz telemetry receiver, a thermodynamic data processor, an onboard personal computer, and a receiver for the radionavigation signal type used, an Advanced Navigation Inc. (ANI) Model 7000 navigator for Loran signals or a Trimble Mini Omega (TMO) Model 8402 receiver for Omega signals.
The dropwindsonde system measures thermodynamic parameters, the horizontal wind, and position. The thermodynamic data - pressure, temperature, and humidity - are transmitted from the sonde in 250 bit data frames at a rate of four frames per second. The position and horizontal wind are obtained from radionavigation signals continuously received and re-transmitted by the sonde. (The sonde does not perform any signal processing on the navaid signals.) Both signals, the 250 bit data frames and the radionavigation signals, are transmitted to the aircraft dropsonde data system on separate subcarriers of the 400MHz transmitter. Upon reception in the aircraft, the two signals are separated and directed to respective processors. The thermodynamic data are transferred to an NCAR thermodynamic data processor while the radionavigation signals are transferred to the appropriate navigator.
The dropsonde thermodynamic data processor decodes the digital bit stream into numerical counts for each parameter measured by the sonde. Those counts are encoded into ASCII (hexadecimal) data frames which are sent via RS-232 to the personal computer. There the data are further processed to give the desired meteorological parameters. The radionavigation data are transferred to the navigator which generates output which is passed to the onboard personal computer for wind and position calculation.
Just after the sonde is ejected from the aircraft through a launch tube, a bias tape unwraps from the sonde delaying parachute deployment by about one second. This delay is required to insure that the parachute isn't destroyed by the forces the sonde is subject to immediately on exit from the aircraft. As the parachute is deployed, the sonde begins to decelerate. At this point, the 400 MHz telemetry antenna is deployed and a mechanism for deployment of the navaid antenna is triggered. (A lanyard attached to the parachute triggers an electrical signal that sends one watt to a one-quarter watt resistor which burns and melts a line holding a spring which kicks out the navaid antenna.) The navaid antenna is deployed several seconds after launch.
The parachute inflates as it falls, taking about fifteen seconds to fully inflate. The initial streaming followed by gradual inflation reduces the overall shock load to the sonde and its electronics. During the time the parachute is inflating, the sonde decelerates from the speed of the aircraft to a speed on the order of 10 meters per second.
The parachute used is a specially designed "square cone" parachute. It is essentially a tethered, inverted, square-based pyramid with the dimension listed in the following table being the length of a side of that square base. That dimension is also the length of the sides of the four equilateral triangles making up the pyramid or "square cone". The parachute is inflated through mesh at the apex of the pyramid.
TABLE 1 Dropsonde Specifications ----------------------------------------------------------------------------------------- Manufacturer - type NCAR - L2D2 - Loran Navaid Sonde NCAR - LOD2 - Omega Navaid Sonde Mass 490 grams 490 grams Dimensions 8.25cm X 41cm 8.25cm X 41cm Parachute 13 inch "Square Cone" 25 inch "Square Cone" Fall Velocity 12km - 20 m/sNote that the LOD2 parachute is significantly larger than the L2D2 parachute. This is designed to slow the descent of the "Omega" sonde to improve the wind data quality and resolution. Omega wind data are calculated over a 240 second smoothing interval using roughly 24 sets of navaid range data. (Loran wind data are calculated over a 60 second smoothing interval using roughly 20 sets of navaid "Time-Of-Arrival" data).
12km - 12 m/s
sea level -9 m/s sea level - 5 m/s Transmitter Frequency 401 - 406 MHz; User selectable 401 - 406 MHz; User selectable Transmitter Power 1.0 W 1.0 W Pressure Sensor Piezoresistive silicon Piezoresistive silicon Temperature Sensor Unicurve Bead Thermistor Unicurve Bead Thermistor Humidity Sensor Carbon Hygristor (heated) Carbon Hygristor (heated) -----------------------------------------------------------------------------------------
TABLE 2 Dropsonde Pressure Sensor Specifications ---------------------------------------------- Manufacturer / Model # Motorola SCX15ANC Sensor Piezoresistive silicon Range 150 to 1050 mb Accuracy +/- 2.0 mb Data System Resolution 0.1 mb Sensor Resolution 0.06 mb @ 1050 mb
0.01 mb @ 250 mb Time Constant 1.0 ms ----------------------------------------------
TABLE 3 Dropsonde Temperature Sensor Specifications ----------------------------------------------------------------- Manufacturer /The manufacturer's specification for the temperature sensor time constant is 3.0 seconds. Evaluation of dropsonde data indicates that this time constant increases with increasing altitude. Time constants were determined from the thermodynamic data files, evaluating the time it took the sensor to come to equilibrium with the outside temperature after release from the aircraft. That point is easily identified, it is when the temperature starts to increase after a period of decrease immediately following launch.
Fenwal Company
Model # 192-103LET-AZ01 Sensor 10K ohm, 60 mil, Unicurve Bead Thermistor Range -55 C to 40 C Accuracy +/- 0.5 C Data System Resolution 0.1 C Sensor Resolution 0.02 C Time Constant 3.0 seconds -----------------------------------------------------------------
The time constant determined at 180mb was nearly 5.0 seconds. In drops made from this altitude, the temperature sensor does not reach equilibrium with its environment until roughly 30 seconds after launch. The time constant determined at 550mb was nearly 4.0 seconds. In drops made from this altitude, the temperature sensor does not reach equilibrium with its environment until roughly 20 seconds after launch. This lag in the temperature sensor will affect the first few ten-second temperature data points calculated from sonde measurements. The number of points affected will depend on the altitude of the drop. (The first data point in the ten-second and interpolated data files is from the aircraft data system.)
The time constant of the thermistor combined with the fall rate of the sonde produce a slight lag in temperature measurement throughout the sounding. However, with typical atmospheric lapse rates the resultant smoothing of the temperature profile is less than the accuracy of the thermistor. The smoothing resulting from the lag time becomes more significant when the sonde crosses frontal boundaries or goes through strong inversions.
Another minor functional shortcoming of the thermistor involves cooling due to evaporation of condensation on the probe or sublimation of ice from the probe. As the sonde falls from colder temperatures to warmer ones, the sensor will be cooler than the environment and be subject to icing or having water condense on the sensor when it passes through clouds or moist layers. As the sonde continues to fall to warmer air there may be subsequent evaporative cooling apparent in the sounding.
TABLE 4 Dropsonde Humidity Sensor Specifications ------------------------------------------------------------- Manufacturer /The VIZ/NCAR hygristor has a heated alumina substrate in the sensing area which prevents condensation from occurring. The temperature of the substrate is controlled by using a humidity set point (typically 75%) that will not allow condensation to occur on the sensor. The temperature of the substrate is controlled using the set point and monitored to allow for calculation of the ambient humidity (and dew point temperature).
VIZ, Inc. Model #1165-200 Model # Sensor Carbon Hygristor (heated) Range 5 to 100% Accuracy +/- 5% for 0 C <= T <= 56 C
+/- 8% for -20 C <= T <= 0 C
+/- 13% for -40 C <= T <= -20 C Data System Resolution 0.1% Sensor Resolution 0.01% Time Constant <= 0.5 seconds @ 25 C
<= 5.0 seconds @ -20 C -------------------------------------------------------------
An intercomparison test of three humidity sensors, a Vaisala humicap, a frost point hygrometer, and the heated carbon hygristor, performed by NCAR in early December 1992 revealed that the VIZ heated carbon hygristor was not measuring low humidities properly (the VIZ registered 25% R.H. where the Vaisala and frost point hygrometer registered 6-7% R.H.). The problem was determined to be an upward drift in the sensor lock-in resistance (i.e. the resistance at the 33% R.H. calibration point supplied by VIZ). It appears that the sensor/substrate resistance does not adequately stabilize and thus subsequent use of the lock-in resistance value supplied by the manufacturer leads to erroneous relative humidity values, particularly in the lower half of the measurement range.
This problem was solved by instigating a baseline calibration procedure just prior to dropsonde launch. In this calibration procedure, a hand-held battery powered humidity/temperature sensor (Vaisala HMP35) was used to calibrate the sonde sensor. The software was modified to adjust the sensor lock-in resistance to where the sonde sensor output agreed with the hand held unit.
This correction procedure was implemented after 1 January 1993. Prior to that time, all drops using a VIZ heated hygristor were affected by this sensor resistance drift. Note that due to the nonlinear nature of the sensor response this problem did not have a significant affect on the measurement of higher humidity values, above roughly 40 to 50%.
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TABLE 5 Navaid Wind and Position Measurement
Specifications
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Manufacturer / Model # Advanced Navigation Inc.
Trimble Mini Omega (TMO)
Model #7000 (ANI 7000) Model #8402
Wind Accuracy 1.0 m/s 2.0 m/s
Averaging Time 60 seconds 240 seconds
Data System Resolution 0.1 meter; 0.1 m/s 0.1 meter; 0.1 m/s
Absolute Position Accuracy 200 meters 2000 meters
Differential Position Accuracy 20 meters 200 meters
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In the L2D2 dropsonde system, the ANI 7000 navigator processes the Loran-C navigation signals relayed from the sonde, automatically acquiring the stations to be tracked for a given geographic location. Reception of at least three Loran stations is required for position and wind calculation. The ANI 7000 outputs an ASCII status message containing field strength, signal to noise ratio, and signal time of arrival (TOA) information for each of up to eight stations being tracked. In the LOD2 dropsonde system, the TMO Model 8402 navigator processes available Omega signals relayed from the sonde to the aircraft. The Trimble navigator selects the strongest Omega signal as the "Master Station". It then detects and outputs phase differences between that "Master Station" and each of the other Omega signals received. (Note that phase data output for the "Master Station" are always zero.) As is the case with Loran, a minimum of three stations is required for wind and position determination. If there are not enough stations for position and wind determination, no data are output.
Regardless of the navigation system used, it is important to note that the winds obtained, in real time and in the final data product, are those that would be obtained if the radionavigation receiver/navigator in the aircraft were connected directly to the falling sonde (it is connected, via the 400MHz telemetry link).
The wind data and relative humidity data are set to 999.0 (Q values set to 99.0) during this "bad data" period just after launch. The length of this period is a function of altitude and has been determined from experience (evaluating numerous drops). The periods when these data are flagged as bad at launch are summarized in Table 6.
The temperature data are bad for a period just after launch in part due to the time constant of the temperature sensor. The temperature sensor time constant is a function of altitude, increasing with increasing altitude, thus the period of bad temperature data increases with increasing launch altitude. (Refer to the time constant discussion in Section 1.4.) Currently, the dropsonde data processing does not flag the bad temperature data in this launch "bad data" period. Table 6 does include information on the length of time that there are bad temperature data at launch.
TABLE 6 Launch Invalid Data Period ----------------------------------------------- Parameter Altitude Range Bad Data Period
(seconds) ----------------------------------------------- Winds All 30 Humidity 0-15,000 ft.
60
15,000-25,000 ft.
90
>25,000 ft. 120 Temperature 15,000-25,000 ft.
20
>25,000 ft. 30 -----------------------------------------------