Radiosonde for Atmospheric Refractive Index Measurements
Transcript of Radiosonde for Atmospheric Refractive Index Measurements
Radiosonde for Atmospheric Refractive Index MeasurementsA. P. Deam Citation: Review of Scientific Instruments 33, 438 (1962); doi: 10.1063/1.1717874 View online: http://dx.doi.org/10.1063/1.1717874 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/33/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Note: Geiger tube coincidence counter for lower atmosphere radiosonde measurements Rev. Sci. Instrum. 84, 076103 (2013); 10.1063/1.4815832 Meteorological radiosonde interface for atmospheric ion production rate measurements Rev. Sci. Instrum. 76, 126111 (2005); 10.1063/1.2149005 Measurement of index of refraction Phys. Teach. 18, 54 (1980); 10.1119/1.2340415 Balloonborne radiosonde for the measurement of the atmospheric thermal structure coefficient Rev. Sci. Instrum. 45, 1563 (1974); 10.1063/1.1686562 Measurements of the Refractive Index of Films J. Appl. Phys. 18, 431 (1947); 10.1063/1.1697671
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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 33, NUMBER 4 APRIL, 1962
Radiosonde for Atmospheric Refractive Index Measurements*
A.P.DEAM The University oj Texas, Electrical Engineering Research Laboratory, Austin, Texas
(Received December 1, 1961; and in final form, February 2, 1962)
An instrument for transmitting a carrier frequency analog of atmospheric radio index of refraction is described, The signal transmitted is received ata remote point through the use of a phase lock type of receiver. It employs a ceramic coaxial cavity, one·half wavelength long, as a sensor, and the nominal operating frequency is 403 Me. The instrument, when operated with other cavities, offers the laboratory gas dielectric investigator a convenient tool for studying real components of gas dielectrics. Operation can be easily extended to higher frequencies by the use of suitable cavities and solid state multipliers.
INTRODUCTION
DURING the past decade, extensive direct measurements of radio index of refraction in the lower atmos
phere have been made by several investigators.1,2 Such measurements have been used to further the basic understanding of wave propagation in the troposphere. Nearly all such measurements have been made with refractometric equipment operating at or near 9400 Mc and with the instrument mounted in an aircraft. These measurements have generally been successful but somewhat limited by the dependence of the equipment on the aircraft.
There has been a need for a reliable and inexpensive instrument that is perhaps more versatile than existing instruments. Since the quantities to be measured are so small (order of a few parts in 106), the design of a reliable and inexpensive instrument is made somewhat difficult. Herein is described an instrument which samples the atmosphere with a resonant cavity nominally resonant at 403 Mc, is light in weight and considerably less expensive than previous equipment. It is primarily designed for battery operation and toleration of the inevitable change in battery voltage. It is capable of providing the necessary information at points remote from the instrument.
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FIG. 1. Radiosonde block diagram.
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* This !mit was developed under Contract AF 33(600)40545, AMC Aeronautical Systems Center, Communications and Reconnaissance Division, Wright-Patterson Air Force Base.
1 C. M. Crain and A. P. Deam, Rev. Sci. Instr. 23, 149 (1952). 2 George Birnbaum.and H. E. Bussey, Proc. I.R.E. 43, 1412 (1955).
438
The instrument is basically a transmitter whose radiated carrier frequency is an analog of the resonant frequency of the cavity sensor, the cavity being vented to the atmosphere. This is accomplished by incorporating the cavity as the frequency control element for a Pound stabilized oscillator.3
EQUIPMENT
Since Pound has adequately described the theory of operation of his stabilizing system, only a block diagram is included to cover this aspect of the problem. Figure 1 is a block diagram indicating the generation of a carrier frequency which is an analog of radio index of refraction. Figure 2 is a photograph of the conversion of this block diagram to a working radiosonde.
Referring to Fig. 2, it will be noted that the unit is built around three aluminum bulkheads which are spaced with aluminum tubing. A coaxial cavity, along with the ring circuit, runs the length of the two compartments formed by the bulkheads. The cavity is open at both ends and loop excited at the center. Electronic components (stabilizing amplifier and stabilized oscillator) are located in the compartment near the parachute container. The stabilizing amplifier and stabilized oscillator are separate modular units which are removable such that either or both units may be removed for service or replacement. Although the stabilized oscillator chassis is not visible, it is just to the rear of the amplifier.
Batteries are assembled in the right-hand compartment and may assume various configurations. Radiation of the
FIG. 2. Sonde---complete except cover.
3 R. V. Pound, Proc. I.R.E. 35, 1405 (1947).
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RADIOSONDE 439
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STABILIZING AMPLIFIER
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FIG. 3. Circuit diagram of radiosonde.
instrument is through the X/4 antenna at the right end. Although the thin rolled aluminum cover has been removed for the photograph, it is easily seen that this cover serves as a ground plane for the antenna.
Figure 3 is a complete circuit diagram of the radiosonde with the exception of the batteries and the ring circuit. The ring circuit is visible in the photograph (Fig. 2) and is constructed of RG-59A coaxial cable, cut to be centered on 403 Me. Interconnections between amplifier, ring circuit, and oscillator are made with standard BNC fittings and RG-58C cable. Exception to this is made in connecting the modulator crystal (CR-3) to the ring circuit where a selected length of RG-62 cable is used to provide the proper time varying reflection coefficient at the ring circuit connection. The only other sensitive length cable is that connecting the cavity to the ring circuit; it must be adjusted for proper phasing of the feedback in the stabilized loop.
No particular elaboration is necessary as regards the stabilizing amplifier other than to say it is a tuned amplifier operating at 4.5 Mc with its output fed into a synchronous detector whose reference is obtained from a 4.5-Mc crys-
tal controlled oscillator. Output from the synchronous detector is compensated and used to control the frequency of the stabilized oscillator with the aid of a solid state variable capacitor (CR-l). Degenerative frequency shift is minimized at the input of this amplifier by "brute force" damping of the crystal detector. Damping is necessary since amplitude variation of the output of the stabilized oscillator causes changes in crystal impedance which in turn can cause undesired shifts in frequency of the stabilized oscillator.
The uhf generator or stabilized oscillator employs a 5703 triode whose tank circuit is a parallel wire system. Mechanical tuning is provided at the end of the line along with electrical tuning by the solid state capacitor. Co and C28
are piston-type capacitors to provide good frequency stability; each is spring loaded to prevent movement. Bifilar coils are used to float the cathode and heaters above ground. A small loop pickup is used to excite the stabilizing circuit and a larger loop for antenna excitation. A resistor is inserted in the antenna pickup loop to minimize pulling of the oscillator's frequency by the antenna while the resistor in the small pickup loop provides damping for waves
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440 A. P. DEAM
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returned from the ring circuit. This oscillator is stabilized against frequency variations due to temperature changes by proper choice of the capacitors connecting the solid state capacitor to the tank circuit. The oscillator delivers 100 mw to the antenna and approximately 4 mw to the stabilizing circuit when the voltages are within the range shown on the circuit.
Stability of this Pound system is principally controlled by the stability of the cavity and this item is the main obstacle in producing an inexpensive sonde. The cavity used and reported here is not considered to be a final solution. Figure 4 is a sketch of the cavity which is constructed
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using Corning Pyroceram and General Electric fused quartz, rendered conductive by the usual silver painting techniques. The parts for this cavity are assembled using Shell Epon VIII epoxy resin which was baked at recommended temperature. Although the length of the center rod can be controlled to a very small tolerance, secondorder effects produced by eccentricity of the outer conductor cause a scatter in resonant frequency. Of 25 cavities constructed, the frequencies were randomly scattered between 402 and 404 Mc or ±0.25%. The unloaded Q of this cavity as constructed and measured is approximately 2000. The temperature coefficient of frequency variation was
DROP"" 5 INDEX SOUNDINGS ON AIRCRAFT NO. 324 USING A TYPE Y MICROWAVE REFRACTOMETER (SE'E' curves A and 8) COMPARED WITH AN INDEx SOUNDING (see curve C) MAOE USING A 400 MCP$ DROP TYPE REFRACTOMETER 600 MILES WEST OF LOS ANGELES, CALIF. 2 AUG UST 1960. CURVES NORMALIZED AT 8000:
!~:8 , ,e (O,OOOf I
J : ..----90 MILES--j AIRCRAFT SOUNDING
FIG: 5. Index soundings-- drop #5.
)80 200 220 240 260 280
RELATIVE INDEX OF REFRACTION 'H.
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RADIOSONDE 441
FIG. 6. Index soundingsdrop #1.
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disappointing since the expansion coefficient of the center conductor was 0.6 ppm;oC. However, a most interesting aspect of the temperature coefficients was that all cavities were found to have coefficients very near the same value -1 ppm;oF. Further, it was found that there was little or negligible hysteresis associated with a temperature cycle.
It has been observed that, with the use of a temperature controlled environment and Invar evacuated cavity, the sonde will maintain a stability of better than ± 1 ppm over a period of a week. This figure is quoted to illustrate that the stability of the instrument is primarily the stability of the cavity.
Practically all the electronic equipment in this instrument is obtainable as "off the shelf" items at any parts wholesaler.
OBTAINABLE RESULTS
Five of these radiosondes have been ejected from an aircraft flying over the Pacific Ocean. This craft was engaged in wave propagation research, the flights under technical direction of Mr. David L. Ringwalt of the Naval Research Laboratory. His cooperation and support made these drops possible.
The craft was equipped with flare ejection tubes capable of launching the sonde and only the phase lock receiver had to be mounted aboard the aircraft. Aboard the aircraft was a recording X-band (9400 Mc) refractometer. Figures 5 and 6 illustrate the results of dropping a sonde and recording index of refraction vs height in the air-
A
10 100 120 140 160
DROP"" I
INDEX SOUNDINGS ON AIRCRAFT NO. 324 USING A TYPE Jr MICROWAVE REFRACTOMETER (IN cur"" A and B) CO",PARED WITH AN INDEX SOUNDING (IN curve C) MAOE USING A 400 MCPS OROP TYPE REFRACTOMETER 600 MILES WEST OF LOS ANGELES, CALIF. 30 JULY 1960. CURVES NORMALIZED AT 9000'.
T.>.::L 10,J:'1't/ B
j C (D4SHEDLINEI
AIRCRAFT SOUNDING
uio . 200 220 240 280 210
RELATIVE I NOEX OF REFRACTION (NI
plane. These graphs also contain X-band recordings made while the airplane sounded in the vicinity of the point of sonde ejection. Altitude information for the X-band recordings comes from an altimeter and altitude information for the sonde was calculated ballistically taking account of the delay in parachute opening. It is to be noted here that a small baro-switch is easily incorporated for altitude indication.
Only two profiles are presented here but five sondes gave five profiles. Figure 5 is the best agreement with the X-band refractometer and Fig. 6 illustrates an important consideration. The large excursion in the profile of Fig. 6 is no doubt water condensation on the conducting surface of the cavity. All five profiles were in good agreement with X-band data above 3000 ft while two were in good agreement froIJ? 9000 ft to the surface. The coaxial mode is more susceptible to liquid water on its surface than the X-band TEoll mode cavity since the electric field at the wall surface is zero in the case of the TEoll mode. This appears to be the only real advantage to be had when using an X-band refractometer as compared to one of the sondes described herein. Balloon ascents have indicated that the water condensation problem is not as severe on ascent as when the sonde is ejected from a high flying aircraft.
Aside from using the instrument to determine atmospheric index of refraction profiles, it has found considerable use in gas polarization studies,4 electron density measurements, and other applications.
4 J. E. Boggs and~A. P. Dearn, J. Phys. Chern. 64, 248 (1960).
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