A Small Tethered Balloon Sounding System

Home , Winch

Alvin L. Morris, David B. Call, a small tethered balloon and Robert B. McBeth National Center for Atmospheric Research 1 sounding system Boulder, Colorado 80303

Abstract A small, portable, tethered balloon system has been developed and proven by use in a variety of field programs by the Field Observing Facility of NCAR. A modernized version is now available. It can be operated by one person in wind speeds up to 10 m/s. It senses pressure, temperature, wet-bulb temperature, wind speed, and wind direction, and transmits data to the surface in a time multiplex format. Provision is made for other sensors, and a temperature structure function sensor has been used. The operational altitude for the system is nominally 750 m.

1. Introduction Demands for data in the earth's boundary layer have in- creased dramatically in recent years. Measurements have been made on towers, from free balloons, and from tethered balloons. No single system meets all requirements and, indeed, each has shortcomings even for mak- FIG. 1. A photograph of the balloon, airborne package, ing the measurements for which it is best suited. and winch of the NCAR BP system. A portable, tethered balloon system has been developed by the Field Observing Facility of NCAR. It has been used for many applications in boundary-layer condition and 3.5 m3 at full inflation. A stinger in the research and monitoring, in settings which range from tail extends during ascent and enables the lift gas to a busy airport to an arctic ice island, and it can be expand, thereby maintaining very nearly constant lift. operated by one person. Data have also been processed At sea level in the U.S. Standard Atmosphere, 1962, the in real time by means of a small programable calcu- balloon will lift 1.9 kg when properly inflated for launch lator, and they can readily be recorded on magnetic with Grade A helium. tape for later processing. The tether line is a bundle of nylon filaments con- Because this small system is inexpensive and fills tained in a braided nylon sheath. It has a mass of 0.9 the gap between the coverage provided by large instru- g/m and a tested breaking strength of 445 N. mented towers and by large tethered balloon systems, The winch shown in Fig. 1 consists of a drum with it has recently been modernized. The new system has level wind, powered by a heavy duty universal drill been operated successfully in the field in temperatures motor. The level wind feature is essential to prevent the ranging as low as — 33°C. line from tangling. The drill motor will operate from either 12 V auto batteries or 110 V ac power. A silicon 2. System description controlled rectifier controller enables the operator to The NCAR Boundary Profile (BP) System is shown in pay out or reel in line at any rate from 0 to 3 m/s when Fig. 1. It consists of a small plastic balloon with a the system is operated on ac power. A separate controller favorable aerodynamic shape, an airborne sensor pack- is used for battery power. age suspended below the balloon, a tether line, a winch The airborne package (Fig. 2) contains the following for controlling the tether, and a ground station. When sensors: an aneroid capsule for pressure, a bead thermis- a smaller winch (now being developed) is substituted tor for temperature, a bead thermistor covered by a for the one shown here, it will be possible to pack the wetted wick for wet-bulb temperature, a sensitive cup entire system in the trunk of a compact sedan. The anemometer for wind speed, and a magnetic compass system can easily be handled by one person. The entire for wind direction. A sensor for measuring the tempera- system can be shipped by air freight without difficulty. ture structure parameter has also been mounted on an The balloon is a Follmer model 115, which has a mass older version, but is not a part of the sensor package 3 of 1.5 kg and a nominal volume of 3.25 m in launch shown. The airborne package also contains a 403 MHz 1 The National Center for Atmospheric Research is spon- transmitter and the circuitry required to condition the sored by the National Science Foundation. sensor output and modulate the transmitter. A second

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FIG. 3. Close-up of the ground station. The receiver and discriminator are both housed in the handy-talky case which is shown in the center, mounted in its battery charger. cord the data. There are jacks on the receiver for both recorders; also, a small speaker on the receiver lets the operator monitor the sounding aurally. Block diagrams of the system electronics are shown in Fig. 4. FIG. 2. This picture of the airborne package shows how the anemometer and the shielded, aspirated psychrometer 3. Operation are mounted. Pressure and wind direction sensors and the electronics are housed in the plastic case. The interior of The principal limitations to operations of the BP sys- the case is made accessible by sliding the side covers down. tem described above are statutory restrictions and strong The package is flown with the psychrometer opening point- winds. The system was designed to present the minimum ing in the direction of the balloon. possible hazard to aircraft. The balloon is small, has a very low density, and is essentially frangible. The tether circuit board is included for optional circuitry that may line is not so strong that it would damage an airplane be added for special purposes. striking it. The airborne sensor package is also low in The ground station consists of receiver and discrimi- density, but it does contain some small hard parts. Thus nator, a strip chart recorder, and an audio tape recorder. the Federal Aviation Administration (FAA) should al- The receiver and discriminator are packaged together ways be notified in advance when flights are planned. in the transceiver case which also originally housed the The system can be made more visible by painting part transmitter, now used in the airborne package. The re- of the balloon an international orange color and by ceiver is shown in Fig. 3, mounted in its battery charger putting thin orange pennants (ribbons) on the tether which also serves as a desk stand. An Esterline Angus line. However, these modifications detract from the chart recorder and audio tape recorder are used to re- operating characteristics of the system.

FIG. 4. Block diagram of system electronics.

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The balloon has been flown repeatedly in winds of 10 m/s without damage. However, in stronger winds and/or turbulence, it can be damaged. Because such damage is usually limited to the opening of seals around the tail surfaces, it rarely results in damage to the airborne package, and it can be repaired. A single balloon can be flown many hours a day for weeks at a time. During such extended operations it is kept in a shelter between flights and occasionally topped off with helium to maintain proper inflation. Minor re- pairs are easily made with pressure-sensitive polyethyl- ene tape. When a balloon is used over a period of weeks, it may sometimes be necessary to change the helium. The gas slowly becomes diluted with air, and the balloon loses some of its lift. As it comes from the manufacturer, the bridle for the tether is attached to the balloon at four points. We FIG. 6. The total suspended mass, mL, which the Follmer have found that the system flies more safely and more Model 115 balloon can support in still air when it is properly inflated for launch is shown as a function of air density, pa, stably if only the two forward lines of the bridle are and height, z, above mean sea level in the U.S. Standard fastened to the tether line and the other two lines are Atmosphere (1962). used to suspend the sensor package. Under no-wind launch conditions, the angle of attack provide usable lift. From 6 to 10 m/s, wind is usually of the balloon should be approximately 25°. To achieve not considered beneficial. this with NCAR's new, light, airborne package, we have Lift and flight altitude characteristics of the balloon shifted the system mass toward the tail by tying exist- system described here are portrayed by Figs. 6 through 8. ing suspension lines to a new attachment point on the The mass, tul, of payload and line which the bal- tail (as shown in Fig. 5). Normally with this arrangement loon can lift is a function of ambient air density, pa. the balloon is more nearly level in flight at high altitudes This function is shown graphically in Fig. 6 for the than at low altitudes because of stronger winds and Follmer Model 115 balloon inflated to a volume of expansion of the gas into the tail. 3.25 m3 with helium. Air density decreases with altitude,

Wind is usually not of particular operational sig- therefore mL is also a function of height, z, above mean nificance unless it is too strong. The balloon has aero- sea level. The relationship between thl and z is also dynamic lift as well as drag. Hence at all speeds, the shown in Fig. 6 for the U.S. Standard Atmosphere, 1962. wind contributes to tether line tension. For wind speeds The length of tether line which can be supported by greater than 10 m/s, drag exceeds lift and causes the aerostatic lift alone is the ceiling of the balloon system attainable ceiling to be lower than it would be with no wind. Steady wind speeds less than 6 m/s appear to

FIG. 7. The length of 445 N test nylon tether line which FIG. 5. Sketch of the balloon and airborne package, show- the Follmer Model 115 balloon can support in still air is ing the method of rigging which NCAR uses for the tether shown as a function of the difference between the total line and the suspended package. supportable mass, mL, and the mass of the payload, mp.

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procedure is to suspend the sensor package about 1.5 m above ground for a short time prior to each flight. Read- ings of wet- and dry-bulb temperatures and pressure from the BP are compared with readings from an Ass- mann psychrometer and digital barometer. Then the balloon is allowed to ascend at whatever rate meets the requirements of the operator. It can ascend safely at rates as high as 3 m/s, but a rate of 0.5 to 1.0 m/s is recommended. (See Table 1 for sensor characteristics.) The balloon can be stopped at any height during ascent to make measurements at that height. Balloon retrieval (descent) is essentially the reverse of ascent. Ground station operation during flight is simple. The zero reference and span on the recorder may require occasional adjustment; otherwise there is little to do except keep the equipment operating. This leaves ample time for observing the record and making operational decisions.

4. Sensor characteristics FIG. 8. The height above the surface at which a properly Temperature and wet-bulb temperature are measured inflated Follmer Model 115 balloon will become fully inflated (dilated) is shown as a function of surface air tempera- by bead thermistors mounted coaxially in the radiation ture and atmospheric lapse rate, 7. It is assumed that air shield (tube) on top of the sensor package. A small and lift gas temperature are equal. electric fan pulls air past the beads. The dry bead is mounted in the airstream 2 cm ahead of the wet bead. in still air. Supportable tether line length, L, is The wet bead is kept wetted by a wick which receives shown as a function of (mL — mP) in Fig. 7. The value water from the small reservoir shown beside the psy- of mL may be obtained from Fig. 6, and mP is the mass chrometer tube (Fig. 2). The psychrometer has been of payload, including its suspension lines. tested extensively in the NCAR automated environ- Another factor may be more restrictive than support- mental chamber and in actual operations. Its character- able tether line. When an ascending balloon becomes istics are listed in Table 1. fully dilated (i.e., when the stinger is fully extended) it Wind speed is measured by a cup anemometer. The becomes essentially a closed, constant-volume vessel. anemometer characteristics shown in Table 1 are wind Continued ascent can occur, however, if the system is not tunnel data, but the anemometer is believed to measure supporting all the line it is capable of lifting. Then at relative, horizontal wind speed quite well when the all levels above the fully dilated level internal gas pres- sensor package is suspended as shown in Fig. 1 and the sure will exceed external air pressure, and this super- air is not turbulent. In turbulent air the balloon may pressure can damage the balloon. Therefore the level, oscillate laterally through appreciable distances. With H, at which the stinger becomes fully extended is usually the data recording format currently in use, apparent regarded as an operational ceiling. Figure 8 may be used wind velocity variations caused by these system motions to estimate H for the Follmer Model 115 balloon oper- have not been separable from the turbulent air motions. ated in a normal manner. Tilting of the sensor package is a source of small error The winch control is placed on the end of a 10 m because it tilts the axis of rotation of the cupwheel out long electrical cable so that one person can handle the of the vertical, but this is not a serious problem in airborne package and winch simultaneously. Normal smooth flow. Motion of the anemometer as it ascends

TABLE 1. Table of NCAR boundary profile system sensor characteristics.

Variable sensed Sensor Electrical Setting element Total range ranges Precision Resolution Temperature Thermistor Resistance -30 to +45°C* 25°C ±0.5°C 0.1 °c Wet-bulb temperature Wick covered thermistor Resistance -30 to +45°C 25°C ±0.5°C o.rc Wind speed Cup anemometer DC generator 0.5 to 10 m/s 0 to 10 m/s ±0.25 m/s 0.1 m/s Wind direction Balloon & magnetic compass Resistance 0-360° 0-360° ±5° ±2° Pressure Aneroid capsule Resistance ** 100 mb ±1 mb 0.5 mb

* Six switch-selectable 25°C ranges make up the total range. ** The pressure sensor is adjusted at the launch site by opening and then closing a small port in the cavity in the capsule. Sensed pressure is the pressure difference between ambient and the air in the capsule. A thermistor is mounted on the pressure sensor and temperature compensation is automatic.

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Unauthenticated | Downloaded 10/08/21 03:31 AM UTC Vol. 56,, No. 9, September 1975 and descends is also a source of error. A correction for full-scale and several zero references in each data cycle, this can be made during periods when the tether takeup the data can readily be recorded on an audio quality or release rate is constant. A correction may also be ob- tape recorder. The frequent zero and full-scale read- tained by stopping the ascent or descent periodically ings provide a scale which can be used to interpret data; to permit the balloon system to come to rest. they also provide check points which are used to assure To measure wind direction, it has been assumed that that data are being transcribed properly when a com- the balloon is an effective vane. The sensor package is puter is connected directly to either the discriminator suspended below the balloon on a wide "rope ladder" or the tape recorder. which is torsionally rigid. When direction is to be sensed, a magnetic field locks the compass needle to a 6. System and operational variations potentiometer that is fixed relative to the sensor pack- With the BP system described here, measurements in age. This technique prevents the recorder chart from the boundary layer can be made frequently, inex- being "painted" as it would be if the position of a trem- pensively, and safely in a wide range of applications. bling, floating needle were being sensed. It gives the im- Small size, light weight, and operational simplicity are pression that directional changes are less variable than its principal attributes. The use of a time multiplex data they actually are, but directions measured by this format with a relatively long cycle time makes it possible system do check quite well with independent measure- to record several variables simply on one chart so that ments of balloon orientation made by viewing the bal- direct, real time interpretation in the field is possible. loon in a horizontally mounted mirror on which a Compromises have been made to achieve these fea- graduated circle has been drawn. An apparent error has tures, and for some uses it is advantageous to change the been noted as a balloon ascended through a zone in system or use nonconventional operational procedures. which wind direction reversed by 180°. The balloon The data cycle can be shortened so that all variables then drifted, nose downwind, until it had crossed over can be sampled virtually continuously without compli- the winch location, after which it turned rapidly into cating the airborne portion of the system. The data the wind again. from a system modified in this way can readily be re- Pressure is sensed by a small, solid-state, aneroid trans- corded on audio magnetic tape or sent directly to a ducer. computer. If this is done, however, some additional ground station equipment is required to make a strip 5. Data format chart record which is usable in the field. A time-multiplex data format was chosen for the BP The NCAR BP system has been used in conjunction system so that several variables can be presented on a with acoustic sounding systems and FM-CW radar in single strip chart. The format is shown in Fig. 9. For boundary layer work. Periodic vertical soundings made facility in reading data from a strip chart, each chan- with the BP system can be used to study the vertical nel records for approximately 1.5 s; hence, the entire structure of features which are sensed by the radar or cycle for eight variables (as shown in Fig. 9) requires 30 acoustic sounder. Alternatively the BP airborne package s. Each variable being sampled will be reported at least can be flown for a long period in a stratum in which the once during each cycle, but a variable may be sampled remote sensor systems show interesting activity. For ex- and reported on more than one channel. The order of ample, breaking waves on the upper surface of a cold sampling can be changed in a few minutes in the field. pool of air in a valley have been observed in this way. Also, by changing the setting of a single switch, it is possible to set the BP to report on one channel con- 7. Summary tinuously. Thus a single variable can be sampled contin- A small, highly portable, tethered balloon sounding uously throughout a flight if that is desired. The change system that can be operated by one person has been cannot be made while the system is in flight. developed and proven through use in a variety of field Since the signal frequency as it comes from the dis- programs by the Field Observing Facility of NCAR. criminator is linear for all variables and there are one New, lighter, more rugged versions of the sensor package and ground station have just undergone field tests in the severe environment of a high mountain valley in winter and they are now considered fully operational. With the ballon and tether line NCAR uses, the system can be flown safely in 10 m/s winds. Its nominal flight ceiling is 750 m; its actual ceiling is a function of the density and speed of the air in which it is flown.

Appendix

FIG. 9. Data format as it appears on a recorder chart. Lift and flight characteristics for the Follmer Model The reporting sequence may be changed in a few minutes 115 balloon were discussed briefly in the text, using by rearranging some plug-in leads in the airborne package. Figs. 6, 7, and 8. The following equations can be applied

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Unauthenticated | Downloaded 10/08/21 03:31 AM UTC Bulletin American Meteorological Society to other balloon systems and so are more general than mL total suspended mass which can be supported the figures. The International System of units (SI) by aerostatic lift should be used. mi mass of a unit length of tether line (kg/m)

When properly inflated for launch, the suspended mv mass of payload (usually the airborne sensor mass the aerostatic force on the balloon is capable of package and its suspension lines) lifting is P atmospheric pressure at balloon level—hence at the sensor package m = p v ( 1 - MjMa) - m . (Al) L a b b Po atmospheric pressure at the ground at the time With helium as the lift gas, this becomes of launch R* the universal gas constant [8314.32 J/K (kg- mL = 0.862paVb - mb. (A2) mol)] In the U.S. Standard Atmosphere (1962) with helium T atmospheric temperature at balloon level— as the lift gas, hence at the sensor package Th atmospheric temperature at level H 5 1.0567& exp( —9.8 X 10~ s) - mb (A3) To atmospheric temperature at the ground at the for z < 5000 m. time of launch The length of tether line a balloon can support is Vb volume of the balloon Vh volume of the balloon at level H—hence the L = (ML — Mp)/tni. (A4) fully inflated balloon volume The height at which a balloon becomes fully inflated Fo volume of the balloon at the ground at the time and starts to experience superpressure is given approxi- of launch mately by the following equations: z altitude above mean sea level (m) y mean lapse rate (K/m) of temperature through

H = 14.64(r0 + Th) \n(ToVH/THVo) (A5) the stratum from the surface to height H 3 pa density of the air (kg/m ) Tjr/Vu\R*yKR*y+9Ma) 1 (A6) H-f[U) -'} Assumptions: At all levels below H, lift gas pressure and tempera- In the U.S. Standard Atmosphere (1962), (A6) may be ture are respectively equal to atmospheric pressure and written temperature. 0 235 H = 153r0[l - (Fo/F^) - ]. (A7) Balloon system volume, lift gas volume, and volume of the displaced air are all equal. Thus the displacement Also a balloon may be expected to experience super- of the balloon material and the payload are assumed to pressure when be negligible.

(.P/T) < (Vo/Vh)(PO/TO). (A8) Atmospheric density decreases exponentially with height through the lowest 5000 m above mean sea Definitions of symbols : level. This applies to (A3) only. g acceleration due to gravity (9.80665 m/s2) Lapse rate of temperature is constant from the H height above the launch site at which the surface to level H. This applies to (A5) through (A7), balloon becomes fully inflated. and in (A7), the lapse rate is assumed to be L length of tether line which can be supported by -0.0065 K/m. aerostatic lift

Ma molecular weight of air (28.9644) Reference M0 molecular weight of lift gas (4.0026 for grade A U.S. Committee on the Extension of the Standard Atmo- helium) sphere, 1962: U.S. Standard Atmosphere, 1962: Washington, mb mass of the balloon D.C., U.S. Government Printing Office, 278 pp.

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