230 RADIO WAVE PROPAGATION EXPERIMENTS 
The Washington State College [WSC] instrument 
has been adapted for airplane measurements and has 
been used on several types of planes during tests in 
Panama.» The elements were housed in a single- 
walled cylinder of aluminum, about 1.75 in. in diam- 
eter, covered on the forward end with a cone. A small 
circular opening (3 in. in diameter) made by cutting 
off the end of the cone reduced the velocity of the air 
across the elements to about one twenty-second of the 
plane’s speed. Comparison of a plane sounding and a 
balloon sounding in the same region at the same alti- 
tude and time gave identical results within reasonable 
experimental error. 
With airplane measurements the determination of 
the plane’s altitude becomes an important task. In 
the experimental flights at the Radiation Laboratory 
the altimeter of the plane itself was used. According 
to the experience obtained in Panama it is desirable 
to have an additional altimeter placed directly before 
the operator in order to facilitate rapid and accurate 
altitude determinations. The nominal accuracy of an 
airplane altimeter is about 20 ft. Over sea it may be 
possible to determine the absolute altitude of the 
plane with about the same degree of accuracy, but 
over land less accuracy is to be expected. 
Measurements from a dirigible (blimp) have been 
carried out by Radiation Laboratory. The instrument 
is suspended on a cable about 100 ft below the ship. 
Captive Balloon Sondes and Kites 
Rapio TRANSMISSION TYPE 
Two different methods have been tried in connec- 
tion with balloons and kites. When first used in prac- 
tice an ordinary radiosonde was attached to the balloon 
(kite) -and the results were recorded on the ground 
by radio in the usual way. This method was used in an 
experimental investigation carried out under the aus- 
pices of the AAF Board, Orlando, Fla.® Although by 
the nature of the instrument the measurements are 
spaced 200 to 300 ft apart, a rough survey of the 
temperature and moisture distribution sufficient for 
some operational purposes was gained in this way. 
The record on the ground was taken by. means of a 
standard U.S. Army radiosonde receiver. 
It was pointed out in this report® that it might be 
advantageous to use a combination of two radiosondes 
in tandem, such that in one instrument the contacts are 
connected to the temperature device, in the other to 
the humidity element. It would then be possible to ob- 
tain simultaneous temperature and moisture readings 
at the same elevation, instead of alternating ones, as 
is the case when only one instrument is used. This 
would, however, require the use of two receivers at 
the ground with two slightly different carrier fre- 
quencies. 
Another adaptation of the standard Weather Bu- 
reau radiosonde was made by WSC.** The ground in- 
stallation was similar to that used by the Weather 
Bureau in its full radiosonde measurements, but the 
standard radiosonde was modified by replacing the 
pressure- (altitude-) actuated switch by a clock-driven 
commutator. The results obtained were quite satisfac- 
tory, and the technique may be appropriate at stations 
where standard radiosonde equipment is available. 
WirED TRANSMISSION TYPE 
The other captive balloon or kite instruments are 
of the wired type with galvanometers or recorders at 
the ground. They may be classified as light and heavy 
types. The light instrument merely carries tempera- 
ture and humidity elements aloft which together with 
the radiation shield do not weigh more than a few 
ounces. To this is added the weight of the cable or 
string carrying the connecting wires. The heavy in- 
strument carries its own aeration equipment in the 
form of a fan driven by a small electric motor. The 
fan and the heavier construction of the frame required 
to accommodate it increase the weight of the airborne 
unit to several pounds. In addition there must be at 
least one more lead on the cable to supply power to the 
fan. 
The first captive balloon instrument was built in 
England several years ago.*° The balloon is anchored 
by an electric cable and the instrument is provided 
with a fan. The overall weight of the instrument with- 
out cable is about 8 lb. Its main part is a piece of poly- 
thene tubing in the shape of an inverted Y with the 
fan placed on top of the tubing while the two legs of 
the Y contain the dry and wet thermopiles. The latter 
are four-junction copper-constantan combinations. 
The cold junctions are enclosed in a small Dewar flask 
filled with melting ice which is located about 10 in. 
below the Y piece. : 
The cable of this instrument has five leads, three 
for the thermocouples and two for the fan (2 to 4 
volts of direct current) ; the instrument is suspended 
from the balloon proper by means of a 100-ft string 
which minimizes the influence of irregular motions of 
the balloon upon the instrument. The ground equip- 
ment consists of potentiometers and a spot galvanom- 
eter with a switch to alternate between the dry and 
wet couples. 
The light type of balloon or kite sounding equip- 
ment was first developed by WSC.°*> The tempera- 
ture and humidity elements are surrounded by a 
double-walled aluminum radiation shield, and the 
whole airborne assembly weighs only a few ounces. 
Originally the standard Weather Bureau temperature 
element was used ; now they use the Sanborn element 
together with the Bureau of Standards humidity ele- 
ments in the circuit shown in Figure 1. 
The sounding procedure used with this instrument 
consists in letting the balloon go rapidly up to a max- 
imum altitude chosen so high that moisture and tem- 
