Comparison of falling sphere and pitot tube atmospheric density measurement

Smith, W. S.

English

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METEOROLOGICAL DATA FROM FALLING SPHERE TECHNIQUE
COMPARED WITH DATA FROM OTHER SOUNDING METHODS
By. Wendell S. Smith
NASA Goddard Space Flight Center
SUMMARY
The objective of the Goddard Space Flight Center's Meteorological Sounding Rocket
Program is the launch and operation of sounding rocket experiments for making synoptic
global measurements of the physical parameters of the atmosphere between 60 and 150 km
and to relate the measurements to those obtained from experiments conducted directly
above and below this region. In the pursuit of this objective, approximately 50 soundings
per year are made of the stratosphere, mesosphere and lower thermosphere utilizing
primarily rocket grenade and pitot-tube experiments. It has been recognized that the
falling sphere technique offers some potential advantage over these other techniques in
terms of economy and operational flexibility provided that the basic accuracy of the
sphere technique can be established and maintained. Opportunities to obtain comparative
data have been sought, most recently utilizing a sphere which is deployed from a pod
mounted to the tail-fin assembly of a Nike Cajun or Nike Apache rocket carrying either a
grenade or pitot-tube experiment. The experiments conducted thus far indicate an aver-
age density difference of 3 percent above 70 km where the fall rate of the sphere is
supersonic and 8 percent below 70 km where the fall rate is subsonic. The difference
above 70 km would appear to be largely random (though the sample is smaller) while
below 70 km the sphere yields results which are consistently closer to the standard atmo-
sphere than the results of either the grenade or the pitot-tube experiments.
I!NTRODUC TION
The potential advantages of the falling sphere experiment over other techniques for
mesospheric and lower thermospheric soundings have been recognized for many years.
The Sensor cost is small when compared with payload instrumentation required to con-
duct grenade or pitot-tube experiments. The small volume and weight required permit
the consideration of a less expensive rocket delivery system. The simplicity of the air-
borne portion of the system would mean an inherently reliable system. In spite of these
potential advantages, the sphere system has not been widely incorporated as a tool in
NASA's research program owing largely to the fact that we have not yet developed confi-
dence in the accuracy or the repeatability of the sphere experiment. For the past decade,
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we have attempted to obtain comparative data either through the nearly simultaneous
launch of sphere and other payloads, through the deployment of spheres from grenade
payloads, or most recently through the use of a "strap-on" sphere pod which is positioned
on the tail-fin assembly of a rocket carrying another experiment. The results of the
earlier work have been reported by Jones and Peterson (ref. 1).
The "strap-on" version of the sphere experiment is an inflatable, 26-inch-dtameter
mylar sphere. It is carried aloft on a Nike Cajun or Nike Apache rocket and, during the
ascent portion of the flight, is housed in a cylindrical pod which is about 13 inches in
diameter and about 231inches long. The mounting arrangement is shown in figure 1.
Each pod (two are required to balance the aerodynamic drag) is capable of housing a
sphere, an ejection system, a timer, and a power supply. Thus the entire sphere exper-
iment is self-contained, requiring no connections to the payload. The increased aerody-
namic drag and weight of the pods degrade the rocket performance by about 10 to 15 per-
cent. Upon reaching the desired time during the flight, the black powder charge in the
ejection system is fired and the sphere is expelled rearward out of the pod. The accel-
eration of the sphere causes the puncture of a small isopentane capsule that releases the
gas required to fully inflate the sphere. The radar systems, which up to this point in the
flight have been tracking the rocket, are then switched to track the sphere.
During 1968 inflatable spheres were successfully deployed from three rockets
carrying grenade experiments, and from one rocket carrying a pitot-tube experiment.
The falling sphere experiments and the pitot-tube experiments were performed by the
High Altitude Engineering Laboratory and the Space Physics Laboratory, respectively,
of the University of Michigan under contract with GSFC. These combined experiments
(one in February, two in July and one in November) were conducted from Wallops Island
and utilized either FPS-16 and/or FPQ-6 radar systems in order to track the spheres.
In each case the grenade or pitot-tube experiment was conducted on the upleg of the tra-
jectory and the sphere experiment primarily on the downleg of the trajectory. Thus, the
time elapsed between the sphere and other measurements is reduced to a few minutes
(on the order of 3 minutes at 100 km, lengthening to about 15 minutes at 30 km). Hereto-
fore, the temporal variation in the atmospheric parameters, in all but a few cases, has
been an uncertain factor in comparisons of two or more techniques.
RESULTS
The data from the four comparative rocket-borne experiments of 1968 are pre-
sented in figures 2, 3, and 4. Figure 2 contains density (in terms of percent of deviation
from the 1962 standard atmosphere) versus altitude information. In the first 3 plots of
figure 2 the sphere data are compared with data from rocket grenade experiments, while
294
in the 4th plot the spheredata are comparedwith a pitot-tube experiment. If one takes
an average of the absolute value of the difference over the entire height range, one finds
that the sphere data agree with those from other techniques within about 6 percent. Fur-
ther, one finds that the two techniques display different characteristics above and below
70 km. Above 70 km, where both the grenade and pitot-tube error functions are increasing
slightly with altitude, the difference between the sphere data and those from other tech-
niques is only about 3 percent. Below 70 km, the average difference between the data
from the spheres and the other techniques is about 8 percent. Whereas the 3 percent
difference above 70 km appears to be largely random in nature, the 8 percent difference
below 70 km appears (within the confidence level that can be obtained with 3 samples) to
be biased in a consistent manner. That is, the sphere data consistently show less devia-
tion from the standard atmosphere than do those from the other techniques. Presuming
that the data from these experiments are not coincidental, an additional examination of
the sphere data handling and reduction appears warranted in an attempt to remove the
bias in the subsonic regime, as has apparently been done in the supersonic regime
above 70 km.
The density data are used in the falling sphere experiment to derive temperatures.
These temperatures are compared with the temperatures measured by the grenade and
derived from the pitot-tube experiment in figure 3. Since the sphere temperatures are
derived by a differentiation process, the temperature differences do not automatically
"track" the density differences. However, the gross features emerge. In the firing con-
ducted on 1 February, the region where the density comparison is very good, naturally,
yields the best temperature comparison, an average difference of 4 ° K. The two firings
in July, in which grehade densities are greater than sphere densities, yield sphere tem-
peratures which tend to be higher than the grenade temperatures. The pitot-tube and
sphere temperatures are consistent; the pitot-tube densities are lower than the sphere
densities, which results in a lower sphere temperature. The average temperature dif-
ference between the four experiments conducted is 7° K.
The zonal and meridional components of wind are shown in figure 4 for the three
grenade and sphere experiments. The pitot-tube experiment, which was the 4th experi-
ment in the density and temperature comparisons, does not have a wind measuring capa-
bility. In general, the wind profile from the falling sphere displays a greater amount of
small-scale structure. This is a predictable result, since the grenade experiment aver-
ages the winds over a 2 or 3 km layer. The averaging process would be expected to
increase the difference between the two techniques to a greater extent with winds than
with the other parameters due to the more variable nature of the winds. In spite of this,
the average difference between the grenade and sphere winds is about 6 meters per sec-
ond. Contrary to the density data, the agreement is very good (differences of only 2 to
295
3 meters per second)at 60km andbelow, with the differences increasing to 10to
15meters per secondbetween60and 70km.
CONCLUSIONS
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The results of the four sphere and grenade or pitot-tube experiments are summa-
rized in figure 5. The salient features of this figure are:
1. Sphere winds which show very good agreement with grenade winds at 60 km and
below, and fair agreement at 65 and 70 km.
2. Sphere temperatures which on the average differ 7° K from grenade and pitot-
tube experiments. The recognized error in the grenade and pitot-tube experiment
accounts for 1.5 to 4 ° of the total difference.
3. Sphere densities which are closer by about 6 percent to the standard atmosphere
than densities derived from either grenade or pitot-tube experiments. The agreement
above 70 km is markedly better than below.
It is further offered that the primary obstacle toward the incorporation of the
falling sphere experiment in a program of synoptic measurements rests not with the
accuracy of the data, but with the requirement for an FPS-16 radar system or one that is
better. It is for this reason that GSFC is currently investigating the concept of utilizing
the relatively low-cost doppler tracking systems with the relatively low-cost falling
sphere payload through the use of a transponder sphere.
REFERENCE
1. Jones, L. M.; and Peterson, J.W.: Falling Sphere Measurements, 30 to 120 km.
Meteorol. Monogr., vol. 8, no. 31, Apr. 1968, pp. 176-189.
296
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...... SPHERE DENSITITES - FINAL DATA
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DEVIATION FROM STANDARD (%)
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-- GRENADE DENSITIES - FINAL DATA
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DEVIATION FROM STANDARD (%)
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....... SPHERE
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FINAL DATA
-- GRENADE
DENSITIES -
FINAL DATA
.( Z 24 JULY 1968
" "')
-40 - 0 20 40
DEVIATION FROM STANDARD (%)
-40-20 0 20 4_0
DEVIATION FROM STANDARD (%)
Figure 2.- Density data, in terms of percent of deviation from 1962 U.S. Standard
Atmosphere, as derived from sphere and grenade or pitot-tube experiments
versus altitude.
298
90
L......
80 "_
70 _ "_'_ 1853z 1 FEB 1968
60 .......SPHERE "'"'_...
-_ TEMPERATURES - \ ""Y'
FINAL DATA "_.., .
_[ 50 -- GRENADE _"_.o"/
TEMPERATURES - ._._
FINAL DATA _'_
40_
160 180 200 220 240 260 280 300
TEMPERATURE (°k)
90 ....... SPHERE TEMPERATURES - FINAL DATA
-- GRENADE TEMPERATURES - FINAL DATA
 ii27o ° ..................TM
__, , , """".-
160 180 200 220 240 260 280 300
TEMPERATURE (Ok)
90
80
70
60
50
40
.10202 24 JULY 1968
"_ GRENADE TEMPERATURES - i/"
FINAL DATA ._/
160 180 200 220 2 0 260 280 300
TEMPERATURE (°k)
/
90 _- "'':"_-_.... ....... SPHERE
....... _ TEMPERATURES
_:":;" ......... _ PRELIMINARY
80 :_ DATA
-::::_ .......... _ -- PITOT
...._ _E_,_T_:_s
20052 19 NOV 1968 .."
70
60
50
40
160 180 200 220 240 260 280 300
TEMPERATURE (°k)
Figure 3.- Temperature data as derived from sphere and grenade or pitot-tube
experiments versus altitude.
299
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WESTERLY WIND (m/see)
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WIND SPEED (m/see)
80[
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IO20Z 24 JULY 1968
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EASTERLY WIND (m/see)
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WIND SPEED (m/see)
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,of.........
80 60 40 20 0
EASTERLY WIND (m/sec)
80_
70I
60t
50t
401
,s
_.--..
NORTH _,_ SOUTH
2'0 .... '0 2O
WIND SPEED (m/sec)
Figure 4.- Wind data, in meridional and zonal components, as derived from
sphere and grenade experiments versus altitude.
50
100.
400 20
DENSITY DIFFERENCE (%)
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........15
TEMPERATURE
DIFFERENCE (°k)
[] STANDARD ERROR IN OTHER TECHNIQUES
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70 _11: .....
60 i ........................
50 ii ''2:
40(_ S I_--- 15_0
WIND DIFFERENCE (m/see)
....... SPHERE DATA
Figure 5.- Summary of density, temperature, and wind data differences.
The zero-line data are an average of the grenade and pitot tube, the
cross-hatched area represents the error estimated with these data, and
the dotted line represents the data from the sphere experiments.
3O0


Meteorological data from falling sphere technique compared with data from other sounding methods

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