Development of method for measuring electron density gradients of plasma sheath around space vehicle during atmospheric entr

Grantham, W. L.

English

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The following information is provided8 
U. S.  Patent No. t 3,473 1) 116 
Government or  
Corporate Employee a Government 
Supplementary Corporate 
Source ( i f  applicable) : N e  A, 
NASA Patent C a s e  No. t XLA-06232 
NOTE - If this  patent covers an invention made by a corporate 
employee of a NASA Contractor, the following is applicable: 
Pursuant t o  Section 305(a) of the N ona% Aeronautics and 
Space A c t ,  the name of the Administrator of NASA appears on 
the first page of the patent; however, the name of the actual  
inventor (author) appears a t  the heading 
Yes No 
. the Specification, following the words Os 
https://ntrs.nasa.gov/search.jsp?R=19710011088 2020-03-17T02:53:55+00:00Z
MEANS FOR MEASURING THE ELECTRON DENSITY GRADIENTS OF THE 
PLASMA SHEATH FORMED AROUND A SPACE VEHICLE 
Filed Jan .  30, 1967 2 Sheets-Sheet 1 
b 
INVENTOR 
BY 
9 
MEANS FOR MEASURING THE ELECTRON DENSITY GRADIENTS OF THE 
PLASMA SHEATH FORMED AROUND A SPACE VEHICLE 
Filed Jan. 30, 1967 2 Sheets-Sheet 7 
d- 
BY 
W cu 
r‘ 
IXVENTOR 
WILLIAM L. GRANTHAM 
z 
teristics of the plasma sheath which can be utilized -to 
3,473,116 solve the reentr; communication problem. Also, this in- 
~ E A N s  ELECTRON DEN- formation is valuable for aerodynamic and structural 
Other objects and advantages of this invention will 
further become apparent hereinafter and in the drawings 
SITU GRADIENTS OF THE PLASMA SHEATH design. FORMED AROUND A SPACE VEHICLE 
William Grantham, Yorktown, va., assignor to 
the United States of America as rertresented bv the 
Administrator of the National Aeronautics- and 
Space Administration 
in which: 
FIG. 1 is a block and schematic diagram showing the 
purpose of the invention; and 
FIG. 2 is a block diagram of the inventiofi. 
In describing the embodiment of the invention illus- 
trated in tlre drawings, specific terminology will be resorted 
to for the sake of clarity. However, it is not intended to 
be limited to the specific terms so selected and it is to ABSTRACT OF THE DISCLOSURE 
Twhnique for measuring electron density gradients of 15 be understood that each specific term includes all tech- 
the plasma sheath formed around the surface of a space nical equivalents which opra te  in a similar manner to 
vehicle as it enters or leaves a planetary atmosphere. accomplish a similar purpose. 
Several different frequencies are emitted into the plasma During the time that a space vericle is either leaving 
sheath and the phase difference between the transmitted or entering a planetary atmosphere the electron density 
energy and reflected energy for each of the frequencies is 20 of the plasma sheath formed around the space vehicle 
measured. This phase difference is proportional to the increases to a maximum value and then decrease$ to  zero. 
distance from the space vehicle to the critical density This invention is a microwave reflectometer system that 
boundary for each frequency, Hence, by emitting each of monitors a t  several different frequencies the occurrence 
the several frequencies from different locations around the and decay of the critical eledron density of the pkisma 
space vehicle electron density gradients of the plasma 25 sheath. The invention sends radio waves into the plasma 
sheath can be determined. A power meter is used to meas- sheath which will propagate into the plasma to its critical 
ure the power of the reflected energy for each frequency density. At the critical density boundaryj the waves are 
to determine if the critical density boundary for the fre- totally reflected due to the sharp impedaoce change a t  
quency exists in the plasma sheath. that point. This type of interaction is typical of plasmas 
30 when the collision frequency is low and the electron den- 
sity gradient is sharp with respect to wavelength. The 
The invention described herein was made by an em- higher the frequency of the radio wave, the deeper the 
ployee of the US. Government and may be manufactured radio wave will penetrate the plasma before it reaches 
and used by or for the Government for governmental pur- the critical density boundary. The wave transmitted into 
poses without the payment of any royalties thereon or 35 the plasma is totally reflected only if its critical density 
therefor. exisits in the plasma. Otherwise, the radio wave passes 
The invention relates generally to  a technique for meas- through the plasma sheath unimpeded. If the critical den- 
uring the electron density of a layer of plasma and more sity exists in the plasma, the phase difference between 
particularly concerns a technique for measuring electron the reflected power and the transmitted power is meas- 
density gradients of the plasma sheath formed near the 40 ured. This phase difference is proportional to the distance 
surface of a hypersonic vehicle as it enters or leaves a from the transmitting-rweiving antenna in the reflectom- 
planetary atmosphere. eter system to the critical density boundary. 
It is well known that when a space vehicle reenters the Turning now to the embodiment of the invention se- 
earth's atmosphere a plasma sheath is formed near the lected for illustration in the drawing, the number 11 
surface of the vehicle. This plasma sheath attenuates the 45 designates a space vehicle and the number 12 indicates 
radio waves that are transmitted to or from the vehicle the plasma sheath formed around the space vehide as 
and hence creates a communications problem. There have it leaves or enters a planetary atmosphere. Frequency 
been several techniques used in the past that provide a Sources 13, which consists of several oscillators, are ap- 
partial solution to the reentry communication problem. 6o plied through switches 14 to antennas 15-18. These 
However, to date there is not an adequate solution. If antennas are flush with the surface of the space vehicle 
the reentry communication problem is to be adequately and emit certain frequencies into the plasma sheath. The 
solved, much more knowledge is going to have to be ob- frequencies for which the critical densities do not exist 
tained about the characteristics of the plasma sheath. Then in the plasma pass on through the plasma. However, 
a systematic approach can be taken which, hopefully, 55 those frequencies for which the critical densities do exist 
will provide an adequate solution to the problem. The will be reflected and received by the same antenna that 
characteristic of the plasma sheath which is most impor- transmitted it. This received energy is applied to a mea- 
tant to the reentry communication problem is the elec- suring system which measures the transmitted power, the 
tron density of the sheath. At what distances from the reflected power and the phase difference between the 
surface of the space vehicle do the critical density bound- 6o transmitted and reflected power for each of the frequen- 
aries for different frequencies occur? How do these dis- cies in frequency source 13. These measurements are 
tances vary relative to the different locations around the recorded and transmitted to earth by means not shown. 
space vehicle? How do these distances vary with time? Even though only four antennas are shown, it should be 
The answers to these and other related questions will pro- realized that any number of antennas at many other loca- 
vide information which will be helpful in solving the re- 65 tions on the surface of the space vehicle can be used 
entry communication problem of space vehicles. without departing from the invention. 
It is therefore the primary purpose of this invention Referring now to FIG. 2, there is shown a block 
to provide a technique for measuring the electron density diagram of an embodiment of the invention that uses 
gradient of the plasma sheath formed around a space only two frequency sources. However, a much larger 
vehicle as it reenters the earth's atmosphere. The measure- ,o number could be used. The two frequency sources, 
ments are made at different times and locations on the oscillators 21 and 22, produce frequencies fl and fz, re- 
surface of a spacecraft during an actual or simulated spectively. The output from oscillator 21 is applied 
flight. Thus, information is provided about the charac- through a bidirectional coupler 23 to a switch and 
Filed Jan. 30, 1967, Ser. No. 612,740 
Int. C1. GOlr 27/04 10 U.S. Cl. 324-58.5 clams 
3,473; 
antenna 24; and the output from oscillator 22 is applied 
through a bidirectional coupler 23 to a switch and 
antennas 26. Each of the switches and antennas 24 and 26 
consists of a switch and several antennas. The switch can 
be used to connect the output from the corresponding 
bidirectional coupler to any one of the several antennas. 
If only one antenna is used the antenna can be connected 
directly to the bidirectional coupler thereby eliminating 
the need for the switch. 
The transmitted power from oscillators 21 and 22 are 
measured by transmitted power meters 27 and 28, rc- 
spectively; and the reflected power is measured by re- 
flected power meters 29 and 30. The phase difference 
between the transmitted and reflected powers are meas- 
ured by phase meters 31 and 32. The outputs from meters 15 
27-32 are applied to a telemeter 33 for transmission to 
earth. 
In the operation of the embodiment of the invention 
shown in FIG. 2, oscillator 21 produces a frequency 
fp  This frequency is applied through bidirectional cou- 20 
pler 23 and switch 24 to  a selected antenna. The fre- 
quency f1 is emitted by the antenna into the sheath of 
plasma surrounding the space vehicle. If the critical 
density for the frequency fl does not exist in the sheath 
of plasma, the frequency will pass through the plasma 25 
unimpeded and none will be reflected. However, if the 
critical density for the frequency fl does exist, the fre- 
quency will be totally reflected back to  the antenna. The 
reflected frequency passes through the antenna and the 
bidirectional coupler 23 to meter 29 where its power is 30 
measured. Meter 27 measures the power of the trans- 
mitted frequency fl. Hence, by comparing the outputs of 
meters 27 and 29 it can be determined if the critical 
density for the frequency fl exists in the plasma sheath. 
If the critical density for the frequency fl does exist then 35 
the phase meter 31 will measure the phase difference of 
the transmitted power and the reflected power. This phase 
difference is proportional to the distance from the sur- 
face of the space vehicle to the critical density boundary. 
Thus, if several different frequency sources are used and 40 
several antennas are located at  difference locations on the 
Burface of the space vehicle the distances of the critical 
density boundaries from the space vehicle 11 for differ- 
ence frequencies, times and locations can be determined. 
Ilt is to be understood that the form of the invention 45 
herewith shown and described is to be taken as a pre- 
ferred embodiment. Various changes may be made in the 
shape, size, and arrangement of parts. For example, equi- 
valent elements may be substituted for those illustrated 
and described herein, parts may be reversed, and ep 50 
,116 
tain features of the invenLion may be utilized inde- 
pendently of the use of other features, all without de- 
parting from the spirit or scope of the invention as de- 
fined in the subjoined claims. Even though only two fre- 
quency sources and four antennas are shown, any desired 
number can be used; switches 24 and 26 can be auto- 
matic with any desired program; and switches 24 and 26 
can be eliminated by providing a complete system for 
each frequency and each antenna location all without 
departing from the spirit and scope of this invention. 
What is claimed is: 
1. Means for measuring the electron density gradients 
of the plasma sheath formed around the surface of a 
space vehicle as it enters or leaves a planetary atmos- 
phere comprising: means on said space vehicle for gen- 
erating a plurality of different frequency signals; antenna 
means mounted at several locations on the surface of 
said space vehicle for emitting said generated signals 
into said plasma sheath and for receiving the reflected 
signals; a bidirectional coupling means and a switch 
associated with each of said plurality of signals for selec- 
tively connecting that signal to said antenna means at 
each of said several locations; and means connected to 
each of said bidirectional coupling means for measuring 
the phase difference between the corresponding generated 
signal and the signal reflected back to the antenna means 
whereby the measured phase differences are proportional 
to the distances from the surface at the selected locations 
on the space vehicle to  the critical density boundary 
for the frequencies of the different signals. 
2. Means for measuring the electron density gradients 
of the plasma sheath formed around the surface of a 
space vehicle as it enters or leaves a planetary atmos- 
phere according to claim 1 wherein means associated 
with each of said different frequency signals is connected 
to  the corresponding bidirectional coupling means for 
measuring the power of the generated and the reflected 
signals whereby determination can be made as to whether 
or not the critical density for the frequency of each of the 
generated signals exists in the plasma sheath. 
References Cited 
UNITED STATES PATENTS 
2,596,530 5/1932 Clarke. 
3,233,172 2/1966 Luoma. 
3,317,826 5/1967 h o p  et al. 
RUDOLPH V. ROLINEC, Primary Examiner 
P. F. W&I& Assistant Examiner 


Means for measuring the electron density gradients of the plasma sheath formed around a space vehicle Patent

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