RIGHTS OF GOVERNMENT
The invention described herein may be manufactured, used, and licensed by or for the Government of the United States for governmental purposes without the payment to us of any royalty thereon.
BACKGROUND OF THE INVENTION
This invention relates to radio communication through a layer of ionized gas, and more particularly to a system for transmitting radio signals through the plasma sheath which normally surrounds a space vehicle upon its reentry into atmosphere.
During space craft reentry into the earth's atmosphere, a well defined though highly inhomogeneous layer of ionized gas surrounds the space craft and creates a serious problem for electromagnetic methods of tracking, communication, fuzing, etc.
The layer of ionized gas, commonly referred to as a plasma sheath, is created through conversion of kinetic to thermal energy during reentry or ascent of the vehicle. The plasma sheath is created by the air passing through the shock wave, thereby producing a nonuniform layer of shock-compressed heated air, which in turn creates a region of positive ions and free electrons. Radio signals transmitted to and from reentry vehicles are adversely affected by their interaction with the plasma particles, primarily the electrons. The interference is characterized by signal reflection, absorption, attenuation losses due to electron collisions and by phase shift and refraction of the signal. The severity of the problem is a function of a plurality of parameters including the velocity of the space craft, its altitude, orientation, nose bluntness, location of the antenna on the space craft as well as numerous other factors.
One important factor is the plasma resonance, a constant of the plasma medium, which varies directly with the square root of the electron density in the plasma. The critical frequency at which blackout of transmission occurs can be expressed as
fc = 1/2π√e2 N/mε= 8979√N
e = electron charge
m = electron mass
N = electron density
ε = dielectric constant of free space
It is apparent from the above equation that as the electron density increases, the critical demarcation frequency for plasma penetration becomes higher. Accordingly, no radio transmission can be achieved unless the transmitting frequency can be raised above the critical plasma frequency. Although communication through the plasma can be achieved with extremely high frequencies, this introduces difficulties in the use of high frequency equipment. More significantly, at high frequencies there exists the problem of atmospheric absorption of the signal.
Most prior art attempts to solve the problem have been directed at techniques for altering the plasma characteristics so as to reduce the electron density concentration in the plasma. One such technique is illustrated in U.S. Pat. No. 3,277,375 to Nelson in which a material is injected into the flow field of the plasma to reduce the electron density within the plasma, the injected material having the effect of cooling the plasma and increasing the recombination rate of the electrons and ions. A different approach is illustrated in U.S. Pat. No. 3,128,965 to Ziemmer in which a fluid stream which is relatively transparent to radio waves is injected into the plasma sheath. Still other techniques for reducing the concentration of electrons within the plasma include magnetic and electrostatic modulators for creating a region within the plasma which contains a smaller electron concentration. These latter techniques are illustrated in U.S. Pat. Nos. 3,295,531 and 3,300,721 to Seaton, U.S. Pat. No. 3,176,228 to Phillips and U.S. Pat. No. 3,176,227 to Bender.
It will be appreciated from the foregoing that prior art techniques have been directed toward reducing the electron density within the plasma. Contrary to this, the present invention achieves greater radio wave transmission through the plasma without any reduction of the electron density within the plasma. Whereas it has previously been thought by persons skilled in the art that increasing transmission power levels tends to compound the plasma sheath problem due to the additional ionization created by radio frequency heating of the plasma, it has been found that the signal transmission coefficient is constant over a large power range and, therefore, increasing the input power level results in increased transmission through the plasma. Additionally, it has been discovered, contrary to previous understanding, that at these breakdown power levels the transmitting antenna system can be impedance matched to a plasma for greater efficiency of transmission.
It is, therefore, a primary object of this invention to provide a system for transmitting radio wave through a layer of ionized gas.
A more particular object of the invention is to provide radio communication through the plasma sheath normally surrounding a space craft reentry vehicle.
Still another object of the invention is to provide radio communication through a layer of ionized gas without artificially reducing the electron density within the ionized layer.
Yet another object is to provide radio communication through an ionized layer of gas without the necessity for using electrostatic or magnetic modulators to change the shape of the ionized flow field.
Still another object of the invention is to provide radio communication through an ionized layer without the necessity for injecting fluids or gases into the ionized medium.
SUMMARY OF THE INVENTION
Briefly, in accordance with this invention, radio communication through a layer of ionized gas is achieved by initially adjusting the impedance of the antenna system to match free space conditions, thereafter applying the radio signal to the antenna system, monitoring the standing wave ratio to detect a change in the ionization level of the ionized gas surrounding the antenna and thereafter adjusting the impedance of the antenna system to match that of the ionized gas. The changing ionization level is continuously monitored and the impedance continuously changed to match the constantly changing plasma conditions. Increased signal transmission is achieved by increasing the power of the signal up to a predetermined level and adjusting the impedance of the antenna system to match the changed ionized gas conditions which result from the increased input power. The increase in input power may be C.W. or may take the form of a series of pulses with subsequent monitoring of the changed ionization level delayed a predetermined time period after each pulse in order to monitor the effects of the increased pulsed power levels. Additional enhancement of signal transmission is achieved by adjusting the phased array of the antenna system in response to vehicle orientation data and remote tracking station signal data. The strength of required power level may also be dependant upon this data.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a box diagram of one embodiment for use with the present invention.
FIG. 2 is a graphic illustration of signal transmission coefficient as a function of input power for three different plasma conditions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, a system for transmitting radio waves through a plasma is depicted generally at 10 as being contained within a space craft housing partially shown at 12. A window 13 is provided along the skin of space craft 12 and an array of flush-mounted antennas 11 direct radiation through window 13 into the atmosphere. Initially, screw tuner 18 is adjusted to match the antenna to free space conditions. During reentry flight, plasma sheath 14 envelops the vehicle thereby causing a mismatch of impedance at the antenna. The impedance level is monitored by means of reflectometer 17 which generally consists of a double stub at fixed known locations and insertion and which feed crystal detectors in signal comparator 20 to determine the standing wave ratio (SWR), a standard microwave technique. Any necessary change is effectuated by servo 19 which derives a signal from comparator 20 and automatically adjusts screw tuner 18. The RF signal is directed to antenna system 11 from power generator 15 through an isolator 16 and thereon to the antenna array in a conventional manner. Power generator 15 may be a conventional continuous wave source or it may, for the purpose of maximizing output power for a specific size generator, be operable in pulsed or C.W. mode. In the case of a pulsed source it is necessary to monitor the ambient conditions of the plasma 14 at a predetermined time after the application of the pulse of power. Accordingly, pulse synchronizer 21 senses the application of a pulse from generator 15 and applies a delayed pulse to signal comparator 20 such that monitoring of the plasma sheath occurs at a predetermined time after the application of the pulse.
Additional enhancement of signal transmission is achieved by adjusting the phased array of the antenna system in response to orientation data and remote station signal data. A signal directivity computer 23 is programmed to provide suitable signals to a servo-motor 27 which in turn adjusts the phased array of antenna system 11 in accordance with the directions of the computer. Computer input data comes from a variety of sources including its own memory bank which may contain stored information based upon predetermined calculations, as well as from tracking station data and vehicle orientation data.
Tracking station data 25 provide information to computer 23 regarding the strength of the signals being transmitted between the vehicle and its tracking stations. Antenna system 11 can be utilized for both signal receiving and transmitting modes with switch 26 enabling the selection of the appropriate mode. The vehicle orientation data is derived from on-board sensors and generally consists of such information as the attitude and roll of the vehicle, its precise trajectory, its angle of attack, etc. Computer designs, programs and calculations for various trajectories, missile shapes and antenna locations can be calculated and provided in advance. Such calculations are currently available in the literature. For example, a NASA Technical Report TM-X-2041 entitled "A Computer Program to Determine Equilibrium-Air Flow-Field Data About a Blunt Body" June, 1970, contains calculations and data of the type necessary for use in accordance with this invention. Similarly, NASA Technical Note D-507 (1961) provides additional information of the type usable in conjunction with this invention. More detailed mathematical analysis of the type which may be programmed into computer 23 can be found in Cornell Aeronautical Laboratory Report No. QM-1626-A-12 (May 1963) as well as in Arnold Engineering Development Center Report No. AEDC-TBR-63-3, "Chemical Kinetic Regimes of Hyparsonic Flight Simulation" (1963). These reports are but a few examples of the type of literature and information available to persons skilled in the art to enable them to properly program a computer capable of utilizing this information. The orientation data 24 and tracking station data 25 which feed into signal directivity computer 23 may also be utilized, through a servo motor 22, to adjust the power level or pulse duration of power source 15. The mathematical calculation necessary for this type of program is also well within the skill of the art.
Referring now to FIG. 2, the relationship of signal transmission coefficient as a function of input power is illustrated. The test data shown was taken across a 0.635 cm section of shock heated air at a frequency of 9.4 GHz and pulse duration of 1.4 microseconds. The RF transmission characteristics of a plasma are represented by its electron density N and its electron collision frequency ν. It will be appreciated from the Figure that for any given plasma characteristics, the transmission coefficient remains substantially constant over a wide range of power inputs.
It will be appreciated from the foregoing that we have described a novel and unique system for transmitting radio waves through an ionized layer of gas without the necessity for artificially reducing the electron density within the gas. Although specific embodiments have been illustrated, we wish it to be understood that we do not desire to be limited to the exact details of construction as shown and described, for obvious modifications will occur to a person skilled in the art.