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CRPL aids missile tracking
NATIONAL BUREAU OF STANDARDS NEWS CRPL AIDS MISSILE TRACKING
Scientists of the National Bureau of Standards' Boulder Laboratories are analyzing errors in radio position measurements and fluctuations in the radio refractive index of the troposphere to determine their correlation. Long-term fluctuations were studied by analysis of punched-card data available in the Radio Refractive Index Data Center 1 of the NBS Central Radio Propagation Laboratory. Shortterm fluctuations are being studied by correlating variations in radio transmission time with variations in radio refractive index, using measurements made with an operating model baseline missile tracking system. The correction factors obtained in these studies will be programmed into the computer of the M ISTRAM baseline missile tracking system, now being built by the General Electric Company for operation near Patrick Air Force Base, in the Cape Canaveral, Fla., area. MISTRAM determines the missile's position by measuring the times required for radio signals to travel from each of several antennas to the missile and back. These antennas are arranged on an orthogonal set of baselines. Translating these transit times to distances (and, hence, to position) requires a knowledge of the speed with which the radio signals travel through the atmosphere. This speed, usually expressed in terms of the refractive index, is a function of the composition of the atmosphere along the signal paths. Tracking inaccuracies result from variations in the earth's atmosphere along these paths. The variations consist of both large-scale changes, caused by air mass movements, and short-term changes, resulting from turbulence. Such tracking errors, introduced by variations in atmospheric refractive index, may become the limiting factor in obtaining the accuracy demanded of new tracking systems by advancing space technology. The development of "second generation" missile tracking systems, such as M ISTRAM, near Patrick Air Force Base, Fla., has necessitated a study of the problem of obtaining correction factors to be programmed into the system's computer. The radio propagation engineering staff of the NBS Central Radio Propagation Laboratory (CRPL) was requested to provide estimates of the nature and extent of the atmospheric effects. The problem was attacked by means of two separate studies. 1 "Radio Refractive Index Data Center," N B S Tech. News Bull., Vol. 46, p. 5 (1962). 330
Apr., I962.]
N A T I O N A L B U R E A U OF STANDARDS
33I
In the first study, a theoretical description of long-term variations of the refractive index for both the homogeneous and inhomogeneous atmosphere was developed by members of the Radio Meteorology section, B. R. Bean, chief. This was done by analysis of all the radiosonde data available in C R P L ' s Radio Refractive Index Data Center. More than 7,000,000 punched cards stored here contain radio refraction measurements made over a long period of time at many points. Application of statistical methods to this pool of information has made possible a systematic correction of tracking data, based on readily available meteorological parameters. This correction has been obtained in a form suited for use in M I S T R A M ' s high speed computer. The other phase of C R P L ' s attack on the problem is a study of the short-term effects of turbulence. The Lower Atmosphere Physics section, Dr. M. C. Thompson, Jr., chief, has constructed a simulated missile tracking system for making this study. The missile is simulated by an antenna mounted on concrete on the side of a mountain behind the Boulder Laboratories. Radio transmissions at microwave frequencies (about 9400 Mc/s) are used between the "missile" and three "tracking station" antennas ten miles to the east, arranged to simulate the relative positions of the tracking antennas under construction in Florida. Radio refractive index measurements at the antenna sites and from aircraft 2 along the transmission path are made simultaneously with radio distance measurements. Dr. Thompson's group has been evaluating the data collected in this experiment in terms of the magnitude of errors due to short-term fluctuations in the atmosphere. To date, atmospheric effects on range determination have been observed which amount to as much as several feet over a 10-mile path. It has also been found that these effects can often be reduced 80 per cent by using appropriate corrections based on simultaneous refractive index measurements. VERSATILE ELECTRON ENERGY ANALYZER
The National Bureau of Standards' investigation of characteristic energy losses of electrons in solids has recently been extended to measurements in the liquid helium and liquid hydrogen temperature range. 3 For use in these low temperature studies, E. M. H6rl and J. A. Suddeth have developed a versatile electrostatic analyzer lens* intended primarily for investigating thin films of solidified gases. However, the lens 2 "Compact Microwave Refractometer for Use in Small Aircraft," by M. C. Thompson, Jr. and M. J. Vetter, Rev. Sci. Instr., Vol. 29, p. 1093 (1958). For a description of phase measurements, see "Measurements of Phase Stability over a Low-Level Tropospheric Path," by M. C. Thompson, Jr. and H. B. Jones, J. Research NBS, Vol. 63D No. 1, p. 45 (1959). 3 For further details, see "Characteristic Energy Loss Measurements at Low Temperatures," by E. M. H6rl and J. A. Suddeth, J. Appl. Phys., Vol. 32, pp. 2521-2525 (1961). 4 G. MOLLENSTADT,Optik, Vol. 5, p. 499 (1949) ; ibid., Vol. 9, p. 473 (1952).
332
NATIONAL BUREAU OF STANDARDS
IJ. F. I.
is suitable for the study of any substance which is obtainable as a thin film. Over the last 30 years, characteristic electron energy losses in gases and solids have been studied using various methods. Most measurements were made at room t e m p e r a t u r e until theoretical considerations indicated possible t e m p e r a t u r e dependence of the energy loss values in metals. Because theoretical studies of this possible temperature influence gave conflicting results, Leder and M a r t o n 5 of the Bureau made the first measurements on metallic a l u m i n u m at increasing temperatures and at temperatures in the liquid helium and liquid hydrogen temperature ranges. Since then, a broader investigation has been undertaken. One of the present areas of interest concerns characteristic energy loss spectra of solidified p e r m a n e n t gases for which the molecular or atomic energy levels are well known in the gas phase and for which some optical d a t a are available, or at least can be expected to be available in the near future, for the solid phase. T h e lens developed at the Bureau for low temperature energy loss studies possesses several useful features. Controls are provided outside the v a c u u m for positioning the slit-lens assembly with respect to the electron beam, positioning the slit with reference to the lens for optimizing resolution and dispersion, rotating the slit with respect to the lens, and for fine and coarse a d j u s t m e n t of slit width. All controls are m o u n t e d on concentric shafts for ease of operation in a darkened room. " I n - v a c u u m " friction is essentially eliminated by the use of phosphor bronze deformation members. 6 In the experimental arrangement used at the Bureau, a thin supporting foil (collodion, Formvar, or aluminum) is m o u n t e d over a hole in a target block of oxygen-free high-conductivity copper which is soldered to the b o t t o m of a cylindrical container filled with liquid helium or liquid hydrogen. The temperature of the target block can be measured with a carbon resistance thermometer. Thin layers of gases are formed by condensation on the supporting foil and investigated by means of an electron beam of 30 key energy. The target is completely surrounded by a radiation shield at a temperature of 20°K. The shield has only three openings--for entrance and exit of the electron beam and for admission of the gas to be solidified on the substrate. After passing through the specimen film, the electron beam enters the cylindrical analyzer lens. By shifting the complete lens assembly off the electron beam path, entire electron diffraction patterns of the samples can be observed and photographed before and after energy loss measurements are made. 5 L. B. LEDER AND L. MARTON, Phys. Rev., Vol. 112, p. 341 (1958). 6 R. V. JoN~:s, J. Sci. Inst., Vol. 28, p. 38 (1951).
Apr., i962. ]
NATIONAL BUREAU OF STANDARDS
333
The electrostatic field of the lens causes two electrons having equal scattering angles but different energies to strike a photographic plate behind the lens at two different positions on a horizontal line. Electrons of equal energies but different scattering angles will hit the plate on a vertical line. The plate therefore becomes a two-dimensional plot of electron intensity (represented by the density of the plate) as a function of scattering angle (vertical coordinate) and energy loss (horizontal coordinate). Patterns of this ideal type are obtained only with very small scattering angles when using the very central region of the lens. Larger scattering angles (up to 0.06 rad) cause bending of the vertical lines of equal electron energy. A calibration of the energy loss (horizontal coordinate) is made by changing the potential of the electron gun filament with respect to the center electrode of the analyzer lens in steps of approximately 6 volts. One of the electron energy loss patterns obtained came from a thin fihn of solid oxygen deposited on a Formvar substrate at 20°K. In addition to the energy loss for the Formvar substrate, an energy loss of 9.5 ev was observed for the oxygen.
Scientists of the National Bureau of Standards' Boulder Laboratories are analyzing errors in radio position measurements and fluctuations in the radio refractive index of the troposphere to determine their correlation. Long-term fluctuations were studied by analysis of punched-card data available in the Radio Refractive Index Data Center 1 of the NBS Central Radio Propagation Laboratory. Shortterm fluctuations are being studied by correlating variations in radio transmission time with variations in radio refractive index, using measurements made with an operating model baseline missile tracking system. The correction factors obtained in these studies will be programmed into the computer of the M ISTRAM baseline missile tracking system, now being built by the General Electric Company for operation near Patrick Air Force Base, in the Cape Canaveral, Fla., area. MISTRAM determines the missile's position by measuring the times required for radio signals to travel from each of several antennas to the missile and back. These antennas are arranged on an orthogonal set of baselines. Translating these transit times to distances (and, hence, to position) requires a knowledge of the speed with which the radio signals travel through the atmosphere. This speed, usually expressed in terms of the refractive index, is a function of the composition of the atmosphere along the signal paths. Tracking inaccuracies result from variations in the earth's atmosphere along these paths. The variations consist of both large-scale changes, caused by air mass movements, and short-term changes, resulting from turbulence. Such tracking errors, introduced by variations in atmospheric refractive index, may become the limiting factor in obtaining the accuracy demanded of new tracking systems by advancing space technology. The development of "second generation" missile tracking systems, such as M ISTRAM, near Patrick Air Force Base, Fla., has necessitated a study of the problem of obtaining correction factors to be programmed into the system's computer. The radio propagation engineering staff of the NBS Central Radio Propagation Laboratory (CRPL) was requested to provide estimates of the nature and extent of the atmospheric effects. The problem was attacked by means of two separate studies. 1 "Radio Refractive Index Data Center," N B S Tech. News Bull., Vol. 46, p. 5 (1962). 330
Apr., I962.]
N A T I O N A L B U R E A U OF STANDARDS
33I
In the first study, a theoretical description of long-term variations of the refractive index for both the homogeneous and inhomogeneous atmosphere was developed by members of the Radio Meteorology section, B. R. Bean, chief. This was done by analysis of all the radiosonde data available in C R P L ' s Radio Refractive Index Data Center. More than 7,000,000 punched cards stored here contain radio refraction measurements made over a long period of time at many points. Application of statistical methods to this pool of information has made possible a systematic correction of tracking data, based on readily available meteorological parameters. This correction has been obtained in a form suited for use in M I S T R A M ' s high speed computer. The other phase of C R P L ' s attack on the problem is a study of the short-term effects of turbulence. The Lower Atmosphere Physics section, Dr. M. C. Thompson, Jr., chief, has constructed a simulated missile tracking system for making this study. The missile is simulated by an antenna mounted on concrete on the side of a mountain behind the Boulder Laboratories. Radio transmissions at microwave frequencies (about 9400 Mc/s) are used between the "missile" and three "tracking station" antennas ten miles to the east, arranged to simulate the relative positions of the tracking antennas under construction in Florida. Radio refractive index measurements at the antenna sites and from aircraft 2 along the transmission path are made simultaneously with radio distance measurements. Dr. Thompson's group has been evaluating the data collected in this experiment in terms of the magnitude of errors due to short-term fluctuations in the atmosphere. To date, atmospheric effects on range determination have been observed which amount to as much as several feet over a 10-mile path. It has also been found that these effects can often be reduced 80 per cent by using appropriate corrections based on simultaneous refractive index measurements. VERSATILE ELECTRON ENERGY ANALYZER
The National Bureau of Standards' investigation of characteristic energy losses of electrons in solids has recently been extended to measurements in the liquid helium and liquid hydrogen temperature range. 3 For use in these low temperature studies, E. M. H6rl and J. A. Suddeth have developed a versatile electrostatic analyzer lens* intended primarily for investigating thin films of solidified gases. However, the lens 2 "Compact Microwave Refractometer for Use in Small Aircraft," by M. C. Thompson, Jr. and M. J. Vetter, Rev. Sci. Instr., Vol. 29, p. 1093 (1958). For a description of phase measurements, see "Measurements of Phase Stability over a Low-Level Tropospheric Path," by M. C. Thompson, Jr. and H. B. Jones, J. Research NBS, Vol. 63D No. 1, p. 45 (1959). 3 For further details, see "Characteristic Energy Loss Measurements at Low Temperatures," by E. M. H6rl and J. A. Suddeth, J. Appl. Phys., Vol. 32, pp. 2521-2525 (1961). 4 G. MOLLENSTADT,Optik, Vol. 5, p. 499 (1949) ; ibid., Vol. 9, p. 473 (1952).
332
NATIONAL BUREAU OF STANDARDS
IJ. F. I.
is suitable for the study of any substance which is obtainable as a thin film. Over the last 30 years, characteristic electron energy losses in gases and solids have been studied using various methods. Most measurements were made at room t e m p e r a t u r e until theoretical considerations indicated possible t e m p e r a t u r e dependence of the energy loss values in metals. Because theoretical studies of this possible temperature influence gave conflicting results, Leder and M a r t o n 5 of the Bureau made the first measurements on metallic a l u m i n u m at increasing temperatures and at temperatures in the liquid helium and liquid hydrogen temperature ranges. Since then, a broader investigation has been undertaken. One of the present areas of interest concerns characteristic energy loss spectra of solidified p e r m a n e n t gases for which the molecular or atomic energy levels are well known in the gas phase and for which some optical d a t a are available, or at least can be expected to be available in the near future, for the solid phase. T h e lens developed at the Bureau for low temperature energy loss studies possesses several useful features. Controls are provided outside the v a c u u m for positioning the slit-lens assembly with respect to the electron beam, positioning the slit with reference to the lens for optimizing resolution and dispersion, rotating the slit with respect to the lens, and for fine and coarse a d j u s t m e n t of slit width. All controls are m o u n t e d on concentric shafts for ease of operation in a darkened room. " I n - v a c u u m " friction is essentially eliminated by the use of phosphor bronze deformation members. 6 In the experimental arrangement used at the Bureau, a thin supporting foil (collodion, Formvar, or aluminum) is m o u n t e d over a hole in a target block of oxygen-free high-conductivity copper which is soldered to the b o t t o m of a cylindrical container filled with liquid helium or liquid hydrogen. The temperature of the target block can be measured with a carbon resistance thermometer. Thin layers of gases are formed by condensation on the supporting foil and investigated by means of an electron beam of 30 key energy. The target is completely surrounded by a radiation shield at a temperature of 20°K. The shield has only three openings--for entrance and exit of the electron beam and for admission of the gas to be solidified on the substrate. After passing through the specimen film, the electron beam enters the cylindrical analyzer lens. By shifting the complete lens assembly off the electron beam path, entire electron diffraction patterns of the samples can be observed and photographed before and after energy loss measurements are made. 5 L. B. LEDER AND L. MARTON, Phys. Rev., Vol. 112, p. 341 (1958). 6 R. V. JoN~:s, J. Sci. Inst., Vol. 28, p. 38 (1951).
Apr., i962. ]
NATIONAL BUREAU OF STANDARDS
333
The electrostatic field of the lens causes two electrons having equal scattering angles but different energies to strike a photographic plate behind the lens at two different positions on a horizontal line. Electrons of equal energies but different scattering angles will hit the plate on a vertical line. The plate therefore becomes a two-dimensional plot of electron intensity (represented by the density of the plate) as a function of scattering angle (vertical coordinate) and energy loss (horizontal coordinate). Patterns of this ideal type are obtained only with very small scattering angles when using the very central region of the lens. Larger scattering angles (up to 0.06 rad) cause bending of the vertical lines of equal electron energy. A calibration of the energy loss (horizontal coordinate) is made by changing the potential of the electron gun filament with respect to the center electrode of the analyzer lens in steps of approximately 6 volts. One of the electron energy loss patterns obtained came from a thin fihn of solid oxygen deposited on a Formvar substrate at 20°K. In addition to the energy loss for the Formvar substrate, an energy loss of 9.5 ev was observed for the oxygen.