RG Herrera - Academia.edu (original) (raw)
Papers by RG Herrera
From March 21 to April 11, 1993, the Galileo, Mars Observer, and Ulysses spacecraft were tracked ... more From March 21 to April 11, 1993, the Galileo, Mars Observer, and Ulysses spacecraft were tracked almost continuously in a coincidence experiment to search for low-frequency (~millihertz) gravitational radiation. We report here a first statistical assessment of the noise characteristics of the data, with particular attention to the performance of the radio science instrumentation itself.
2003 IEEE Aerospace Conference Proceedings (Cat. No.03TH8652)
... In order to achieve this value, the phase fluctuations of the radio beams crossing the solar ... more ... In order to achieve this value, the phase fluctuations of the radio beams crossing the solar corona must be fully calibrated to ensure a good ... also generates a reference signal to the Ka-band Exciter (KEX) for the X/Ka link; furthermore a coherent frequency translator (KaT) is used ...
Journal of Geophysical Research, 2002
An ionosphere has been detected at Callisto by the Galileo spacecraft, using the radio occultatio... more An ionosphere has been detected at Callisto by the Galileo spacecraft, using the radio occultation technique. There were four usable occultations by Callisto, providing eight observation opportunities, all equatorial and near the terminator (entry and exit observations). Detectable electron densities were obtained from six of the eight opportunities. It was found that a detectable ionosphere was only present at the observed location when the trailing hemisphere of Callisto, which is the one that is impacted by the corotating plasma of Jupiter's magnetosphere, was illuminated by the Sun. Two of these observations yielded well-defined electron density profiles, having peak densities of 15,300 and 17,400 cm À3 at altitudes of 27.2 and 47.6 km and topside plasma scale heights of 29.6 and 49.0 km. Four different methods, based on both photoionization and electron impact ionization, were used to obtain estimates of the corresponding neutral densities at the surface. The various assumptions inherent in these methods required using a variety of parameters, (cross sections, rate constants, etc.) all with their associated uncertainties. It was rather surprising and reassuring to find that all of the methods used to estimate the surface neutral density gave very similar results in each of the eight cases. The estimated values fall between 1 and 3 Â 10 10 cm À3 , leading to an estimate for the column density of from 3 to 4 Â 10 16 cm À2 .
Geophysical Research Letters, 1997
The Galileo spacecraft passed 'behind Jupiter on December 8, 1995, allowing the first radio occul... more The Galileo spacecraft passed 'behind Jupiter on December 8, 1995, allowing the first radio occultation measurements of its ionospheric structure in 16 years. At ingress (24øS, 68øW), the principal peak of electron density is located at an altitude of 900 km above the 1-bar pressure level, with a peak density of 105 cm-3 and a thickness of-200 km. At egress (43øS, 28øW), the main peak is centered near 2000 km altitude, with a peak density of 2x104 cm-3 and a thickhess of-1000 km. Two thin layers, possibly forced by upwardly propagating gravity waves, appear at lower altitudes in the ingress profile. This is the first in a twoyear series of observations that should help to resolve long-standing questions about Jupiter's ionosphere.
Journal of Geophysical Research, 1998
Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detai... more Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake w•s measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s-1 at a distance of 3 Io radii from the center of Io to 57 km s-• at 7 Io radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceedin• 50,000 cm-3 at 9 out of 10 locations and reaching a maximum of 277,000 cm-ø. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0øW) and away from (180øW) Jupiter and minima near the center of the downstream (90øW) and upstream (270øW) hemispheres. This pattern m•y be related to the Alfv•nic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from-•200 km near the center of the upstream hemisphere to-•400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202 4-14 K if Na + is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location. 1. Introduction Studies of Io are difficult to confine within the boundaries of traditional scientific disciplines. Interior dynamics produce active and widespread volcanism, which profoundly changes the nature of its atmosphere. Gases escaping from the atmosphere populate a plasma torus, which in many respects determines the character of
Journal of Geophysical Research, 1998
Space, 2002
International collaboration between the National Aeronautics and Space Administration (NASA) and ... more International collaboration between the National Aeronautics and Space Administration (NASA) and the Italian Space Agency (ASI) resulted in the development of spacecraft and ground instrumentation to enable a set of Radio Science experiments on the Cassini mission that include a search for gravitational waves, a solar conjunction experiment, atmospheric occultations and gravity field measurements of Saturn and several of its satellites. Key to these experiments is the addition of a Ka-band translator to the Cassini payload to augment the telecommunications system at X-band and the Radio Science downlink at S-band, the most sophisticated radio system in deep space to date. The Deep Space Network instrumented one 34-meter diameter beam-waveguide station in California with systems capable of simultaneous transmission and reception at X-and Ka-bands along with special antenna pointing techniques. Upgraded high-stability digital open-loop receivers augmented the tracking receivers. In order to fully take advantage of the superior stability radio link at Ka-band where the effect of interplanetary plasma noise is less than the effect at X-band, modifications were made to the frequency and timing subsystem and advanced water vapor radiometers were developed and installed as part of a media calibration system to calibrate the tropospheric effect. A multi-frequency scheme can be used to model and calibrate the intervening media of charged particles. The first formal operational use of this new Radio Science system took place between November 2001 and January 2002 where the Cassini spacecraft was in solar opposition and a 40-day search for gravitational waves was conducted. Two additional opportunities of such searches are planned at oppositions to be followed six months later by solar conjunction experiments measuring the relativistic deflection of the radio beams as they pass near the sun. This paper describes the new DSN science capability and highlights of the engineering work that lead to its development. It will also discuss experience with operations along with statistics and data quality.
Aerospace …, 2003
... data from the DSN stations to JPL 5. Transfer the RSR data from the DSN stations to JPL 6. Pr... more ... data from the DSN stations to JPL 5. Transfer the RSR data from the DSN stations to JPL 6. Process the raw Media Calibration System ... and down-link capability, where the Open Loop receivers acquired and sampled the down-link carrier in three bands (X/X, X/Ka and KaKa). ...
From March 21 to April 11, 1993, the Galileo, Mars Observer, and Ulysses spacecraft were tracked ... more From March 21 to April 11, 1993, the Galileo, Mars Observer, and Ulysses spacecraft were tracked almost continuously in a coincidence experiment to search for low-frequency (~millihertz) gravitational radiation. We report here a first statistical assessment of the noise characteristics of the data, with particular attention to the performance of the radio science instrumentation itself.
2003 IEEE Aerospace Conference Proceedings (Cat. No.03TH8652)
... In order to achieve this value, the phase fluctuations of the radio beams crossing the solar ... more ... In order to achieve this value, the phase fluctuations of the radio beams crossing the solar corona must be fully calibrated to ensure a good ... also generates a reference signal to the Ka-band Exciter (KEX) for the X/Ka link; furthermore a coherent frequency translator (KaT) is used ...
Journal of Geophysical Research, 2002
An ionosphere has been detected at Callisto by the Galileo spacecraft, using the radio occultatio... more An ionosphere has been detected at Callisto by the Galileo spacecraft, using the radio occultation technique. There were four usable occultations by Callisto, providing eight observation opportunities, all equatorial and near the terminator (entry and exit observations). Detectable electron densities were obtained from six of the eight opportunities. It was found that a detectable ionosphere was only present at the observed location when the trailing hemisphere of Callisto, which is the one that is impacted by the corotating plasma of Jupiter's magnetosphere, was illuminated by the Sun. Two of these observations yielded well-defined electron density profiles, having peak densities of 15,300 and 17,400 cm À3 at altitudes of 27.2 and 47.6 km and topside plasma scale heights of 29.6 and 49.0 km. Four different methods, based on both photoionization and electron impact ionization, were used to obtain estimates of the corresponding neutral densities at the surface. The various assumptions inherent in these methods required using a variety of parameters, (cross sections, rate constants, etc.) all with their associated uncertainties. It was rather surprising and reassuring to find that all of the methods used to estimate the surface neutral density gave very similar results in each of the eight cases. The estimated values fall between 1 and 3 Â 10 10 cm À3 , leading to an estimate for the column density of from 3 to 4 Â 10 16 cm À2 .
Geophysical Research Letters, 1997
The Galileo spacecraft passed 'behind Jupiter on December 8, 1995, allowing the first radio occul... more The Galileo spacecraft passed 'behind Jupiter on December 8, 1995, allowing the first radio occultation measurements of its ionospheric structure in 16 years. At ingress (24øS, 68øW), the principal peak of electron density is located at an altitude of 900 km above the 1-bar pressure level, with a peak density of 105 cm-3 and a thickness of-200 km. At egress (43øS, 28øW), the main peak is centered near 2000 km altitude, with a peak density of 2x104 cm-3 and a thickhess of-1000 km. Two thin layers, possibly forced by upwardly propagating gravity waves, appear at lower altitudes in the ingress profile. This is the first in a twoyear series of observations that should help to resolve long-standing questions about Jupiter's ionosphere.
Journal of Geophysical Research, 1998
Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detai... more Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake w•s measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s-1 at a distance of 3 Io radii from the center of Io to 57 km s-• at 7 Io radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceedin• 50,000 cm-3 at 9 out of 10 locations and reaching a maximum of 277,000 cm-ø. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0øW) and away from (180øW) Jupiter and minima near the center of the downstream (90øW) and upstream (270øW) hemispheres. This pattern m•y be related to the Alfv•nic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from-•200 km near the center of the upstream hemisphere to-•400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202 4-14 K if Na + is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location. 1. Introduction Studies of Io are difficult to confine within the boundaries of traditional scientific disciplines. Interior dynamics produce active and widespread volcanism, which profoundly changes the nature of its atmosphere. Gases escaping from the atmosphere populate a plasma torus, which in many respects determines the character of
Journal of Geophysical Research, 1998
Space, 2002
International collaboration between the National Aeronautics and Space Administration (NASA) and ... more International collaboration between the National Aeronautics and Space Administration (NASA) and the Italian Space Agency (ASI) resulted in the development of spacecraft and ground instrumentation to enable a set of Radio Science experiments on the Cassini mission that include a search for gravitational waves, a solar conjunction experiment, atmospheric occultations and gravity field measurements of Saturn and several of its satellites. Key to these experiments is the addition of a Ka-band translator to the Cassini payload to augment the telecommunications system at X-band and the Radio Science downlink at S-band, the most sophisticated radio system in deep space to date. The Deep Space Network instrumented one 34-meter diameter beam-waveguide station in California with systems capable of simultaneous transmission and reception at X-and Ka-bands along with special antenna pointing techniques. Upgraded high-stability digital open-loop receivers augmented the tracking receivers. In order to fully take advantage of the superior stability radio link at Ka-band where the effect of interplanetary plasma noise is less than the effect at X-band, modifications were made to the frequency and timing subsystem and advanced water vapor radiometers were developed and installed as part of a media calibration system to calibrate the tropospheric effect. A multi-frequency scheme can be used to model and calibrate the intervening media of charged particles. The first formal operational use of this new Radio Science system took place between November 2001 and January 2002 where the Cassini spacecraft was in solar opposition and a 40-day search for gravitational waves was conducted. Two additional opportunities of such searches are planned at oppositions to be followed six months later by solar conjunction experiments measuring the relativistic deflection of the radio beams as they pass near the sun. This paper describes the new DSN science capability and highlights of the engineering work that lead to its development. It will also discuss experience with operations along with statistics and data quality.
Aerospace …, 2003
... data from the DSN stations to JPL 5. Transfer the RSR data from the DSN stations to JPL 6. Pr... more ... data from the DSN stations to JPL 5. Transfer the RSR data from the DSN stations to JPL 6. Process the raw Media Calibration System ... and down-link capability, where the Open Loop receivers acquired and sampled the down-link carrier in three bands (X/X, X/Ka and KaKa). ...