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Aseel Anabtawi

Aseel Anabtawi
NASA · Jet Propulsion Laboratory

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62
Publications
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Publications

Publications (62)
Article
We present the results of the remaining ten soundings of Titan's atmosphere by radio occultations using the Cassini spacecraft that have not been previously reported. Three were in 2008 and 2009, and used the Ultra Stable Oscillator onboard the spacecraft. The rest were in 2014 and 2016 after the USO had failed and were executed in two-way mode. Co...
Article
Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et...
Presentation
During six of the 22 Grand Finale orbits, Cassini acquired Doppler data for the determination of the gravity field of the gas giant. In addition, the unique geometry of the last phase of the mission, when the spacecraft flew between the planet and its rings, broke the correlation between the weak gravitational signal from the rings and Saturn’s eve...
Article
The Ultra Stable Oscillator aboard the Cassini spacecraft failed in late 2011, which means that all radio occultations after that date have to be done in two way mode, using a ground based signal transmitted to the spacecraft as the frequency reference. Here we present the numerical technique we use to analyze the data from the two way atmospheric...
Article
The Cassini spacecraft has accumulated over 60 individual radio occultation measurements of the vertical electron density profile in the ionosphere of Saturn from 2005 to 2013. These measurements extended from the Cassini Prime Mission from 2005 t0 2008, the Equinox Mission (2009-2010), and the present Solstice Mission (2011-2013). The rings of Sat...
Article
The results of the Cassini radio occultation measurements of the electron density profiles in the Saturn ionosphere during the Cassini Prime mission (2005-2008) have been presented previously( Nagy, A.F., et al., JGR, 2006; Kliore, A.J., et al., JGR, 2009). These measurements covered mostly low- and mid-latitudes, with some high-latitude measuremen...
Article
We present the results of six soundings of the atmosphere of Titan by the radio occultation technique using the Cassini spacecraft currently in orbit around Saturn. These occultations occurred during four separate targeted Titan encounters in both the Prime and Equinox missions of Cassini over 3 years. They cover a wide range of latitude from 75°S...
Article
Likely evidence of nearly six centuries old collision of captured cometary material with Saturn's Ring C is uncovered in recent Cassini Radio Science ring observations. Three Cassini ring occultation experiments were especially designed so that radio signals transmitted by Cassini to the Earth pass through the rings when the rings are nearly closed...
Article
We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34°S near the surface, and the fourth at 52.8°S. Temperature-altitude profiles for all four occultation soundings are presented, and...
Article
Full-text available
Quasi-periodic ring structure of mean wavelength ? ? 1.3 km is uncovered in Cassini radio occultation optical depth (? ) profiles of the innermost region of Ring C (∼74,480-77,740 km). The structure is characterized by two interfering "tones" separated in ? by few 100 meters. The mean ? increases slowly with ring radius. The observed behavior is co...
Article
A series of near-equatorial radio occultations of Cassini by Saturn occurred in 2005 and again in 2009-2010. Comparison of the temperature-pressure profiles obtained from the two sets of occultations shows evidence of a descending pattern in the stratosphere that is similar to those associated with equatorial oscillations in Earth's middle atmosphe...
Article
The Cassini orbiter spacecraft flew behind the Enceladus plume, as observed from the Earth, on 26 January, 2010. At that time, Cassini was abut 577000 km behind Enceladus, and the radio line of sight passed through the plume about 52 km from the South pole. The occultation was observed by two Deep Space Net tracking stations near Canberra, Australi...
Article
Seven Cassini radio occultations have provided 13 electron density profiles in the Titan iono-sphere. The first four were conducted in 2006(T12 and T14), and 2007(T27 and T31).The results are described in: Kliore A.J., et al.,JGR (2008). The 2006 observations probed the Titan ionosphere at low-to mid-S. latitudes, while the 2007 measurements were a...
Article
Nineteen new radio occultations of the ionosphere of Saturn have been obtained since 2006. Sixteen of these occultations were from midlatitude and high latitudes and thus provided important, new information of the ionosphere for these regions. A high degree of variability in the electron densities were observed, but grouping and averaging the obser...
Article
Previous Cassini radio occultations, which provided measurements of the vertical electron density profiles in the Titan ionosphere, were conducted on March 26 and May 28, 2006, and March 19 and May 20, 2007 ( Kliore,A.J., et al. (2008), J. Geophys. Res., 113, A09317, doi:10.1029/2007JA012965)The 3006 observations probed the Titan ionosphere at low-...
Article
Four out of six Radio Science bistatic scattering (bistatic-radar) observations of Titan's surface completed during the Cassini nominal mission yielded detectable quasi-specular 3.6 cm-lambda (X-band) surface echoes, making Titan the most distant solar system object for which bistatic echoes have been successfully detected. Right circularly polariz...
Article
Twenty-four one-sided Cassini radio occultation observations have been successfully completed. Two types of observations provide information about physical ring properties. The first is differential extinction of three sinusoidal signals simultaneously transmitted through the rings (0.94, 3.6, and 13 cm-wavelength). The observations reveal remarkab...
Article
The Cassini spacecraft performed a series of radio occultation observations of the near-equatorial ionosphere of Saturn during orbits 7 to 14 in 2005 ( Nagy A.F., et al., J. Geophys. Res..111, 2006). They provided 12 vertical profiles of electron density at latitudes between 10oN to 10oS, and solar zenith angles ranging from 84 to 96 ° The results...
Article
Since the 2006 COSPAR meeting, , Saturn radio occultation observations have been conducted with the Ccassini spacecraft on orbits 28(5oS, 56oS), 44 (75oN, 5oN), 46 (5oS ), 47 (51oS ), 51 ( 41oS, 39oS ), 54 (18oS, 68oS ), 56 (18oS, 70oS ), 58(75oS), 68(22oN,58oS), and 70(37oN,68oS). These occultations provided data on the structure of the ionosphere...
Article
Eight vertical profiles of the electron density in Titan's ionosphere were derived from the Cassini radio occultations of March 26, and May28, 2007 (T27 and T31), as well as those of March 19, 2006 (T12), and May 20,2006 (T14) . The 2006 occultations occurred at low Southern latitudes of 14.7S, 36.2S, 19.8S, and 21.9S. The 2007 occultations were ne...
Article
Cassini radio occultation observations of the extinction and near-forward scattering of 0.94, 3.6, and 13 cm- wavelengths sinusoidal signals have shed much new light on the structure and physical properties of Saturn's main ring system, especially enigmatic Ring B. As of June 2007, the occultations covered 15 distinct ring longitudes and roughly tw...
Article
Eight vertical profiles of the electron density in Titan's ionosphere were derived from the Cassini radio occultations of March 26, and May28, 2007 (T27 and T31), as well as those of March 19, 2006 (T12), and May 20,2006 (T14) . The 2006 occultations occurred at low Southern latitudes of 14.7S, 36.2S, 19.8S, and 21.9S. The 2007 occultations were ne...
Article
Radio occultations of Saturn's rings during the Cassini prime mission fall into three main groups, depending on the rings opening angle B. The first is a set of eight diametric occultations completed early in the mission (March-September/2005) when |B| was relatively large (19.5 to 23.5°). They permitted multiple-longitude profiling of relatively o...
Article
Gravity results are available from radio Doppler data acquired by the Deep Space Network during the encounter of the Cassini spacecraft with Enceladus in February 2005. We report the mass of Enceladus to be (1.0798±0.0016)×1020 kg, which implies a density of . For a core made of hydrated silicates with a density of 2500 kg m−3 the core radius is ∼1...
Article
A series of eight Cassini radio occultation observations were completed from May 3 to September 5, 2005. Four were ingress-egress occultations and four were either ingress or egress occultation, for a total of 12 full cuts of the ring system. Diffraction reconstructed optical depth profiles reveal significant wealth of information about ring struct...
Article
We report results on Titan's ionosphere from the Cassini radio occultation of March 26, 2007 (T27), as well as those of March 19, 2006 (T12), and May 20,2006 (T14) . The 2006 occultations occurred at low Southern latitudes of 14.7S, 36.2S, 19.8S, and 21.9S. The 2007 occultation was nearly polar, at latitudes of 81S and 59N. The solar zenith angles...
Article
A fortuitous Cassini radio occultation by Enceladus plume occurs on September 15, 2006. The occultation track (the spacecraft trajectory in the plane of the sky as viewed from the Earth) has been designed to pass behind the plume (to pass above the south polar region of Enceladus) in a roughly symmetrical geometry centered on a minimum altitude abo...
Article
Cassini conducted the first two Radio Science bistatic scattering observations of Titan's surface on March 18 (T12) and May 20 (T14), 2006. The experiment was designed to search for mirror-like (quasi-specular) reflections from relatively flat surface regions. Three sinusoidal signals (0.94, 3.6, and 13 cm-wavelength; Ka-, X-, and S-band) transmitt...
Article
The dynamic interplay of gravitational and collisional forces lead to the formation of gravitational wakes in Saturn's rings. Wakes, which were first studied on larger scales as they exist in galactic discs, manifest themselves as elongated clusters of ring particles. Dynamical studies have predicted the formation of wake structures in Saturn's A a...
Article
We report results on Titan s ionosphere from the first two Titan radio occultations of the Cassini spacecraft on March 19 and May 20 2006 The latitudes of the occultations were 29 S on entry and 49 S on exit for the first one and 32 S and 34 S for the second one The Cassini radio science system is unprecedented in having three frequencies that can...
Article
The first two Cassini radio occultations of Titan's atmosphere occurred on March 18 (T12) and May 20 (T14), 2006. The atmosphere was probed on the ingress and egress sides, yielding observations at four mid-southern latitudes. Titan's surface was also probed using bistatic-scattering during the inbound period on T12 and the inbound and outbound per...
Article
Much new knowledge about Ring B has been acquired by a series of eight Cassini radio occultations completed from May 3 to September 5, 2005. The optimized occultation geometry and the relatively large ring opening angle (19 to 23.6 degrees) allowed detailed structure profiling, revealing remarkable variability with radius. Four regions of distinct...
Conference Paper
The Huygens probe was released by the Cassini spacecraft and landed onto the surface of Saturn's moon Titan. During the descent on 14 January 2005, radio signals were transmitted from the probe to the Cassini orbiter. One radio link, referenced to an ultra-stable oscillator was the prime link to conduct a Doppler experiment for the purpose of deriv...
Article
The first set of near-equatorial occultations of the Saturn ionosphere was obtained by the Cassini spacecraft between May and September of 2005. The occultations occurred at near-equatorial latitudes, between 10°N and 10°S, at solar zenith angles from about 84° to 96°. The entry observations correspond to dusk conditions and the exit ones to dawn....
Article
Eight completed Cassini radio occultation observations of Saturn s rings have yielded high spatial resolution 1 km X-band 3 6 cm-wavelength optical depth profiles at twelve distinct ring longitudes The profiles provide a rich resource of information about radial ring structure and its azimuthal variability Additional acquired Ka- and S-band 0 94 an...
Article
The Cassini orbiter has provided 12 opportunities for three-frequency radio occultations of the rings, atmosphere, and ionosphere of Saturn between May and September of 2005. By virtue of being optimized for the ring observations, these occultations provided measurements of the Saturnian ionosphere in a range of latitudes from about 7 deg. S to abo...
Article
We present results from 12 ingress and egress soundings done within 10 degrees of Saturn's equator. Above the 100-mbar level, near the tropopause, the vertical profiles of temperature are marked by undulatory structure that may be associated with vertically propagating waves. We determine the properties and spectra of these waves, and speculate on...
Article
Cassini orbits around Saturn were designed to provide eight optimized radio occultation observations of Saturn's rings during summer, 2005. Three monochromatic radio signals (0.94, 3.6, and 13 cm-wavelength) were transmitted by Cassini through the rings and observed at multiple stations of the NASA Deep Space Network. A rich data set has been colle...
Article
On May 3, 2005, the first of a series of eight nearly diametric occultations by Saturn's rings and atmosphere took place, observed by the Cassini Radio Science (RSS) team. Simultaneous high SNR measurements at the Deep Space Network (DSN) at S, X, and Ka bands (lambda = 13, 3.6, and 0.9 cm) have provided a remarkably detailed look at the radial str...
Article
Eight Cassini radio occultations of Saturn's rings were conducted from May 3 to September 5, 2005. During any given occultation, Cassini transmits Ka-, X-, and S-band sinusoidal signals (0.94, 3.6, and 13 cm-wavelength) through the rings. Spectral analysis of the perturbed signals received at stations of the Deep Space Network (DSN) reveals two dis...
Article
On May 3, 2005 occurred the first of a series of seven occultations of Cassini by Saturn which have taken or will take place during the spring and summer of 2005. These near diametric occultations cover a latitude range of between 8 N and 10 S planetocentric latitude, and will give us a detailed look at the structure of the low latitude neutral atm...
Article
We will present the determination of the masses of Phoebe, Iapetus, Dione, and Enceladus. The mass and the volume yield the density, a crucial quantity in the modeling of the evolution of the Saturn system. Cassini gravity experiments were designed to benefit from two new technologies. First, Ka-band and multi-frequency radio links allow for a red...
Article
The first Cassini radio occultation observations of the atmosphere and ionosphere of Saturn were made on May 3rd, 2005. The path of Cassini as seen from Earth (the occultation track), which had been designed to cross Saturn's rings almost diametrically, has also provided two near-equatorial occultations of the atmosphere and ionosphere at longitude...
Article
We present preliminary results expected from the first planned Cassini radio occultation observation of Saturn's rings, to be conducted on May 3rd, 2005. The path of Cassini as seen from Earth (the occultation track) has been designed to cross the rings from the west to the east ansa almost diametrically, allowing for occultation of all major ring...
Article
We present results from 12 ingress and egress soundings done within 10 degrees of Saturn's equator. Above the 100-mbar level, near the tropopause, the vertical profiles of temperature are marked by undulatory structure that may be associated with vertically propagating waves. Below the 200-mbar level, in the upper troposphere, the vertical profiles...
Article
The first set of near-equatorial occultations of the Saturn ionosphere was obtained by the Cassini spacecraft earlier this year. The occultations occurred at low Southern latitudes (< 10 deg.) and covered solar zenith angles from about 84o to 97o. The exit observations correspond to dusk and the entry ones to dawn. An initial look at the data appea...
Article
The results from the first Cassini Radio Science Subsystem(RSS) occultation, which occurred at the Rev 7 periapse, are being used to derive profiles of the atmospheric constituents encountered by the three frequency (S-, X-, and Ka-band) radio link. A computer model has been developed to simulate ray paths and the ray path parameters in the atmosph...
Article
A close Galileo flyby of Jupiter's inner moon Amalthea (JV) occurred on 5 November 2002. The final aimpoint was selected by the Galileo Radio Science Team on 5 July 2002. The closest approach distance for the selected aimpoint was 221 km from the center of mass, the latitude was - 45.23 Deg and the west longitude was 266.41 Deg (IAU/IAG/COSPAR cart...
Article
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 opportunitie...
Article
Between 1997 and 1999, the S-band (2.5 GHz)radio signal of the Galileo orbiter was used to obtain radio occultation data on the ionospheres and plasma environments of Io, Europa, Ganymede, and Callisto. These data revealed the existence of ionospheres on Europa(Kliore,et al.,Science,277,355(1997)), Callisto(Kliore,et al.,JGR-Sp.Phys.,in press), and...
Article
During the last close flyby of Io on 17 January 2002, the Galileo spacecraft automatically shut down the science sequence in response to the detection of a possible fault. However, because shutting down the telecommunications system could be suicidal, Doppler tracking with the S-band radio carrier wave proceeded as scheduled. As a result, coherent...
Article
The U.S. Galileo spacecraft, which has been in orbit around Jupiter since December, 1995, has provided opportunities to collect s-band radio occultation data using the 70 meter antennas of the NASA/JPL Deep Space Net(DSN) at Goldstone, California, Madrid, Spain, and Canberra, Australia. To date, four occultations and one near-occultation by Europa...
Article
The U.S. Galileo spacecraft, which has been in orbit around Jupiter since December, 1995, has provided opportunities to collect s-band radio occultation data using the 70 meter antennas of the NASA/JPL Deep Space Net(DSN) at Goldstone, California, Madrid, Spain, and Canberra, Australia. Ganymede (J3) has been observed five times by Galileo radio oc...
Article
The Galileo Europa Mission (GEM) provided three occultations by Callisto, in addition to the single occultation during the prime mission, which occurred on orbit 9 (C9, June 25, 1997). The new occultations occurred on C20 (May 5, 1999), C22 (Aug. 14, 1999), and C23 (Sept 16, 1999). The C9 occultation observations, which occurred very close to the e...
Article
The Galileo orbiter has provided radio occultation measurements of the electron density profiles of the plasma surrounding Io and Europa. There have been six occultations of Io, providing twelve electron density profiles at various locations relative to the ram direction of the impinging particles of the Jupiter magnetosphere on Io, and eight profi...
Article
The Galileo orbiter has provided radio occultation measurements of the electron density profiles of the plasma surrounding lo and Europa. There have been six occultations of Io, providing twelve electron density profiles at various locations relative to the ram direction of the impinging particles of the Jupiter magnetosphere on Io, and eight profi...
Article
The Galileo orbiter has provided radio occultation measurements of the electron density profiles of the plasma surrounding Io and Europa. There have been six occultations of Io, providing twelve electron density profiles at various locations relative to the ram direction of the impinging particles of the Jupiter magnetosphere on Io, and eight profi...
Conference Paper
The residual-carrier tracking performance for PCM/PSK/PM/NRZ for square-wave and sine-wave subcarriers is evaluated as a function of the symbol SNR degradation. Explicitly, the RF residual carrier tracking performance is defined in terms of the subcarrier-to-symbol rate ratio, n. The upper and lower bound values for n are derived, and used to deter...
Article
The Cassini spacecraft, which has been in orbit about Saturn for over a year, is the first Radio Science platform to provide three downlink frequencies. In addition to the X-band telemetry link (3,56 cm w.l.), two other frequencies, S-band (13.04 cm), and Ka-band (0.94 cm) are available. This, plus the high SNR (>50 dBHz at X-band) afforded by the...
Article
As a world-class instrument for science research, the Deep Space Network is credited with many discoveries in planetary science reached via data acquired by the Radio Science Receiver. Various generations of this receiver (independent of the tracking receiver used for all mission support) have existed in the Network for about three decades, under v...

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