Engineering topics

Edenhofer, P.

Publications and source records attributed to Edenhofer, P..

At least 19 records

NASA NTRS2005

The Cassini/Huygens Doppler Wind Experiment: Results from the Titan Descent

The primary objective of the Doppler Wind Experiment (DWE), one of the six scientific investigations comprising the payload of the ESA Huygens Probe, is a determination of the wind velocity in Titan's atmosphere. Measurements of the Doppler shift of the S-band (2040 MHz) carrier signal to the Cassini Orbiter and to Earth were recorded during the Probe descent in order to deduce wind-induced motion of the Probe to an accuracy better than 1 m s-1. An experiment with the same scientific goal was performed with the Galileo Probe at Jupiter. Analogous to the Galileo experience, it was anticipated that the frequency of the Huygens radio signal could be measured on Earth to obtain an additional component of the horizontal winds. Specific secondary science objectives of DWE include measurements of: (a) Doppler fluctuations to determine the turbulence spectrum and possible wave activity in the Titan atmosphere; (b) Doppler and signal level modulation to monitor Probe descent dynamics (e.g., spinrate/spinphase, parachute swing); (c) Probe coordinates and orientation during descent and after impact on Titan.

Bird, M. K.↗

NASA NTRS1997

Rubidium Ultra-Stable Oscillators at Titan: The Huygens Doppler Wind Experiment

Rubidium↗

NASA NTRS1997

Rubidium Ultra-Stable Oscillators at Titan: The Huygens Doppler Wind Experiment

The Doppler Wind Experiment (DWE) is one of six investigations to be performed during the Titan atmospheric descent of the ESA Huygens Probe. The primary scientific objective is to measure the direction and strength of Titan's zonal winds with an accuracy better than 1 m/s. The Probe's wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects, from the beginning of the mission (altitude: approx. 160 km) down to impact on the surface. The DWE Instrumentation consists of Rb-based Ultra-Stable Oscillators used to: (1) generate the transmitted signal from the Probe and (2) extract the frequency of the received signal on the Orbiter. The capabilities of these USOs under the rugged experimental conditions on Titan and some results from the DWE pre-launch test program are described.

Bird, M. K.↗

NASA NTRS1997

Analysis of Galileo Doppler Measurements During the Solar Occulations in 1994 and 1995

Measurements of S-band downlink frequency (Doppler) shift were collected for intervals of about 30 days during the 1994 and 1995 solar conjunctions of the Galileo spacecraft. The occulation geometries enabled coronal radio sounding of the heliographic equatoral region over a heliocentric range from about 5 to 20 R(sub s) (solar radii) for the 1994 conjuctions and from about 4 to R(sub s) for the 1995 conjunction.

Galileo Doppler Solar Occulations↗

NASA NTRS1995

Radio sounding of the solar corona during 1995 solar conjunction of the Ulysses spacecraft

The Ulysses spacecraft will pass through superior solar conjunction on March 5 1995, a few days before its perihelion and passage through the ecliptic plane. Dual-frequency S/X-band ranging and Doppler observations will be conducted in support of the Ulysses Solar Corona Experiment (SCE) during a three-week interval centered on the conjunction. The occultation geometry is unique in the annals of interplanetary exploration. As viewed from Earth, the spacecraft will appear to cut diagonally through the southwest quadrant of the solar corona from the South Pole to the equator. The minimum proximate distance to the Sun of the radio ray path will be 21.6 solar radius. The entire latitude scan from pole to equator occurs for a limited range of solar offset distances (is less than 30 solar radius thus facilitating the separation of latitudinal from radial variations in the coronal density and associated parameters of interest.

Bird, M. K.↗

NASA NTRS1994

The coronal electron density distribution determined from dual-frequency ranging measurements during the 1991 solar conjunction of the Ulysses spacecraft

Dual-frequency ranging and Doppler measurements were conducted in support of the Ulysses Solar Corona Experiment (SCE) at and around the spacecraft's first solar conjunction in 1991 August. The differential group delay time between range codes on the two downlink carrier signals at the wavelengths 13.1 and 3.6 cm, a direct measure of the total electron content between spacecraft and ground station, was used to derive the electron density distribution in the solar corona. Linear power-law representations of the coronal electron density were derived for the range of solar distances from 4 solar radii to 40 solar radii on both sides of the Sun. The corona was found to be very nearly symmetric; the radial falloff exponent being 2.54 +/- 0.05 for occultation ingress (east solar limb) and 2.42 +/- 0.05 for egress (west limb), respectively. The departure of these exponents from the inverse equare relation implies that significant solar wind acceleration is occurring within the radial range of the observations. The electron density level was found to be considerably lower than that observed during the 1988 December solar occultation of Voyager 2. Although the smoothed sunspot number R(sub z) (a standard indicator of solar activity) was almost the same in 1988 December and 1991 August, the mean electron density at 20 solar radii was found to be 1.7 +/- 0.1 x 10(exp 3)/cu cm during the Ulysses conjunction, a decline by almost a factor of 4 from the value obtained during the Voyager conjunction.

Bird, M. K.↗

NASA NTRS1992

Ulysses radio occultation observations of the Io plasma torus during the Jupiter encounter

Radio signals from Ulysses were used to probe the Io plasma torus (IPT) shortly after the spacecraft's closest approach to Jupiter. The frequencies of the two downlinks at S-band (2.3 gigahertz) and X-band (8.4 gigahertz) were recorded, differenced, and integrated in order to derive the columnar electron density of the IPT. The measurements agree qualitatively with contemporary models of the IPT based on Voyager data, but significant differences are apparent as well. The overall level of the IPT electron density is approximately the same as the prediction, implying that the amount of gas (or plasma) injected from Io is similar to that observed during the Voyager era. On the other hand, the IPT seems to be less extended out of the centrifugal equator, implying a smaller plasma temperature than predicted.

Bird, M. K.↗

NASA NTRS1992

The coronal-sounding experiment

The main science objective of the Ulysses Solar Corona Experiment is to derive the plasma parameters of the solar atmosphere using established coronal-sounding techniques. Applying appropriate model assumptions, the 3D electron density distribution will be determined from dual-frequency ranging and Doppler measurements recorded at the NASA Deep Space Network during the solar conjunctions. Multi-station observations will be used to derive the plasma bulk velocity at solar distances where the solar wind is expected to undergo its greatest acceleration. As a secondary objective profiting from the favorable geometry during Jupiter encounter, radio-sounding measurements will yield a unique cross-scan of the electron density in the Io Plasma Torus.

Bird, M. K.↗

NASA NTRS1992

Coronal sounding with Ulysses - Preliminary results from the first solar conjunction

Radio-sounding observations of the solar corona between 4 and 115 solar radii were performed during the first superior solar conjunction phase of the Ulysses spacecraft in August/September 1991. As a first result of this Solar Corona Experiment, the total electron content inferred from dual-frequency ranging observations is presented here as a function of solar distance.

Paetzold, M.↗

NASA NTRS1987

Measurement technique of the Giotto radio science experiment

The paper describes the technique used to record time delay and waveform measurements for the Giotto radio science experiment of ESA's mission to comet Halley. The data were taken by using either two-way measurements (during pre- and post-encounter) or one-way measurements (during encounter with comet Halley), the downlink of the radio signal of the Giotto spacecraft being received at 8.4 GHz by the 64 m tracking stations of NASA's Deep Space Network (DSN). The waveform measurements were obtained at a sampling frequency of 50 kHz with an open-loop receiver assembly at DSN station Canberra as recently used for the Voyager/Uranus fly-by. Performance and calibration data are given as relevant to the radio subsystems on the ground and aboard Giotto.

Edenhofer, P.↗

NASA NTRS1986

Dust distribution of Comet Halley from the Giotto radio science experiment

Measurements from the Giotto radio science experiment representing the Doppler frequency shift and the intensity level of the X-band downlink signal of the Giotto spacecraft during its encounter with comet Halley are shown. Continuous data reception was maintained throughout the encounter. The Doppler shift measured within a time interval of 100 sec is due to drag effects in the cometary atmosphere causing a deceleration of the spacecraft. The total change of velocity of Giotto is 23.2 cm/sec resulting in a best estimate of 0.32 g for the total cometary mass impacting Giotto during the Halley flyby. A dust jet structure for the inner coma of comet Halley is derived. Several sharply confined dust jets are distinguishable; for the most prominent one, characteristic properties are deduced. Features of this jet structure are shown to correlate with measurements from on-board Giotto experiments and with Earth-based observations.

Edenhofer, P.↗

NASA NTRS1986

First results from the Giotto radio-science experiment

A definite deceleration of the comet probe Giotto due to drag in the comet Halley atmosphere has been noted by means of Doppler and ranging measurements based on the radio signals of the Giotto spacecraft. The total radial velocity change is 16.7 cm/s over a 100-s interval; this corresponds to a Doppler frequency of 4.7 Hz. This velocity change is used to estimate a total cometary mass striking the spacecraft of between 0.1 and 1.0 g.

Edenhofer, P.↗

NASA NTRS1986

The Giotto radio-science experiment

The scientific objectives of the Giotto Radio Science Experiment (GRE) are to determine the columnar electron content of Comet Halley/s ionosphere and the cometary mass fluence from atmospheric drag by using the radio signals from Giotto during the Halley encounter. The radio science data (S and X-band Doppler and range measurements) will be collected at NASA/s deep-space 64 m tracking antenna at Tidbinbilla near Canberra, in Australia. In order to separate the effects of the terrestrial ionosphere and the interplanetary plasma, S-band Doppler measurements will also be taken at Tidbinbilla along the line-of-sight of Japan/s cometary probe Sakigake during the Giotto-Halley Encounter. The measurements of cometary electron content and mass fluence will be inverted to derive the spatial distribution of the electron and mass (dust and gas) density within Halley/s coma. The GRE is the only experiment on Giotto capable of measuring the low-energy (10 eV) electron bulk population of Halley/s ionosphere and the total cometary mass flow impacting upon the spacecraft.

Edenhofer, P.↗

NASA NTRS1985

Preliminary results of the Giotto radio science experiment

Doppler and ranging measurements using the radio signal of the Giotto spacecraft were taken before, during, and after the encounter with Comet Halley on Mar. 13, 14, 1986. The spacecraft velocity was found to decrease by a total of 23.3 cm/s due to impacting gas and (primarily) dust in the cometary atmosphere. A preliminary dust production rate of 1000 kg/s is found to be consistent with this deceleration. Power spectra of the carrier phase fluctuations reveal an increase in level and a flattening of the spectrum just prior to encounter, presumably associated with the enhanced dust impact rate. Finally, simulated Doppler time profiles are computed using the radial dependence of plasma density observed by the Giotto in situ investigations. It is shown that the cometary electron content profile would have been clearly seen if a dual-frequency downlink radio configuration had been available at encounter.

Edenhofer, P.↗

NASA NTRS1982

Possible evidence for coronal Alfven waves

A statistical ray analysis is used to analyze observed electron content and Faraday rotation fluctuations in the 2.29 GHz S band carrier signals of the two Helios spacecraft probing the magnetic and density structures of the solar corona inside 0.05 AU. It is found that (1) the observed Faraday rotation fluctuations cannot be due only to electron density fluctuations in the corona, unless the coronal magnetic field is about five times stronger than suggested by current estimates; and (2) the observed Faraday rotation fluctuations are consistent with the hypothesis that the sun radiates Alfven waves whose energies are great enough to heat and accelerate high-speed solar wind streams.

Hollweg, J. V.↗

NASA NTRS1980

Solar corona electron density distribution

The paper discusses the three and one-half months of single-frequency time delay data which were acquired from the Helios 2 spacecraft around the time of its solar occultation. The excess time delay due to integrated effect of free electrons along the signal's ray path could be separated and modeled following the determination of the spacecraft trajectory. An average solar corona and equatorial electron density profile during solar minimum were deduced from the time delay measurements acquired within 5-60 solar radii of the sun. As a point of reference at 10 solar radii from the sun, an average electron density was 4500 el/cu cm. However, an asymmetry was found in the electron density as the ray path moved from the west to east solar limb. This may be related to the fact that during entry into occultation the heliographic latitude of the ray path was about 6 deg, while during exit it was 7 deg. The Helios density model is compared with similar models deduced from different experimental techniques.

Esposito, P. B.↗

NASA NTRS1980

Hydromagnetic wavelike phenomena from Helios time delay measurements by remote sensing

A survey of the electron content measurements during solar occultations of the Helios A and B spacecraft is presented, and a spectral analysis using the method of maximum entropy is discussed. Typical variations measured are on the order of 0.1-1.8 x 10 to the 18th/sq m, while typical values for the rate of change are 0.7-50 x 10 to the 13th per sq m per sec. Numerical results in agreement with findings from Helios radio science, reveal a fundamental period of about 70 minutes superimposed by minor spectral peaks corresponding to shorter time periods such as 35 and 25 minutes. In addition, the periodicities observed in electron content are discussed in terms of fast hydromagnetic waves excited by nonlinear Alfven waves via coupling terms before crossing the Helios ray path. It is noted that for the first time experimental evidence is presented that hydromagnetic waves may actually be propagating from the solar corona into the interplanetary medium.

Edenhofer, P.↗

NASA NTRS1978

Time delay occultation data of the Helios spacecraft for probing the electron density distribution in the solar corona

S-band time delay measurements were collected from the spacecraft Helios A and B during three solar occultations in 1975/76 within heliocentric distances of about 3 and 215 earth radius in terms of range, Doppler frequency shift, and electron content. Characteristic features of measurement and data processing are described. Typical data sets are discussed to probe the electron density distribution near the sun (west and east limb as well) including the outer and extended corona. Steady-state and dynamical aspects of the solar corona are presented and compared with earth-bound-K-coronagraph measurements. Using a weighted least squares estimation, parameters of an average coronal electron density profile are derived in a preliminary analysis to yield electron densities at r = 3, 65, 215 earth radius. Transient phenomena are discussed and a velocity of propagation v is nearly equal to 900 km/s is determined for plasma ejecta from a solar flare observed during an extraordinary set of Helios B electron content measurements.

Edenhofer, P.↗