These two benchmarks required the development of a virtual prototype and a hardware prototype, respectively, of a Synthetic Aperture Radar processor. The two RASSP Developers chose different approaches: one used COTS components on custom boards with a methodology emphasis on detailed VHDL prototyping and board design and one used COTS computer boards with a methodology emphasis on efficient VHDL modeling and automatic code generation. Both efforts are briefly described. A preliminary assessment of Benchmark-1, which has been completed, is offered with emphasis on the experience with VHDL modeling. Based on this assessment, some recommendations for improvement are made.

Radar images of Venus with best resolution near 1.3 km have been formed using the Goldstone Deep Space Stations as a ground‐based synthetic aperture radar system. Twelve days of observations were successful during the 1986 inferior conjunction, and we have selected three observations for initial processing to demonstrate the new capability. The three images show several remarkable surface features including regions of high Fresnel reflectivity in the plains region.

view Abstract Citations (117) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS JPL Pulsar Timing Observations. V. Macro- and Microjumps in the VELA Pulsar 0833-45 Cordes, J. M. ; Downs, G. S. ; Krause-Polstorff, J. Abstract In this paper, 14.5 yr of JPL timing data are analyzed to study the spindown of the Vela pulsar. Discontinuities in the spin frequency and its derivatives have bimodal amplitude distributions. Macrojumps are characterized by relative changes in spin rate of about 10 to the -6th with a derivative of 0.01, while microjumps have relative changes in spin rate of less than about 10 to the -9th with derivative of less than about 0.0001. Macrojumps display a (+,-) signature with a well-defined ratio of the amplitudes. Microjumps, however, show all possible signs of events and a larger range in signature. Six macrojumps and their associated transients are quantified using a 10-parameter model for each jump. Amplitude and decay time parameters vary by a factor of about five over the set of jumps. The results are discussed in terms of superfluid vortex models. The macrojumps are consistent with a model in which angular momentum is suddenly transferred to the crust from a more rapidly rotating superfluid core, followed by relaxation of the spin-down rate. Publication: The Astrophysical Journal Pub Date: July 1988 DOI: 10.1086/166518 Bibcode: 1988ApJ...330..847C Keywords: Neutron Stars; Pulsars; Stellar Rotation; Astrometry; Random Walk; Spin; Stellar Interiors; Superfluidity; Time Dependence; Vortices; Astrophysics; PULSARS; STARS: NEUTRON full text sources ADS | data products SIMBAD (1) Related Materials (4) Part 1: 1981ApJ...249..687D Part 2: 1983ApJS...53..169D Part 3: 1985ApJS...59..343C Part 4: 1986ApJS...62...81D

Mars topography in the 23 to −22° latitude band has been measured by the Goldstone λ13-cm radar during the 1971–1982 oppositions. A total of 108 scans were produced. Each scan spans several tens of degrees in longitude. The longest scan spans 133° (∼8000 The vertical resolution generally is better than 200 m. With some exceptions, the areal resolution is 0.16° (∼8 km) in longitude and 1.30° (∼80 km) in latitude. On regional scales, elevation differences of as much as 3 km over less than 10° longitude have been observed (Aeolis, Memnonia). Most local relief is provided by structural elements of impact craters. One hundred fifty-two complex, degraded craters, some as small as 25 km in diameter, have been identified in this data set. Repeated scans of the same craters yield nearly identical elevation profiles. Measured crater depths rarely exceed 2.5 km. Depths of the most degraded craters in the sample are a weak function of crater diameter. Depths of the largest craters D>125km show no functional relationship to diameters. Measured rim heights rarely exceed several hundred meters. Structures with no discernible rim (e.g., Ladon) have retained a basin shape. Low exterior relief rather than overall low relief seems to be the distinguishing characteristic of large, degraded craters included in the Goldstone sample. Craters or basins that serve as sinks for major channel systems (e.g., Gusev, Ladon) appear to be as deep as comparable structures not associated with such systems, suggesting that processes other than fluvial sedimentation contributed to crater shallowing.

view Abstract Citations (22) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS JPL Pulsar Timing Observations. IV. Excess Phase Noise Downs, G. S. ; Krause-Polstorff, J. Abstract Previously published tables of geocentric arrival times for 24 pulsars covering a 12 year span are extended here to 14.5 years. The list of pulsars is extended by nine, most of which were observed for about 4 years. Known positins of these new objects are confirmed, and limits on the proper motions are obtained. Large phase excursions in PSR 0525 + 21 are found. The orbital parameters of the binary pulsar 0820 + 02 are tentatively confirmed. Short-term timing noise in excess of that expected from receiver considerations alone is established. Variations in the timing residuals for the original 24 pulsars are analyzed for correlations with other observable parameters. Little significant correlation with changes in pulse shape or energy or with the drift correction is found on time scales of 500 pulses or longer. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1986 DOI: 10.1086/191134 Bibcode: 1986ApJS...62...81D Keywords: Binary Stars; Periodic Variations; Pulsars; Stellar Activity; Ephemeris Time; Geocentric Coordinates; Radio Astronomy; Tables (Data); Astrophysics; PULSARS; STARS: BINARIES full text sources ADS | data products SIMBAD (33) Related Materials (4) Part 1: 1981ApJ...249..687D Part 2: 1983ApJS...53..169D Part 3: 1985ApJS...59..343C Part 5: 1988ApJ...330..847C

The 1971/1973 Mars data set acquired by the Goldstone Solar System Radar was analyzed. It was established that the seasonal variations in radar reflectivity thought to occur in only one locality on the planet (the Solis Lacus radar anomaly) occur, in fact, over the entire subequatorial belt observed by the Goldstone radar. Since liquid water appears to be the most likely cause of the reflectivity excursions, a permanent, year-round presence of subsurface water (frozen or thawed) in the Martian tropics can be inferred.

Although many radar profiles and images of the areas within 20 deg of Mercury's equator had been obtained from 1971, at both Goldstone and Arecibo radar facilities, surprisingly little geological analysis had been done with these data until recently. Topographic profiles and radar roughness reflectivity images which can be derived from these data will be crucial in completing the geological mapping of Mercury now underway at the U.S. Geological Survey. Earth based radar observations of Mercury and the other terrestrial planets as well are a potentially very valuable tool in the determination of the physical nature of their surfaces. Processing of available radar data must be completed to establish any systematic relationship between raar reflectivities and roughness, density, dielectric constant, and other related geological parameters.

view Abstract Citations (202) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS JPL pulsar timing observations. III. Pulsar rotation fluctuations. Cordes, J. M. ; Downs, G. S. Abstract Pulsar arrival-time data obtained with the NASA Deep Space Network from 1968 to 1982 are subjected to variance and structure function analyses to achieve empirical descriptions of rotational activity for 24 objects. A wide range of activity is seen, including fluctuations caused by identifiable step functions in rotational frequency and frequency derivative in some cases, and by processes with unresolvable events in others. Timing activity is found to be correlated with Pand νbut is uncorrelated with period P. The authors calculate the Allan variance of pulsar rotation frequencies and compare pulsars with terrestrial clocks. Most pulsars are inferior clocks, having a frequency stability worse than 1 part in 1011 on a 1 year time scale. Publication: The Astrophysical Journal Supplement Series Pub Date: November 1985 DOI: 10.1086/191076 Bibcode: 1985ApJS...59..343C Keywords: Pulsars; Stellar Rotation; Angular Velocity; Periodic Variations; Statistical Tests; Stellar Activity; Time Measurement; Astrophysics full text sources ADS | data products SIMBAD (24) Related Materials (4) Part 1: 1981ApJ...249..687D Part 2: 1983ApJS...53..169D Part 4: 1986ApJS...62...81D Part 5: 1988ApJ...330..847C

A pulsar and the earth may be thought of as end masses of a free-mass gravitational wave antenna in which the relative motion of the masses is monitored by observing the Doppler shift of the pulse arrival times. Using timing residuals from PSR 1133 + 16, 1237 + 25, 1604-00, and 2045-16, an upper limit to the spectrum of the isotropic gravitational radiation background has been derived in the frequency band 4 x 10 to the -9th to 10 to the -7th Hz. This limit is found to be S(E) = 10 to the 21st f-cubed ergs/cu cm Hz, where S(E) is the energy density spectrum and f is the frequency in Hz. This would limit the energy density at frequencies below 10 to the -8th Hz to be 0.00014 times the critical density.

Monitoring of the behavior of naturally pulsating galactic radio sources, or pulsars, through regularly spaced measurements of pulse arrival times, has been conducted by several laboratories. A tabular presentation is here made of pulse arrival time measurements from the NASA Deep Space Network between late 1968 and early 1981. By expressing the measurements in ephemeris time, and referring them to the geocenter, usable tables of results have been generated for each pulsar listed in the first of the tables given. The considerations addressed by the tables are: (1) a necessary step in the study of pulsar dynamics is the reduction of topocentric arrival times to the barycenter of the solar system; (2) the tabulated data are accessible to all opinions as to the procedures to be used in interpreting arrival time data; and (3) different observing programs can usually be combined to produce an improvement in the total data set.

view Abstract Citations (71) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Intensity dependence of the pulse profile and polarization of the VELA pulsar. Krishnamohan, S. ; Downs, G. S. Abstract Polarization measurements of the Vela pulsar PSR 0833-45 at 2295 MHz revealed that the radio pulse profile is composed of four components. The four component configuration was found to fit the observed dependence of the pulse profile on the intensity of the pulsar, and the dependence of the modulation index and power spectrum of the intensity fluctuations on the pulse-longitude. Deviations of the observed polarization position angles from those predicted by a rotating vector model were explained by the emission regions for each component being located at different distances from the star. The components are affected by retardation, aberration, and magnetic field curvature effects, all of which contribute to arrival of the components at different longitudes even though the components actually carry the position angle signature predicted by the rotating vector model. Publication: The Astrophysical Journal Pub Date: February 1983 DOI: 10.1086/160682 Bibcode: 1983ApJ...265..372K Keywords: Polarization (Waves); Pulsars; Stellar Radiation; Circular Polarization; Electromagnetic Pulses; Linear Polarization; Modulation; Polarization Characteristics; Power Spectra; Probability Density Functions; Astrophysics full text sources ADS | data products SIMBAD (1)

Earth-based r a d a r measurements o f t h e topography o f Mars have been made over t h e l a s t decade a t t h e J e t P ropu ls ion L a b o r a t o r y ' s Goldstone F a c i l i t y . Using t h e JPL ephemeris DE-114, these measurements have r e c e n t l y been reduced t o t h e e l e v a t i o n datum cor respond ing t o t h e 6.1 mb pressure sur face (1 ) . Here we p resen t a r e v i s e d topograph ic map o f t h e m a r t i a n e q u a t o r i a l reg ions based on these data (F ig .1 ) and p resen t some f e a t u r e s t h a t a r e now apparent . Tha rs i s : E a r l i e r es t ima tes o f t h e T h a r s i s Dome (2,3) gave a mean e l e v a t i o n o f 5-8 km f o r t h i s 4000 km wide topograph ic r i s e . Our data show i t t o be 2-3 km lower , w i t h t h e g r e a t e s t d i s p a r i t y between data s e t s o c c u r i n g a t 1 1 ° ~ , 1 1 0 0 ~ where t h e l a v a p l a i n s sur round ing Ascraeus Mons a re 6 km lower than p r e v i o u s l y be l i eved . A l l f o u r o f t h e l a r g e T h a r s i s s h i e l d volcanoes a r e found t o be on a s l o p i n g p l a i n which d i p s down toward t h e nor thwest ( F i g . 2 ) . A topographic h i g h (5 km above t h e datum) i s assoc ia ted w i t h Cer raun ius Fossae ( 2 0 ° ~ , 102 W1, w h i l e t h e r e i s a l s o evidence f o r a p e r i p h e r a l t r o u g h around Olympus Mons ( 1 ) . Lunae Planum: Rather than be ing a p l a t e a u a t an e l e v a t i o n o f 4-5 km above t h e datum ( 3 ) , Lunae Planum now seems t o have an a l t i t u d e o f 1-2 km above t h e 6.1 mb sur face. I n t e r e s t i n g l y , t h e p l a i n s m a t e r i a l s i n t h i s area a r e e l e v a t e d 1-1.5 km above the ad jacen t h e a v i l y c r a t e r e d t e r r a i n ; e q u i v a l e n t t h i ckness es t ima tes f o r t h e Lunae Planum l a v a f l o w s have been d e r i v e d ( 4 ) , suggest ing t h a t these flows may have been s imp ly emplaced on t o p o f p r e e x i s t i n g c r a t e r e d t e r r a i n . Chryse P l a n i t i a : The southern p o r t i o n o f t h e b a s i n has an e l e v a t i o n of -2 km, o r about 1 km lower than r e p o r t e d e a r l i e r . The channels w i t h i n Simud and T i u Va l l es appear t o have carved channels t o about t h e same l e v e l as Chryse. The c l o s e s t r a d a r data t o V i k i n g Lander 1 ( a t 22.71oN, 46-480W) g i ves a mean e l e v a t i o n o f -1.77 km, approx imate ly 700 meters h i g h e r than p a r t s of Simud V a l l i s . N o c t i s Laby r in thus : The l a v a p l a i n s sur round ing t h e western end o f V a l l e s Mari n e r i s rep resen t t h e h i g h e s t area o f t h e p l a n e t except f o r t h e v o l c a n i c cons t r u c t s . E l e v a t i o n s i n excess o f 9 km a r e found i n S i n a i Planum and t h e r e l i e f c o r r e l a t e s w e l l w i t h t h e source r e g i o n f o r l a v a f l o w s ( 5 ) . The r i m m a t e r i a l a t t h e western end o f t h e canyon i s found t o be more than 7 km h i g h e r than the e q u i v a l e n t u n i t s t o t h e e a s t such as t h e r i m o f Eos Chasma ( F i g s . 1 & 2 ) . Nor thern P l a i n s : Ex tens i ve p l a i n s m a t e r i a l s i n Elysium, Amazonia, I s i d i s and Chryse P l a n i t i a e a l l have a common e l e v a t i o n of about -2 km. Such an a t t r i b u t e may be c o i n c i d e n t a l , b u t i t migh t a l s o be t h e r e s u l t o f l a r g e s c a l e ( l ow-ha r monic) i s o s t a t i c read jus tment a f t e r i n i t i a l emplacement o f t h e p l a i n s u n i t s . North-South Hemispher ic Asymmetry: Prev ious g e n e r a l i z a t i o n s t h a t t h i s subd i v i s i o n can be made p u r e l y on t h e b a s i s o f abso lu te r e l i e f need r e v i s i o n . P a r t s o f t h e "southern h igh lands" i n t h e Ox ia Palus Quadrangle (MC-11) a r e 1500 meters below datum, w h i l e p a r t s o f t h e " n o r t h e r n lowlands" i n S i n a i Planum r i s e t o 9 km. A s l i g h t t opograph ic h i g h ( 4 km above datum) can be recogn ized t o t h e n o r t h o f H e l l a s Bas in (270-320°w; F i g . l ) , b u t i t i s u n c l e a r i f these two f e a t u r e s a re r e l a t e d . We a r e c u r r e n t l y work ing on a more d e t a i l e d a n a l y s i s o f t h e data t o r e s o l v e t h i s and o t h e r c h a r a c t e r i s t i c s o f t h e data s e t .

view Abstract Citations (36) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS JPL pulsar timing observations : spinups in PSR 0525 +21. Downs, G. S. Abstract Observations of the very slow objects PSR 0525+21, in which two small spin-ups occur, are presented. Twelve years of observation at 2.4 GHz reveal that the ratio of small jumps to the period is about one billionth, where the period is about 3.745 s; the analysis that arrived at these results is shown. The short-term behavior following the larger jump suggests an exponential decay, and long-term changes in the period derivative of about 0.03 percent clearly persist after each jump. It is argued that the spin-ups are not due to starquakes, but may be caused by uncoupling of a lighter crust from the core. Publication: The Astrophysical Journal Pub Date: June 1982 DOI: 10.1086/183810 Bibcode: 1982ApJ...257L..67D Keywords: Neutron Stars; Pulsars; Stellar Rotation; Stellar Structure; Angular Momentum; Numerical Analysis; Radio Sources (Astronomy); Starquakes; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (1)

Earth‐based radar altimetry data for the northern equatorial belt of Mars (6°S–23°N) have recently been reduced to a common basis corresponding to the 6.1‐mbar reference surface. A first look at these data indicates that the elevations of Tharsis, Elysium, and Lunae Planum are lower (by 2–5 km) than has been suggested by previous estimates. These differences show that the required amount of tectonic uplift (or constructional volcanism) for each area is less than has been previously envisioned. Atmospheric or surficial conditions are suggested which may explain the discrepancies between the radar topography and elevations measured by other techniques. The topographies of Chryse Planitia, Syrtis Major, and Valles Marineris are also described.

The directions of lava flows in the Tharsis region of Mars are used to identify regional palaeo-slopes, vent areas and local topography. A comparison is made between these flow directions and the present day radar-measured topography; good agreement between these data sets indicates that little infra-regional tectonic deformation has occurred following the emplacement of the preserved lavas.