An analysis of Martian photometry and polarimetry

The physical parameters that influence the photometric and polarimetric properties of a solid are enumerated and used to guide a comparison of laboratory measurements with observations of Mars. Both the bright and dark areas of Mars are found to be covered by a fine powder. Furthermore, they appear to have a very similar chemical composition. It is argued that goethite is a major constituent of both regions. The particles on the bright areas are characterized by an average particle radius of 25 , while those on the dark areas have a mean size of 100 outside of the period of seasonal darkening and about 200 near the peak of the darkening. The seasonal darkening of the dark areas is the result of a change in the average particle dimension without an accompanying chemical change.The Martian atmosphere has much less of an influence on the photometric and polarimetric observations than was previously supposed. The observed lack of contrast in the blue appears to be largely the result of an intrinsic loss of surface contrast, and not an effect of a hypothetical atmospheric blue haze.

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Résumé Les paramètres physiques qui influencent les propriétés photométriques et polarimétriques d'un solide sont énumérés et utilisés pour conduire une comparaison entre des mesures de laboratoire et des observations de Mars. On trouve que les régions brillantes et les régions sombres de Mars sont couvertes d'une fine poudre. En outre, elles paraissent avoir des compositions chimiques très semblables. Il est soutenu que la goethite est un constituant majeur des deux régions. Les particules des régions brillantes sont caractérisées par un rayon moyen de 25 , tandis que celles des régions sombres ont une taille moyenne de 100 en dehors de la période d'assombrissement saisonnier, et d'environ 200 près du maximum d'assombrissement. L'assombrissement saisonnier des régions sombres est le résultat d'une variation de la dimension moyenne des particules, non accompagné d'un changement chimique.L'influence de l'atmosphère Martienne sur les observations photométriques et polarimétriques est bien inférieure à ce qui était supposé antérieurement. Le manque de contraste que l'on observe dans le bleu, parait être principalement une conséquencede la perte de contraste de surface, et non pas un effet d'un hypothétique halo atmosphérique bleu.

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This work was supported in part by grants NGR-09-015-023 and NGR-33-010-082 from the National Aeronautics and Space Administration. A preliminary account was published as Smithsonian Astrophysical Observatory Special Report 258 (1967). This paper is dedicated to the memory of V. V. Sharonov.     

Comments on Stabilization of a Satellite

An expression is derived for the cost resultingfrom the stabilization of a satellite using aspecific optimal control (SOC). An exampleis given in which the optimal and SOC yieldvirtually identical costs.     

Search radar detection and track with the Hough transform. II.detection statistics

For pt.I see ibid., vol.30, no.1, (Jan.1994). This paper describes the calculation of P_F and P_D for the Hough transform technique when the primary threshold crossings are weighted by their power before transforming (i.e., noncoherent integration). After expressions for P_F and P_D are derived, we examine the question of optimal granularity of the Hough accumulator space. We also investigate the relationship between primary and secondary thresholds and its effect on detectability     

EUV spectroscopy as a plasma diagnostic

The EUV wavelength regions is rich in emission lines from the transition region and the corona. Spectroscopic techniques have been used extensively to determine the physical conditions in the solar atmosphere for such diverse phenomena as coronal holes, active regions, sunspots, flares, etc. The diagnostics and dynamics of plasmas, both homogeneous and inhomogeneous plasmas, are reviewed. The future projects such as the CDS and SUMER instruments on SOHO have been discussed as they cover EUV wavelength region and will provide a wealth of observational data with excellent spatial, spectral, and temporal resolution.     

Carrier phase navigation architecture for shipboard relative GPS

Carrier phase differential GPS (DGPS) navigation architectures and algorithms for automatic shipboard landing of aircraft are described. Processing methodologies are defined to provide high integrity carrier phase cycle estimation and positioning by optimally exploiting the complementary benefits of measurement filtering and satellite geometric redundancy for the terminal navigation problem. Navigation performance sensitivity to the standard deviations of raw carrier and code phase measurement errors, measurement error correlation times, and the filtering duration is quantified. Necessary conditions to ensure acceptable terminal navigation availability are specifically defined.     

Robust adaptive beamforming for HF surface wave over-the-horizon radar

Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia.     

Langmuir Turbulence and Solar Radio Bursts

Langmuir waves and turbulence resulting from an electron beam-plasma instability play a fundamental role in the generation of solar radio bursts. We report recent theoretical advances in nonlinear dynamics of Langmuir waves. First, starting from the generalized Zakharov equations, we study the parametric excitation of solar radio bursts at the fundamental plasma frequency driven by a pair of oppositely propagating Langmuir waves with different wave amplitudes. Next, we briefly discuss the emergence of chaos in the Zakharov equations. We point out that chaos can lead to turbulence in the source regions of solar radio emissions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solar wind corotating stream interaction regions out of the ecliptic plane: Ulysses

Ulysses plasma observations reveal that the forward shocks that commonly bound the leading edges of corotating interaction regions (CIRs) beyond 2 AU from the Sun at low heliographic latitudes nearly disappeared at a latitude of S26°. On the other hand, the reverse shocks that commonly bound the trailing edges of the CIRs were observed regularly up to S41.5°, but became weaker with increasing latitude. Only three CIR shocks have been observed poleward of S41.5°; all of these were weak reverse shocks. The above effects are a result of the forward waves propagating to lower heliographic latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a global model of solar wind flows that originate in a simple tilted-dipole geometry back at the Sun.     

Energetic ion emission for active spacecraft control

Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. In the near-Earth space these spacecraft are usually solar-cell powered; consequently, parts of their surface are most of the time exposed to solar photons. Outside the plasmasphere, a positive surface potential due the dominance of surface-emitted photoelectrons over ambient plasma electrons is to be expected. Photo- and ambient electrons largely determine the potential and positive values between a few Volts up to 100 V have been observed. Active ion emission is the obvious solution of this problem. A liquid metal ion emitter and a saddle field ion emitter are nearing the stage of flight unit fabrication. We will attempt to clamp the spacecraft potential to values close to the plasma potential. We present first results from vacuum chamber tests and describe the emission behaviour and characteristics of emitters producing, respectively, In⁺ and N₂⁺ beams with an energy of ≥ 5 keV.