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Recent advances with space navigation technologies developed by NASA in space-based atomic clocks and pulsar X-ray navigation, combined with past successes in autonomous navigation using optical imaging, brings to the forefront the need to compare space navigation using optical, radiometric, and pulsar-based measurements using a common set of assumptions and techniques. This review article examines these navigation data types in two different ways. First, a simplified deep space orbit determination problem is posed that captures key features of the dynamics and geometry, and then each data type is characterized for its ability to solve for the orbit. The data types are compared and contrasted using a semi-analytical approach with geometric dilution of precision techniques. The results provide useful parametric insights into the strengths of each data type. In the second part of the paper, a high-fidelity, Monte Carlo simulation of a Mars cruise, approach, and entry navigation problem is studied. The results found complement the semi-analytic results in the first part, and illustrate specific issues such as each data type’s quantitative impact on solution accuracy and their ability to support autonomous delivery to a planet.

We describe the first demonstration of a Southern Hemisphere planetary radar system to detect two near‐Earth asteroids (NEAs). The demonstration was conducted in a bistatic manner, with the 70 m antenna of the Canberra Deep Space Communications Complex transmitting at 2.1 GHz and reception at the Parkes Radio Telescope, outfitted with multiple receivers, and the Australia Telescope Compact Array. This initial system was used to detect the NEAs (43577) 2005 UL5 and (33342) 1998 WT24 during their close approaches in 2015 November and 2015 December, respectively. We describe the performance of the system and consider future possibilities using other antennas of the Canberra Deep Space Communications Complex as transmitters. Planetary radar uses large radio dishes to transmit and receive radar signals off distant objects in the solar system. Careful processing of these signals over periods of several hours allows researchers to very accurately determine orbits, measure rotation rates, and in some cases to map surface features. In the past all of these activities have been performed with assets in the Northern Hemisphere. This paper details the first use of a Southern Hemisphere network for this task. The 70 m dish at the NASA/JPL/CSIRO complex at Tidbinbilla in Canberra, Australia, was used as the transmitter, and the Parkes and Narrabri radio telescopes were used as receivers to successfully detect and track asteroids. A Southern Hemisphere network allows asteroids approaching from the south to be observed, filling in a blind spot in our existing capabilities. This is the first detection of a Southern Hemisphere planetary radar systemFuture Southern Hemisphere radar observations will augment existing Northern Hemisphere capabilities

The search for extraterrestrial intelligence (SETI) at optical wavelengths counts photons from target stars. The rationale is that the number of photons received from a solar-type star in a nanosecond is typically much less than unity and that an excess number of photons may be indicative of a laser pulse from a technological civilization. Extreme magnification gravitational microlensing is a possible contaminant to optical SETI programs as it would increase the photon rate and could masquerade as an optical pulse. We show that extreme magnification gravitational microlensing is unimportant, both because the required amplifications are difficult to obtain and because the time scales are too large. This conclusion holds both for the case of background stars in the field of view of the target star and from objects (such as planets) orbiting the target star.

We present results derived from a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz (λ = 90 cm). With a resolution of ~7" × 12" and an rms noise of 1.6 mJy beam⁻¹, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency (eg. LaRosa et al. 2000). The improvement in sensitivity has significantly increased the census of small diameter sources in the region, resulted in the detection of two new Non-Thermal Filaments (NTFs) and 18 new NTF candidates, and resulted in the lowest frequency (tentative) detection of Sgr A*.

We report on a search for transient and variable radio sources in the Galactic center using a number of 327 MHz VLA observations made during the 1990's, and a series of monthly VLA observations made during Spring and Summer 2002. A typical yield of compact sources in a given epoch is roughly 200. We have detected one new bright radio transient, GCRT J1746–2757, located only 1.1 degrees north of the Galactic center. We discuss our on-going transient monitoring program and the implications of this work for constraining the Galactic center population of transients.

Radio pulsars in the Galactic center would serve as useful probes of its magnetoionic medium, and possibly its spacetime structure. However, of the roughly 1200 known pulsars, no more than 1% are within 1° of Sgr A* and none have distances consistent with being within or behind the Galactic center. This deficit of pulsars is due to interstellar scattering so severe as to smear together individual pulses. We are engaged in multiwavelength surveys of the GC to identify radio pulsars. Using the VLA at 1.4 GHz we are searching for compact radio sources, with the objective of finding sources on which a periodicity search can be conducted at frequencies high enough to defeat the interstellar pulse broadening (>10 GHz). With the Chandra X-ray survey of the Galactic center, we have also discovered a number of X-ray point sources with radio counterparts that may be radio pulsars. Our current total census of promising radio pulsars candidates is roughly 10 sources. We are pursuing periodicity searches of these objects on the 100 m Efflesberg and Green Bank Telescopes. The number of candidates also allow us to constrain the supernova rate, and therefore star formation history, in the GC.

The large-scale topology and strength of the Galactic Center magnetic field have been inferred from radio imaging of the nonthermal filaments (NTFs). These objects, which seem to be unique to the Galactic center, are defined by extreme aspect ratios and a high degree of polarization. Recent high resolution, wide-field VLA imaging of the GC at 90 cm has revealed new candidate NTFs with a wide range of orientations relative to the Galactic plane. We present follow up 6 cm polarization observations of 6 of these candidates and confirm 4 as new NTFs. Together the new 90 and 6 cm results complicate the previous picture of largely perpendicular filaments that trace a globally ordered magnetic field. NTF observations in general do not rule out any particular models for the origin of the NTFs. Hence we explore the idea that the NTFs are local, individual structures: magnetic wakes generated through the interaction of molecular clouds with a Galactic Center wind. Numerical simulations of the evolution of a magnetized wake will be discussed and compared with NTF observations.

This paper considers scattering screens that have arbitrary spatial variations of scattering strength transverse to the line of sight, including screens that are spatially well confined, such as disks and filaments. We calculate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include (1) source image shapes that are determined by the physical extent of the screen rather than by the shapes of much smaller diffracting microirregularities (these include image elongations and orientations that are frequency dependent); (2) variation with frequency of angular broadening that is much weaker than the trademark ?-2scaling law (for a cold, unmagnetized plasma), including frequency-independent cases; and (3) similar departure of the pulse-broadening time from the usually expected ?-4scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scattering of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active galactic nuclei by intervening galaxies. We also address the consequences for inferences about the shape of the wavenumber spectrum of electron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (<100 MHz) array observations will also be strongly affected by the Galactic structure of scattering material. Our formalism is derived in the context of radio scattering by plasma density fluctuations. It is also applicable to optical, UV, and X-ray scattering by grains in the interstellar medium.

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the source's diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the source's diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 ?s, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 ?m emission.

Radio wave scattering is enhanced dramatically for Galactic center sources in a region with radius 15. Using scattering from Sgr A* and other sources, we show that pulse broadening for pulsars in the Galactic center is at least6.3?-4s (? = radio frequency in GHz) and is most likely 50-200 times larger because the relevant scattering screen appears to be within the Galactic center region itself. Pulsars beyond-but viewed through-the Galactic center suffer even greater pulse broadening and are angularly broadened by up to ~2'. Periodicity searches at radio frequencies are likely to find only long-period pulsars, and then only if optimized by using frequencies 7 GHz and by testing for small numbers of harmonics in the power spectrum. The optimal frequency is ? ~ 7.3 GHz[?0.1P(?1/2)]-1/4, where ?0.1is the distance of the scattering region from Sgr A* in units of 0.1 kpc, Pis the period (in seconds), and ? is the spectral index. A search for compact sources using aperture synthesis may be far more successful than searches for periodicities because the angular broadening is not so large as to desensitize the survey. We estimate that the number of detectablepulsars in the Galactic center may range from ?1 to 100, with the larger values resulting from recent, vigorous starbursts. Such pulsars provide unique opportunities for probing the ionized gas, gravitational potential, and stellar population near Sgr A*.

The scattering diameters of Sgr A* and several nearby OH masers (? 1'' at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. We combine radio-wave scattering data and free-free emission and absorption measurements in a likelihood analysis that constrains the following parameters of the GC scattering region: The GC-scattering region separation, ?GC; the angular extent of the region, ?and ?b; the outer scale on which density fluctuations occur, l0; and the gas temperature, Te. The maximum likelihood estimates of these parameters are ?GC=133?80+200pc, 05??1°, and (l0/1 pc)2/3Te?1/2=10-7±0.8. The parameter ?bwas not well constrained, and we adopt ?b= 05. The close correspondence between ?GCand ?DGCsuggests that the scattering region encloses the GC. As host media for the scattering, we consider the photoionized surface layers of molecular clouds and the interfaces between molecular clouds and the 107K ambient gas. We are unable to make an unambiguous determination, but we favor the interface model in which the scattering medium is hot (Te~ 106K) and dense (ne~ 10 cm-3). The GC scattering region produces a 1 GHz scattering diameter for an extragalactic source of 90'', if the region is a single screen, or 180'', if the region wraps around the GC, as appears probable. We modify the Taylor-Cordes model for the Galactic distribution of free electrons in order to include an explicit GC component. We predict that pulsars seen through this region will have a dispersion measure of approximately 2000 pc cm-3, of which approximately 1500 pc cm-3arises from the GC component itself. We stress the uniqueness of the GC scattering region, probably resulting from the high-pressure environment in the GC.

We present the geometrical optics for refraction of a distant background radio source by an interstellar plasma lens, with specific application to a lens with a Gaussian profile of free-electron column density. The refractive properties of the lens are specified completely by a dimensionless parameter ?, which is a function of the wavelength of observation, the free-electron column density through the lens, the lens-observer distance, and the diameter of the lens transverse to the line of sight. A lens passing between the observer and a background source, due to the relative motions of the observer, lens, and source, produces modulations in the light curve of the background source. Because plasma lenses are diverging, the light curve displays a minimum in the background source's flux density, formed when the lens is on-axis, surrounded by enhancements above the nominal (unlensed) flux density. The exact form of the light curve depends only upon the parameter ? and the relative angular sizes of the source and lens as seen by the observer. Other effects due to lensing include the following: (1) the formation of caustic surfaces, upon which the apparent brightness of the background source becomes very large; (2) the possible creation of multiple images of the background source; and (3) angular position wander of the background source. If caustics are formed, the separation of the outer caustics can be used to constrain ?, while the separation of the inner caustics can constrain the size of the lens. We apply our analysis to two sources, which have undergone extreme scattering events: (1) 0954+658, a source for which we can identify multiple caustics in its light curve, and (2) 1741-038, for which polarization observations were obtained during and after the scattering event. We find general agreement between modeled and observed light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies between the modeled and observed light curves may result from some combination of substructure within the lens, an anisotropic lens shape, a lens which only grazes the source rather than passing completely over it, or unresolved substructure within the extragalactic sources. Our analysis also allows us to place constraints on the physical characteristics of the lens. The inferred properties of the lens responsible for the scattering event toward 0954+658 (1741-038) are that it was 0.38 AU (0.065 AU) in diameter with a peak column density of 0.24 pc cm-3(10-4pc cm-3), an electron density within the lens of 105cm-3(300 cm-3), and a mass of 6.5 × 10-14M?(10-18M?). The angular position wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. In the case of 1741-038, we can place an upper limit of only 100 mG on the magnetic field within the lens.

We describe a new protocol deployed at the National Radio Astronomy Observatory’s Karl G. Jansky Very Large Array (VLA) to support the distribution of data in support of commensal data analysis. The protocol, vys, is designed to provide access to a high time resolution data stream while a primary observation continues with the typical (lower) time resolution data stream. This form of dual time resolution, commensal observing has been implemented to enable the search for millisecond astrophysical transient events by a new, dedicated compute cluster located at the VLA. The fast transient detection system, realfast, performs real-time analysis in situto detect events of interest and record relatively short duration data “cut-outs” of those events. By selectively recording high time resolution data, provided by vysat rates of up to 1.4GBs−1, realfastwill reduce the recorded data volume by an estimated factor of up to 1000. This makes it possible to search for transients commensally in a high data rate stream over the thousands of hours needed to find the rarest events.