Frequent, low-latency measurements of the Earth’s rotation phase, expressed as UT1 $$-$$ UTC  critically support the current estimate and short-term prediction of this highly variable Earth orientation parameter (EOP). Very long baseline interferometry (VLBI) Intensive sessions provide the required data. However, the Intensive UT1 $$-$$ UTC measurement accuracy depends on the accuracy of numerous models, including the VLBI station position. Intensives observed with the Maunakea (Mk) and Pie Town (Pt) stations of the Very Long Baseline Array (VLBA) illustrate how a geologic event (i.e., the $$M_w$$ 6.9 Hawai‘i Earthquake of May 4th, 2018) can cause a station displacement and an associated offset in the values of UT1 $$-$$ UTC measured by that baseline, rendering the data from the series useless until it is corrected. Using the nonparametric Nadaraya–Watson estimator to smooth the measured UT1 $$-$$ UTC values before and after the earthquake, we calculate the offset in the measurement to be 75.7 ± 4.6 $$\upmu $$ s. Analysis of the sensitivity of the Mk-Pt baseline’s UT1 $$-$$ UTC measurement to station position changes shows that the measured offset is consistent with the 67.2 ± 5.9 $$\upmu $$ s expected offset based on the 12.4 ± 0.6 mm total coseismic displacement of the Maunakea VLBA station determined from the displacement of the co-located global navigation satellite system (GNSS) station. GNSS station position information is known with a latency on the order of tens of hours and thus can be used to correct the a priori position model of a co-located VLBI station such that it can continue to provide accurate measurements of the critical EOP UT1 $$-$$ UTC as part of Intensive sessions. In the absence of a co-located GNSS receiver, the VLBI station position model would likely not be updated for several months, and a near real-time correction would not be possible. This contrast highlights the benefit of co-located GNSS and VLBI stations in support of the monitoring of UT1 $$-$$ UTC with single-baseline Intensives.

The NSF's Karl G. Jansky Very Large Array (VLA) is used to observe 122 magnetic cataclysmic variables (MCVs) during three observing semesters (13B, 15A, and 18A). We report radio detections of 33 stars with fluxes in the range 6--8031 uJy. Twenty-eight stars are new radio sources, increasing the number of radio detected MCVs to more that 40. A surprising result is that about three-quarters (24 of 33 stars) of the detections show highly circularly polarized radio emission of short duration, which is characteristic of electron cyclotron maser emission. We argue that this emission originates from the lower corona of the donor star, and not from a region between the two stars. Maser emission enables a more direct estimate of the mean coronal magnetic field of the donor star, which we estimate to be 1--4 kG assuming a magnetic filling factor of 50%. A two-sample Kolmogorov-Smirnov test supports the conclusion that the distribution function of radio detected MCVs with orbital periods between 1.5-5 hours is similar to that of all MCVs. This result implies that rapidly-rotating (Pspin < 10 days), fully convective stars can sustain strong magnetic dynamos. These results support the model of Taam & Spruit (1989) that the change in angular momentum loss across the fully convective boundary at Porb = ~3 hours is due to a change in the magnetic field structure of the donor star from a low-order to high-order multipolar field.

The U.S. Naval Observatory has, since late 2011, been observing a UT1−UTC Intensive series using the Very Long Baseline Array (VLBA). The primary baseline for these sessions is Mauna Kea – Pie Town (MkPt), using Los Alamos – St. Croix (LaSc) as secondary stations. The session observation times are offset by eight to 16 hours from the IVS Intensives. The VLBA Intensives series are observed at the standard S/X bands. In addition to the daily single baseline VLBA Intensives, VLBA fortnightly sessions using up to four VLBA antennas are observed to characterize the additional baselines. The stations that participate in the fortnightlies are Mauna Kea (Mk), Pie Town (Pt), Los Alamos (La), and St. Croix (Sc). During the period when Sc was offline due to Hurricane Maria damage, a tertiary Intensive was observed using Hancock – Owens Valley (HnOv). Hn and Ov were added to the fortnightlies. We present results of these UT1−UTC series to date and discuss future observing. One issue with the current observations is that the S-band can often be affected by RFI, which usually leads to dropped channels and increased noise in the data. To address the RFI in S-band, test observations are being done to characterize the performance of the VLBA using its more sensitive C-band. Potential future goals include extending the Intensives using three-station baselines to maximize the east-west array.

The US Naval Observatory makes daily UT1 Intensive observations on the Mauna Kea–Pie Town (MkPt) baseline of the Very Long Baseline Array (VLBA) using the standard S/X bands in a bandwidth synthesis mode. These observations are increasingly negatively impacted by RFI in the S-band. The frequency range of the S-band receiver is no more than 256 MHz wide, restricting the ability to place the 32 MHz wide channels to avoid the RFI. The VLBA C-band receiver is more sensitive and has a wide frequency range (3.9–7.9 GHz) which allows for more flexibility in the placement of channels. To see if the difficulties encountered in the S-band can be overcome by using the C-band, we have undertaken two experiments using the C-band receiver on the Hancock–Owens Valley baseline of the VLBA. The first is a standard group delay observing setup accomplished by placing channels at the low and high ends of the C-band frequency range as analogs of the Sand X-bands. A major question here is whether the smaller frequency separation is sufficient for ionosphere calibration. The second is an attempt at broadband group delay measurement across the width of the C-band. Here we present the design of these sessions and preliminary results.

The Jansky Very Large Array was used to observe 121 magnetic cataclysmic variables (MCVs). We report radio detections of 18 stars. Thirteen are new radio sources, increasing the number of MCVs that are radio sources by more than twofold, from 8 to 21. Most detections are at 8.7 GHz (X-band) with a lesser number at 5.4 and 21.1 GHz (C- and K-bands). With the exception of AE Aqr, whose flux density is typically >5 mJy, the flux densities are in the range of 24–780 μ Jy. Thirteen of the detections show highly circularly polarized emission, which is characteristic of electron-cyclotron maser emission. The data suggest that MCVs could possibly be divided into two classes of radio emitters: those dominated by weakly polarized gyrosynchrotron emission and those by highly polarized electron-cyclotron maser emission.

As part of the USNO radio-optical reference frame link project, data were taken with the CTIO 1.0 m telescope in 2009. First position results of 5 ICRF sources are presented as part of this primarily photometric investigation of optical counterparts.

Quasometry is precision measurement of celestial positions and apparent motion of very distant extragalactic objects, such as quasars, galactic nuclei, and QSOs. We use this term to identify a specific area of research, the methodology of which differs from that of general astrometry. The main purpose of quasometry is to link the sub-milliarcsecond radio frame (ICRF) with the existing and emerging optical reference frames of similar accuracy, constructed by astrometric satellites. Some of the main difficulties in achieving this goal are discussed, e.g., the extended structures of quasar hosts, apparent motion on the sky, optical variability, galactic companions, faintness. Besides the strategic purpose, quasometry is undoubtedly useful for global astrometric surveys, as it helps to verify or even correct the resulting reference frames. There are two options of using measurements of distant quasars in a global astrometric solution: 1) hard constraints embedded in the fabric of observational equations; 2) {\it a posteriori} fitting of zonal errors. The relative benefits and shortcoming of the two options are reviewed. A relatively small set of about 200 carefully selected reference quasars can go a long way in improving the astrometric value of a space mission, if they are sufficiently bright, stable, fairly uniformly distributed on the sky, and are defining sources in the ICRF. We present an ongoing program at the USNO to construct a quality set of optical quasars with the required properties and to enhance the ICRF with new sources in the areas where known, well-observed quasars are scarce.

The Joint Milliarcsecond Pathfinder Survey (JMAPS) is a small, space-based, all-sky, visible wavelength astrometric and photometric survey mission for 0(th) through 14(th) I-band magnitude stars with a planned 2013 launch. The primary objective of the JMAPS mission is the generation of an astrometric star catalog with 1 milliarcsecond (mas) positional accuracy or better, and photometry to the 1% accuracy level or better at 1(st) to 12(th) mag. Achieving this level of accuracy in the final catalog requires a demanding attention to reducing systematic effects.We present our findings on distortion, signal to noise, and the astrometric bandpass necessary to obtain the desired accuracy for JMAPS.

The software development process for many space observatories is often disjoint and inefficient due to the use of multiple languages during the different phases of mission development. Code and algorithms that are often developed using an interactive, array language during the pathfinding efforts of Phase A are often rewritten in a non-interactive, compiled language for use in the production code for Phase C. This approach leads to inefficiency in both development time and cost and can introduce errors during the rewriting process. Python is one programming language that can be used as a high-level, array language and as an efficient, production language. This paper shows how Python will be used during the different phases of development of the Joint Milli-Arcsecond Pathfinder Survey (JMAPS) space mission with an emphasis on code and algorithm reuse from one phase to the next.

: JMAPS is a small, space-based, all-sky visible wavelength astrometric and photometric survey mission for oth through 14th V-band magnitude stars with a 2012 launch. The primary objective of the JMAPS mission is the generation of an astrometric star catalog with better than 1 milliarcsecond positional accuracy and photometry to the 1% accuracy level or better at 1st to 12th mag. A I-mas all-sky survey will have a significant impact on our current understanding of galactic and stellar astrophysics. JMAPS will improve our understanding of the origins of nearby young stars, provide insight into the dynamics of star formation regions and associations, investigate the dynamics and membership of nearby open clusters, and discover the smallest brown dwarfs at distances up to 5 pc after a 2-year mission, and Jupiter-like planets out to 3 pc after 4 years.