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In this paper, autonomous pulsar-based spacecraft navigation problem is formulated in terms of a nonlinear filtering problem. The spacecraft dynamics, pulsar-timing model, and clock deviation model are combined to form the navigation system. An extended Kalman filter that uses epoch folding and leverages the timescale of the system is developed to more efficiently estimate the spacecraft states from four simultaneous X-ray pulsar measurements. Simulation results of an orbiter mission and a deep space mission are presented to show the accuracy of pulsar-based navigation in space.

A new generation of observational science instruments is dramatically increasing collected data volumes in a range of fields. These instruments include the Square Kilometer Array (SKA), Large Synoptic Survey Telescope (LSST), terrestrial sensor networks, and NASA satellites participating in "decadal survey"' missions. Their unprecedented coverage and sensitivity will likely reveal wholly new categories of unexpected and transient events. Commensal methods passively analyze these data streams, recognizing anomalous events of scientific interest and reacting in real time. Here, the authors report on a case example: Very Long Baseline Array Fast Transients Experiment (V-FASTR), an ongoing commensal experiment at the Very Long Baseline Array (VLBA) that uses online adaptive pattern recognition to search for anomalous fast radio transients. V-FASTR triages a millisecond-resolution stream of data and promotes candidate anomalies for further offline analysis. It tunes detection parameters in real time, injecting synthetic events to continually retrain itself for optimum performance. This self-tuning approach retains sensitivity to weak signals while adapting to changing instrument configurations and noise conditions. The system has operated since July 2011, making it the longest-running real-time commensal radio transient experiment to date.

Future large radio astronomy arrays, particularly the Square Kilometre Array (SKA), will be able to generate data at rates far higher than can be analyzed or stored affordably with current practices. This is, by definition, a "big data" problem, and requires an end-to-end solution if future radio arrays are to reach their full scientific potential. Similar data processing, transport, storage, and management challenges face next-generation facilities in many other fields. The Jet Propulsion Laboratory is developing technologies to address big data issues, with an emphasis in three areas: 1) Lower-power digital processing architectures to make highvolume data generation operationally affordable, 2) Date-adaptive machine learning algorithms for real-time analysis (or "data triage") of large data volumes, and 3) Scalable data archive systems that allow efficient data mining and remote user code to run locally where the data are stored.1

The NANOGrav project's goal is to make a direct detection of gravitational waves taking advantage of the extraordinary rotational stability of millisecond pulsars. The project has identified 80 pulsars for regular timing over the next 10–20 years. The fantastic timing precision at the level of 100 ns necessary for detection of gravitational waves imposes strict requirements on understanding and correcting for systematic errors at the same precision level. A major source of systematic error has been identified as the changing interstellar medium density, which manifests itself in the dispersion correction applied to each observation. In this presentation we describe a new program to monitor a subset of NANOGrav pulsars using a large radio telescope in Goldstone, CA. The wideband capability of this instrument combined with excellent sensitivity provides an opportunity to develop and maintain a long-term precision monitoring program of interstellar plasma along the lines of sight to these pulsars.

At present spacecraft angular position with Deep Space Network (DSN) is determined using group delay estimates from very long baseline interferometer (VLBI) phase measurements employing differential one way ranging (DOR) tones. As an alternative to this approach, we propose estimating position of a spacecraft to half a fringe cycle accuracy using time variations between measured and calculated phases as the Earth rotates using DSN VLBI baseline(s). Combining fringe location of the target with the phase allows high accuracy for spacecraft angular position estimate. This can be achieved using telemetry signals of at least 4–8 MSamples/sec data rate or DOR tones.

Compact radio sources are routinely used as reference calibrators in VLBI observations of radio sources to accurately determine the latter's position. Similar techniques have been developed over the years to determine the position of spacecrafts with remarkable accuracy at the level of a few nano-radians. We have carried out measurements at the DSN (Deep Space Network) that show great promise to further improve the accuracy below one nano-radian by removing many sources of calibration errors. This level of improvement is achieved by taking advantage of calibrators that are within one degree of the radio source. However, such improvement can be counted on only if there are sufficient numbers of compact radio sources near the position of the spacecraft. Over the last two years, we have carried out small surveys of the sky with the VLA and subsequently with the VLBA to empirically determine the compact radio source density at 8 GHz for sources with flux density above 1 mJy. We will report the results of the latest observations and their implication for precision spacecraft astrometry.

The deep space network (DSN) Array of the future provides an intriguing possibility of using the techniques of in-beam phase referencing to determine the angular position of spacecraft with accuracy at the level of 0.1 nano-radian (nrad). In this paper, we discuss the prospects for carrying out such measurements at both 8.4 GHz (X-band) and 32 GHz (Ka-band). Our study suggests that at X-band in-beam calibration may be available as an astrometric tool over 20-30 percent of the sky. The prospects at Ka-band, on the other hand, are not very hopeful. We point out that these estimates depend strongly on the number density of compact sources at the 1-mJy level. We also present the results from our recent VLBA (very long baseline array) observations at X-band, which has determined the compact source density at the 10-mJy level and discuss future observations to probe weaker source population. Finally, we discuss issues related to phase cycle ambiguity resolution and potential techniques to resolve them.

Precision measurements are now capable of determining the angular position of spacecrafts with accuracies of 2-5 nanoradians. To achieve this level of precision, compact radio sources with flux density of at least a few hundred milli-Jansky (at 8.4 GHz) are used for calibration purposes. Further improvements in position measurement accuracy may be possible with use of appropriate calibrators near the direction of the spacecrafts even if the calibrators are much weaker (a few milli-Jansky) in flux density. In this paper we discuss the calibrator flux density required to achieve sub-nanoradian astrometric accuracy and attempt to estimate the density of suitable calibrators, using existing source count surveys. We point out, however, that the fraction of these sources that are suitable for use as calibrators is not well understood and requires further study.