Frequently Asked Questions


The next-generation VLA (ngVLA) is a future centimeter-to-millimeter wave interferometer that builds on the legacy of the JVLA, ALMA and the VLBA, as the next major facility in ground-based U.S. radio astronomy. The ngVLA is optimized for observations at wavelengths between the superb performance of ALMA at sub-mm wavelengths, and the future SKA1-MID at longer (decimetric) wavelengths.

 

The ngVLA opens a new window on the Universe through ultra-sensitive imaging of thermal line and continuum emission down to milliarcecond resolution, as well as unprecedented broad band high-angular-resolution continuum polarimetric imaging of non-thermal processes. The ngVLA will perform transformative science covering areas of terrestrial planet formation to the first generation of molecules in the early Universe, as detailed in the recently published ngVLA Science Book.

The ngVLA reference design consists of 244 dishes of 18 m diamter. Of these, 214 are concentrated in the U.S. Southwest and comprise the main array which provides 10x the effective collecting area of the JVLA at 40GHz. Thirty ngVLA dishes are distributed at very long baselines up to 9000 km reaching across North America, and are referred to as the Long Baseline Array or LBA. Presently, an unblocked offset-Gregorian geometry is preferred, with its feed-arm low, for optimal science and operational performance.

 

An additional 19 dishes of 6 m diameter will make up a short baseline array (SBA) and will be sensitive to a portion of the larger angular scales undetected by the main array. The SBA can be combined with four 18 m (main-array) antennas (defined above) used in total power mode to completely fill in the central hole in the (u, v)-plane left by the 6 m dishes.

The ngVLA will nominally operate from 1.2 – 116 GHz (25 – 0.26 cm). Technical details about the ngVLA can be found in the recently published Reference Design. Key performance parameters of the ngVLA are available on the Performance Estimates page.

 

The U.S. Low Frequency Radio Community is exploring an option for a co-located array that would use the ngVLA infrastructure to cover 20 MHz to 150 MHz. An option to add a prime focus feed to the ngVLA antennas to cover 150 MHz to 800 MHz is also possible. Funding and design for these options (e.g., ngLOBO) is outside the scope of the ngVLA project.

The resolution of an interferometer is determined by the observed wavelength and its longest baseline. The ngVLA will be designed to implement multiple subarrays, with each subarray covering a range of baselines to provide a resolution and surface brigthtness sensitivity appropriate for the object under study. With up to ~1000 km baselines, the ngVLA main array will achieve an angular resolution ranging between ~0.5 and 50 mas at 2.6 mm (116 GHz) and 25 cm (1.2 GHz), respectively. Using the full extent of the ngVLA in a configuration including the LBA antennas on continental scales, even higher angular resolution imaging will be possible, achieving ~60 mas and 6 mas at 2.6 mm (116 GHz) and 25cm (1.2 GHz), respectively.

 

Key performance parameters of the ngVLA are available on the Performance Estimates page.

The ngVLA core will be centered near the location of the VLA site on the plains of San Agustin, with additional mid-baseline stations currently spread over greater New Mexico, Arizona, Texas, and Mexico. The 10 long baseline array stations are currently located at six VLBA sites plus four existing radio facilities that will take advantage of present infrastructure: Washington, California, Iowa, Massachusetts, New Hampshire, Puerto Rico, the U.S. Virgin Islands, and Canada. New observing sites in Hawaii are under consideration.

The VLA has been the scientific powerhouse of radio astronomy since its inception in the late 1970s, consisting of 27 movable 25 m symmetric antennas, with maximum baselines of 36.4 km. The VLA underwent a major electronics upgrade, completed in 2011, which provided continuous frequency coverage between 1 – 50 GHz. The ngVLA, by comparison, will consist of 214 dishes of 18 m diameter extending up to ~1000 km baselines and an additional 30 (18 m) antennas on scales up to 9,000 km, delivering an order of magnitude improvement in both sensitivity. The ngVLA dishes will not be movable, so different angular scales are achieved using subarray selection. The ngVLA will also extend the operational frequency range from 1.2 –116 GHz (25 – 026 cm).

ALMA is a radio interferometer optimized to take advantage of the sub-mm (THz) windows that are only accessible at high, dry sites such as the Chajnantor plateau, which is at ~5000 m altitude. While ALMA operates in the 3mm band and above, the ngVLA main array (at an altitude of ~2100 m) can both access the 3mm atmospheric window and will provide a factor of ~10 times better sensitivity and baselines that are ~60x longer than ALMA.

 

As currently envisioned, SKA1-MID will be the premier radio interferometer at decimetric wavelengths, consisting of up to 133 x 15m (+ 64 x 13.5m MeerKAT) dishes with a maximum baseline of up to ~150 km and eventually covering a frequency range spanning 350 MHz (85 cm) to 14 GHz (2 cm). The ngVLA, on the other hand, is being optimized to cover and complement the frequency range above the highest SKA1-MID band, on continental-scale baselines, providing much higher angular resolution and sensitivity at cm wavelengths.

The scientific and technical differences between the arrays are apparent, with the ngVLA focusing on cm/mm emission, in particular thermal gas/dust emission on milliarcsecond scales and redshifted molecular line emission. SKA1 is planning to observe at meter through centimeter wavelengths, expanding on the scientific legacy of instruments like WSRT, ATCA and the VLA.

 

 

The ngVLA strongly complements SKA1 and ALMA by providing enhanced sensitivity and resolution to pursue new scientific goals and interests in the parameter space bridging those facilities.

Yes.  The project office is currently in the process of exploring international and domestic (academic and industrial) partnerships. International and domestic collaborating institutions have already contributed to community studies and baseline system designs. They also currently participate in project advisory councils. If you are interested in discussing partnership options, please contact our Project Director. We anticipate a name change for the instrument once the project enters into its design and development phase, allowing our international partners to participate in this activity.

NRAO and its international partners will be building upon their ALMA experience and success in developing the ngVLA project. Many projects undergo scoping revisions before and during construction to address planning shortcomings, changes in assumptions or external conditions, new constraints, etc. ALMA was rebaselined in 2005, eventually completing the project within 0.5% of the revised budget estimate. NRAO has a long tradition of excellence in construction of radio astronomy instruments and facilities.

The U.S. government has never been asked to fund the SKA in any significant way; the SKA was unable to gather scientific community support in the Astro 2010 Decadal Survey due to uncertainties in the scientific requirements and technical definition of the project at that time. NSF’s withdrawal from supporting overall SKA development in the U.S. was a consequence of funding shortfalls in U.S. science early this decade and a recognition that the project was not broadly supported in the U.S. astronomy community. Since 2010, the scientific goals of SKA1 have narrowed, and engagement with the broader U.S. community has further declined. The fact that U.S. science funding agencies are not engaging with the SKA arises from several scientific and cultural differences.

The ngVLA concept has been developed to explore new horizons in cm radio astronomy, synergistic with other existing, under construction, or planned instruments in the U.S. in coming years, and focuses on our PI-driven science interests. The ngVLA can address many of the goals of the Astro 2010 Decadal Survey, and will be even more relevant to the science priorities identified for the 2020s in Astro 2020.

 

The ngVLA may eventually be considered as a partner with an SKA global program, thereby engaging all parts of the U.S. Radio/Millimeter/Sub-Millimeter community in new opportunities in the 2020s. ngVLA and SKA leadership are currently discussing these possibilities.

 

The ngVLA is being designed as a proposal-driven instrument. The science program will be determined via a competitive peer review process similar to other large PI-driven observatories (e.g., ALMA, VLA, HST, JWST etc.), adopting best practices for minimizing selection biases. In this way the ngVLA will be a different style of instrument than other facilities on the horizon, such as LSST and SKA1, which have science programs focused on carrying out large surveys. Further details on the facility Science Operations can be found in the Operations Concept document.

Building on the current model for ALMA science operations, we anticipate ngVLA users will be delivered Science Ready Data Products (SRDPs; e.g., images and data cubes), rather than raw data, due to the sheer volume of data and computing resources that will be required to reduce data. However, raw visibilities will still be available upon request. Using ALMA and the VLA as test beds, we are building up our expertise and knowledge base of the best methods/practices/deliverables for SRDPs in preparation for the ngVLA.

 

The ngVLA data processing implementation will be based on the CASA package, providing continuity with JVLA and ALMA and continuing to provide a flexible data reduction package for experts in the community. As part of the construction project, CASA will evolve both in the implementation of cutting edge algorithms and the infrastructure needed for broad scale parallelization in order to support the ngVLA. We anticipate data processing and analysis features being split out into separate CASA packages, with data analysis capabilities being most commonly used by users at their home institutions.

The Science Ready Data Products (SRDPs) produced by the ngVLA will be available through the NRAO Archive interface, providing a single access point for ALMA and ngVLA data products. After the proprietary period, these SRDPs will be available to the full community, providing a rich archive of images ready for study. Capabilities for inspection and selection prior to download will assist in mitigating download times. This approach is being used successfully already for ALMA science.

Depending on the recommendation for the Astro 2020 Decadal Survey, we expect to enter the NSF Major Research Equipment and Facilities Construction (MREFC) design phase in late 2021 and have a final design completed by late 2024. Procurement and construction would then commence in 2025 and should be completed by 2034.

With construction commencing in 2025, we would anticipate a notional Early Science start date in 2028, with full array operations beginning in 2034. Prior to early science, the VLA will likely see a period of reduced capabilities as the ngVLA commissioning efforts ramp up. We are currently starting to develop a transition plan in consultation with the broader local and scientific community that will bridge VLA and ngVLA operations.

Current estimates for ngVLA are $1.9B (2018) for construction funds ($2.25B risk adjusted), and an operational cost cap of $93M (2018) per year (i.e., <3x current VLA + VLBA operations). Of the $1.9B in construction, we anticipate the U.S. to be the majority partner, with the remaining piece covered by a combination of international and multi-agency partnerships that are actively being pursued. For context, construction costs for the ngVLA are anticipated to be comparable to those for ALMA, allowing for inflation.

The ngVLA project includes a Science Advisory Council (SAC) and Technical Advisory Council (TAC) to guide the science case and technical implementation of the array. To date, the ngVLA Science Working Groups (SWGs) have prepared over 80 science use cases that were subsequently prioritized by the SAC. These form the highest-level requirements for the system, and have significantly impacted the design concept as it has matured from 2015 through 2019. Examples of community driven changes are the inclusion of 1.2 – 10 GHz frequency coverage, a Short Baseline Array of 6m apertures plus total power antennas, and the Long Baseline Array.

Likewise, the ngVLA TAC has also influenced key design decisions, including the feed design for Band 1 and 2, the aperture size, and the selection of the optical design for the ngVLA antenna reference design.

If you are interested in providing scientific or technological input to help guide the design of the ngVLA, please contact the respective council chairs.

The ngVLA reference design includes an assessment of the technological readiness of each major element in the system. All ngVLA systems could be built with 2019 technology. The imaging pipeline and archive will require continued advances in processing power and storage technology from 2019 through full operations in 2034 in order to achieve our current science goals within the allowable cost.

For other systems, while they can be achieved with present technology, a core goal with ngVLA development is to find solutions that allow for volume manufacturing, improved performance, increased reliability, and reduced lifecycle cost. There is ample scope for advances in technology to improve the value of the array over the course of the design.

Some enabling technologies for ngVLA are discussed on the Technology page. The technologies and approaches developed for ngVLA will have application well beyond the project, enabling many different futures.

Depending on detailed science interests, processing requirements are often set by the lowest observing frequency (i.e., fully imaging large primary beams); as our primary science missions are at higher frequencies, the processing challenges are much more tractable. We expect to benefit from additional technological development due to the later delivery of the ngVLA. Even if expected improvements fail to materialize by the beginning of operations, because we store the raw visibility data, reprocessing of data with better algorithms or with increased fidelity can be done when the computational resources become available.

 

The current ngVLA compute size model, based on a set of defined science use cases, is available in the Project Documentation page. The average compute capacity required, inclusive of expected parallelization inefficiencies, is of order 60 PFLOP/s. This is comparable to the recently constructed Frontera system at the Texas Advanced Computing Center. Note that ngVLA would not require such capacity until approaching full operations in 2034, when such systems will be common and affordable.

The average total power load is estimated at 6.5MW for the array, central infrastructure, and off-site buildings combined. This is approximately four times the current VLA load. Significant savings are achieved in the design of the antenna electronics, correlator system, and computing cluster when compared to existing facilities. The main power source on the plains is expected to be grid power provided by the local utility company. Green power sources (photovoltaic and wind turbines) are being considered, and are increasingly attractive to reduce operating costs.. Remote ngVLA antennas are being located in part to use local infrastructure (e.g., grid power).

NRAO started the ngVLA project as an internally-funded R&D effort in 2015. NSF provided support of order $2M for development in 2015 through 2017. Based on clear community engagement and support in developing the key science goals, NSF reprofiled an additional $11M for 2018 and 2019, which has supported the definition of the ngVLA science case and the development of the ngVLA reference design. In-kind partner contributions of approximately $1M have also been made over the 2016-2018 period.

Most recently, the NSF has awarded NRAO/AUI an additional $4M for 2020 design & development activities, while additionally creating a new cooperative support agreement specifically for ngVLA. Such agreements are long-term funding arrangements, put in place to support existing facilities and key initiatives.

 

NRAO will continue to pursue funding from the NSF at the ~$8-10M/yr level through 2021 when the Astro 2020 Decadal Survey concludes. The design effort would then ramp up through final design in 2024, and progress to construction in 2025.

Yes! The NRAO recognizes the interests and concerns of local and regional communities and that these must be addressed in a respectful and collaborative manner. We also value the unique knowledge, experience and insights of local communities and their critical influence and role in enabling the project to realize its potential. The NRAO is currently developing an authentic and deliberate ngVLA stakeholder engagement strategy, which will open and maintain a number of channels for inclusive, in-depth local and regional engagement during all phases of the project.

Yes! The NRAO is committed to incorporating the potential broader impacts (BI) of our work for society in our planning, and has appointed a BI Lead to research, develop and manage the anticipated multi-faceted BI efforts for the ngVLA throughout the life of the project. The NRAO will invest significant funding in BI projects that promote Collaboration and Partnerships; Education and Diversity; Infrastructure; Natural and Cultural Heritage; and Technology and Commercialization. Implementation plans to use the ngVLA to drive broadening participation by under-represented minorities in STEM, as well for education and public outreach, are included in the BI strategy. Contact odi@nrao.edu for more information.

Yes, we are planning to operate the ngVLA as an Open Skies observatory, as is the case for all other NRAO facilities, subject to the general principle of reciprocity outlined by the NSF/NASA-sponsored Astronomy & Astrophysics Advisory Committee. Investigators may be awarded ngVLA time based on scientific merit regardless of institutional affiliation.