GBT commissioning has been
defined by three phases: operation
to 15 GHz (Phase I), to 50 GHz (Phase II), and to 115 GHz (Phase III).
We are now approaching the final stages of Phase I and II commissioning
that will establish routine operation to 50 GHz.
At the present time, we are focusing our commissioning effort on
improving spectral baselines over wide bandwidths, and on K-band, Spectrometer,
and observing program checkouts. We
are also placing considerable emphasis on the Precision Telescope Control System
project, which will provide the control facilities necessary for operation at
The spectral baseline
program is designed to improve baseline flatness over bandwidths of up to 800
MHz, which are much broader (by factors of 10-20) than those available at the
140 Foot and previous centimeter-wave telescopes.
The offset optics of the GBT greatly reduce spectral standing wave
artifacts, but the wide bandwidths expose the effects of small, but cumulative
system gain instabilities, mismatches, frequency resonances, etc. Many of the
scientific programs proposed for the GBT require very flat spectral baselines
over wide bandwidths. To meet this
requirement, the baseline project team is systematically examining all
constituents of the signal path and is making improvements whenever possible.
The team has set a goal of significant progress by March.
K-band commissioning has
concentrated on beam offset measurements, focus position determination, active
surface performance, and general characterization of both the lower (18-22.4
GHz) and upper (22-26.5 GHz) frequency bands.
The system is ready for some K-band observing now, and further program
checkouts are planned for January and February.
There has been good
progress on both the engineering and software side of the Spectrometer over the
past few months. A number of fixes
to hardware and firmware problems have been made or are underway, and many modes
have passed their engineering checkouts. The
reliability of Spectrometer control software has been greatly improved.
In addition, support for multi-bank operation of the Spectrometer was
provided in the December release of the Monitor and Control system.
Multi-bank operation allows use of all four digital quadrants and up to
eight sampler inputs. Astronomical
checkouts of these modes are underway.
Throughout the late summer and early autumn, about 30% of total time was allocated to scientific observations. Observations have included HI spectroscopy, pulsar observing, VLBI, and planetary radar reception. Scheduled observing time in January is smaller owing to the commissioning requirements described above, although we expect to make additional time available to low-frequency backup programs when weather conditions are not suitable for K-band observing. Available observing time will increase again in February and March, and by mid-spring, the focus will shift from commissioning to routine, scientific operation. For about three months in the summer, up to one-third of the time will be needed for structural inspections and painting and other heavy engineering tasks, which will likely be an annual requirement. In the autumn, we will need some additional time to complete Q-band commissioning, improvements in operation of the active surface, and some general commissioning. These activities have not been scheduled in detail, but should continue to leave the majority of time for scientific observations.
On 12 November, an internal review of the GBT was held in Green Bank. Scientists and engineers from Green Bank, Charlottesville, Socorro, and Tucson, including NRAO Director Fred Lo, participated. The review was highly successful in clarifying status and priorities, and broadening participation in GBT activities by staff from around the Observatory. The priorities outlined above -- spectral baseline improvements, Precision Telescope Control, K-band commissioning, and azimuth track work (described below) – were defined at the review. We also took the opportunity of the review to improve our project management system to allow better planning and scheduling of both short-term and intermediate-range goals.
As described in previous
GBT news updates and in the NRAO Newsletter, the GBT azimuth track has been
exhibiting significant, premature wear. The
principal problem has been fretting wear on the interior faces of the wear plate
and base plates, particularly near the ends of track segments at the splice
joints. The fretting is wearing out
cavities near the segment ends. In
turn, these cavities are causing tilts of the azimuth wheels as they pass over
the joints. The bogie flex plates
from which the wheels are suspended are designed to accommodate some tilting,
but if the cavity wear continues, the tilts may eventually exceed allowable
limits and overstress the flex plates. Observatory engineers have been investigating the problem and
have taken several steps to slow the wear and extend the life of the track.
Nevertheless, it has been clear for some months that more extensive
action would be required eventually.
On 31 October and 1
November, the Observatory convened a review panel of engineering experts to
examine the track issue and to recommend appropriate, corrective measures.
The panel recommended three tasks, to be done concurrently.
These were (1) an analytical program to improve the engineering analysis
of the wheel assembly and the track joint; (2) a trial retrofit and evaluation
program to test methods of stiffening the track joints through braces,
partial-penetration welds, and by bridging the joint with a staggered wear
plate; (3) a program to develop design concepts for a new azimuth track, should
that ultimately prove necessary.
The GBT antenna engineering
team is presently developing a work plan to address the recommendations of the
panel. We anticipate that this
plan, including the evaluation period for modifications, will require a year or
more to complete. In the meantime,
we are confident that we can shim the wear cavities and slow the rate of
fretting so that the GBT can remain in operation.
It is likely that a significant modification or replacement of the
azimuth track will be required at some time within the next 1-3 years.
P. R. Jewell