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very long baseline interferometry ( vlbi ) observations , and vlbi surveys in particular , have contributed significantly to our understanding of active galaxies .
for example , the discovery of superluminal motion in these systems provided the first clue that viewing angle was an important parameter in interpreting and eventually unifying different classes of active galaxies , including radio galaxies and quasars ( urry & padovani 1995 ) .
more recently , vlbi has provided direct evidence for an evolutionary scenario wherein very young compact sources are the precursors of giant extended sources associated with radio galaxies ( taylor et al .
numerous other advances are summarized in these proceedings .    at ghz frequencies ,
vlbi surveys usually target sources distributed across the sky .
the sources are also often pre - selected to be strong ( @xmath0  mjy ) and to exhibit flat spectra ( e.g. , beasley et al.2002 ) .
such samples are largely dominated by intrinsically luminous active galaxies at moderate redshifts @xmath1 .
self - calibration techniques can be used for such bright sources so calibration overhead is minimal .
but the observation time @xmath2 per source is reduced by needing to slew the array from source to source .
also , most observers impose a typical image field of view ( @xmath3 ) of @xmath4  milliarcseconds , ensuring that only one source is detected in any given observation time @xmath2 .
such vlbi surveys are therefore slow and the biases introduced are substantial .
recent attempts have been made to survey weaker ( @xmath5  mjy ) sources using phase referencing in the switching style .
such vlbi surveys take advantage of the huge catalog of first sources now available at 1.4  ghz with a resolution of 5 ( white et al.1997 ) .
these vlbi surveys are usually localized to a region subtending a few square degrees and including a strong phase calibrator close to weaker targets .
example surveys are presented in these proceedings by gurvits et al .
( 2004 ) and by wrobel et al.(2004 ) .
these surveys are both unbiased as no spectral pre - selection is made and efficient as they minimize array slewing .
but phase referencing in the switching style causes substantial calibration overhead , leaving a typical observation time per source of @xmath6  minutes . for the weaker vlbi detections , the dynamic range can be 10 or less . for the stronger vlbi detections ,
the dynamic range is often limited by residual phase - referencing errors .
these limitations mean that jets and other secondary features can easily be missed , making morphological classification difficult .
although a @xmath7  milliarcseconds is required to accommodate first s astrometric accuracy , this is still small enough to ensure only one target per @xmath3 .
moreover , the 10-millijy sources from first still involve essentially the same radio source populations that are targeted in the bright surveys ( ivezi et al .
2002 ) .
indeed , at 1.4  ghz most sources slightly stronger than @xmath8  mjy are energized by active nuclei ( e.g. , condon et al .
2003 ) . what steps can be taken toward studying the more diverse sub - millijy population at resolutions of , say , 10  milliarcseconds ( 80  pc at @xmath9 for @xmath10 , @xmath11 , and @xmath12  km  s@xmath13  mpc@xmath13 ) ?
sources brighter than 0.1  millijy have an areal density of 0.2 per square arcminute , so about a dozen are expected within the primary beam of a 100-m class antenna .
therefore , progress can be expected if two technical goals can be reached : achieve wider fields of view and lower detection thresholds , such that many sources can be observed simultaneously with the vlbi array .    for a connected array with short baselines , the @xmath3 is often set by the primary - beam response of the antennas in the array . for vlbi
this is rarely the case . in vlbi
, a more demanding limitation is set by the frequency and time sampling used during data correlation .
this sampling must be fine enough to mitigate the effects of both bandwidth and time - average smearing in images . also , since preserving @xmath3 scales computationally with the square of the baseline length , generating wide - field vlbi images is often limited by the off - line computing resources available .
this latter restriction has introduced a psychological barrier , which is that most vlbi observers over - average their data in both the time and frequency domains to make data sets more manageable .
data - averaging collapses both the @xmath3 and the total information content , leaving us with the postage - stamp vlbi images with which we are all so familiar .
the sub - millijy population in a region containing the hubble deep field - north ( hdf - n ) was observed using the evn at 1.6  ghz , 25-milliarcsecond resolution , and 64-mhz bandwidth ( garrett et al.2001 ) .
the observations spanned two 16-hour segments but phase referencing in the switching style left an observation time of only @xmath14  hours .
a @xmath3 a few arcminutes in diameter was set by bandwidth and time - average smearing , and by the primary - beam responses of the largest antennas in the array .
the evn clearly detected sources stronger than 210  microjy per beam ( 5@xmath15 ) in two of the five target patches imaged within the @xmath3 .
one detection locates the active nucleus and an adjacent feature in an fr - i galaxy at @xmath16 , while the other detection is a candidate active nucleus embedded in an obscured starburst at @xmath17 .
the observed rms noise levels were higher than the expected thermal levels , due to residual phase - calibration errors .
we have improved upon the hdf - n survey with a vlbi observation in the noao botes field using the nrao vlba and gbt at 1.4  ghz and 64-mhz bandwidth ( garrett , wrobel , & morganti , in preparation ) .
the vlba is described by napier et al .
the observations spanned three 8-hour segments .
phase referencing in the in - beam style led to an observation time @xmath18  hours .
two correlation passes were made within the gbt primary beam of fwhm 8.6 , shown as the circle in the figure .
the passes were centered on the in - beam calibrator and on a region with six faint wsrt targets ( de vries et al .
2002 , morganti & garrett 2002 ) . for each pass , a @xmath19 4 in diameter was preserved and the output data rate of 1  mbyte per second yielded a data set of size 60  gbytes .
the data from the calibrator pass were edited , averaged , and calibrated , and its gain parameters applied to the target - pass data .
the aips task imagr was used to make dirty images / beams from the target - pass data after editing and calibration .
each dirty image subtended a square 6 on a side . for each target , three such dirty images were made from the three observation segments and then simply co - added .
this was a slow computational task : about 8 hours were required to produce one dirty image / beam on a 2-ghz linux pc with dual processors .
for the stronger sources , cleaned images were produced with aips task apcln by subtracting the dirty beam from the dirty image .
visibility - based cleaning is currently prohibitively expensive in terms of computing requirements .
three sources were detected within the gbt primary beam , at a resolution of 10  milliarcseconds and above the detection threshold of 63  microjy per beam ( 7@xmath15 ) .
these detections include the 20-millijy calibrator and two sub - millijy sources .
each detection is unresolved and has noao photometric data available ( januzzi & dey 1999 ) . the calibrator is a quasar .
one sub - millijy detection locates the active nucleus with @xmath20  millijy within an fr - i galaxy with @xmath21  millijy .
the other sub - millijy detection has @xmath20  millijy and recovers most of the @xmath22  millijy .
this is a candidate active nucleus because , at the comological distance suggested by the host s blue drop - out , ( 1 ) the spectral luminosity is too high for prompt emission from a sn and ( 2 ) the wsrt photometry spanning 20 months shows no variability , arguing against emission from a grb afterglow .
finally , the disk galaxy ngc5646 at @xmath23 with @xmath24  millijy was not detected at 10-milliarcsecond resolution ; its wsrt emission follows the optical isophotes and obeys the fir / radio correlation , suggesting a star formation origin .    by tapering the visibility data , portions of the vlba primary beam of fwhm 29 were imaged at poorer sensitivity and resolution to yield five further detections .
these detections involve brighter radio sources probably hosted by active galaxies and quasars , but the available noao photometric data have not yet been analysed .
the long - term goal for these new vlbi surveys is to constrain the contribution that active galaxies make to the faint radio source population in general , and the optically faint radio / sub - mm source population in particular ( garrett 2003 ) . to have an impact in this area , it is essential that many sources be surveyed over large regions with rms sensitivities of a few microjy .
global vlbi arrays employing disk - based recordings should be able to achieve such sensitivities .
the proposed pcint program at jive will enable output data rates as high as 160  mbytes per second , remarkably expanding the accessible @xmath3 .
it will thus be possible to sample simultaneously the summed response of all milliarcsecond - scale emission within ( and beyond ) the primary - beam response .
after some phase - referencing to improve the coherence time , simple self - calibration of most fields should be possible at frequencies of 1 - 3  ghz .
access to grid - like computing resources may be the best way to analyse the enormous data sets generated .
huge and unbiased vlbi surveys will be conducted , with the bulk of the targets being faint sources with flux densities of only a few tens of microjy .
this work made use of images provided by the ndwfs , which is supported by noao .
noao is operated by aura , inc .
, under a cooperative agreement with the nsf .
nrao is a facility of the nsf operated under cooperative agreement by aui .
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a 20-hour vlbi observation with the nrao vlba and gbt in the noao botes field reaches an rms noise of 9 microjy per beam at 1.4  ghz . 
 three sources were detected at 10-milliarcsecond resolution within the gbt primary beam of fwhm 8.6 , including the 20-millijy calibrator and two sub - millijy sources . by tapering the visibility data , portions of the vlba primary beam of fwhm 29 were imaged at poorer sensitivity and resolution to yield five further detections . 
 new developments at jive will permit deeper and wider vlbi surveys at full sensitivity and resolution , enabling new types of survey science .    
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