[a,b,c]María J. Rioja

# The Readiness of EVN Telescopes for the SKA-VLBI Era

Richard Dodson

###### Abstract

The application of VLBI to scientific problems has undergone a relentless
expansion since its conception, yet the potential for further expansion is
still large. We are on the cusp of revolutionary progress given the arrival of
a host of next-generation instruments. Over the last few years the community
has been working hard to ensure the SKA design includes the capability to
enable multiple simultaneous tied-array beams, which is a crucial technology
to deliver ultra-precise astrometry and improve survey speed capabilities.
However, to reach the full potential requires that the network of antennas is
upgraded to match the SKA capabilities. We identify multiple-pixel technology,
on large telescopes and connected arrays, as a crucial missing component and
here will make recommendations for the upgrade path of the partner EVN (and
other network) telescopes. Our feasibility studies on SKA-VLBI suggest an
order of magnitude improvement in the precision and also in the frequency
range at which astrometry can be performed today, if the full network has the
required capabilities.

##  1 Introduction

Precision astrometry measurements add a new dimension to the research of many
astrophysical fields. It has provided deep insight into the astrophysical
processes in a huge range of environments, and is a probe for fundamental
properties of the Universe. VLBI has traditionally provided the highest
astrometry precision measurements and the next-generation of instruments hold
the potential for an order of magnitude of improvement. However, the current
conventional astrometric methods are limited by systematics in most cases.
Hence we need a commensurate improvement in methods as well as in sensitivity
for the next-generation of instruments.

Since the beginnings of VLBI, there has been a relentless quest for increase
of astrometric accuracy and wider applicability. In the last decade there have
been huge strides in achieving the full astrometric potential, arising from
advanced phase referencing (PR) calibration strategies [i.e. GeoBlocks: 8]
that accurately compensate for the dominant tropospheric propagation residual
errors that have led to a few tens of micro-arcsecond precision measurements,
most notably at $\sim$22 GHz.

Instead the field of astrometry at lower frequencies ($<$8 GHz) has lagged
behind, with an order of magnitude larger errors at L-band ($\sim$1.6 GHz).
This regime is dominated by residual ionospheric propagation effects that pose
a rather different set of challenges. Chief of those challenges is the fact
that the ionospheric effects have a strong spatial structure (i.e. they are
direction-dependent), which limits the use of observations of a reference
source (necessarily along a different line of sight than that of the target)
to correct for the atmospheric errors. We note as an aside the multi-frequency
advanced PR calibration strategy [3], where ionospheric residuals along the
line of sight from the target are removed with “ICE-blocks” without using a
reference source. Thus the direction-dependent corrections are not required.

The arrival of the SKA, currently under construction, will focus on the lower
frequencies and will revitalise all aspects of VLBI astronomy at these
wavelengths with joint observations with EVN telescopes. Among these, the
ultra precise astrometric capability is of great importance and is a
scientifically driven motivation in the SKA-VLBI era; the goal is to improve
by an order of magnitude both the astrometric accuracy and the range of
applicable frequencies [7, 4]. The high sensitivity and long baselines of SKA-
VLBI observations will result in a much reduced thermal noise level and high
spatial resolution. Therefore this goal is achievable, as long as a
sufficiently accurate ionospheric phase-calibration strategy is in place.

This paper discusses the way to achieve this using a next-generation method,
namely MultiView [12], and an upgrade of the network telescopes to implement
its requirements for optimal performance. Section 2 describes the basics of
MultiView, presents current empirical demonstrations with existing instruments
and our estimates for the expected performance in the SKA-VLBI era. Section 3
describes the technological developments relevant to astrometry for the
telescope network, namely multiple-pixel capabilities. Section 4 are the
conclusions.

##  2 cm/m-VLBI Microarcsecond astrometry using MultiView methods

MultiView [12] is a next-generation calibration method that has the potential
for optimum correction of the dominant ionospheric residual errors, which are
the main challenge and limit the measurements at the SKA frequency range with
PR. The result is ultra high precision astrometry measurements with wide
applicability, that is, to many sources and across a much wider frequency
range than can be used today [10].

Standard PR strategies using a single calibrator result in uncorrected
systematic residual phase errors caused by the spatial structure in the
atmospheric propagation effects (even when the sources are close i.e. $\sim
1^{o}$). These systematics strongly affect the analysis and impose limits in
the astrometric accuracy. Figure 2 illustrates these astrometric limits across
the frequency range. Instead, MultiView calibration uses observations of
multiple calibrators surrounding the target and combines their phases with
spatial interpolation to effectively result in using a virtual calibrator at
$\sim 0^{o}$ angular separation. PR performance increasingly deteriorates
towards the low frequency regime, with errors much larger than at the higher
frequencies, such as 22 GHz, limiting its application. A clear example of the
residual ionospheric propagation effects becoming the dominant source of
errors at observing frequencies less than $\sim\,8$ GHz is to be found in the
analysis of 6.7 GHz methanol maser observations in the BeSSeL project, using
similar PR with GeoBlocks strategies than for 22 GHz water maser observations.
The precision of the results from 2016 [14], are significantly worse than the
re-analysis [13] with a variation on the MultiView strategy [9].

MultiView calibrators can be further away than for PR because the solutions
are interpolated rather than transferred. This has been demonstrated at 1.6
GHz [12], 8 GHz [2, 6] and 6.7 GHz [5] with calibrators up to 6o from the
target. Encouraged by the outstanding performance, explorations in PR corner
cases are under way, with observations at very low elevations, very low
(0.3GHz) and high (43GHz) frequencies, where we are exploring the application
for next-generation instruments such as SKA, ngVLA-LONG and FAST. For maximum
precision one requires simultaneous observations of (nearby) calibrators
uniformly distributed surrounding the target. Nominally, following from
empirical ionospheric spatial structure studies with MWA [11], the
expectations of residual errors with MultiView would be about 1 mTECU. See
Figure 1 for an example of the ionospheric phase screens as observed with the
MWA. Residuals of this level would result in MultiView systematic astrometric
errors at the 1 micro-as level above $\sim$5GHz (see Table 1) with the final
error to be comprising of the additional contributions set by the thermal
noise or measurement errors, the dynamic range of the image and the stability
of the reference points selected within the sources for multi-epoch
comparisons. This precision is more than an order of magnitude improvement
over the current limits.

The improvement that MultiView can provide to VLBI astrometric observations
has been demonstrated with an increasing number of empirical astrometric
measurements, showing outstanding performance compared to standard PR methods,
reaching the thermal noise limit of current VLBI networks, as predicted by our
error analysis. Thus we are confident in our estimates of an order of
magnitude improvement for SKA-VLBI, assuming an upgraded network of antennas
that match the SKA capabilities. This is predicated on the sensitivity
improvement from the increased collecting area (and bandwidth) and the quasi-
perfect compensation of systematic atmospheric effects, as provided by
MultiView (see Figure 3). Note that MultiView is expected to achieve an order
of magnitude improvement compared to in-beam PR.

As stated above, the ultimate limit, after correcting for the currently
dominant atmospheric propagation errors, we expect to be related to intrinsic
source structure effects and the definition and stability of the reference
points over time. To alleviate their impact, and other potential phase
ambiguity related issues in the analysis, we plan for an over-determined fit
to the phase plane, i.e. to use more than the minimum number of three
calibrators surrounding the target, and a dense network (to provide uv-
coverage) of moderate to large sized telescopes for joint MultiView
observations.

To provide simultaneous observations of all sources we identify multiple-pixel
technology, on large telescopes and connected arrays, as the crucial missing
component. In the next section we describe the technology and make
recommendations for the upgrade path of the partner EVN (and other network)
telescopes.

Figure 1: Example of an ionospheric residual phase screen as observed over the MWA at 88MHz after direction independent and bandpass calibration. Shown as a mesh are the station-based calibration phases above the site for one direction, color-coded in degrees, with the X and Y axis in meters for the 128 stations (at the nodes of the mesh). The wireframe shows the second order fit to the data, underlining the high curvature of the surface. The typical residuals to a planar fit are about 1mTECU. See Rioja & Dodson [11] for details. Figure 2:  Illustration of theoretical systematic limits for a range of astrometrical techniques (solid lines: for conventional PR in black and brown for in-beam; Advanced Tropospheric Calibration (ATC) in green; Advanced Ionospheric Calibration (AIC) in blue) compared to actual observational results from the literature shown with symbols, taken from Rioja & Dodson [10]. This emphasises how these limits are greater than the thermal limit for a dynamic range 100:1 (dotted line) and that the systematics dominate observationally except for the next-generation methods, MultiView (red) and SFPR (pink). See Rioja & Dodson [10] for details.  Figure 3: The theoretical thermal (dotted line) and systematic (solid lines) limits for the next-generation of instruments and methods, taken from Rioja & Dodson [10]. This illustrates the astrometric improvements of MultiView, which matches the thermal limit of a dynamic range 1000:1. Some suggested astrometric projects are indicated. See Rioja & Dodson [10] for details.  Table 1:  Table to characterise the performance of MultiView and its feasibility for current and next-generation instruments, across the SKA spectrum, taken from Rioja & Dodson [10]. Col. 1 is the observing frequency, Col. 2 is the estimated systematic astrometric error using MultiView, as discussed in Rioja & Dodson [10]. Col. 3 is the number of 100$\sigma$ calibrator sources for current arrays, expected within the primary beam of a single pixel 20 m antenna if FoV< $1^{o}$, otherwise 1o (marked with †), calculated using the source count prediction from Bonaldi et al. [1]. Col. 4 is the same as Col. 3, but using the sensitivity for SKA-VLBI Phase-1 (in brackets for Phase-2 at the higher frequencies) [4] that are strong enough to exceed the MultiView systematic limits ($\sim\,1000\sigma$, see Rioja & Dodson [10] for details). We note that the number of in-beam sources for ngVLA-LONG observations would fall between SKA Phase 1 and 2 estimates. Based on Col. 3 & 4, simultaneous (e.g. within primary beam, in-beam) MultiView would be feasible at frequencies $<$1.4, $<$2 and $<$6.7 GHz, with the sensitivities of current VLBI, SKA-VLBI Phase 1 and Phase 2, respectively. At higher frequencies, MultiView is possible using nodding observations or using simultaneous observations with sites with multi-antennas (e.g. ngVLA-LONG) or subarraying (e.g. SKA) capabilities.  Frequency | MultiView error | No. in-beam | No. in-beam
---|---|---|---
$\nu$ | $\sigma\Delta\theta^{MV}$ | sources for | sources for
(GHz) | ($\mu$as) | current arrays | SKA-VLBI
0.3 | 150 | 1.2† | 14†
0.9 | 17 | 3.5 | 15
1.6 | 6 | 2.9 | 5.5
5.0 |  $\sim$1 | 0.4 | 0.4 (6)
8.0 |  $\sim$1 | 0.1 | 0.1 (2)
15.0 |  $\sim$1 | 0.0 | 0.0 (0.4)

##  3 Implementation of MultiView Technological requirements for the telescope
network

Figure 4: Left: The CSIRO MKII PAF installed at the Effelsberg telescope prime
focus. MPIfR are now developing their own version with increased capabilities.
Of particular importance for multiple-pixel VLBI will be the ability for the
PAF beams to track the same point in the sky during the parallactic angle
rotation. Right: The APERTIF PAF in WRST.

Optimum MultiView performance comes from simultaneous high-SNR observations of
the multiple sources involved. This translates to observational requirements,
namely sensitive observations with large collecting areas and (relatively)
wide FoVs. Multiple-pixel technology is fundamental to bring the two
requirements together, for large telescopes and connected arrays across the
network. These MultiView observational requirements have driven the community
efforts that resulted in an Engineering Change Proposal to ensure that the SKA
design includes the innovative capability to enable multiple simultaneous
tied-array beams from the connected phased-up array and/or from subarraying,
which has been approved. These support a number of concurrent pencil beams
ranging in number between 4 full-sensitivity VLBI beams with 2.6 GHz bandwidth
and up to 46 beams in total, with bandwidth tradeoffs [for details see 4].
These beams can simultaneously point in any direction within the individual
single-antenna FoV with the full sensitivity of the connected array, or have
even wider separations by using subarraying.

The remaining missing component is to upgrade the rest of the network of large
telescopes and connected arrays, to match the capabilities of SKA. To deliver
this upgrade path for the partner EVN (and other) telescopes we need to both
define the technological multiple-pixel VLBI beam requirements and carry out
practical end-to-end demonstrations to discover the key issues. These two
activities necessarily depend on each other, as one sets what can be done and
the other sets what must be done. The benefits of innovative multiple-pixel
technologies available to increase the FoV such as Phased Array Feeds (PAFs)
and Multi-Beam receivers are recognized within the EVN, with the largest
telescopes already equipped or with plans to do so. For example, Effelsberg
(100m, Germany) and Lovell (76m, UK) telescopes are equipped with CSIRO MKII
PAFs, the Sardinia Radio Telescope (64m, Italy) has multi-beam receivers, and
WSRT array is equipped with the APERTIF PAFs system. A 100m telescope such as
Effelsberg with a 25-beam PAF has the same FoV as a 20m and a connected array
with multiple tied-array beams, such as WRST, has the same FoV as the
individual elements. Nevertheless, its application to VLBI has not been
implemented so far, and plans to do it have a very low priority. Figure 4
shows the current PAFs on Effelsberg and WSRT.

Such an upgrade will benefit the operations of the EVN as an stand alone
instrument as well as providing an order of magnitude improvement in
astrometric precision for SKA-VLBI observations. Other than SKA, the multiple-
pixel capability is a part of the design of other next generation instruments,
such as FAST and ngVLA-LONG.

##  4 Conclusions

The arrival of sensitive next generation instruments brings exciting
opportunities and challenges for VLBI observations [10]. Among the former, the
realization of ultra precise astrometric measurements that will enable the
addressing of a host of innovative open questions in astrophysics. The list of
challenges includes the readiness of the telescope network to reach the
astrometric potential in joint observations with existing telescopes. This
paper is mainly concerned with an upgrade of the EVN telescopes, for
improvement and benefits of joint observations with SKA and also as a stand-
alone instrument. The proposed multiple-pixel capability for large telescopes
and arrays is predicated on the next-generation calibration methods and their
observational requirements.

MultiView was originally conceived to address the poor astrometric performance
of conventional PR methods at low (<8 GHz) frequencies. MultiView has clearly
demonstrated superior performance, with increased astrometric precision from
removing the dominant ionospheric errors and wide applicability, to many
sources, at frequencies $<$8 GHz using existing instruments. Based on this
outstanding performance a number of efforts are ongoing to extend the
MultiView method beyond its original scope, to higher and lower frequencies,
as well as to better characterise the performance limits in corner cases.
These include astrometric observations at very low elevations and with ever
wider angular separations. The on-going investigations at higher frequencies
show very promising outcomes for the correction of the tropospheric
propagation medium effects as well as the ionospheric effects.

SKA has adopted the new technologies required for the next-generation of
calibration method MultiView, i.e. multiple tied-array beam technologies in
this particular case. It is imperative that we ensure that the keystone
telescopes that make up the EVN are equally prepared for these new techniques.
Thus it is urgent that a roadmap for the telescope network, that includes
fleshing out the requirements of individual EVN partners and for the various
technological multiple-pixel solutions, is developed. As part of this, further
practical end-to-end demonstrations are vital to define what is needed (e.g.
number of beams, angular range) that will affect the technological options.
These technological upgrades will impact the VLBI observations of the EVN with
the SKA, but also the capabilities of the EVN as a stand alone array both for
astrometry and survey speed. Furthermore, these considerations are also of
interest to FAST-VLBI and ngVLA-LONG and space VLBI astrometry.

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