We must compare our K-band astrometric solutions using independent geodetic VLBI analysis software packages, e.g. VieVS and CalcSolve (e.g. Krásná et al., 2023).

Troposphere: ongoing investigations on high-airmass, low-elevation scans are present in our data. We plan to refine the elevation-dependent weighting used in our geodetic analysis. We are also investigating appropriate models for atmospheric conditions.

Ionosphere calibration: a challenge for K-band is the imperfect ionosphere calibration based on GNSS data due to a lack of dual-band observations. We plan to incorporate the JPL R&D 15-min temporal resolution GNSS calibrations and 3D modelling (e.g. Soja et al., 2019) and improved ionosphere mapping functions in our geodetic analysis (e.g. Krásná et al., 2023). A collaborative effort between South Africa (SARAO), Switzerland (ETH), and Austria (TU Wien), funded through Leading House Africa and Swiss TPH, will drive the development of a machine learning-based ionospheric model for K-band. Known as KOSMIC (K-band VLBI Observations with Improved Scheduling and Ionospheric Corrections), this project is set to commence in February 2023.

We aim to complete the imaging of all VLBA astrometric K-band sessions (from 2019 to date) to characterise the source structure and its variation over time. 

Source structure: CRF sources generally appear more compact at K versus X-band, but can still exhibit measurable extended emission. Considering the effect of source structure in the modelling will become necessary. This will require continuous imaging and monitoring of source structure (e.g. de Witt et al., 2023) and correcting for the corresponding effects in the astrometric and geodetic data calibration and analysis (e.g. de Witt et al., 2023).

We plan to include full Stokes polarisation imaging for our VLBA  K-band 4-Gbps, dual-polarisation sessions. The fraction of polarised light may help to identify which component is the core (or base of the jet) and thus the point of reference for the source. The core region of AGN (closer to the black hole and origin/base of the jet) are often characterised by weak polarisation, which tends to increase with greater distances from the jet base.  

Currently, the VLBA has geodetic-quality GNSS receivers at only five of their ten sites.  Plans are underway to install geodetic-quality stations at the five VLBA sites without GNSS, thereby improving the ionospheric calibration.

In the longer term, higher radio frequencies may benefit from a broad-band receiver, covering 8 to 36 GHz,  being prototyped for the VLBA (Kooi et al., 2023).  This prototype, installed at the Owens Valley VLBA site, achieved its first fringes at X and K-bands in July 2023. A full 24-hour pass including astrometry and imaging was successfully conducted on 2023 Aug 31. If this system is funded for operational use, it has the potential to enable simultaneous X/K (8/24 GHz) and X/Ka-band (8/32 GHz)  observations.

Higher data rates: the VLBA is planning to upgrade to a data rate of 8 Gbps (current VLBA K-band sessions are conducted at 4 Gbps). There are plans to upgrade southern K-band sessions and K-band sessions on the Korean VLBI Network (+HartRAO+Yebes+Mopra) from 2 Gbps to 4 Gbps. 

Additional antennas: There are plans to further increase the sensitivity of our K-band astrometric observations, e.g. to augment the VLBA with the 50-meter Large Millimeter Telescope (LMT) in Mexico (Kurtz et al., 2020) which would almost double the sensitivity of VLBA baselines and to add the 40-meter Thai National Radio Telescope (TNRT) in Thailand (Jaroenjittichai et al., 2022) to the KVN+ network. There are also plans to add more European VLBI Network (EVN) antennas with K-band capability to the current K-band network.

Wider bandwidth: The possible deployment of a broadband (8-36 GHz) receiver on the VLBA (Kooi et al., 2023) would provide almost a factor of three increase in sensitivity by allowing analog bandwidth as large as 4 GHz per band.

The biggest challenge for improving the precision of the K-CRF is the need for a more uniform global network geometry. The current K-band CRF exhibits significant weaknesses in precision along the declination direction compared to the right ascension direction. 

More Southern stations (e.g. in Australia, Thailand, and South America) and more North-South baselines (e.g. the recently added Korea to Australia and South Africa to Spain baselines) are needed to improve the declination precision by at least a factor of two. 

Adding additional EVN stations with K-band capability to the K-CRF network will significantly improve the u,v-coverage.

Our goal is to merge all global high-frequency astrometric/geodetic VLBI efforts into one service;
(1) Ongoing K-CRF imaging/astrometric observations (K-CRF adopted as part of ICRF3 in 2018 Charlot et al., 2020).
(2) EVN K-band geodetic observations for station location maintenance (Gomez et al., 2021).
(3) KVN K-band calibrator catalog (e.g. Jung 2018) and the ongoing K-band geodesy campaign on the EAVN (e.g. Xu et al., 2021). 

Our goal is to make our astrometric and geodetic K-band products available to the community:
(1) K-CRF astrometric solutions, for inclusion in the next generation ICRF and comparisons with S/X and  X/Ka-band CRFs and the Gaia optical CRF.
(2) K-band geodetic products such as EOP and station positions (e.g.  Krásná et al., 2022 & Gomez et al., 2021), for multi-frequency and  multi-technique comparisons and combination approaches.

Increased collaboration between the fields of astronomy and geodesy:
(1) Make our database of high-resolution, multi-epoch K-band images (e.g. de Witt et al., 2023) available to the astronomical community.
(2) Combine astrometric/geodetic and astronomical observing sessions, where possible, to be more efficient and to make better use of available resources. 

Observations to explore the construction of CRFs at even higher radio frequencies, such as Q (43 GHz) and W (86 GHz) bands, are being investigated. At Q-band, a recent exploratory imaging-astrometry project to compare the structure of ICRF sources at S-, X-, K-, and Q-band (e.g. de Witt et al., 2022 &  Hunt et al., 2023) showed promising results for the construction of a CRF at Q-band.

The K-CRF needs dual-band X/K observations for improved ionosphere calibration. See the above comments on the possible use of the newly designed JPL broadband receiver for simultaneous X/K-band observations (Kooi et al, 2023).

At Q-band the ionosphere is negligible, but the sources are weaker. Recent studies show that Q-band can be a viable option for CRF work (e.g. de Witt et al., 2022).

At W-band the ionosphere is not a problem, but observations may be limited by efficiency and pointing issues.

The Korean K/Q/W/D-band (22/43/86/129 GHz) receiver system will empower K-band and perhaps Q and W-band CRF work at many new sites (e.g. Jung 2023).