2022

[1]Matthias Schartner

These authors contributed equally to this work.

These authors contributed equally to this work.

These authors contributed equally to this work.

These authors contributed equally to this work.

These authors contributed equally to this work.

[1]Institute of Geodesy and Photogrammetry, ETH Zurich, Robert-Gnehm-Weg 15,
Zurich, 8093, Switzerland

2]NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, 20771,
Maryland, USA

3]Bundesamt für Kartographie und Geodäsie, Sackenrieder Str. 25, Bad Kötzting,
93444, Germany

#  aVGOS VLBI Intensives between macgo12m and wettz13s for the rapid
determination of UT1-UTC

<EMAIL_ADDRESS>Leonid Petrov<EMAIL_ADDRESS>Christian Plötz
<EMAIL_ADDRESS>Frank G. Lemoine<EMAIL_ADDRESS>Eusebio Terrazas<EMAIL_ADDRESS>Benedikt Soja<EMAIL_ADDRESS>*  [  [

###### Abstract

In this work, we present a status update and results of the designated
research and development VLBI Intensive program VGOS-INT-S, observed between
macgo12m and wettz13s for the rapid determination of the Earth’s phase of
rotation, expressed via UT1-UTC. The main novelty of these sessions is the use
of a special observation strategy, rapidly alternating between high- and low-
elevation scans, enabling an improved determination of delays caused by the
neutral atmosphere. Since 2021, 25 Intensive sessions have been observed
successfully. In early 2022, VGOS-INT-S was among the most accurate Intensive
programs with an average formal error $\sigma_{\text{UT1-UTC}}$ of
$3.1\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$ and a bias w.r.t. IERS C04
of $1.1\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$. Later, the session
performance decreased due to multiple technical difficulties.

###### keywords:

VLBI, Intensives, VGOS, IVS

##  1 Introduction

Among the space geodetic techniques, Very Long Baseline Interferometry (VLBI)
is able to provide the most accurate and unbiased estimates of the angle of
the Earth’s orientation with respect to the rotation axis, expressed via
UT1-UTC. Since 1984, regular observing campaigns have been launched for
monitoring Earth orientation parameters (EOP) including UT1-UTC. They are now
coordinated by the International VLBI Service for Geodesy and Astrometry (IVS)
Nothnagel2017 . Most sessions of the IVS observing campaigns for EOP
determination either run for 24 hours to determine the full set of EOP, or for
1 hour to determine solely UT1-UTC. The 24-hour programs run 2–3 times a week
with a latency between observations and delivery of EOP estimates of about
15–20 days. The 1-hour programs on average run 2–3 times a day and the latency
between observations and delivery of UT1-UTC estimates is 1–3 days. For that
reason, these campaigns are called Intensives. Nowadays, a number of VLBI
Intensive programs dedicated to the estimation of UT1-UTC run in parallel.

VLBI observations commenced in 1967. Since then, the VLBI technique went
through a number of upgrades. The most recent upgrade is called the VLBI
Global Observing System (VGOS) Niell2018 . The changes in that upgrade,
relevant to the present study, are faster slewing speeds of $12^{\circ}$ over
azimuth and $6^{\circ}$ over elevation, combined with an increased data rate
of currently $8\text{\,}\mathrm{G}\mathrm{b}\mathrm{p}\mathrm{s}$ distributed
among four bands. The fast slewing rates reduce the time when the antenna is
slewing and thus not recording signals. The higher data rate allows for
shorter observation times to reach the desired signal-to-noise ratio (SNR).
Combined, this leads to a significantly increased number of scans per hour, up
to 100, allowing for faster sampling of the atmosphere, which is considered
one of the major error sources in VLBI.

In the current geodetic catalogs, there are over 200 extragalactic radio
sources that can be observed by VGOS radio telescopes and up to 100 sources
can be observed in one hour. Thus, the number of combinations of sources that
can be selected for observations is extremely large. We are interested in
developing new techniques for the generation of optimal observing plans, the
so-called schedules, which provide UT1-UTC with minimum errors. The
theoretical basis of the development of an optimal schedule was described in
(r:sch_phd, ).

In order to verify the optimized scheduling algorithm, we launched a research
and development Intensive VLBI observing program, named VGOS-INT-S, on the
$8418\text{\,}\mathrm{k}\mathrm{m}$ long baseline between macgo12m and
wettz13s in 2021. Station macgo12m, also known as Mg, is located in Western
Texas, USA, and station wettz13s, known as Ws, is located in Northeast
Bavaria, Germany. Here, we outline the design of the observing program and
discuss preliminary results.

##  2 Methods

The major error source in geodetic VLBI is mismodeling of the path delay in
the neutral atmosphere. The a priori atmospheric path delay can be computed
either using a regression model of surface atmospheric pressure and air
temperature or by direct integration of equations of wave propagation through
an inhomogeneous refractivity field that can be derived from the output of
numerical weather models. In both cases, the accuracy of the a priori path
delay is still insufficient and we have to estimate the residual path delay in
the zenith direction from the VLBI data themselves.

During the analysis, atmospheric delays are commonly divided into a
hydrostatic and a non-hydrostatic (wet) part. While the hydrostatic part can
be modeled with sufficient accuracy, the wet part has to be estimated due to
its higher variability. Historically, one zenith wet path delay (ZWD) per one-
hour observing session was estimated. With the fast slewing VGOS antennas, we
can develop a scheduling strategy that would allow us to estimate atmospheric
path delay with segments as short as 5 minutes. To enable a more frequent
estimation of ZWD, special emphasis has to be laid on providing observations
at different elevation angles within the estimation interval.

Following this idea, a new VLBI observation strategy has been developed for
VGOS-INT-S and applied using VieSched++ (Schartner2019, ). Due to the special
geometry of the Mg/Ws baseline with its baseline length of
$8418\text{\,}\mathrm{k}\mathrm{m}$, observations at high elevation on one
station naturally result in low elevation at the other station as depicted in
Figure 1.

Figure 1: Mutual visibility color-coded by the elevation of the partner
telescope. The black lines represent the station horizon masks while the
dashed gray line marks the theoretical horizon.

Although longer baselines are potentially more sensitive to UT1-UTC, they also
have a limited mutually visible sky. For example, the frequently observed
kokee/wettzell baseline has a length of
$10\,358\text{\,}\mathrm{k}\mathrm{m}$, resulting in a maximum observable
elevation of only $\sim 65\text{\,}{}^{\circ}$. This can potentially result in
a worse determination of the ZWD and thus UT1-UTC.

The new observation strategy is based on rapidly alternating between high and
low-elevation scans to allow for a higher frequent ZWD determination.
Therefore, the following scan sequence is repeated:

  * •

scan with high elevation at Mg (low elevation Wz)

  * •

scan without constraints

  * •

scan with high elevation at Wz (low elevation Mg)

  * •

scan without constraints

Thus, every other scan is especially dedicated to measuring ZWD. The remaining
scans are selected in a way to increase the sensitivity towards UT1-UTC, e.g.
by observing sources located at the corners of the mutually visible sky
(Schartner2021a, ), or by reducing potential systematic errors caused by
source-structure effects via observing a high number of different sources.

The effect of this special observing strategy is illustrated in Figure 2,
which depicts the distribution of observations in azimuth and elevation.

Figure 2: Scan distribution for station Ws. Left: new observation strategy.
Right: old Intensive observing strategy. The darker the color, the higher the
number of observations in this area. The black lines represent the station
horizon mask while the dashed gray line marks the theoretical horizon.

While the distribution is more balanced using an old observing strategy, two
clear clusters are visible with the new observing strategy, one at high
elevation and one at low elevation. This confirms that the observing strategy
is working as intended.

##  3 Data

The VGOS-INT-S observing program started on December 7th, 2021 with session
S21341. In the years 2021 and 2022, 25 sessions have been observed
successfully. Visibility data, geodetic databases, and results of the analysis
are available at the IVS Data Centers111https://ivscc.gsfc.nasa.gov/sessions/.

The first sessions S21341–S22011 were scheduled conservatively, using a fixed
integration time of 30 seconds independent of the source brightness and thus
mimicking the current 24-hour VGOS-OPS mode to gather some experience on the
new baseline. Afterward, the integration time was reduced to increase the
number of observations per session, and an SNR-based observing time based on
the source brightness and antenna sensitivity was utilized. For S22018–S22053
the minimum integration time was set to 15 seconds, while it was lowered to 12
seconds in the remaining sessions while the maximum allowed integration time
(except for calibrator scans) was set to 30 seconds in all sessions. The
target SNR per band was set to 15 for all sessions until S22053, while it was
lowered to 10 between S22060–S22095, before being increased to 12 again for
all sessions after S22109.

To troubleshoot existing hardware-related problems at the stations, two
special sessions have been designed. First, S22277 was split into two
sections. The first 30 minutes were observed regularly with an SNR-based
integration time and a minimum of 12 seconds while the second 30 minutes were
scheduled using a fixed 30-second long integration time. Second, S22284 was
scheduled including onsa13ne (Oe; Sweden) to provide the independent baseline
Mg/Oe.

Furthermore, two sessions, namely S22213 and S22215, were scheduled using a
different scheduling software instead of VieSched++.

Figure 3 depicts the number of scheduled and successful (defined as analyzed
by the NASA analysis center) observations per session.

Figure 3: Number of scheduled observations (blue bars), analyzed observations
(blue hatched), and percentage of analyzed observations (orange)

Some sessions suffered from a significant number of non-detections, mostly
explained by hardware failures. For S22067, 23 scans were not recorded at
station Mg (from 19:45 until 20:00 UTC). For S22074, 12 observations were
rejected during analysis due to large residuals. Similarly, 22 observations
were rejected for S22214 and 12 observations for S22263.

A list of the most important technical problems affecting VGOS-INT-S is
provided in Table 1. Please note that the dates listed in this table are
approximations. In some cases, it is no longer possible to find out exactly
when a problem first occurred. For some of the listed problems, it is also
unclear how much they affected the performance of VGOS-INT-S in general.

Table 1: List of technical difficulties encountered during VGOS-INT-S sessions. date |  | issue | effect
---|---|---|---
2022 Jan–May | Ws | encoder error | short downtimes
2022 Feb | Mg | compressor shutdown | downtime
2022 May | Mg | elevation motor coupler failed | downtime
2022 May–2023 Feb | Ws | LNA failure | reduced sensitivity
2022 Jun | Mg | power failure of azimuth motors | downtime
2022 Jul | Mg | problems with azimuth motor stop | downtime
2022 Jul–2023 Feb | Ws | 2nd LNA failure | reduced sensitivity
2022 Aug | Mg | HubPC disk failure | downtime
2022 Aug–Sep | Mg | failure of M700 compressor unit | reduced sensitivity
2022 Jul–Aug | Mg | phase cal signal failure | reduced sensitivity
2022 Nov–2023 Feb | Ws | dewar failure | downtime
2023 Jan | Mg | compressor failure | downtime

##  4 Results

Figure 4 depicts the VGOS-INT-S precision and accuracy from the GSFC
operational solutions produced with the Solve/$\nu$Solve software package.

Figure 4: VGOS-INT-S accuracy (top) and precision (bottom) extracted from the
GSFC analysis reports. Accuracy: UT1-UTC estimate w.r.t. IERS C04, the
3$\sigma$ value is depicted in the error bars. Precision: UT1-UTC formal error
$\sigma$.

We use two measures of UT1-UTC errors: formal uncertainties derived from the
observation SNR following the law of error propagation, called here precision,
and the differences of the UT1-UTC estimates and the IERS C04 time series,
called here accuracy. To interpolate the daily IERC C04 UT1-UTC values to the
Intensive reference epoch, first, tidal effects with periods $<35$ days were
subtracted, followed by a Lagrangian interpolation of order four and re-adding
the previously subtracted tidal effects to ensure minimal interpolation error.
It is clearly visible that the formal errors between S22025 – S22081 are
significantly smaller compared to the remaining sessions. Within this period,
the average formal error $\sigma_{\text{UT1-UTC}}$ is
$3.1\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$ while the offset w.r.t.
IERS CO4 is $1.1\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$ and the root
mean square error (RMSE) is
$31.7\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$. For the remaining
sessions, the average formal error is increased to
$9.8\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$, the offset w.r.t. IERS
C04 is increased to $-2.1\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$, and
the RMSE is increased to $80.7\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$.
The increase in uncertainty can be explained by the technical problems
encountered at the stations as listed in Table 1. Very small formal
uncertainties and good agreement with IERS C04 between S22025 and S22081
suggest that superior precision in UT1-UTC determination at the Mg/Ws baseline
can be achieved. For comparison, during the same time, the VGOS-INT-A sessions
at the kokee12m/wettz13s baseline achieved an average formal error of
$4.3\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$ with an offset w.r.t. IERS
C04 of $-8.6\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$ and a RMSE of
$28.7\text{\,}\mathrm{\SIUnitSymbolMicro}\mathrm{s}$, although according to
simulations, it is expected that based on the baseline geometry alone, VGOS-
INT-A should be $40\text{\,}\%$ more sensitive towards UT1-UTC compared to
VGOS-INT-S (Schartner2021a, ).

Here, it is to note that the GSFC operational analysis presented above does
not yet make use of a more frequent estimation of ZWD enabled by the special
observation strategy. Instead, ZWD is parameterized as a constant offset only,
similar to all other Intensive sessions. This highlights that the proposed
scheduling strategy, while having potential benefits through allowing a more
frequent parameterization of ZWD, still provided highly accurate UT1-UTC
estimates even when a traditional parameterization was used.

Figure 5: Comparison of various VGOS and S/X Intensive operation programs.
Accuracy (top) and precision (bottom) were extracted from the GSFC analysis
reports. Accuracy: UT1-UTC estimate w.r.t. IERS C04, the 3$\sigma$ value is
depicted in the error bars. Precision: UT1-UTC formal error $\sigma$.

Finally, Figure 5 compares the VGOS-INT-S performance with other Intensive
programs, including the previously mentioned VGOS-INT-A series, the VGOS-INT-B
and VGOS-INT-C series observed between the Onsala twin stations onsa13ne,
onsa13sw and ishioka, and several well established S/X series IVS-INT-1, IVS-
INT-2, and IVS-INT-3 Baver2020 ; Schartner2022 . From this comparison, the
especially good performance of VGOS-INT-S until April 2022 is visible. During
this time, VGOS-INT-S performed best compared to all other Intensive programs.

##  5 Summary

We presented a design of a research and development VLBI observing program for
the determination of UT1-UTC at a single baseline between fast slewing
radiotelescopes wettz13s (Germany) and macgo12m (Texas, USA).

Although the telescopes suffered a number of technical failures, the results
are very encouraging. Despite the shorter baseline length compared to more
typical Intensive sessions and the resulting theoretical lower sensitivity
towards UT1-UTC, the VGOS-INT-S sessions performed exceptionally well during
the first part of 2022. We plan to continue the campaign and investigate
errors of UT1-UTC determination in detail. We further plan to make use of a
more frequent ZWD estimation during analysis, enabled by the new scheduling
strategy presented in this work.

## References

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