Summary of the Fourth ALMA Phasing Project (APP)
Commissioning and Science Verification Mission:2016 April 3-8
ALMA Technical Note: Number 19
Status: Final

Prepared by:

Organization

Date

L. D. Matthews
G. B. Crew
V. L. Fish

MIT Haystack Observatory2017 March 28

Memorandum

To: The ALMA Community
From: L. D. Matthews, G. B. Crew, & V. L. Fish (MIT Haystack Observatory)
Date: March 28, 2017
Subject: Summary of the Fourth ALMA Phasing Project (APP) Commissioning andScience Verification mission: 2016 April 3-8

1

Background and Objectives

The fourth Commissioning and Science Verification (CSV) mission for the ALMA Phasing
Project (APP) was carried out during the dates of April 3-8, 2016. Descriptions of prior
APP CSV campaigns can be found in ALMA Technical Notes1 16 through 18 and references
cited therein.
The primary objectives of the fourth APP CSV campaign were twofold: (1) to execute Very Long Baseline Interferometry (VLBI) observing mode (VOM) observations using
Schedule Blocks (SBs); (2) to carry out the first end-to-end testing of intercontinental
VLBI sessions in both Bands 3 and 6. While intercontinental VLBI fringes with ALMA
have already been obtained during previous CSV campaigns (Matthews & Crew 2016b, c),those sessions were not conducted in a manner identical to future VLBI science campaigns(i.e., they used manual execution of observing commands rather than SBs) and did not
involve observations of a full suite of ALMA and VLBI calibrators. Secondary objectives ofthe fourth CSV mission included further development work on an ALMA Phasing System(APS) graphical user interface (GUI), additional testing of the fast phasing loop (under awider variety of weather conditions), tests of the phasing system in Band 7 (in support ofan ongoing North America ALMA Study award), and the training of ALMA staﬀ in theoperation of the VOM and the APS hardware and software.
At the time of the fourth CSV mission, proposal calls announcing the availability of
the APS for VLBI observations in Cycle 4 had already been issued. Despite this, criticalaspects of APS operations necessary for successful VLBI science campaigns had not yet
been fully commissioned. These included:• Operation of VLBI-mode observing using SBs generated with VEX2VOM.2
1https://almascience.nrao.edu/documents-and-tools/alma-technical-notes/
VEX2VOM is an APP software tool that translates information from a standard VLBI Experiment(VEX) file (including information about VLBI sites, scan timing, and other ancillary information) into
information needed to successfully execute the VLBI Observing Mode (VOM) at ALMA.
21• Successful execution of a full >3-hour3 VLBI session, including observations of allnecessary ALMA and VLBI calibrators.
• Full (post-correlation) testing of the PolConvert software (Martı́-Vidal et al. 2014,2016a, b) to convert ALMA’s linearly polarized correlation products into circularpolarizations.
• End-to-end calibration and analysis of a VLBI data set that includes phased ALMA
as a VLBI station.

2

Personnel

APP personnel on site during the April 2016 CSV campaign included Geoﬀrey Crew and
Lynn Matthews (MIT Haystack Observatory), Helge Rottmann (Max Planck Institut für
Radioastronomie = MPIfR), and Alejandro Saez (Joint ALMA Observatory = JAO). The
ALMA Science lead during the week was Lars-Åke-Nyman (JAO). The APP team received
additional on-site support during the mission from Violette Impellizzeri (JAO), who has
been designated as ALMA’s “Friend of VLBI”, and Akihito Hirota (JAO), who had beenpreviously designated as the APP’s contact for assistance with the implementation of VLBISBs.
The APP was also supported by colleagues stationed at other remote sites, including
Eﬀelsberg (operated by MPIfR), the Submillimeter Array (SMA; operated by the Smithsonian Astrophysical Observatory), the James Clerk Maxwell Telescope (JCMT; operated by
the East Asian Observatory), the Heinrich Hertz Submillimeter Telescope (SMT, operated
by the University of Arizona), and the Large Millimeter Telescope (LMT; operated by the
University of Massachusetts, Amherst and the Instituto Nacional de Astrofisica Óptica yElectronónica). Lastly, Vincent Fish (MIT Haystack) was responsible for preparation of
VLBI observing schedules.

3

Resources and Set-Ups Employed

The observations were prepared with the Cycle 4 Observing Tool (OT), which had justbeen released for the Call for Proposals, in the same manner as planned for Cycle 4 science
projects. For the VLBI sessions, a “science” target was selected (i.e., a relatively brightquasar of flux density 1-2 Jy), along with a suite of VLBI and ALMA calibrators of thetype needed to conduct a fully calibrated VLBI science experiment.

3.1

Software Release

The APP used software release R2015.8 during the mission. This release had only limited
on-sky testing by ALMA staﬀ prior to the time of the campaign and had not been tested
3

The minimum duration of a VLBI session is set by the requirement to obtain adequate parallactic
angle coverage on a polarization calibrator in order to enable robust conversion of the ALMA data fromlinear to circular polarization (Martı́-Vidal et al. 2016a, b).

2

at all in the FDM correlator mode used for APS operations. Because ongoing ALMA
Cycle 3 science observations required an earlier software release, all APP activities during
the mission necessitated a software switch. This procedure results in ∼1 hour of downtime
during which ALMA cannot observe.

3.2

Antennas

The ALMA antennas were in configuration C36-2/3 during the mission, providing baselines
between 15 m and 460 m. Arrays of up to approximately 40 antennas (including both 12-m
and 7-m antennas) were used by the APP for testing.

3.3

Remote VLBI Stations and Observing Dates

Partner sites for the series of VLBI experiments to be conducted during the mission werearranged in advance. For 1.3 mm (Band 6) VLBI testing, the APP coordinated with the
Event Horizon Telescope Consortium (EHTC), which was conducting a science campaign
during dates overlapping with the APP CSV mission. Stations participating in the 1.3-mm
network included the LMT in Mexico, the SMA and JCMT in Hawaii, and the SMT in
Arizona. However, only two of these sites were able to participate in VLBI during ALMA
CSV time (see Section 5.6).
To enable 3 mm (Band 3) VLBI testing, the APP submitted a Director’s Discretionary
Time proposal to the National Radio Astronomy Observatory (NRAO) for time on the VeryLong Baseline Array (VLBA). This proposal was awarded 4 hours of coordinated VLBA
observing time (program BM452A). Unfortunately, because of poor weather at ALMA, thisprogram could not be executed during the mission. A make-up observation was scheduled
on 2016 July 10. Results from that campaign will be described in a future ALMA TechnicalNote. Despite the postponement of the VLBA session, a 4-hour Band 3 VLBI test session
was executed between ALMA and Eﬀelsberg on 2016 April 5. However, it did not resultin the detection of VLBI fringes (see Section 6.1).

3.4

Time Allocation

In total, approximately 30 hours of ALMA time were devoted to APP CSV activities
and APP-related testing during the April 2016 campaign. Approximately 30% of these
activities were conducted during engineering time, and the remainder were during regular
ALMA science time. However, a large fraction of the “science” time used by the APP wasin fact during intervals when the weather was unsuitable for carrying out any of the ALMA
science programs in the Cycle 3 observing queue.

3.5

Observing Targets

VLBI targets selected for the mission comprised well-known bright (>1 Jy) quasars andother compact sources from the ALMA Source Catalogue that are known to be unresolved
on scales of the longest ALMA baselines. Standard ALMA calibrators (including phase,3

bandpass, flux, and polarization calibrators) were also observed as part of the global VLBI
sessions.

3.6

Data Collected

Approximately 480 GB of standard ALMA interferometry (ASDM) data were collected inBands 3 and 6, and ∼6 TB (per quadrant) of VLBI-mode data were recorded.

4

Background: Known Issues Leading Up to the Mission

As described in Matthews & Crew (2015c), a problem with the ALMA station cards impacted the quality of the phasing data obtained during the last major APP CSV campaign
in July/August 2015. A solution to that issue was subsequently identified and implemented
in late August 2015 (see http://ictjira.alma.cl/browse/ICT-5401). Following that fix, APS
data acquired during test sessions executed by JAO staﬀ in late August and early September were found to be of excellent quality, with high phasing eﬃciency (≥90%; Crew &Matthews 2015).
On November 11, 2015, some additional observations with the APS were carried out by
JAO staﬀ to test a software modification that enabled implementation of 16 or 32 spectral
windows (spws) across a single baseband instead of the previously permitted maximum ofeight.4 Two observations of several minutes duration were obtained in Band 3 on the brightquasar 1924-292. There were 38 antennas in the array, 37 of which were phased. Baselines
ranged from ∼250 m to ∼ 14.2 km.
Examination of the data sets obtained on November 11 displayed poor phasing quality
despite what should have been satisfactory Band 3 observing conditions. Figure 1 showsphases as a function of time for all baselines between phased antennas for one of the data
sets. Properly phased data should show all points clustering near zero phase. Instead, thephases are seen to scatter throughout the range ±180◦ for nearly the entire observation.
Figure 2 shows the behavior of phase versus time for several baselines plotted individually. Inspection shows that upon phase-up, the dispersion in the data points (caused byphase oﬀsets between the individual spws in each baseband) drops markedly. However,instead of converging near zero, the corrected phases in all four basebands cluster near aseemingly random phase, after which the phases drift with time. This behavior is seen on
all baselines despite the continuous application of phasing corrections.
We performed additional oﬀ-line tests in CASA, computing corrections to the phases
using a single phase-only iteration of self-calibration. Solution intervals as coarse as 60
seconds dramatically improved the phases, suggesting that neither the weather nor thelong baselines alone could fully account for the behavior in the raw data.
Our next opportunity to examine data acquired with the APS did not occur until March
2016, at which time data were taken by JAO staﬀ for the purpose of testing VLBI SB
4

See http://ictjira.alma.cl/browse/ICT-4656.

4execution. Observations were again performed in Band 3. In this case, the array contained
45 antennas, 37 of which were phased. Baseline lengths ranged from 15 m to 460 m.
Poor phasing quality and drifting of phases with time were again seen in those data.
Figure 3 shows phase as a function of baseline length for all pairs of phased antennas.
Only for the few shortest baselines do the phases lie near the expected values near zero.
For longer baselines, not only does the phase dispersion increase, but points appear tobe preferentially absent from the region around zero phase. This behavior suggests thatthe phases are not merely random (as would occur, for example, if the phasing systemwere turned oﬀ, or the weather were extremely unstable), but instead that either invalid
corrections are being applied, or else that the corrections are someone being systematicallycorrupted. However, no log messages that implied incorrect commanding or missed phasing
applications were found. Since no part of the phasing software was modified following the
successful phasing observations in September 2015, this suggests that the computed phasingcorrections are being corrupted elsewhere in the signal chain.
Subsequent APS observations obtained during the same March 2016 session showedfurther deteriorating phasing behavior. Examination of the phase as a function of frequency
for these data revealed phase wrapping and steep phase slopes, indicative of delay errors.
Furthermore, these slopes changed as a function of time (Figure 4).
In an attempt to diagnose the source of the problem, further tests on the data was
performed in “oﬀ-line” mode—i.e., by manually applying the phasing algorithm on the
newly obtained ALMA data sets. In the event the phasing software had simply not been
operational during the real-time observations, this oﬀ-line processing should have resulted
in well-phased data. Instead, these tests showed that while the phasing software could
sometimes find correct-looking solutions post facto, at other times, both the raw and oﬀline corrected data looked poor.
Several noteworthy changes occurred at ALMA between mid-September 2015 (whenphased data were seen to be of excellent quality) and the next APS test on November 12,
making it challenging to diagnose and isolate the source of the recurring phasing issues.
First, ALMA began a switchover from 32-bit to 64-bit computing systems. Second, theyswitched python version from 2.6 to 2.7. Finally, the November 12 test was the first
APS test performed with software R2015.8 instead of R2015.6. While in principle, regularregression testing of the APS by ALMA staﬀ would have helped to pinpoint any ALMA
updates that produced changes in the behavior of the phasing system, in practice, such
regression tests were largely precluded during Cycle 3 because of the diﬀerent software
version requirements for APS operations compared with normal science operations. There
were a number of debugging investigations launched, with no “smoking-gun” explanation
for the problems. In particular, the Data Capture and Delay systems were checked overthoroughly. At the time of this memo, these behaviors remained unexplained. Importantly,
they did not appear again subsequently, including APS data taken in July 2016 or January
2017.

5

5

Summary of Activities and Results During the April
2016 CSV Campaign

Upon arrival, it quickly became clear that it would not be possible to run the project SBs
directly from the Archive in a dynamic/interactive array as was originally envisaged, andwhich had already been demonstrated in simulation during the previous December. The
issue was related to the hand-over of the Archive software from Cycle 3 (for operations)
to Cycle 4 (for new proposals); the SBs in question were not (yet) compatible with theArchive available in the Cycle 4 software candidate. Fortunately, the SSR testing harness
included a script to execute an SB as found in a disk file (e.g., SchedBlock1.xml), so it wasstraightforward to run this script with a manual array.

5.1

April 3: Standalone ALMA Testing

Because of the issues we had seen leading up to the April mission (Section 4), we began on
the morning of April 3 with a series of VLBI regression tests and DelayCal measurements
at ALMA using the Cycle 3 software release (R2016.6) and both TDM and FDM correlator
modes. During these tests, which included a phased array of 25 antennas, a fairly large
dispersion in the phases was seen (Figure 5). However, this appeared to be consistent withthe weather conditions [∼3 mm of precipitable water vapor (PWV) and quite unstable],and we saw no clear evidence for the types of behaviors seen during tests between November
2015 and March 2016 (Section 4). This tentatively suggested that the earlier problem(s)were linked with software release R2015.8, although we did not obtain suﬃcient data with
the Cycle 3 release to establish this with certainty.
Later in the afternoon of April 3, a switchover from the Cycle 3 software to R2015.8
was performed, and the APP Team carried out a series of tests over the course of several
hours. These were standalone ALMA tests (i.e., no other VLBI sites were recorded). Tests
included:• An initial regression test in the Cycle 4 candidate.
• Variation of the total number of antennas in the array, as well as the number andtypes of of antennas designated for phase-up, to assess impact on phasing quality.
• The execution of calibrator scan measurements as part of a VLBI SB.
Phasing behavior appeared acceptable during the initial regression test scan. However,in subsequent observations, evidence was seen that DelayEvents (DGCK) were not being
properly applied. (Subsequent detailed investigation of the DGCK logs turned up no obvious errors, and so the issue is either un-logged bugs, or weather, with the latter appearing
most likely.)
In some of the subsequent observations, phase slopes as a function of frequency were
present in data obtained on ALMA calibrators, indicating a problem with the application ofdelays. It was determined that the restoration of the baseband delay (BBD) was not being
done prior to the final polarization calibrator scans in the ALMA calibrator (“calalma”)
sequence. A revision was subsequently made to the way ALMA-specific calibrations were6

handled in order that the BBDs would get properly reset. [The default BBD corrections are
not applied during APS operations (i.e., are not applied during VOM scans; see Matthews& Crew 2015b), but are applied during the ALMA calibrator observations taken during a
VLBI session with ALMA.]
Despite this fix, in subsequent scans taken with the VOM, we continued to see generally sub-optimal performance by the phasing system, even in scans where the array size
and number of phased antennas were reduced. Signatures of delays issues were also seen
during some of the VLBI scans (e.g., Figure 6). No obvious problems were seen in thecorresponding CDP logs, suggesting that the issues were most likely weather-related.

5.2

April 4: More Standalone ALMA Tests

Additional standalone ALMA testing of the APS was undertaken on April 4. The APP teamwas granted an extended session during the afternoon (engineering time) and continuing
into the evening because the weather was unsuitable for executing any of the queued Cycle 3
science projects. The types of APS tests performed were similar to those on the previous
day, including additional tests of the observing script to insure that it would not skip key
calibrator observations because of timing issues. A forecast for poor weather at ALMA
led to the postponement of the scheduled 3-mm VLBI tests with the VLBA and Eﬀelsberg
until April 7 (but see Section 5.3).

5.3

April 5

Despite postponement of the 3-mm VLBI session with the VLBA, staﬀ at Eﬀelsberg agreed
to record a VLBI session with ALMA during a 4-hour window from 07:00-11:00 UTC,
providing a useful “dress rehearsal” of end-to-end VLBI operations.
Conditions at Eﬀelsberg were poor, with high humidity and drizzle (PWV∼18 mm).
PWV at ALMA ranged from ∼5-6 mm during the session, but conditions were nonethelessquite stable. The ALMA array included 40 antennas (both 7-m and 12-m dishes), 33 ofwhich were part of the phased array.
At ALMA, preparations for the VLBI session got underway approximately two hours
before the scheduled start time of the VLBI recording. The R2015.8 software was alreadyactive, so no software change was needed. However, several issues were encountered upon
start-up. First, for unknown reasons, the system failed to accept the first manually-definedantenna array. Subsequently, a correlator reset was required. The timing of the phasing
interface cards (PICs) was found to be oﬀ by 48 ms initially, and both PICs associatedwith correlator quadrant 4 were unresponsive.
Once these issues were resolved and all systems were operational, standard focus andpointing operations were performed. However, because of the earlier issues, the first VLBI
SB was launched slightly late (i.e., <15 minutes before the first VLBI scan). This caused
the observation of the ALMA polarization calibrator to be skipped over. Steps were taken
to prevent this from occurring in future executions, since the polarization calibrator measurements are critical for performing the linear-to-circular polarization conversion of ALMA7data for all APS observations, even those where the science objectives do not involve polarization measurements (see Section 7).
Following this series of start-up issues, the April 5 session with Eﬀelsberg allowed successful testing of the option to record only select correlator quadrants (in this case, quadrant 1) instead of all four quadrants. This recording option will be exercised during Cycle 4
science observations, since during Cycle 4, no other VLBI partner sites will match ALMA’s
total bandwidth. [We note, however, that even when only select quadrants are recorded in
VLBI mode, data from all four quadrants are still present in the standard interferometry
data (SFI) from the ALMA correlator and will be present in the ALMA ASDM file.]
Examination of ALMA SFI data from the first ALMA-Eﬀelsberg VLBI scans in nearreal time showed good phasing behavior, although it was discovered that the associated
CalAppPhase.xml table was missing entries for various subscans. This was found to berelated to the fact that some of the subscans during the session were never processed. A
subsequent investigation revealed that this was caused by a combination of two factors: (1)creation of the metadata took too long for some of the scans5 ; (2) the WVR calibration
scan prior to each VLBI scan encroaches on the first subscan. Minor adjustments to the
SSR script quickly solved this latter problem, and the metadata bug has since been fixed.
In all, approximately one hour of good data was obtained during the early part of the
VLBI session before a series of software problems disrupted the remainder of the session.
One issue was that “exceptions” were being periodically reported to TelCal by the pointing
software. It was recognized that this could be mitigated in the future by increasing thevalue of “MaxPointingSeparation” to avoid automatic pointing after every large slew (since
default ALMA pointing is suﬃcient for VLBI observing, particularly in Band 3).
Following the VLBI session with Eﬀelsberg, additional ALMA-only testing was performed during the afternoon, including installation of some software patches. Plans werealso made for Band 6 VLBI tests during the morning of April 6, in concert with the EHT.
However, later in the afternoon of April 5, weather conditions deteriorated at the ALMA
high site. Precipitation was reported, and operations were shut down around 17:00 local
time (20:00 UTC). Operations resumed ∼30 minutes later, but worsening conditions (freezing fog and sleet) led to the ALMA dishes being stowed for the night at around 23:00 UTC.
The planned Band 6 VLBI session was consequently postponed until April 8.

5.4

April 6: Simulated Band 6 VLBI

Owing to the shutdown status of the array, a Band 6 VLBI schedule was exercised insimulation mode during the early morning hours of April 6. (In this mode, actual receivernoise is recorded, but the antennas do not point). Two of the four SBs that comprised thefull schedule were successfully executed, after which the test was terminated. No aborts
were encountered, but the ALMA polarization calibrator was not “observed”. It was foundthat this was because the polarization calibrator that had been selected through a “query”
was no longer observable. To avoid this issue in the future, the decision was made that
the ALMA calibrators should be specified explicitly in the ALMA OT rather than being
selected through queries at the time of observation.
5

See http://ictjira.alma.cl/browse/ICT-7124.

8

5.5

April 7: Simulated Band 3 VLBIThe late afternoon/evening hours of April 6 exhibited a repeat of the weather patternof the preceding day, namely increasing clouds and humidity, culminating in freezing fog
conditions after sunset. This led to another shutdown of the Array during the early morning
hours of April 7, forcing a cancellation of the scheduled 3-mm VLBI session with the VLBA.
During the shutdown, a portion of the 3-mm VLBI schedule was run in simulation
mode (see Section 5.4). A time-out error occurred upon the initial start-up, but this waseliminated by reducing the array size from 59 to 40 antennas. The cause of this issue
remains unresolved at the time of this writing, but it appears to occur only during the useof simulated mounts (see http://jira.alma.cl/browse/PRTSIR-9441).

5.6

April 8: Band 6 VLBI Session with the EHT

A four-hour Band 6 (1.3-mm) global VLBI session was carried out on the morning of April
8 (local time). Because of weather (rain) and equipment issues (problems with receiverstabilization), respectively, the SMT and JCMT were unable to participate, leaving the
LMT and SMA as the two available partner sites.
The nominal schedule ran from 07:00-11:00 UTC, but the ALMA schedule was launchedapproximately 35 minutes prior to this to perform pointing checks and execute ALMAspecific calibrations, including observations of gain, bandpass, polarization calibrators instandard (non-VLBI) mode, as well as system temperature (Tsys ) measurements. The first
ALMA SB was launched on time and without any issues.
Weather at ALMA at the start of the session was nominal for Band 6 (PWV∼2.2 mm
and relatively stable). The ALMA array contained 39 antennas, 37 of which were partof the phased array. The SMA and the LMT sites both reported good weather and nomajor technical problems during the session. The SMA recorded 5 phased dishes in dual
polarizations, along with two single dishes in opposite polarizations.

6

Correlation

Following the April 2016 mission, the modules containing ∼6 TB of recorded ALMA VLBI
data were shipped to Haystack and arrived in late April. The subsequent correlation andanalysis of these data is described in the sections that follow.

6.1

Band 3

The Band 3 scans involving ALMA and Eﬀelsberg were copied oﬀ the disk modules andtransferred electronically from Haystack to Bonn in late May 2016. W. Alef (MPIfR) subsequently reported that a zero baseline correlation between the two backends at Eﬀelsberg
produced fringes, confirming that the telescope was able to see the target sources despite
the poor weather at that site. However, no fringes were found between Haystack andEﬀelsberg during multiple correlation attempts in late May and early June 2016.

9It was subsequently discovered that the Band 3 frequency settings at ALMA did notmatch the intended values owing to improperly specified “weighting” values in the spectral
specification. (Strictly speaking, this is an OT bug, but it has been resolved in VEX2VOM
by updating the Band 3 weights.) The underlying problem is that the frequencies of thefour bands may not be arbitrarily, independently assigned, and the configuration provided
to the LO system did not specify which band mattered. The fix was to weight the band
being recorded as 100% and downgrading the others to smaller values.
As of this writing, further fringe searches have not been attempted with the ALMAEﬀelsberg data, as eﬀorts instead were focused on the multi-baseline Band 3 VLBI data set
obtained in 2016 July. Results from the July Band 3 tests will be described in a forthcoming
ALMA Technical Note.

6.2

Band 6

6.2.1

Fringe Detection

The Band 6 global VLBI data obtained during April 2016 mission were correlated at
Haystack using the DiFX software correlator (Deller et al. 2007). “Zoom bands” wereused to compensate with the sampling diﬀerence between ALMA and the other EHT sites(Deller et al. 2011). Fringe-fitting was subsequently performed using fourfit, a componentof the Haystack Observatory Postprocessing System (HOPS).6 A full report on the correlation procedures used for the Band 6 data will be described in a forthcoming memorandum
by one of us (GBC).
High signal-to-noise ratio fringes were obtained on multiple sources between phased
ALMA, the LMT, and the SMA in both the “lo” and “hi” bands (e.g., Figure 7). These twobands are centered at 227.1 GHz and 229.1 GHz, respectively, each with a total bandwidthof 1.8 GHz. At the SMA site, fringes were obtained between ALMA and both the phased
SMA array and an independent SMA reference antenna.
Following the initial fringe detection, B. Corey (MIT Haystack) computed an improved site position for ALMA of X =2225061.55210 m, Y = −5440056.66740 m, Z =−2481685.66880 m which was then used during a re-correlation of the data. There wasno obvious improvement in the quality of the results, implying there may still be residual
errors in positions for all of the sites of up to ∼1 meter.

7

Polarization Conversion

The April 2016 global VLBI experiment represented the first ALMA VLBI experiment
during which a full suite of calibration measurements was obtained. This data set therefore enabled the most complete testing and evaluation to date of the PolConvert softwaredeveloped by the APP team for the purpose of converting ALMA’s linear polarization
products to circular polarization (Martı́-Vidal et al. 2014, 2016a, b). PolConvert is run
post-correlation, and for future ALMA VLBI science programs, this processing step will be6

http://www.haystack.mit.edu/tech/vlbi/hops.html

10performed for each Principal Investigator (PI) by ALMA staﬀ as part of the QA2 process
prior to the delivery of the VLBI data to the PI.
Results of the PolConvert testing performed with the April 2016 data are described indetail in Martı́-Vidal et al. (2016a) and will not be repeated here. In short, this crucial
calibration procedure is now fully operational (e.g., Figure 8).

8Band 6 VLBI Data Analysis

A preliminary analysis of the Band 6 VLBI data from the April 8 experiment was performedby one of us (VLF), primarily using tools available in HOPS. A summary of some initial
findings is provided below. While some of these findings merit follow-up investigation with
future data sets, no issues were identified that are expected to have a significant impact on
the scientific usefulness of the data. Indeed, the overall data quality from the experimentis found to be consistent with design requirements of the APS and suitable to meet thescience requirements of Cycle 4 VLBI observers.
Phase-Up: It was found consistently across all scans that the first 3 or 4 seconds of dataon ALMA baselines have much lower correlation coeﬃcients (“amplitudes”) than the restof the scan. This suggests that for optimal observing eﬃciency, the phasing loop should
be started approximately 5 additional seconds earlier than it was during this experiment.
This adjustment has now been implemented.
Dropouts: On many scans (e.g., Figures 9), the correlation coeﬃcient on ALMA baselines is seen to dip significantly for short periods of time. During these “dropouts”, thecorrelation coeﬃcients become progressively lower over a period of 15–35 seconds before
rather sharply recovering. As these drops occur simultaneously on the ALMA-LMT and
ALMA-SMA baselines, it is clear that the eﬀect originates at ALMA. This behavior indicates a deterioration of the phasing eﬃciency, the most likely explanation for which appears
to be atmospheric eﬀects (e.g., highly variable opacity).
Examining the phase as a function of time for baselines between phased ALMA antennas
during the scans shown in Figures 9, it is seen that indeed, scatter in the phases is highat the times corresponding to the “dropouts” in VLBI data (Figure 10). Furthermore,it can be seen in that the quality of the phasing is a clear function of baseline length;
phasing eﬃciency is consistently high for baselines shorter than 50 m (Figure 10, left), butshows much greater variation for baselines >150 m (Figure 10, right). This trend is further
underscored in Figure 11, which plots phase as a function of u-v distance for the samescans.
Late Starts: During a handful of scans, ALMA got oﬀ to a late start. In the example
shown in Figure 12, it appears that ALMA was completely oﬀ source for nearly the firstminute, then was unphased for a while, and then phased up. Indeed, running fourfit on
the ALMA-LMT baseline over the first 45 seconds fails to find a fringe, while running
fourfit over the last 45 seconds finds the fringe with SNR >420. Both the ALMA-LMT and
11

ALMA-SMA baselines are aﬀected, but not the LMT-SMA baseline (for which the SNR
over the first 45 seconds is 11.7).
Delay/Rate Drifts: When fourfit is used to find a fringe, the algorithm looks for a single
value of the residual multiband delay and a single value of the residual delay rate to fit
the data. However, these values are seen to be changing on ALMA baselines. When fourfit
is restricted to run individually on each of the 32 channels across the 1.8-GHz band, it
locks in on diﬀerent values of the delay rate (Figure 13). Alternatively, if fourfit is run on
all channels but restricted to consecutive 10-second segments of the data, it locks in on
diﬀerent values of the multiband delay. In both cases, the trend is linear.
These two eﬀects are manifestations of the same underlying issue. A delay is a changeof phase with frequency: ∂φ/∂ν, while a delay rate is a change of phase with time: ∂φ/∂t.
The fact that there is a drift in the residual multiband delay with time indicates that∂/∂t(∂φ/∂ν) ̸= 0. Likewise, the change in the residual delay rate with frequency indicatesthat ∂/∂ν(∂φ/∂t) ̸= 0.
We do not see evidence that these second-order partial derivatives are zero on nonALMA baselines. However, the eﬀect is subtle, requiring both very high SNR and verylarge spanned bandwidth to be noticeable.
Linear drifts of delay rate with channel are apparent on a number of high-SNR scans
on ALMA baselines (Figure 14). The plotted scan is typical of these drifts, showing an
increase of approximately 3 × 10−9 µs/s from one end of the ∼ 1.8 GHz frequency band tothe other. This produces a small but noticeable eﬀect, which will produce a corresponding
small loss of amplitude.

9

Progress on Secondary Objectives

Tests of a prototype python-based APS GUI designed by H. Rottmann were performed
during the mission and demonstrated the promise and utility of such a tool. Such a GUI
is envisioned to provide status on various aspect of VLBI and APS operations, as well as
to oﬀer some online commanding capabilities to adjust APS operations in real time (e.g.,changes in the specification of which antennas to include in the phased array). Based on
experience gained, updates and improvements to the GUI will be implemented and tested
during future observing sessions, subject to the availability of funding for the APP team.
Poor weather conditions during the mission precluded planned testing of the phasingsystem in Band 7 and further testing of the fast phasing loop. Neither of these capabilities
will be oﬀered during ALMA Cycle 4, and this work will be rescheduled as opportunities
allow.

10

Summary

A successful 4-hour Band 6 (1.3 mm) global VLBI session with phased ALMA was carriedout in April 2016 as part of APP CSV. To the extent possible, all aspects of the observations
were carried out in a manner identical to what will be used for conducting VLBI science12

observations with phased ALMA during ALMA Cycle 4. An exception is that an archiving
bug precluded the automated launch of the VLBI SBs. (This problem has since been
remedied). The session included observations of all calibration sources needed for end-toend processing of both the VLBI data and the standard ALMA interferometry data.
The resulting Band 6 data set resulted in fringe detections for multiple bright continuum sources (>1 Jy) on all three baselines of the ALMA-SMT-LMT triangle. Following
correlation of the VLBI data, the ALMA VLBI data were successfully converted from linear
to circular polarization products using the PolConvert software developed by the APP team(Martı́-Vidal et al. 2016a). A Band 3 (3 mm) global VLBI experiment scheduled for April
2016 was postponed until July 2016 because of poor weather at ALMA.
Additional information on the April 2016 APP CSV mission is available on the mission
wiki page. 7

11

References

Crew, G. B. & Matthews, L. D. 2015, “ALMA Phasing Project Phasing Eﬃciency Status
Report”, https://wikis.alma.cl/twiki/pub/DSO/VLBI/StatusReviewOctober2015/ALMA05.11.63.03-0001-A-REP.pdf
Deller, A. T., Brisken, W. F., Phillips, C. J., Morgan, J., Alef, W., Cappallo, R., Middelberg, E., Romney, J., Rottmann, H., Tingay, S. J., & Wayth, R. 2011, PASP, 123,
275
Deller, A. T., Tingay, S. J., Bailes, M., & West, C. 2007, PASP, 119, 318
Martı́-Vidal, I., Conway, J., Lindqvist, M., Roy, A. L., Alef, W., & Zensus, A. J. 2014,
PoS, http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=230
Martı́-Vidal, I., Crew, G., Matthews, L., and the APP Team. 2016a. https://wikis.alma.cl/
twiki/pub/DSO/VLBI/VLBITelecon15November2016/IMV MEMO APP VLBA B6 v6.pdf
Martı́-Vidal, I., Roy, A., Conway, J., & Zensus, A. J. 2016, A&A, 587, 143
Matthews, L. D. & Crew, G. B. 2015a, “Summary of the First ALMA Phasing Project
Commissioning and Science Verification Mission: 2015 January 6-13”, ALMA Technical
Note 16, https://almascience.nrao.edu/documents-and-tools/alma-technical-notes/
Matthews, L. D. & Crew, G. B. 2015b, “Summary of the Second ALMA Phasing Project
Commissioning and Science Verification Mission: 2015 March 24-30”, ALMA Technical
Note 17, https://almascience.nrao.edu/documents-and-tools/alma-technical-notes/
Matthews, L. D. & Crew, G. B. 2015c, “Summary of the Third ALMA Phasing Project
Commissioning and Science Verification Mission: 2015 July 28-August 3”, ALMA Technical
Note 18, https://almascience.nrao.edu/documents-and-tools/alma-technical-notes/
7

https://ictwiki.alma.cl/twiki/bin/view/Control/AppMissionChileCommissioning06.

13

Figure 1: Band 3 observations with the APS obtained on November 11, 2015. The target was the bright quasar 1924-292. Phase as a function of time is plotted for data forbaselines between all phased antennas (37 in total) from the first correlator quadrant,
XX and YY polarizations. Approximately 360 seconds of data are shown. Data source:
uid://A002/Xacba6b/X15.

14

Figure 2: As in Figure 1, but showing several baselines plotted individually. Here, only XX
data are shown, and the the four colors correspond to the four correlator quadrants. Oncephase-up is achieved near 19:26:55 UT, the dispersion in the phases within in baseband
decreases markedly, with values converging near zero phase, as expected. However, thephases subsequently drift, with the four quadrants varying in lock-step. A complete loss ofcoherence occurs near the end of the scan.

15

Figure 3: Band 3 data obtained with the APS on March 24, 2016. Phase is plotted asfunction of u-v distance for all baseline pairs of phased antennas. Note that only for the
few shortest baselines do the phases lie near the expected values near zero. For longer
baselines, not only does the dispersion increase, but points appear to be preferentiallyabsent from zero phase. This behavior suggests that the phasing corrections are being
corrupted. Data source: uid://A002/Xb0dfe8/X6a2.

Figure 4: Band 3 data obtained with the APS on March 24, 2016. Phase is plotted asfunction of frequency for several baselines in Stokes XX. Diﬀerent colors indicate the four
correlator quadrants. During an early scan (scan 3, left panel), some of the data in spw 2
are clearly corrupted. However at later times (e.g., scan 26, right panel), severe delay errors
are evident in all four quadrants as evidenced by the steep slopes in phase as a function of
frequency. Data source: uid://A002/Xb0dfe8/Xeed.
16

Figure 5: Band 3 regression test data obtained with the APS on April 3, 2016 using
the Cycle 3 software release. Phase is plotted as function of time for all four correlator
quadrants, XX polarization. While the scatter in the phases is high, it appears to beconsistent with the unstable weather conditions. There is therefore no evidence for thetypes of phasing problems seen in test data from November 2015 and March 2016 (cf.
Figures 1-4). Data source: uid://A002/Xb1586e/X4b2f.

17

Figure 6: Band 3 test data obtained with the APS on April 3, 2016 using software release
R2016.6. In each subpanel, phase is plotted as function of frequency for all four correlator
quadrants, XX polarization, during a single scan on a baseline between phased antennas.
Phased slopes are visible in several of the panels, indicating delay errors. Data source:
uid://A002/Xb16072/X1cc.

18

Figure 7: Band 6 (228.1 GHz) VLBI fringe on the quasar 3C273 obtained between phased
ALMA and the phased SMA on April 8, 2016. The plot shows a correlation between
ALMA’s X polarization and the SMA’s R polarization based on 4 minutes of data.

Figure 8: Delay rate fringe amplitudes for the source 3C279 on a baseline between ALMA
and the LMT (Band 6). The data were obtained on April 8, 2016. The four left panels show
the “mixed” linear and circular polarization products prior to the application of PolConvert.
The four right panels show the data after PolConvert. The signal in the cross-hand products
is negligible after the conversion, indicating robust operation of the conversion software.
From Martı́-Vidal et al. (2016a).
19

Figure 9: Amplitude vs. time during a Band 6 VLBI scan on 3C273 for the ALMA-LMT(left) and ALMA-SMA (right) baselines, RR polarization, obtained on April 8, 2016. The
correlation coeﬃcients are seen to decay frequently, reflecting a loss of phasing eﬃciency,
but then recover to a nominal level. These dropouts occur at the same time on two diﬀerent
ALMA baselines, indicating that the issue originates at ALMA.

Figure 10: Phase as a function of time for ALMA Band 6 observations of the quasar
3C273, taken during the global VLBI CSV experiment on April 8, 2016. Baselines between
the 37 phased antennas are shown for the XX polarization in a single correlator quadrant(corresponding to the VLBI “hi” band; see text). The left panel is restricted to baselines≤50 m, while the right panel is restricted to baselines between 105-305 m. Scatter is thephases is consistently smaller on the shorter baselines. Periods of higher phase scatter inthe longer baselines correspond in time to the correlation coeﬃcient (amplitude) dropouts
seen in the VLBI data (Figure 9). Data source: uid://A002/Xb187bd/X2d5.
20

Figure 11: Phase as a function of u-v distance for the same Band 6 data as shown in
Figure 10. Scatter in the phases increases systematically with increasing baseline length.
Data source: uid://A002/Xb187bd/X2d5.

21

Figure 12: Additional results from the Band 6 VLBI data of April 8, 2017. Amplitude vs.
time for one scan on the ALMA-LMT (top left), ALMA-SMA (top right), and LMT-SMA(bottom) baselines (RR polarization). ALMA has issues for approximately the first minuteof the scan (red points). The LMT-SMA baseline produces acceptable quality data during
part of this minute. LMT-SMA data are averaged to 10 seconds because of the much lower
signal-to-noise ratio on this baseline.

22Figure 13: Additional results from the Band 6 VLBI data of April 8, 2017. Left: The
residual delay rate as a function of frequency channel for the first scan on 3C279 on the
ALMA-LMT baseline, LL polarization. There is a linear drift in the residual delay ratewith frequency. Right: The residual multiband delay found by fringe-fitting diﬀerent 10second segments of the data. There is a linear drift of residual multiband delay with time.

23

Amp. and Phase vs. time for each freq., 6 segs, 125 APs / seg (40.00 sec / seg.), time ticks 60 sec-5˚47’21.525"

U

Figure 14: Additional results from the Band 6 VLBI data of April 8, 2017. Top: The redpoints show the phase vs. time on the ALMA-LMT baseline (LL polarization) on the firstscan of the experiment for each of the 32 frequency channels (labeled by letter). Bottom:
The same plot, but with a line drawn in connecting the first and last time points within eachchannel. The eﬀect of diﬀerent residual delay rates (i.e., time-varying phases) is evident ineach channel.

24