Summary of the Second ALMA Phasing Project (APP)
Commissioning and Science Verification Mission:2015 March 24-30
ALMA Technical Note: Number 17
Status: Final

Prepared by:

Organization

Date

L. D. Matthews

Massachusetts Institute ofTechnology Haystack Observatory2015 September 4

G. B. Crew

Memorandum

To: The ALMA Community
From: Lynn D. Matthews and Geoffrey B. Crew, MIT Haystack Observatory
Date: September 4, 2015
Subject: Summary of the second ALMA Phasing Project (APP) Commissioning andScience Verification mission: 2015 March 24-30

1BackgroundThe second Commissioning and Science Verification (CSV) mission for the ALMA Phasing
Project (APP) was carried out during the ALMA Extension and Optimization of Capabilities (EOC) Week from 2015 March 24-30. The first APP CSV mission took place in2015 January (hereafter referred to as the “January mission”) and is summarized in ALMA
Technical Note 16.1 A highlight of the January mission was the first successful detection of
a Very Long Baseline Interferometry (VLBI) fringe between ALMA and a fully independent
station (the Atacama Pathfinder Experiment (APEX) telescope), demonstrating that all
major hardware components of the ALMA Phasing System (APS) are fully operational.
However, the signal-to-noise ratio of the detected fringe implied that ALMA was effectively
operating as a single 12-m antenna rather than a phased array, a problem that was ultimately traced to a difference in the way various delays are applied to the data stream used
to form the phased sum signal compared with the normal ALMA antennas (see ALMA
Technical Note 16 for additional discussion).
Following the January mission, the APP Team developed a workaround for the delay
problem. As a first step, an option was implemented to turn off the baseband delay
applications in the correlator data processor (CDP) when executing an observing block(i.e., “useRDC=False”). This patch was tested during software regression testing in late
January and verified to work as expected. As a second step, the APP’s TelCal code wasmodified to create an arbitrary (power of two) number of channel averages in any given
spectral window.2 ,3 The appropriate phase corrections for each individual antenna arecomputed independently for each of these channel averages, effectively allowing the APS
software to solve for phase as a function of frequency. This in turn removes the phase ramp
across each baseband caused by turning off the default delay corrections.
1

https://almascience.nrao.edu/documents-and-tools/alma-technical-notes/
While the APP software now allows an arbitrary number of channel averages per baseband, the maximum number permitted by ALMA Observing Tool (OT) is currently set to 10. An increase in this number
to a value of 32 is slated to be implemented in late 2015.
3
An alternative would be to use a larger number of spectral windows; however, the correlator does not
currently perform well when using large numbers of spectral windows.
2

1

1.1

Objectives

The primary objective for the second APP CSV campaign was to test and characterize
the phasing system, including the recent changes in the handling of the front-end delays.
Secondary goals were to repeat the local VLBI test between ALMA and an antenna at
the Operations Support Facility (OSF) (also attempted during the January mission) andto obtain a short VLBI recording on a calibrator source with ALMA and one or morestations operating as part of the Event Horizon Telescope (EHT) network, thus allowing
demonstration of an intercontinental VLBI fringe.

1.2

Personnel

APP personnel on site during the 2015 March APP CSV mission included: Geoffrey Crew
and Lynn Matthews (MIT Haystack Observatory); Matias Mora (NRAO); Alejandro Saez
(JAO); and Helge Rottmann (MPIfR). The ALMA Science lead on site during the week
was Anthony Remijan.

1.3

Impacts of Severe Weather

Northern Chile received record-setting rainfall during the early part of the 2015 March
24-30 EOC week. Blizzard conditions were present at the Array Operations Site (AOS),
leading to a shutdown of operations. All internet and phone service were also disabled in
Northern Chile for approximately one day. Clearing of the roadway leading to the AOS
began on March 26, but owing to the roughly one meter of accumulated snow, ALMA
personnel were not able to access the plateau and begin performing health checks on theantennas until March 28. Consequently, observations were possible only during the lasttwo days of the EOC week.

2

Resources and Set-Ups Employed

2.1

Software Release

ALMA EOC activities during the week employed software version R2014.6. While the APP
was nominally working on developments under R2015.2, to negate the need for softwarechanges when switching between EOC and APP CSV activities, the APP opted to employ
instead a backported branch of R2014.6 (APP-BACKPORT-2014-06-B). This branch contained all of the features needed by the APP. This branch was tested prior to the mission
during the ALMA Department of Computing (ADC) regression test on March 23.

2.2

Antennas

ALMA was in a compact configuration during the mission, with maximum baselines of afew hundred meters. (The configuration was the same as during the January Mission).
Because of snowfall (see §1.3), all antennas were stowed during the first five days of the
EOC week and were unavailable for observing, except in simulation mode. On Day 6, 132

antennas became available, and 25 antennas became available on Day 7, but one of theselacked a Band 3 receiver and was excluded from the array used by the APP.
In addition, to the AOS antennas, antenna PM01 was parked on pad TF01 at the OSF
and was nominally available for use by the APP for VLBI testing. However, ultimately it
was not used during the March mission because of time constraints following the weatherrelated shutdown.

2.3

Remote VLBI Stations

During late March and early April 2015 (including dates overlapping with the March mission), the Event Horizon Telescope (EHT) project was engaged in a global VLBI campaignthat included observations at 1.3 mm and 0.87 mm. Participating stations included the
Combined Array for Research in Millimeter-wave Astronomy (CARMA), the James Clerk
Maxwell Telescope (JCMT), the Submillimeter Array (SMA), the Submillimeter Telescope(SMT), the Large Millimeter Telescope (LMT), and the Institut de Radioastronomie Millimétrique (IRAM) 30-m telescope. ALMA was not formally part of this experiment. However, since obtaining intercontinental fringes in Band 6 between ALMA and other VLBI
stations is part of the formal APP CSV plan, the staffing of these remote sites in parallel with the APP CSV campaign provided an ideal testing opportunity for the APP (seebelow).

2.4

Time Allocation

During the first five days of the EOC week, the APP observed in simulation mode only,
collecting approximately two hours of simulation mode data on each day. (In this mode,
the antennas do not point, but the receivers are powered on, and receiver noise is recorded).
Approximately 10 hours of on-sky array time were allocated to the APP on Day 6 and 2hours on Day 7, for a total of 12 hours on sky during the mission.

2.5

Data Collected

Observing targets for the mission comprised bright quasars selected from the ALMA Source
Catalogue. Approximately 20 GB of ALMA standard interferometry mode (ASDM) data
were collected in a combination of Bands 3, 6, and 7, and ∼1 TB of useful VLBI-mode
data were recorded simultaneously with the ALMA interferometry mode data at selected
times.

3

Results

3.1

Simulated Observations and Recorder Testing

Preliminary tests of the APP software used during the mission were performed using the
SCO2 simulator in Santiago just prior to the start of the mission. Because of inclement3

weather, the first five days of CSV activities were also limited to simulation mode observations, albeit with the real array. In contrast to the data produced by SCO2, which
comprise only zeros, the simulated array data contain recorded receiver noise. These simulation mode tests allowed verification that the latest APP software could run without
problems under normal observing conditions and that the system could be commandedin the manner required for phased ALMA observations. Various APP observing modes
were tested (including those that exercised the fast and slow phasing loops independently
and together). Other parameters were also varied, including the number of antennas in
the array, the scan durations, the spectral set-ups, the numbers of spectral windows, andthe number computed spectral averages. The readability and formatting integrity of theresulting uid data were then verified in CASA.
Among the things verified in simulation is that it is possible to carry out observations
with up to 32 spectral averages per baseband and produce ASDM format data that canbe read into CASA and processed in a normal manner, without any data formatting or
CASA modifications. To test this, the baseline correlator Validator was patched to accept
up to 32 channel average regions per spectral window. Following this, we were able tosuccessfully execute manual mode simulated observations with 16 and 32 spectral averages
per baseband, respectively. TelCal worked as expected with the output data, and we werealso able to import the resulting ASDM files into CASA and perform standard operations
on these data, demonstrating that no change to the ASDM model will be required toaccommodate this change. We note that the ALMA Observing Tool (OT) currently limits
the number of channel averages to 10, and the consequences of the increased data rateinherent in using larger numbers of channel averages has not yet been tested under real
observing conditions, although no problems are anticipated once this capability becomes
available in software release 2015.6 (see http://ictjira.alma.cl/browse/ICT-4656).
In addition to the software testing, the APP Mark 6 recorders were exercised during
the shutdown time. During these recorder tests, we established that as a result of humanerror, the optical fiber link (OFL) had been disconnected. This was difficult to diagnose,as the network monitoring port on the device had been previously damaged, either due toaccident or human error. Finally, a failing disk in one of the recorder modules required
replacement. After correcting these issues, a 9.5-hour recording session showed nominal
loss rates (∼1 ppm).

3.2

On-Sky Observations: Checkout of the Phasing System –Session 1

The first on-sky tests became possible on March 29/30 (Day 6 of the mission), between
19:00-02:00 local time (22:00-05:00 UT; hereafter Session 1). An array of 12 12-m antennas
was available; consequently, either 9 or 11 antennas were used in the phased array, alongwith either 1 or 3 unphased comparison antennas, respectively. The weather was stableand dry; the precipitable water vapor (PWV) at the start of the session was ∼1.1 mm anddropped to 0.56 mm near the end.
The highest priority goal of Session 1 was validation of the phasing software following
several changes related to the handling of delays and the maximum number of spectral
4averages for which phasing correlations can be computed per baseband (see §1 and §??).
3.2.1

USB/LSB Handling Issue

Examination of the first on-sky data obtained with the phasing system operating during
Session 1 revealed that while the basebands associated with the lower sidebands (USB) were
behaving as expected, the upper sidebands (USB) were applying phases incorrectly when using multiple TelCal phases (as is required when computing phase corrections for more than
one channel average across a given baseband). It was quickly determined that this problem
originated because of an ordering inconsistency between TelCal phases, tunable filterbank(TFB) filter frequencies, and channel average region center frequencies; VLBIObsControlHelper was reverting channel average frequencies according to USB/LSB, while AppUtilities::unpack was expecting them to be in the same (ascending) order as the twoother parameters. A fix was implemented, installed, and tested during the session and wascommitted to software release 2014-06-B. The fix consisted of keeping all three parametersin ascending order before passing them to AppUtilities::unpack. After the full phasesequence is received back, the sign is reverted for USB before applying the correction.
3.2.2

Delay Corrections

Session 1 marked the first on-sky tests of the APP’s workaround to the delay issue describedabove (§1) and in ALMA Technical Note 16. To exercise this option, the default delay
corrections were turned off, and a modified version of the APP’s TelCal code was deployedthat allows solving for the phases in arbitrary numbers of channel averages within eachcorrelator quadrant. During Session 1, tests with up to 8 channel averages per baseband
were undertaken.
As seen in Figures 1 & 2, with this new method of handling the front-end delays implemented, acceptable phasing performance is now found in all four correlator quadrants,
both polarizations, and in both Band 3 and Band 6. The left panels of Figures 1 & 2
show that for all polarization/baseband combinations, phases on baselines between antennas in the phased sum show a clear convergence toward zero (as expected for an unresolved
point source) and a significant drop in RMS dispersion when the phasing solutions converge. At the same time that the phases converge toward zero, a marked increase in thecorrelated amplitude is observed in all quadrants and both polarizations on baselines between the phased sum and the (unphased) comparison antennas (right panels). Prior to
the March campaign, a similar convergence of the phases toward zero was routinely seenin all quadrants/polarizations on baselines between antennas used to form the phased sum(e.g., Figure 3 of EOC Technical Memo 16); however, the corresponding jump in correlated
amplitude was not present. The reason is that originally, the front-end delays were being
applied in a correct manner to the “normal” ALMA antennas (i.e., to those shown in theleft panels of Figures 1 & 2), but not to the phased sum signal or “sum antenna” itself,
which is used to form the cross-correlation signals depicted in the right-hand panels of
Figures 1 & 2.
In the case of the Band 6 data (Figure 2) we see that scatter in the phases is lowest
immediately after phase-up, but then the data temporarily undergo some decorrelation,5

leading to a systematic decrease in correlated amplitude, followed by a “recovery”. It is
unclear whether this behavior was due to a fluctuation in atmospheric conditions or someother cause.
3.2.3

Fast+Slow Loop Corrections

In addition to testing of the standard “slow” phasing loop (as for Figure 1 and 2), tests
were also performed during Session 1 using the slow loop in conjunction with the “fast”
phasing loop. The latter applies phase corrections on a shorter (∼1-second) cadence basedon the path delays computed from the water vapor radiometer measurements obtainedat each station. As seen in Figure 3, during the weather conditions of the March 30
observations, the RMS dispersion in the phases in Band 6 in each quadrant/polarization is◦lower ( <∼ ± 5 ) compared with the slow loop alone (Figure 2), suggesting that the phasingalgorithms themselves are performing with an efficiency approaching ∼99%.4 .
We note that Figure 3 exhibits two time intervals where the phases are identically zero.
Such behavior was seen intermittently during the March campaign. Its origin is unclear,but it appears to be an ALMA software or firmware issue that is unrelated to the APS.
The amplitudes and phases of the spectral data are also corrupted during affected scan.
This problem is different from one that was documented previously in ICT JIRA ticket(ICT-3886), where only the channel-averaged data were zeroed, while the spectral data
appeared to be normal.
3.2.4

Use of Multiple Channel Averages Per Baseband

One drawback of the method that the APP team has implemented to handle front-end
delay corrections is that while the gross delays are well removed by solving in real time forphase as a function of frequency, these corrections are computed for only a finite numberof frequency points across each baseband (i.e., one correction for each of the N0 channel
averages, where N0 ≤8 during our March tests). A consequence is that in some cases, thecorrected cross-correlation spectra exhibit small residual phase ramps across each of theN0 band segments, resulting in a “sawtooth” pattern (Figure 4). In practice, these phaseslopes were found to range from roughly zero (i.e., imperceptible) to ∼0.1 turns across eachof the individual segments. Because these phase ramps are relatively small, the resulting
decorrelation losses are less than a few per cent. Consequently, we do not anticipate that
this decorrelation will inhibit our ability to meet our formal phasing efficiency requirements.
Nevertheless, we plan to explore further mitigating this effect by increasing the number ofchannel averages per baseband from ≤8 to 16 or 32. As noted above, we have successfully
tested this in simulation, and this capability is expected to become available for routine
on-sky testing in late 2015 (see ICT JIRA ticket ICT-4656).
4

Because of quantization errors and other losses, this number does not reflect the total phasing efficiencyof the systems; see §3.4.1

6

3.3

On-Sky Observations: Session 1 VLBI Fringe Tests

Within the first two hours of Session 1 it was established that the phasing system was
performing well following the latest APP software updates. Given this satisfactory performance, coupled with favorable weather conditions, a decision was made to shift the focusof the session to attempting some VLBI recordings in parallel with the ongoing worldwide
EHT observing campaign described above (§2.3). ALMA therefore joined various EHT
sites in recording observations of the quasar 3C273, thereby providing an opportunity for
VLBI fringe tests on >5000 km baselines with ALMA.5 Indeed, such tests are specified aspart of the formal APP CSV plan and are a natural follow-on to the successful fringe tests
performed in 2015 January on the ∼2 km baseline between ALMA and APEX (see ALMA
Technical Note 16).
APP observations that included VLBI recording were first attempted utilizing only twoof the four correlator quadrants, each tuned to a center frequency commensurate withthose adopted at other EHT sites. The other VLBI peer sites utilize a range of VLBI
backends, but none presently matches the data rate of ALMA, hence this choice was madeto minimize ALMA VLBI data volumes while producing data that could be correlatedwith peer sites in a straightforward manner. Unfortunately, two initial attempts to executesuch VLBI scans failed for unknown reasons. During a subsequent attempt, real-time
displays on the CorrGUI showed that the latest data exhibited artifacts in the form of a“porcupine” pattern in the cross-correlation amplitudes during certain scans, coupled withspurious phase values. As seen in Figure 5, these spurious amplitude and phase behaviors
repeat within each of the 32 TFB channels. The origin and trigger of this behavior remains
unknown. It is documented in an ICT JIRA ticket (ICT-4815) and has since been shown tobe linked with use of the frequency division (FDM) observing modes, and to be independentof APP hardware and software.
Following these failures, observations were attempted again, this time commanding all
four correlator quadrants, each tuned to the same center frequency of 227.10 GHz. Thismode of operation at last produced data free of the corruption seen in the 2-quadrant
mode data.6 In the end, several successful VLBI scans were obtained on 3C273, and two
short segments of data overlapped with successful VLBI recording on this source at othersites (04:05:00-04:07:16 UT and 04:40:00-04:43:12 UT). The stations with overlapping data
included the IRAM 30-m antenna (Pico Veleta, Spain), the SMT (Mt. Graham, Arizona),
and CARMA (Owens Valley, California).

3.4

On-Sky Observations: Session 2 Step Scans

A second APP observing session was carried out from 23:00-01:00 UT on March 30/31.
Weather conditions were slightly less dry and stable than the previous night, with PWV ∼1.4 mm. During Session 2 there were 23 12-m antennas available, up to 19 of which were
5

The data on baselines involving ALMA were obtained solely for APP CSV purposes and will not be
distributed outside the APP team nor used in any manner by the EHT consortium.
6
In fact, the number of correlator quadrants in use may have been immaterial; following the Marchmission, “porcupine” artifacts have also been seen in FDM data where all four quadrants were commanded(see ICT-4815).

7

used as part of the phased array. The objectives of this session were threefold: (1) validationof the performance of the phasing system with a sufficiently large number of antennas (≥15)
to permit testing of compliance with our formal requirements;7 (2) additional testing of thefast loop phase corrections; (3) performance of “step scan” tests, as described in our formal
CSV plan. For the latter tests, antennas are systematically added to or subtracted fromthe phased array, permitting evaluation of how the correlated amplitude and the phasing
efficiency scale with the number√ of phased antennas in the array. In theory, the correlated
amplitude should increase as NA , where NA is the number of phased antennas; however,
the absolute value of the amplitude will also be affected by the overall phasing efficiency.
During Session 2, two “step scans”, each of approximately 12 minutes duration, were
executed in Band 6. These step scans began with an array of 19 phased antennas in all four
correlator quadrants. Subsequently, pairs of antennas were sequentially removed from thephased array at arbitrary times during the execution block. The four correlator quadrants
were modified independently, resulting in different numbers of phased antennas, NA , asa function of time for each quadrant. This type of exercise allows us to test how theperformance and overall phasing efficiency of the system changes with different numbers ofphased antennas.
Figure 7 shows sample step scan results for one baseband and polarization. The crosses
indicate the correlated amplitude as a function of the number of phased antennas for abaseline between comparison antenna DA45 and the phased sum “antenna” during each
16-second scan. We see that the correlated amplitude scales as expected with the squareroot of the number of antennas in the phased sum for NA < 19, while for NA = 19, themeasured amplitude appears to level off. However, given the limited number of antennas
and measurements, additional observations will be need to confirm and fully evaluate thisbehavior. Further, while the relative amplitudes show approximately the expected “squareroot of NA ” behavior as a function of the number of antennas in the phased array, a much
more detailed and sophisticated analysis is required to gauge the actual phasing efficiency.
Losses in efficiency occur at two different stages in the APS. First, imperfect phasing
solutions (e.g., due to atmospheric fluctuations, residual delay errors, and latency in theapplication of the phasing corrections) lead to a residual scatter, σφ , in the phases of all thecorrected antennas used for form the phased sum. The decorrelation resulting from thisresidual scatter will produce a reduction in the correlated amplitude of the phased sum by−σφa factor "0 ≈ e 2 . For the data set shown in Figure 7, "0 ∼0.93.
Ignoring gain differences between antennas, if we compute a vector average over all
baselines between the phased antennas, the correlated amplitude that we derive, Ā! , will
therefore be "0 times the correlated amplitude expected for an ideal array of 2 phased
antennas. The value of Ā! for baseband 1, polarization XX is shown as an asterisk on
Figure 7. Assuming that correlated amplitude scales as the square root of the number ofphased antennas, we may therefore extrapolate from this point the correlated amplitude
expected for larger phased arrays—assuming that no further losses occur in the phasingsystem. This predicted relation in shown on Figure 7 as a dotted curve.
To compare with the dotted curve, the solid curve shown on Figure 7 shows a fit to7

One of the formal APP requirements is to demonstrate phase-up of with ≥90% efficiency for ≥15
antennas with baselines ≤2 km.

8the actual measured values of the correlated amplitude on the baseline between the phased
sum antenna and a comparison antenna for 11 ≤ NA ≤ 19. We see that the solid curve liesa factor of 0.89 below the dotted curve, implying that additional losses in efficiency haveindeed occurred in forming the phased sum signal. To within uncertainties, this numericalfactor matches the theoretical efficiency loss of 0.88 expected from the 2-bit quantizationof the signal. We emphasize that the measurement represented by the asterisk on Figure 7
is unaffected by this loss, since it was derived using the signals from individual ALMA
antennas, which unlike the sum signal, do not incur these extra quantization losses. In
conclusion, for the particular baseband and polarization shown in Figure 7, total losses
in phasing efficiency are a combination of a ∼7% loss from imperfections in the phasing
solutions and a numerical loss of ∼11% in forming the phased sum signal. We note that
there are some small uncertainties in these estimates resulting from the fact that gain
optimization was not applied during most of the observations executed during the March
campaign. Furthermore, we are still working on carefully confirming that normalization of
the sum antenna is being handled correctly.
For some of the other baseband/polarization combinations in the data set shown in
Figure 7, estimation of the phasing efficiency is complicated by at least one additional
factor, namely that the comparison antenna—used to form cross-correlation prodcuts fromwhich the correlated amplitude of the sum antenna is evaluated—receives no front-end
delay corrections under our current implementation of the phasing system. Consequently,
in the event that there happens to be a significant residual delay error for a particular
baseband/polarization combination, the correlated amplitude will appear artificially low
and the inferred phasing efficiency will appear lower than its true value. For the baseband/polarization combination shown in Figure 7, the phase gradient across the 1.875 GHzband is ∼ 5◦ , leading to negligible decorrelation. However, this is not the case for all of
the baseband/polarization pairs, two of which show roughly half a turn of phase acrossthe band. After correcting for these delay errors, we find similar efficiencies in all 8 baseband/polarization combinations.

4

Correlation of VLBI Data

Immediately following the mission, preliminary attempts to find VLBI fringes between
ALMA and one of the other EHT sites using the DiFX software correlator proved unsuccessful. However, in mid-August, an issue was uncovered in the way that DiFX was
handling the specified Earth Orientation Parameter. Following resolution of this issue, afringe was detected in a segment of data on a baseline between ALMA and Pico Veleta(Figure 6). This detection marks the first successful intercontinental VLBI fringe between
ALMA and another station in any band. Furthermore, it is the first successful VLBI fringe
between a phased ALMA and an independent station. (During the VLBI fringe test withAPEX in 2015 January, ALMA was effectively operating as a single antenna; see EOC
Technical Memo 16).

9

5

Future and Ongoing Work

Subsequent to the fringe detection between ALMA and Pico Veleta with dual polarizations
recorded at both stations, the correlated VLBI data were transferred to I. Martı́-Vidal(Onsala) to allow testing of PolConvert, a tool that the APP team has been developing
to handle the “mixed” polarizations obtained on VLBI baselines between ALMA (which
records dual linear polarizations) and other VLBI sites (most of which record single ordual circular polarizations). PolConvert will operate by correcting the ALMA data at
the post-correlation stage by computing and applying necessary corrections to the VLBI
fringes and outputting circularly polarized data products in FITS-IDI format.
In parallel, analysis of the remaining VLBI scans obtained in 2015 March remains
ongoing. Fringes were also detected with a second segment of data on the baseline between
ALMA and Pico Veleta, but the signal-to-noise ratio is somewhat lower than in the scan
shown in Figure 6. Most likely this is due to the low elevation of the source as observed from
Pico Veleta (∼ 20◦ elevation compared with ∼ 27◦ in the earlier data segment), but otherpossible causes should be ruled out. So far no fringes have been detected between ALMA
and SMT, and the reason for this is also under investigation. Finally, only preliminary
attempts have been made so far to search for fringes on the ALMA-CARMA baseline.

6

Summary

Despite extreme weather conditions that lead to a 5-day shutdown of ALMA operations,
the APP was able to make important progress toward its CSV goals during the 2015 March
24 EOC week. Prior to this campaign, an issue with the manner in which front-end delays
were applied to the data used to form the phased sum lead to significant decorrelationin all baseband/polarization/observing frequency combinations outside of quadrant 1, polarization XX in Band 3. A software workaround to this delay problem was successfully
implemented, and the phasing system now appears to be working satisfactorily in both the
XX and YY polarizations in all four correlator quadrants in Bands 3 and 6. Successful testsof the phasing system were executed with up to 19 phased antennas using both the slow andthe fast phasing loops, as well as a combination of the two. Another major milestone was
the first successful detection of a VLBI fringe with phased ALMA on an intercontinental
baseline. Additional information on the 2015 March APP CSV mission is available on the
mission wiki page:https://ictwiki.alma.cl/twiki/bin/view/AppMissionChileCommissioning02.

10

Figure 1: Left: phase as a function of time during Band 3 observations of the quasar 0522364 on 2015 March 30. Only the slow phasing loop was active during this observation. All
baselines between the 11 antennas in the phased sum are shown. Different colors indicate
data from the four independent correlator quadrants. Data from both the XX and YY
polarizations are shown within each quadrant. Right: correlated amplitude as a function oftime for the same observation, for the baseline between the phased sum and an unphased
comparison antenna. As seen in the left panel, phase-up occurs roughly 01:08:52 UT, orapproximately 20 seconds after the start of the observation. At the corresponding timein the right-hand panel, the correlated amplitude significantly increases. Data source:
uid://A002/X9cdda2/X245.

Figure 2: Same as Figure 1, but in Band 6. Data source: uid://A002/X9cdda2/X25e.

11

Figure 3: Phase as a function of time during Band 6 observations of the quasar 3C273
on 2015 March 30. Both the fast and slow phasing loops were active during this observation. Baselines between all phased antennas are shown. Colors indicate different
baselines. Data from both the XX and YY polarizations and all four independent correlator quadrants are shown. Data on all baselines were corrupted during two scans (near
02:25:00 UT and 02:27:40 UT), resulting in spurious zero-valued phases. Data source:
uid://A002/X9cdda2/X3b8.

Figure 4: Example of a Band 3 cross-power spectrum on a baseline between the reference
antenna and an antenna that was part of the phased sum. Data are shown for a single
correlator quadrant (baseband 2), XX and YY polarization. Two scans (each comprised offour subscans) are shown. The orange symbols represent data prior to phase-up and showa phase gradient as a function of frequency as a result of the suppression of the default
front-end delay corrections. The brown points show the data after phasing corrections
were computed and applied in each of eight spectral regions. Because the corrections arecomputed from subsets of the data averaged over one-eighth of the band, small residual
phase slopes remain across each of these eight regions in the spectral data. Data source:
uid://A002/X9cdda2/X37.
12

Figure 5: Example of a corrupted cross-correlation spectrum obtained on March 30 duringa Band 6 VLBI fringe test. Amplitudes as a function of frequency are shown on the left
and phases as a function of frequency on the right for the baseline between antennas DA41
and DA49. The two colors designate the data from the two different commanded correlator
quadrants. All baselines and both polarizations were corrupted during the affected scan.
Data source: uid://A002/X9cdda2/X553.

13

Figure 6: Band 6 VLBI fringe detection on the source 3C273, obtained on a baseline
between the IRAM 30-m antenna on Pico Veleta, Spain and phased ALMA. The data
were obtained on 2015 March 30. There were nine 12-m ALMA antennas in the phasedarray. Approximately 136 seconds of data are shown. The high signal-to-noise ratio (∼49)
of the correlation between the Pico Veleta left circular polarization and the ALMA linear(XX) polarization on this very long baseline (∼9770 km) is evident from the clear peaksin the delay rate (red curve, top panel), in the single-band delay (green curve, center-leftpanel) and in the multi-band delay (blue curve, top panel). The center-right panel shows
amplitude (blue) and phase (red), averaged over all baseband channels at a resolution of
100 kHz. (The VLBI correlation was performed at 3.125 kHz resolution). The lower plotshows amplitude (blue) and phase (red) as a function of time for each of the 32 51-MHz
frequency channels. (The wings of the 62.5 MHz ALMA TFB channels were not included).
Data source: uid://A002/X9cdda2/X578.

14

Figure 7: Sample results from a “step scan” test, carried out on 2015 March 31 on thesource 0522-364 in Band 6. Data for baseband 1, polarization XX are shown. At the
beginning of the ∼12-minute observation, the phased array contained 19 antennas; as
the observation progressed, pairs of antennas were sequentially removed from the phasedarray. Crosses indicate measurements of the correlated amplitude on a baseline between
the comparison antenna DA45 and the phased sum “antenna” on 16-second intervals of
data. For comparison, the asterisk shows the mean correlated amplitude measured overall baselines between antennas used in the phased sum; this effectively measures the mean
correlated amplitude for an array of two phased antennas. The dotted line shows thepredicted amplitude increase for larger arrays on antennas. In contrast, the solid curveshows a fit to individual√ measurements between comparison antenna DA45 and the phased
sum of the form y = !q NA , where !q =0.89 is scaling factor and NA is the number of phased
antennas. In this example, all of the plotted points (asterisk and crosses) suffer inherent
amplitude losses of ∼7% owing to residual RMS fluctuations in the phases of each antenna.
However, the offset between the two plotted curves shows that an additional efficiency lossof ∼11% occurs during computation of the phased sum signal from the individual antenna
signals. See §3.4.1 text for additional discussion. Data source: uid://A002/X9ceec1/Xb0.

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