FringeRotationTask

FringeRotationTask does delay and/or phase tracking under control of ingest pipeline. This is an interim solution used for BETA and early ADE with ingest pipeline running at MRO. We expect to do fringe rotation outside of SDP (i.e. by the Telescope Operating System configuring the hardware fringe rotator) following the transition of ingest pipeline to the Pawsey centre (i.e. this tight real time control loop is not expected to extend across the MRO-Pawsey boundary). The task itself is generic for various ways of doing fringe rotation. It implements a simple delay model (using casacore’s measures and simple geometry/frame conversions) and computes delays and rates for each antenna present in the ANTENNA table with respect to the Earth’s centre. These delay and rates are passed to chosen fringe rotation method class (see method keyword) which do the low level work (different methods do tracking via software, via hardware or both with various degrees of accuracy). The parameters required depend on the method used. All currently available methods apply delay and rates with respect to a chosen reference antenna to minimise the adjustments needed. The task relies on the correct phase centre being set (either via metadata or parset, depending on the source task used), accurate timing of the data and, in most cases, correct frequency axis. Note, this task currently supports a single data stream only. Therefore, it should always be after merge task in the processing chain.

Configuration Parameters

Ingest pipeline requires a configuration file to be provided on the command line. This section describes the valid parameters applicable to this particular task. These parameters need to be defined only if this task is used. As for all tasks, parameters are taken from keys with tasks.name.params prefix (not shown in the tables described below) where name is an arbitrary name assigned to this task and used in tasklist. The type of the task defined by tasks.name.type should be set to FringeRotationTask.

General Parameters

The table below contains general parameters which are required for all fringe rotation methods. Parameters specific to some methods are given in the following subsections. The swdelays method is fully software-based and does not require any additional parameters to those listed in this table.

Parameter Type Default Description
method string None The choice of the fringe rotation method to physically apply the delay model. See subsections below for the list of available methods.
fixeddelays vector<double> empty vector This is the obsolete way of specifying fixed delays. This parameter is understood, but not used. A warning is given in the log if this parameter is present (but value is simply ignored).
calcuvw bool true If true (default), this task also calculates UVWs rendering CalcUVWTask unnecessary. The math required to calculate UVWs and delay model is very similar (just the reference frame is different). Therefore, doing these calculations at the same place allows us to improve the performance.
refant string None Reference antenna used in delay and rate application. This parameter is actually defined at the method level, but all currently available methods work with differential delay and rates (w.r.t. a chosen antenna), so this keyword is described in the general section. The antenna should be one of names defined in the array layout. Note, for the swdelays method (which does not talk to the hardware at all) the reference antenna is not required to be physically present, but must be defined in the antenna table. For other methods, the ingest receiver script should be able to talk to the appropriate hardware.

Available methods

The table below lists available methods. Note, some methods are BETA-specific and some are ADE-specific. The difference arises because of two factors. First, only BETA DRx has a functionality to apply delays through the offset pointer in the memory buffer. And second, there are substantial differences in the frequency conversion (ADE system doesn’t have LOs) affecting phase tracking. Note also, that any changes to the frequency handling (for example to benefit from flexibility supported by ADE hardware which is currently not covered by software) would require changes in the fringe rotation methods.

Method Needs ingest receiver Tracks phases Tracks delays Tracks rates Description
drxdelays Yes Yes Yes No BETA-specific method to correct for delays using DRx 1.3ns steps. Corresponsing phases are corrected in software, phase rates are ignored. Optionally, the residual delay (e.g. <1.3 ns) can be tracked in software.
hwanddrx Yes Yes Yes Yes BETA-specific method to correct for delays using 1.3ns DRx steps and for phase rates using the hardware fringe rotator. Phases and residual delays (e.g. <1.3ns) are corrected in software.
swdelays No Yes Yes No Software-based phase and delay tracking. Phase rates are ignored. Tested with ADE only, but should work with small BETA baselines too as it works with sky freqiuencies and does not touch the hardware ( and, therefore, differences in frequency conversion should not matter).
hwade Yes Yes Yes Yes ADE-specific method to correct for delays and phase rates using hardware fringe rotator. Phases and residual delays (0.2ns is the smallest possible delay step of the current hardware) are corrected in software.

Common parameters for hardware-based methods

This subsection summarises common parameters for all methods communicating with the ingest receiver, i.e. all but swdelays. Methods interacting with the hardware are doing so by sending a message over Ice to the OSL script called ingest receiver (note, the ADE and BETA versions are slightly different). This script decodes the message and runs an appropriate low-level OSL script (also called auto-script) setting the values to either DRx or hardware fringe rotator for a particular antenna. Antenna names written into ANTENNA table of the measurement set (i.e. see the array layout section of the main Ingest pipeline page for the information how to define the names) are used in the message to the ingest receiver and, therefore, in the OSL scripts. Therefore, the names should be that recognised by the Telecope Operating System (TOS). All methods which require communication with ingest receiver (see the table above), need appropriate Ice configuration in the parameters. These parameters are summarised in the table below. Also, it is the nature of the control loop based on the timestamp supplied by the correlator that it is always at least a couple of cycles late. In addition, some elements of the hardware (e.g. BETA DRx) or software (current asynchonous implementation of the auto-scripts for ADE) make it difficult to perfectly synchronise application of the new hardware settings with the visibility data. Therefore, there is an option to flag a given additional number of correlator cycles following the update of hardware. Note, the tolerances are likely to be different for the DRx and the hardware fringe rotator, but only a single waiting period is implemented for simplicity.

Parameter Type Default Description
cycles2skip unsigned int 5 Number of additional correlator cycles to flag following the receipt of the reply message from ingest receiver. This parameter is required to account for additional latencies in the system. Set it to zero to avoid any extra flagging (if the data happen to be good).
ice.locator_host string None Host name for the machine running Ice locator service.
ice.locator_port string None Port number for the Ice locator service.
icestorm.topicmanager string None Topic manager for communication channel to the ingest receiver
icestorm.outtopic string None Topic name for the ingest pipeline to the OSL script messages, i.e. outgoing traffic.
icestorm.intopic string None Topic name for the OSL script to the ingest pipeline or reply messages (sent when the request is fulfilled with the actual time of application).

Additional parameters for drxdelays

The table below describes configuration parameters specific to the drxdelays fringe rotation method.

Parameter Type Default Description
drxdelaystep unsigned int 0 Tolerance in DRx delay steps (i.e. in 1.3ns steps) describing when the old DRx delay setting can be reused. If desired diverges from the current setting by more than the tolerance DRx update is initiated (may take several correlator cycles). Higher value ensures less data are flagged for the price of larger residual delay. The default of zero forces update every time the delay correction changes by 1.3 ns.
trackresidual boolean true If true, the task will correct for the residual delay (i.e. up to 1.3ns or more, if the previous parameter is set) in software. The accuracy of software-based correction is limited by the spectral resolution.

Additional parameters for hwanddrx

The table below describes configuration parameters specific to the hwanddrx fringe rotation method. Note, in constrast to drxdelay method, residual delays are always corrected in software.

Parameter Type Default Description
drxdelaystep unsigned int 0 Tolerance in DRx delay steps (i.e. in 1.3ns steps) describing when the old DRx delay setting can be reused. If desired diverges from the current setting by more than the tolerance DRx update is initiated (may take several correlator cycles). Higher value ensures less data are flagged for the price of larger residual delay. The default of zero forces update every time the delay correction changes by 1.3 ns.
frratestep unsigned int 20 Tolerance in phase rate (in the hardware units). When the desired rate diverges more than this value from the current a fringe rotator update is initiated for a particular antenna. Descreasing the value will lead to more aggressive flagging ( due to more frequent updates), but less decorrelation in the visibility data).
drxmidpoint int 2048 Middle of the range for DRx delays. Delay tracking via DRx is implemented using the memory pointer offset in the buffer. This offset is an unsigned quantity while the delay has the sign. As only the relative delay matters, the signed delay is implemented by offsetting the midpoint. An adjustment to midpoint may be handy in the case of equatorial sources (as delay may fall out of range, otherwise).
flagoutofrange boolean true If true, antennas with out of range hardware settings are flagged. Otherwise, the software will apply as large as (or as small as) the value which is supported by the hardware.
updatetimeoffset int None A fudge factor subtracted from the timestamp when the hardware fringe parameters were updated. The hardware fringe rotation is done in the beamformer which has a different clock domain to the correlator. This parameter allows to adjust update time for possible differences. The value can be both positive and negative with the units of microseconds.

Additional parameters for hwade

The table below describes configuration parameters specific to the hwade fringe rotation method. It is very similar to hwanddrx method but adapted to ADE hardware with the following differences:

  • Delay tracking via hardware fringe rotator (as DRx no longer has this functionality)
  • No out of range flagging as the library works with floating point and should support all settings which could arise in operations

The minimum increments of fringe rotator parameters are exactly the same for ADE and BETA. These hardware units are used in the interface between ingest receiver and the task (so the code can be reused) and define the meaning of the tolerance parameters (as for BETA).

Parameter Type Default Description
delaystep unsigned int 0 Tolerance in delay steps, one step is approximately 0.206ns. The fringe rotator setting is updated if desired delay is off by more than this tolerance from the actual setting. Zero means update delay every time it changes by a minimal step. The residual delay is always tracked in software. Reducing this tolerance might be little bit more accurate, but more data will be flagged. This is a similar parameter to drxdelaystep understood by the previous method, but it has different units (as it applies to fringe rotator as opposed to digital receiver.
frratestep unsigned int 20 Tolerance in phase rate (in about 0.0248 deg/s steps). When the desired rate diverges more than this value from the set value the update is initiated for a particular antenna. Descreasing the value will lead to more aggressive flagging ( due to more frequent updates), but less decorrelation in the visibility data due to wrong rate applied).
updatetimeoffset int None A fudge factor subtracted from the timestamp when the hardware fringe parameters were updated. The hardware fringe rotation is done in the beamformer which has a different clock domain to the correlator. This parameter allows to adjust update time for possible differences. The value can be both positive and negative with the units of microseconds.

Example

########################## FringeRotationTask ##############################

tasks.tasklist = [MergedSource, Merge, CalcUVWTask, FringeRotationTask, MSSink, TCPSink]

# immediately unflag the data when the reply is received from ingest receiver
tasks.FringeRotationTask.params.cycles2skip = 0

# update delays when they diverge by more than 500 hardware units
tasks.FringeRotationTask.params.delaystep = 500

# just to give an example, this is the obsolete way of specifying fixed delays. It is not supported any more.
# Use delay definition in the antenna section (see the main page of ingest pipeline documentation for details)
# fixed delays in nanoseconds, in the order of increasing antenna indices
# values below are fixed delays used for antennas ak02, ak04, ak05, ak12, ak13 and ak14
# (in that order) in the November commissioning run
#tasks.FringeRotationTask.params.fixeddelays = [-198.004385, 0, 275.287053, -1018.02295, -1077.35682, 2759.82581]

# update rates when they diverge by more than 50 hardware units
tasks.FringeRotationTask.params.frratestep = 50

# Ice parameters to communicate with ingest receiver OSL script
tasks.FringeRotationTask.params.ice.locator_host = aktos10
tasks.FringeRotationTask.params.ice.locator_port = 4061
tasks.FringeRotationTask.params.icestorm.intopic = frt2ingest
tasks.FringeRotationTask.params.icestorm.outtopic = ingest2frt
tasks.FringeRotationTask.params.icestorm.topicmanager = IceStorm/TopicManager@IceStorm.TopicManager

# fringe rotation method class (hwade = ADE h/w fringe rotator + tracking residual delays in s/w)
tasks.FringeRotationTask.params.method = hwade

# reference antenna, the name should be one of the defined antenna names
tasks.FringeRotationTask.params.refant = AK04

# assume that the fringe rotator and correlator are perfectly synchronised
tasks.FringeRotationTask.params.updatetimeoffset = 0

# type of the task
tasks.FringeRotationTask.type = FringeRotationTask