SKA PST RECV Interfaces

Overview

RECV implements the external interface between PST and the Correlator Beamformer (CBF) as described in the CBF to PSR Interface Control Document.

Interface Details

The UDP packet format described in the ICD includes 3 main components:

• packet meta-data (96 bytes)

• relative weights (variable)

• channelized voltage data (variable)

The sizes of the relative weights and channelized voltage data depend on the Beam Former (Low or Mid), the Band (Low, 1, 2, 3, 4, 5) and the Array Assembly. The sizes for each packet are shown in the table below.

Band	AA	Nchan	Nsamp	Nweight	Nbit	Packet Size
Low	All	24	32	1	16b+16b	6288
Mid 1,2	0.5,1	185	4	4	16b+16b	6432
Mid 1-3	2+	1 *	2048 *	64 *	8b+8b *	8416
Mid 4-5	2+	1 *	1024 *	32 *	16b+16b	8352

* To be Confirmed with Mid.CBF

Packet Details

The meta-data in the UDP packets is interpreted by RECV using a C struct that is mapped onto the packed bytes received in the UDP payload. The layout of the packet meta-data is shown below.

The Validity Flags field is comprised of single bit flags indicating validity of the data in the packet arising from upstream signal processing functions performed by the CBF. The validity bits are shown below.

A description of each of the fields is summarized below.

Parameter	Comments
packet sequence number	counts from 0 at the start of the scan
samples since ska epoch	samples since SKA epoch
sample period numerator	sample period numerator in microseconds
sample period denominator	sample period denominator in microseconds
channel separation	frequency spacing between channels in the packet in units of millihertz
first channel freq	channel centre frequency of the First Channel Number in the packet in units of millihertz
scale 1	scale factor #1 - value by which CBF voltage data is scaled before conversion to the values in the packet
scale 2	scale factor #2 - Stokes Q [Mid.PSS only]
scale 3	scale factor #3 - Stokes U [Mid.PSS only]
scale 4	scale factor #4 - Stokes V [Mid.PSS only]
first channel number	first frequency channel index within the packet
channels per packet	number of contiguous frequency channels contained in the packet
valid channels per packet	number of valid frequency channels in the packet, always <= channels per packet
time samples per packet	number time samples for each frequency channel contained within the packet
beam number	beam (of 16 possible beams for PST and 501 for Low.PSS and 1500 for Mid.PSS) to which the data in the packet belongs
magic word	enables a basic check of the packet contents during decoding [BEADFEED]
packet destination	0=Low.PSS, 1=Mid.PSS, 2=Low.PST, 3=Mid.PST
data precision	number of bits per integer value stored in the Channel/Sample data section of the packet
num power samples averaged	number voltage time samples, whose power is averaged, for each time sample in the packet. Valid only for Mid.PSS
num time samples per relative weight	number of times samples, for each frequency channel, over which a relative weight is calculated
station beam polynomials applied	flag indicating if the station beam delay polynomials have been applied
pst beam polynomials applied	flag indicating if the PST beam delay polynomials have been applied
jones polarisation corrections applied	flag indicating if the Jones polarisation corrections have been applied
reserved 1	reserved byte
cbf version major	major release number for the beam-former version
cbf version minor	minor release number for the beam-former version
scan id	scan ID specified by the LMC
offset 1	Offset #1 - to be added to the corresponding Stokes parameters before applying scale [1..4]
offset 2	Offset #2 - Stokes Q [Mid.PSS only]
offset 3	Offset #3 - Stokes Q [Mid.PSS only]
offset 4	Offset #4 - Stokes Q [Mid.PSS only]

The relative weights are each stored as a 16-bit unsigned integer where the decimal point is interpreted to be after the most significant bit (value range: 2-2^-16 to zero) and 1 is encoded as 8000hex.

The channelised voltage data are stored as 16-bit, signed integers in Twos Complement binary with Little Endian encoding.

Behaviours

RECV’s primary function is to receive the UDP packets, inspect them and then write the data, weights and scales to the Shared Memory Ring Buffer (SMRB). To achieve this RECV will

• Inspect the meta-data of each packet to determine the location in SMRB to write the packet data and weights

• Write the scale_1 meta data and relative weights to the SMRB weights ring buffer

• Write the channelised voltage data to the SMRB data ring buffer

Validity Flags

RECV supports the inspection and application of the validity flags. RECV supports different validity flag processing policies that affect the treatment of the relative weights block in a packet. The policy, in combination with the validity flags, will control whether RECV will overwrite the relative weights to 0.

• IgnoreAll: ignore all validity flags, no changes to relative weights

• StationBeamDelayPolynomials: zero the relative weights if the Station Beam Delay Polynomials have not been applied

• PSTBeamDelayPolynomials: zero the relative weights if the PST Beam Delay Polynomials have not been applied

• JonesMatrices: zero the relative weights if the Jones Matrices have not been applied

• AnyDelayPolynomials: zero the relative weights if the Station or PST Beam Delay Polynomials have not been applied

• All: zero the relative weights if any of the validity flags are not set.

The default policy for RECV is IgnoreAll, this can be changed programmatically via ska::pst::recv::UDPFormat::set_validity_flags_policy, but is not currently configurable via the control interface with CSP-LMC, this is future work.