Full-Polar Folded Profile Archives

• Full-Polar Folded Profile Archives are scientific data products that encapsulate time-, frequency-, and polarization-resolved pulsar profiles with complete full-Stokes calibration.
• They integrate advanced instrumentation and real-time processing techniques, including both coherent and incoherent dedispersion to correct dispersive delays.
• The archives are meticulously organized in PSRFITS format with comprehensive metadata, enabling automated RFI cleaning and precise pulsar timing analysis.

Full-polar folded profile archives are scientific data products that encapsulate time-, frequency-, and polarisation-resolved pulse profiles of pulsars, processed with full-Stokes parameterization across multiple subintegrations. These archives preserve all calibration, metadata, and radio-frequency interference (RFI) cleaning, typically stored in the PSRFITS FITS format. A prominent implementation is found in the GBD-DART-II pipeline developed for the Diamond Array Radio Telescope at the Gauribidanur observatory, optimized for routine, high-cadence polarimetric pulsar studies at 175 MHz (B et al., 31 Jan 2026).

1. Instrumentation and Data Acquisition

The GBD-DART-II system comprises 32 off-axis dual-polarised log-periodic dipole arrays (LPDAs), combined into a phased tile conveying ≃22 dBi gain throughout 130–350 MHz (HPBW ≃15° at 175 MHz). Each LPDA is connected via a balun and a low-noise amplifier (20 dB gain, 1.35 dB noise figure), phase-matched to ±5° to ensure coherency across the array. A “Tubular Receiver” provides ≃51 ± 0.5 dB total gain and extracts a selectable 16 MHz sub-band (typically 170–196 MHz), using high-pass, band-pass, and low-pass filtering, referencing a GPS-disciplined 10 MHz clock and 1 PPS signal for precision timing.

Digitisation uses two 8-bit ADCs, sampling X and Y polarisations at 33 MSPS. Baseband packets (UDP, 512 samples per packet, alternating polarisations) are streamed over Gigabit Ethernet. The high-throughput setup is buffered with GULP on a 10 GB ring buffer, periodically writing 2 GB PCAP files (~30 s recording blocks, ≈ 2×33 MSPS data). A 20 GB staging buffer computes and archives 1 s/64.5 kHz raw spectra for continuous monitoring, while a 70 GB transient buffer holds approximately 5 minutes of recent PCAP data for triggered retrieval.

2. Polarimetric Signal Processing

Signal processing begins by extracting complex voltages $X(t, \nu)$ and $Y(t, \nu)$, which are Fourier transformed or channelised. Full-Stokes parameters are calculated as:

$I = \langle X X^* \rangle + \langle Y Y^* \rangle$

$Q = \langle X X^* \rangle - \langle Y Y^* \rangle$

$U = 2\,\mathrm{Re}\{\langle X Y^* \rangle\}$

$V = 2\,\mathrm{Im}\{\langle X Y^* \rangle\}$

Instrumental polarisation is calibrated using a front-end-injected artificial pulsed-noise source during dedicated calibration scans. PSRCHIVE's pac tool derives the complex Jones (or equivalent Mueller) matrix $J$ relating observed and true Stokes vectors by

$\vec S_{\rm true} = M \vec S_{\rm meas}$

where $M$ is constructed from $J \otimes J^*$. This calibration addresses differential gain, phase, and cross-coupling between receptor polarisations.

3. Dedispersion Approaches

Dedispersion corrects cold-plasma dispersive delays using either coherent or incoherent schemes:

• Coherent Dedispersion: The known dispersion measure (DM, from ATNF ephemeris) is applied in the Fourier domain by multiplying the spectrum by the inverse transfer function

$$H(f) = \exp[+2\pi i\,K\,\mathrm{DM}/f], \quad K \approx 4.148808 \times 10^6~\mathrm{MHz}^2\,\mathrm{pc}^{-1}\,\mathrm{cm}^3\,\mathrm{s}$$

and transforming back to the time domain. DSPSR performs this operation on large data segments (typically 10–30 s blocks).

• Incoherent Dedispersion: After channelising to 256–512 frequency channels (Δν ≈ 31–64 kHz), each channel is delayed by

$$\Delta t = K \,\mathrm{DM}\,(\nu_1^{-2} - \nu_2^{-2})$$

so as to align pulses, executed via DIGIFITS, DSPSR, or PRESTO. Subintegration durations for folded outputs may range from 1 to 60 s, depending on science goals.

4. Folding, Subintegration, and Archive Structure

Folding aligns the dedispersed time series by pulsar period ($P_0$), as well as its derivative ($P_1$), both drawn from ATNF-provided ephemerides. PRESTO’s prepfold and DSPSR's folding modules condense the data into $N_\text{bin}$ phase bins per rotation.

Folded archives segment the data along frequency (Nchan) and time (Nsub). For instance, Nchan=256 and Nsub=60 yields 1 s subintegrations for a 60 s total observation at 64.5 kHz/channel, each with a full set of Stokes phase bins. This configuration trades time resolution against signal-to-noise ratio.

5. Metadata and PSRFITS Archive Format

Processed folded profile archives are stored in the PSRFITS format (both search and folded modes). The FITS header includes telescope, receiver, observer, observation date (MJD), central frequency (FREQ), bandwidth (BW), channel count (NCHAN), phase bins (NBIN), polarisation states (NPOL), start time (TSTART), sampling time (TSAMP), and ephemeris parameters (POLYCO, P0, P1, DM, pulsar name). The primary data extension structures the multidimensional data as phase × subint × channel × Stokes.

Calibration metadata, including the Jones/Mueller matrices and rotation measure (RM) solutions, are embedded in header-data units or HISTORY records such as:

HISTORY “RM_CORR applied: psi(lambda^2)=psi_0 + RM*lambda^2, RM=–39.4±1.8 rad/m^2”

with the RM relationship

$\psi(\lambda^2) = \psi_0 + \mathrm{RM}\,\lambda^2$

applied to polarisation angle corrections.

6. Data Storage, Retrieval, and Access

After primary data reduction, raw PCAP voltage recordings (~220 GB/hr) are deleted, with only the folded PSRFITS archives (on the order of tens of MB per hour) and occasional search-mode filterbanks (≤ 100 MB) being retained. The archive is arranged hierarchically by observation date and source, exemplified as:

/data/GBD-DART-II/2025-11-21/J0534+2200/folded/J0534+2200_MJD61000.fits

Nightly automated backups synchronize all new archives to remote repositories. Data access is provided via a web interface or standard UNIX tools (rsync/SSH); real-time VOEvent triggers can prompt immediate buffer dumps for transient follow-up.

7. Technical Capabilities and Workflow Integration

Key strengths of GBD-DART-II’s full-polar folded profile archive system include:

• Real-time data capture and processing at a 1:1 time ratio on an Intel i9/AMD R9 server pair.
• Unified pipeline supporting both coherent and incoherent dedispersion modes.
• Immediate production of fully calibrated, frequency-resolved, full-Stokes folded archives in PSRFITS, compatible with major pulsar analysis toolchains (PSRCHIVE, tempo, tempo2, PRESTO).
• Automated RFI rejection (iterative_cleaner), polarisation calibration (pac), RM fitting (rmfit), and single-pulse analysis.
• Ongoing system health monitoring at 1 s/64 kHz and a persistent transient buffer for triggered events.

This cohesive architecture ensures that researchers utilizing GBD-DART-II folded profile archives are provided with products that are polarimetrically calibrated, dispersion-corrected, and enriched with comprehensive metadata—suitable for high-precision timing campaigns and detailed emission morphology studies (B et al., 31 Jan 2026).