The LWA-SV Station

The telescope

The Long Wavelength Array station at the Sevilleta National Wildlife Refuge (LWA-SV) is located at latitude $34.348^{\circ }$ N, longitude $106.886^{\circ }$ W, in central New Mexico. It is the second station of the Long Wavelength Array, following LWA1. Its main characteristics are:

• Antennas. 256 stands, each consisting of a pair of crossed dipoles that measure orthogonal linear polarizations, denoted X and Y.
• Layout. The stands occupy a pseudo-random elliptical footprint of approximately 110 m × 100 m, elongated north–south. A small number of outlying stands extend beyond it.
• Frequency range. 10–88 MHz.

Each dipole responds to nearly the entire visible sky, and the angular resolution of the ~100 m aperture at wavelengths of 3.4–30 m is of order one degree. The instrument is therefore intrinsically an all-sky imager. With a large number of elements in a compact, filled aperture, it lies in the regime in which direct imaging is competitive with FX correlation (Thyagarajan et al. 2017).

Below 100 MHz, the sky brightness temperature is dominated by Galactic synchrotron emission. The variable sky at these frequencies is populated by coherent emitters:

• solar radio bursts
• Jovian decametric emission
• pulsar giant pulses
• fast radio bursts

Propagation through the ionosphere imposes further variability. An instrument that images the whole sky at millisecond resolution is well matched to these phenomena.

Digital signal processing

The station digitizes each dipole at 196 MHz. The Advanced Digital Processor (ADP), the station F-engine, channelizes the band into 25 kHz channels. The parameters relevant to EPIC are listed below (src/ex/constants.h).

Quantity	Value
Sampling rate $f_s$	196 MHz
Channel width $\Delta \nu$	25 kHz
Spectrum interval	$1/\Delta \nu =40\mu \text{s}$ (25 000 spectra s⁻¹)
Inputs per channel	256 stands × 2 polarizations = 512
Sample format	4-bit + 4-bit two's-complement complex
Channels per sub-band	132 (3.3 MHz)
Processing boards	16, each serving 16 stands

The F-engine output is multicast on the station network in the CHIPS packet format, organized as follows:

• Sub-bands. Up to six sub-bands are multicast to the groups 239.168.40.11–16 on ports 4015 and 4016.
• Header. Each packet carries a 16-byte header containing the board index, the first channel index chan0, the number of channels, and a 64-bit sequence counter.
• Payload. The payload contains $32n_{\mathrm{chan}}$ bytes: 16 stands × 2 polarizations × 1 byte per channel.
• Spectrum. A complete spectrum from all 256 stands at one time sample therefore comprises 16 packets, one from each board.

The sky frequency of a channel is

$$\nu =\text{chan0}\times 25\text{kHz}.$$

The absolute time of a spectrum follows from its sequence number and from the start time of the ADP, which the imager obtains from the station monitor-and-control system (MCS):

$$t=t_{\mathrm{start}}+\frac{\text{seq}}{25000{\text{s}}^{-1}}.$$

Commensal operation

EPIC operates commensally with the other station back ends. The imager subscribes to the F-engine multicast groups using NIC-level flow steering through the ibverbs interface. Its only control-plane interaction with the station is a read-only query of the ADP start time. Commensal operation has two consequences.

Observing frequency. The observing frequency is determined by the station schedule, not by EPIC. When the tuning changes, the imager detects the change in chan0 and reconfigures for the new tuning:

• antenna positions in wavelengths
• gridding kernels
• phase corrections
• database session

Health checks. The station periodically steps through a fixed set of health-check tunings: chan0 ∈ {440, 1160, 1880, 2600, 3320, 4040}, corresponding to 11–101 MHz. The imager suspends imaging at these tunings.

Each contiguous interval at a fixed tuning is recorded as an imaging session. Gaps between sessions, and changes of frequency between them, are an expected property of the data.