Observation Modes#

Different observing strategies require different reference selection and data preparation. This page describes the supported modes and how to add new ones.

Mode Detection#

The engine auto-detects the observation mode from the data:

  1. Check the instmode attribute (e.g., "OTF")

  2. Examine sobsmode strings across subscans

  3. Fall back to TotalPower if ambiguous

This can be overridden with --mode otf or --mode totalpower on the command line.

Implementation: cal-core/src/config.rs → ObsMode enum

Supported Modes#

TotalPower#

Single-point position-switched observations with discrete ON/OFF pairs.

        graph LR
  OFF1[OFF] --> ON1[ON]
  ON1 --> OFF2[OFF]
  OFF2 --> ON2[ON]
  ON2 --> OFF3[OFF]

  style OFF1 fill:#bbdefb
  style OFF2 fill:#bbdefb
  style OFF3 fill:#bbdefb
  style ON1 fill:#c8e6c9
  style ON2 fill:#c8e6c9

  • ON subscans: single dump per pointing

  • Reference: nearest-time OFF (first dump only)

  • Matching: each ON is paired with the temporally closest OFF by MJD

Implementation: cal-core/src/modes/totalpower.rs

OTF TotalPower#

On-the-fly scanning where multiple dumps trace a path across the source.

        graph LR
  OFF1[OFF] --> OTF1[OTF-ON<br/>D dumps]
  OTF1 --> OFF2[OFF]
  OFF2 --> OTF2[OTF-ON<br/>D dumps]
  OTF2 --> OFF3[OFF]

  style OFF1 fill:#bbdefb
  style OFF2 fill:#bbdefb
  style OFF3 fill:#bbdefb
  style OTF1 fill:#c8e6c9
  style OTF2 fill:#c8e6c9

  • ON subscans: multiple dumps along scan direction (D > 1)

  • Reference: nearest-time OFF, averaged over all dumps (NaN-aware)

  • OTF coordinates: otf_lon and otf_lat are propagated to the output as [D, S] arrays

Implementation: cal-core/src/modes/otf.rs

Data Preparation#

File: cal-core/src/modes/prepare.rs

The preparation phase decouples reference selection from calibration math, enabling vectorized processing:

  1. Identify ON subscan indices from sobsmode

  2. For each ON, find the nearest-time OFF by MJD

  3. Deduplicate OFF references (compute average once, reuse for all ONs sharing the same reference)

  4. Build ref_data[C, R, A, S_on] from deduplicated OFF averages

  5. Compute integration time t_int[S_on]

The ON data is not copied — PreparedData stores indices into ScanData.data, and the calibration loop reads from there directly. This avoids a ~1 GB allocation for large OTF datasets.

Future Modes#

The ObsMode enum includes placeholders for:

  • FrequencySwitch — reference from frequency-shifted spectra

  • BeamSwitch — reference from offset beam position

These will be implemented when test data becomes available. See /source/developer/adding-observation-mode for the extension guide.