Target position and celestial movement

In astronomical observations, the location of a celestial object is primarily defined using Right Ascension (RA) and Declination (Dec) coordinates. These are represented in ODS data by the src_ra_j2000_deg and src_dec_j2000_deg fields, indicating the position in Degrees for the J2000 epoch.

Sidereal targets

Sidereal targets are objects like stars and galaxies whose apparent positions in the sky remain essentially constant over the duration of an observation. For these targets, the initial RA and Dec coordinates are sufficient, and no updates are needed during the observation interval.

Slow-moving non-sidereal targets

Non-sidereal targets are objects that show noticeable movement across the sky. This motion is often referred to as proper motion. For these targets, multiple ODS data entries, or Mitigation Requests (MRs), may be needed to account for their changing positions. The update cadence of these MRs depends on how significantly the RA/Dec coordinates change relative to the size of the telescope and downlink beams.

As a general guideline, position updates for moving targets should not exceed the inverse of the proper motion (1/pm), where pm is expressed in Degrees per Second (dps). This ensures that updates are frequent enough to track the object accurately without being pushing too many MRs to the API at once.

For many Solar System bodies, such as objects in the asteroid belt or the Voyager spacecrafts, the proper motion is relatively slow, often in the order of tens of Arcseconds per Hour, or about one micro-Degree per Second (0.000001 dps). At these slow rates, position updates are generally unnecessary and can be kept as a single RA/Dec value in the MR.

Fast-moving non-sidereal targets

However, some observation techniques involve rapidly scanning the sky, which necessitates more frequent position updates. Examples include:

1. Tipping scans, which involve quickly moving the telescope to observe different points in the sky to measure atmospheric opacity;
2. Drift scanning, where the beam of the antenna sweeps across the sky due to Earth's rotation (slewing or not the antenna);
3. On/Off source calibration when the telescope has to point at a random patch of sky near the target of interest to measure off-source flux level.

For these rapid scanning methods, the observation needs to be divided into multiple MRs. A few position updates per minute are typically sufficient to maintain accurate pointing for mitigation.

About

Contact: Bang Nhan (NRAO)

Acknowledgment

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"The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The ODS system was developed by NRAO with support from the National Science Foundation's SII NRDZ: Dynamic Protection and Spectrum Monitoring for Radio Observatories (AST-2232159), and the SWIFT-SAT: Observational Data Sharing (AST-2332422)."