Observation Handling

In Tudat, observations, whether real or simulated, are stored in an ObservationCollection object. This class manages the storage of all observations in a sorted manner and handles all related bookkeeping.

A single observation is defined by the following quantities:

• Its type, which also defines the size of the observation (e.g., a range observation has a size of 1, while an angular position has a size of 2).

• Its link ends, which define the transmitter, receiver, and any transponders involved in the observation.

• The actual observable value.

• The time of the observation. While original data may be time-tagged in any scale (e.g., UTC), all times are converted to and stored in the Barycentric Dynamical Time (TDB) scale within Tudat.

• The weight assigned to the observation, used during least-squares estimation to account for observation uncertainty (defaults to 1).

• Any dependent variables calculated for the observation.

• The observation residual, computed as the difference between the observed and computed values. This value is only available after being explicitly computed.

• Optional ancillary settings required for the simulation of an observation, such as the integration time of a Doppler observation.

Within an ObservationCollection, observations are not stored individually but are grouped into SingleObservationSet objects. SingleObservationSet objects store any number of observations that share:

• the same observable type

• the same link ends

• the same ancillary settings

Each SingleObservationSet contains vectors of observation times, values, and weights. Residuals and dependent variables are also stored here once they are computed. The ObservationCollection acts as a container for these SingleObservationSet objects, organizing them in an internal data structure.

Note

The internal dictionary structure of the ObservationCollection is a low-level implementation detail that is typically not needed for most user interactions.

Working with Observations: A Workflow Guide

The following sections of this guide will walk you through the workflow of working with observations in Tudat. We will cover:

• Creating an Observation Collection: How to generate observations by simulation or load them from real tracking data files, as well as how to create pseudo-observations from external ephemerides.

• Interaction with an Observation Collection: How to manipulate and interact with an existing ObservationCollection, including filtering, splitting, and retrieving auxiliary quantities.