Glossary of Terms
Definitions for terms that are commonly used across the LeoLabs Platform

Bias (e.g., range, doppler) A constant offset between sensor measurements and a truth reference used to provide a corrected measurement.
Collection A user-defined group of Objects. Users may create a Collection with any grouping of objects they have permission to receive data products for, on the LeoLabs platform.
Conjunction A close approach event between two orbiting objects that pass within a relatively close distance to one another, typically a few kilometers or less. Not to be confused with "collision", which implies physical contact is made between the two objects. Upcoming conjunctions are analyzed to assess the likelihood that a collision may occur.
Conjunction Data Message (CDM) A standardized message format providing risk quantification information for orbital conjunction events. Full CCSDS specification available here:
Conjunction Plane The 2D plane that is normal to the relative velocity vector at TCA for two objects in a conjunction event.
Constellation A list of an existing on-orbit satellite group. This list is created and regularly updated by LeoLabs, and is available for view without a user login.
Covariance Matrix A square matrix describing the covariance of elements of a state vector. The diagonal elements of the matrix represent the standard variance.
Detection Probability The likelihood of success in collecting radar measurements for an upcoming radar pass on the corresponding object. Detection probability is calculated from historical data, and computes the percentage of passes where measurements were collected divided by total number of radar passes over a period of time.
Ephemeris A data file showing the positions and velocities of an orbiting object propagated over a specific time span at known intervals. Ephemerides may be historical, or predictive. LeoLabs ephemerides also contain propagated covariances.
Epoch The time associated with a particular event or data set. In the LeoLabs Platform, epochs most often refer to the time and date corresponding with positions and velocities for state vectors and ephemerides.
Error Ellipse A 2D projection of miss distance and covariances at TCA between two objects involved in a conjunction event onto the conjunction plane. The origin (0,0) corresponds to the center of Object 1. The center of the ellipse corresponds to the center of Object 2. The size of the ellipse represents the combined covariances of both objects together at TCA. The ellipse is commonly drawn at the 1 and 3 sigma levels.
Mahalanobis Distance The statistical distance between a data point and a relative distribution. LeoLabs utilizes Mahalanobis distance as a metric to convey how the distance between two state vectors corresponds to the covariances of the two objects. A large Md (>~3) implies that the difference is outside of the expected distribution. Mahalanobis distance is a reported value in both CDMs, as well as state vector comparisons for individual objects.
Measurement An individual observation on an object from a LeoLabs radar. A single measurement collects range, doppler, RCS, and time-stamp information. Depending on the radar site, some measurements may include angles information as well (azimuth / elevation). Measurements are calibrated with bias and residual adjustments before being processed through LeoLabs' OD algorithms to fit a state vector and covariance.
Miss Distance In a conjunction event, the miss distance refers to the closest distance that the two objects will come to one another at TCA. Miss distance is calculated as center-of-mass to center-of-mass between the two objects.
Observed Passes An object passing through the field of view of one of LeoLabs' ground radar sites, during which time measurements are collected on the object.
Orbit Ephemeris Message A standardized message format providing ephemeris information on an object. Full CCSDS specification available here:
Prioritization Assigning increased relative weighting or "importance" of one object in the LeoLabs catalog relative to other objects for collecting additional measurements in radar passes. Increased prioritization of an object often yields higher quality state vectors with lower covariances.
Probability of Collision (PoC) The computed probability of a collision occurring between two orbital objects involved in a conjunction event. LeoLabs platform utilizes the Alfano (2005) algorithm for computing short-term encounters, and the Coppola (2012) method for computing long-term encounters.
Probability of Collision (PoC) Sensitivity The sensitivity of the PoC value to variations in input parameters. PoC sensitivity may vary significantly among different conjunctions. Factors influencing PoC sensitivity for a given conjunction include miss distance, covariance size, and hard-body radius.
Radar Cross Section (RCS) LeoLabs radars estimate the radar cross section on observed targets based on the measured signal strength. The RCS is related but not equal to the physical cross-sectional areas. It is important to note that RCS estimations can and do change, depending on the scattering mechanisms, as well as the orientation of the object being measured. LeoLabs utilizes a statistical average RCS taken from many measurements over different periods of time to produce an RCS estimate. Not all objects in the LeoLabs catalog will list RCS.
Residuals The estimated uncertainties on corrected sensor measurements. Residual values are derived using the distribution of differences between measurements and a truth reference.
Screening Performing a comparison of one ephemeris against one or more ephemerides for other objects, and searching for potential conjunction events.
State Vector The position and velocity of an orbiting object at a specific moment in time known as the solution epoch.
Time of Closest Approach (TCA) In a conjunction event, the time at which the two objects pass by one another at their closest point, or minimum miss distance. TCA is shown in UTC (GMT+0) time zone.
Two Line Element set (TLE) A compact representation of an object's orbit using mean Keplerian elements and the simplified general perturbations (SGP4) propagator. TLEs do not contain covariances, and should not be used for operational conjunction risk assessment or collision avoidance activities.