> ## Documentation Index > Fetch the complete documentation index at: https://docs.valar.space/llms.txt > Use this file to discover all available pages before exploring further. # Measurement Types > Overview of tracking measurement types > For the complete documentation index, see [llms.txt](/llms.txt). VALAR supports multiple types of measurements, each providing different information about spacecraft position and motion. Understanding these measurement types is essential for effective orbit determination and data management. ## AZEL (Azimuth-Elevation) **Azimuth-Elevation measurements** are angular observations from ground-based tracking stations that describe where a spacecraft appears in the local sky. * **Azimuth**: The horizontal angle measured clockwise from true north (0° to 360°) * **Elevation**: The vertical angle above the horizon (0° to 90°) AZEL measurements use a **topocentric coordinate system** centered at the observer's location on Earth. These measurements are commonly produced by: * Ground-based radar systems * Radio frequency tracking stations * Optical telescopes with alt-azimuth mounts **Use cases**: Initial acquisition, tracking passes, antenna pointing ## RADEC (Right Ascension-Declination) **Right Ascension-Declination measurements** describe a spacecraft's position on the celestial sphere using astronomical coordinates. * **Right Ascension (RA)**: The celestial longitude, measured eastward along the celestial equator from the vernal equinox (0h to 24h or 0° to 360°) * **Declination (DEC)**: The celestial latitude, measured north or south from the celestial equator (-90° to +90°) RADEC measurements use an **inertial celestial coordinate system** that doesn't rotate with Earth. These measurements are typically produced by: * Optical telescopes with equatorial mounts * Star tracker systems * Space surveillance optical sensors **Use cases**: Optical tracking, catalog maintenance, uncorrelated target observation ## RANGE **Range measurements** provide the direct distance between a tracking station and the spacecraft. * Measures **line-of-sight distance** from sensor to spacecraft * Typically expressed in kilometers (km) or meters (m) * Can be one-way or two-way (round-trip time-of-flight) Range measurements are obtained through: * Two-way radio ranging (transponder systems) * Laser ranging (SLR - Satellite Laser Ranging) * Radar time-of-flight measurements **Use cases**: High-precision orbit determination, station-keeping, rendezvous operations ## PVT (Position-Velocity-Time) **Position-Velocity-Time measurements** are complete state vectors that provide the full kinematic state of a spacecraft at a specific epoch. A PVT measurement includes: * **Position**: Three-dimensional Cartesian coordinates (X, Y, Z) * **Velocity**: Three-dimensional velocity components (Ẋ, Ẏ, Ż) * **Time**: Precise epoch of the measurement PVT measurements commonly originate from: * Onboard GPS/GNSS receivers * Satellite navigation solutions * Processed measurement products * Telemetry-derived state vectors **Use cases**: Direct orbit initialization, autonomous navigation, real-time state updates ## DOPPLER (Range-Rate) **Doppler measurements** are line-of-sight range-rate observations — the time derivative of the slant range between a ground station and a spacecraft. A Doppler sample reports how fast the geometric distance to the target is changing at a given epoch. * Measures **line-of-sight range-rate** from sensor to spacecraft * Reported in metres per second (m/s) on the internal record (km/s on the wire in TDM) * One-way (downlink) or two-way (round-trip) modalities supported Doppler measurements are obtained through: * Coherent radar tracking * RF transponder ground stations (one-way downlink and two-way coherent ranging) * Modern commercial and institutional Earth-orbit receivers (KSAT, SSC, Leaf Space, Viasat RT Logic, ESTRACK) **Use cases**: High-precision orbit determination, especially on radial-velocity-rich passes; complementing range and angle observations to constrain along-track and radial state components. For ingestion details — supported keywords, two-way convention rules (MEAN / SUM / UNDECLARED), default sigma values per band, and error code reference — see [Doppler (Range-Rate) Measurements](/file-formats/tdm#doppler-range-rate-measurements) on the TDM page. ## Measurement Selection The choice of measurement type depends on several factors: | Factor | Considerations | | ----------------- | --------------------------------------------------------------------------------------------------------------- | | **Observability** | AZEL/RADEC for angles, RANGE for distance, DOPPLER for range-rate, PVT for complete state | | **Sensor Type** | Ground radar (AZEL, RANGE, DOPPLER), Optical (RADEC), RF transponder ground station (DOPPLER, RANGE), GPS (PVT) | | **Accuracy** | RANGE, DOPPLER, and PVT typically provide highest precision for orbit determination | | **Availability** | PVT requires onboard systems, ground measurements (AZEL, RADEC, RANGE, DOPPLER) depend on pass geometry | | **Processing** | PVT is simplest to process, angular and Doppler measurements require station location modeling | ## Reference Frames Different measurement types naturally align with specific reference frames: * **AZEL**: Local topocentric frame (station-centered) * **RADEC**: Inertial celestial frame (Earth-Centered Inertial) * **RANGE**: Can be provided in various frames depending on processing * **DOPPLER**: Local topocentric frame (station-centered); the observable is a scalar range-rate along the station–spacecraft line of sight * **PVT**: Typically ITRF (Earth-fixed) or ECI (Earth-Centered Inertial) For more information on coordinate systems, see the [Reference Frames](/platform-overview/reference-frames) documentation. ## Next Steps Learn how to submit measurements Process measurements to determine orbits Understand coordinate systems Explore the platform data architecture