Measurement Types

Documentation Index

Fetch the complete documentation index at: https://docs.valar.space/llms.txt

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For the complete documentation index, see 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.

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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

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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

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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

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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

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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 on the TDM page.

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Measurement Selection

The choice of measurement type depends on several factors:

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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 documentation.

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