Measurement Types

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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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
• 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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