The Tracking Data Message (TDM) is a standardized ASCII text format for exchanging spacecraft tracking observations between ground stations, control centers, and orbit determination systems. Unlike OEM which contains computed ephemeris, TDM contains raw or processed measurement data from tracking sensors such as radar, optical telescopes, and ranging systems.
VALAR Doppler Support: While the CCSDS TDM standard includes Doppler measurement types, VALAR does not currently process Doppler data. TDM files containing Doppler measurements can be imported, but the Doppler observations will be ignored. Only angle measurements (AZEL, RADEC) and range measurements are processed.
Key Components
A TDM file contains a hierarchical structure:
- Header: File-level metadata including format version, creation date, and originator
- Segment(s): One or more independent tracking data segments, each containing:
- Metadata Section: Observation context (participants, time system, measurement types, reference frame)
- Data Section: Time-tagged measurements in chronological order
Each segment represents a distinct tracking pass, observation session, or measurement configuration. A single TDM file can contain multiple segments for different tracking stations or observation periods.
Structure Overview
TDM Hierarchy
TDM File
├── Header (format version, creation date, originator)
└── Segment 1
├── Metadata Section (META_START to META_STOP)
└── Data Section (DATA_START to DATA_STOP)
└── Segment 2
├── Metadata Section
└── Data Section
└── ... (additional segments)
Segment Structure
Each segment is self-contained with its own metadata and data:
- Metadata defines the observation context and applies to all measurements in the data section
- Data contains timestamped observations in the format:
KEYWORD = EPOCH VALUE
| Field | Description | Example Values |
|---|
| TIME_SYSTEM | Time system for all epochs | UTC, TAI, GPS |
| PARTICIPANT_1 | First participant in tracking path | Ground station ID, spacecraft ID |
| PARTICIPANT_2 | Second participant in tracking path | Spacecraft ID, ground station ID |
| MODE | Data transmission mode | SEQUENTIAL, SINGLE_DIFF |
| PATH | Signal path between participants | 1,2 (one-way), 2,1,2 (two-way) |
Angle Observations
- ANGLE_TYPE: Geometry convention (AZEL, RADEC, XEYN, XSYE)
- REFERENCE_FRAME: Coordinate frame for angles (EME2000, ITRF, topocentric)
Range Observations
- RANGE_UNITS: Units for range measurements (km, m, s, RU)
- TRANSMIT_BAND: Frequency band (S, X, Ka)
- RECEIVE_BAND: Receiving frequency band
- TIMETAG_REF: Time tag reference point (TRANSMIT, RECEIVE, BOUNCE)
- INTEGRATION_INTERVAL: Data averaging period (seconds)
- CORRECTION_ANGLE_1: Correction applied to first angle component
- CORRECTION_ANGLE_2: Correction applied to second angle component
- CORRECTION_RANGE: Range correction applied
Corrections fields indicate whether measurements have been preprocessed to remove systematic errors, biases, or atmospheric effects.
Measurement Types
TDM supports a wide variety of tracking observations:
Angle Measurements
| Keyword | Description | Units | ANGLE_TYPE |
|---|
| ANGLE_1 | First angle component | degrees | Azimuth (AZEL), RA (RADEC), X-East (XEYN) |
| ANGLE_2 | Second angle component | degrees | Elevation (AZEL), Dec (RADEC), Y-North (XEYN) |
- AZEL: Azimuth and elevation (local horizontal frame, typical for radar and RF tracking)
- RADEC: Right ascension and declination (inertial frame, typical for optical telescopes)
- XEYN: X-East and Y-North (topocentric Cartesian)
- XSYE: X-South and Y-East (alternative topocentric)
Range Measurements
| Keyword | Description | Units |
|---|
| RANGE | One-way or two-way range | km, m, s, or range units (RU) |
- One-way: Signal travels from transmitter to receiver (PATH = 1,2 or 2,1)
- Two-way: Signal transmitted, reflected, and received (PATH = 1,2,1 or 2,1,2)
Frequency Measurements
| Keyword | Description | Units |
|---|
| RECEIVE_FREQ | Received signal frequency | Hz |
| TRANSMIT_FREQ_1 | First transmitter frequency | Hz |
| TRANSMIT_FREQ_2 | Second transmitter frequency | Hz |
Additional Observations
| Keyword | Description | Units |
|---|
| MAG | Visual or radar magnitude | dimensionless |
| RCS | Radar cross-section | m² |
| PRESSURE | Atmospheric pressure | hPa |
| TEMPERATURE | Atmospheric temperature | K |
| HUMIDITY | Relative humidity | % |
Critical Keywords
MODE — Transmission Mode
Defines the signal transmission configuration:
- SEQUENTIAL: Measurements taken in chronological sequence (most common)
- SINGLE_DIFF: Single-difference measurements (VLBI, GPS)
- COHERENT: Coherent ranging
- CONSTANT: Constant frequency transmission
PATH — Signal Path
Specifies the route of the signal between participants:
- 1,2: One-way from Participant 1 to Participant 2
- 2,1: One-way from Participant 2 to Participant 1
- 1,2,1: Two-way round trip starting from Participant 1
- 2,1,2: Two-way round trip starting from Participant 2
PATH order matters for interpreting range measurements. Two-way range includes the round-trip distance, while one-way range is the direct distance.
- TRANSMIT: Time when signal was transmitted
- RECEIVE: Time when signal was received
- BOUNCE: Time when signal reflected off target (two-way ranging)
This is critical for correctly processing light-time corrections in orbit determination.
Common Use Cases
- Orbit Determination: Processing radar and optical tracking to estimate spacecraft state
- Orbit Solution Validation: Comparing predicted ephemeris against ground-based observations
- Navigation Support: Providing ranging data for real-time navigation
- Conjunction Assessment: Sharing raw observations for independent orbit solutions
- Data Archiving: Long-term storage of historical tracking campaigns
- Multi-Station Campaigns: Combining observations from multiple ground stations
Sample TDM Files
Example 1: Optical Angle Measurements (RADEC)
This example shows right ascension and declination measurements from an optical telescope tracking a spacecraft in an inertial reference frame:
CCSDS_TDM_VERS = 2.0
CREATION_DATE = 2024-12-10T16:30:14.352701
ORIGINATOR = VALAR
COMMENT Optical telescope tracking data
COMMENT Measurements have not been corrected for atmospheric refraction
META_START
TIME_SYSTEM = UTC
PARTICIPANT_1 = OPTICAL-STATION-01
PARTICIPANT_2 = 2023-001A
MODE = SEQUENTIAL
PATH = 2,1
TIMETAG_REF = RECEIVE
ANGLE_TYPE = RADEC
REFERENCE_FRAME = EME2000
INTEGRATION_INTERVAL = 2.0
META_STOP
DATA_START
ANGLE_1 = 2024-12-10T02:49:31.512 172.016040
ANGLE_2 = 2024-12-10T02:49:31.512 -3.931946
MAG = 2024-12-10T02:49:31.512 14.535494
ANGLE_1 = 2024-12-10T02:49:33.513 172.024207
ANGLE_2 = 2024-12-10T02:49:33.513 -3.931761
MAG = 2024-12-10T02:49:33.513 14.621805
ANGLE_1 = 2024-12-10T02:49:35.514 172.032289
ANGLE_2 = 2024-12-10T02:49:35.514 -3.931576
MAG = 2024-12-10T02:49:35.514 14.708923
DATA_STOP
Example 2: Radar Tracking (AZEL + Range)
This example shows azimuth-elevation angles with two-way range from a ground-based radar:
CCSDS_TDM_VERS = 2.0
CREATION_DATE = 2024-12-15T08:00:00.000
ORIGINATOR = GROUND_STATION_ALPHA
COMMENT Two-way radar tracking pass
COMMENT Range corrected for tropospheric delay
META_START
TIME_SYSTEM = UTC
PARTICIPANT_1 = RADAR-SITE-A
PARTICIPANT_2 = SPACECRAFT-BETA
MODE = SEQUENTIAL
PATH = 1,2,1
TIMETAG_REF = TRANSMIT
ANGLE_TYPE = AZEL
REFERENCE_FRAME = ITRF
RANGE_UNITS = km
TRANSMIT_BAND = X
RECEIVE_BAND = X
CORRECTION_RANGE = YES
META_STOP
DATA_START
ANGLE_1 = 2024-12-15T12:30:00.000 87.234
ANGLE_2 = 2024-12-15T12:30:00.000 45.678
RANGE = 2024-12-15T12:30:00.000 842.156
ANGLE_1 = 2024-12-15T12:30:02.000 87.456
ANGLE_2 = 2024-12-15T12:30:02.000 46.123
RANGE = 2024-12-15T12:30:02.000 845.892
ANGLE_1 = 2024-12-15T12:30:04.000 87.678
ANGLE_2 = 2024-12-15T12:30:04.000 46.567
RANGE = 2024-12-15T12:30:04.000 849.734
DATA_STOP
Multi-Segment TDM Example
A single TDM file can contain multiple segments from different stations or observation sessions:
CCSDS_TDM_VERS = 2.0
CREATION_DATE = 2024-12-25T20:00:00.000
ORIGINATOR = MISSION_CONTROL
COMMENT Multi-station tracking campaign
META_START
TIME_SYSTEM = UTC
PARTICIPANT_1 = STATION-NORTH
PARTICIPANT_2 = SPACECRAFT-GAMMA
MODE = SEQUENTIAL
PATH = 1,2,1
ANGLE_TYPE = AZEL
REFERENCE_FRAME = ITRF
RANGE_UNITS = km
META_STOP
DATA_START
ANGLE_1 = 2024-12-25T10:00:00.000 120.234
ANGLE_2 = 2024-12-25T10:00:00.000 30.567
RANGE = 2024-12-25T10:00:00.000 1250.123
DATA_STOP
META_START
TIME_SYSTEM = UTC
PARTICIPANT_1 = STATION-SOUTH
PARTICIPANT_2 = SPACECRAFT-GAMMA
MODE = SEQUENTIAL
PATH = 1,2,1
ANGLE_TYPE = AZEL
REFERENCE_FRAME = ITRF
RANGE_UNITS = km
META_STOP
DATA_START
ANGLE_1 = 2024-12-25T14:00:00.000 240.678
ANGLE_2 = 2024-12-25T14:00:00.000 25.123
RANGE = 2024-12-25T14:00:00.000 1380.456
DATA_STOP
Usage in VALAR
When working with TDM files in VALAR:
-
Sensor Configuration Required: Ground station or sensor metadata must be configured in VALAR before importing TDM files. This includes station location, antenna characteristics, and measurement biases.
-
Automatic Processing: VALAR validates TDM structure, parses metadata, and associates measurements with the correct spacecraft and sensors.
-
Orbit Determination: TDM measurements are used as observations in weighted least-squares orbit determination to estimate spacecraft state vectors.
-
Residual Analysis: After orbit determination, VALAR computes observation residuals (measured minus computed) for quality assessment.
-
Multi-Station Support: TDM files with multiple segments from different stations are automatically handled.
Sensor configuration in VALAR must match the PARTICIPANT_1 field in TDM metadata. If the sensor ID is not found, the TDM import will fail.
Best Practices
Data Quality
- Include COMMENT fields: Document observation conditions, corrections applied, and data quality issues
- Specify corrections: Use CORRECTION_* keywords to indicate preprocessing
- Provide metadata: Include INTEGRATION_INTERVAL and measurement uncertainties when available
File Organization
- One segment per pass: Separate distinct tracking passes or observation sessions into different segments
- Consistent time systems: Use UTC for operational tracking data
- Appropriate reference frames: Use ITRF for ground-based AZEL, EME2000/GCRF for RADEC
Measurement Types
- Match ANGLE_TYPE to sensor: Use AZEL for radar/RF, RADEC for optical telescopes
- Specify PATH correctly: Ensure PATH matches actual signal propagation
- Use TIMETAG_REF appropriately: TRANSMIT for radar, RECEIVE for optical
Interoperability
- Follow CCSDS conventions: Adhere to standard keyword names and units
- Validate before sharing: Ensure TDM syntax and semantics are correct
- Document non-standard fields: If using custom keywords, explain them in comments
| Feature | TDM | OEM | OPM |
|---|
| Data Type | Raw observations | Computed ephemeris | State snapshot |
| Temporal Scope | Observation period | Time series | Single epoch |
| Source | Ground sensors | Propagation/OD | OD output |
| Purpose | Input to OD | Trajectory sharing | State exchange |
| Measurement Types | Angles, range | Position, velocity | Position, velocity, elements |
| Multi-Segment | Yes (multiple stations) | Yes (multiple intervals) | No |
Reference Frames and Time Systems
Supported Reference Frames
TDM supports any reference frame appropriate for the measurement type:
- ITRF/ITRF2020: Earth-fixed frame for ground-based AZEL measurements
- EME2000/GCRF: Inertial frames for RADEC measurements
- Topocentric: Station-centered frames (local horizontal)
Time Systems
Standard time systems for TDM:
- UTC: Coordinated Universal Time (most common for operational tracking)
- TAI: International Atomic Time
- GPS: GPS Time
- TDB: Barycentric Dynamical Time (deep space missions)
Ensure your time system matches the precision requirements of your orbit determination. For sub-meter accuracy, distinguish between UTC, TAI, and dynamical time systems.
Technical References
VALAR’s TDM processing is built on industry-standard astrodynamics models, following:
- CCSDS 503.0-B-2: Tracking Data Message standard
- CCSDS 502.0-B-3: Navigation Data Messages (for reference frame definitions)
- IERS Conventions: Earth orientation and time systems
- Vallado’s Fundamentals of Astrodynamics and Applications
For detailed measurement modeling and observation types, consult the CCSDS TDM Specification. 