ASTERIX CAT 048 vs CAT 062: A Comparison

The ASTERIX (All Purpose STructured Eurocontrol SuRveillance Information EXchange) standard is the global de facto standard for sharing surveillance sensor data (e.g., radar). Developed by EUROCONTROL, ASTERIX defines structured message formats for exchanging surveillance information between sensors, processing systems, and operator displays.

Two of the most critical categories are Category 048 (CAT-48/CAT 048) and Category 062 (CAT-62/CAT 062). While both carry target (e.g. aircraft) position, velocity and identification data, they represent two entirely different stages of the surveillance processing chain.

This article provides a brief comparative analysis of their architecture, data resolution, and operational use cases. (You may also be interested in our other article: Introduction to ASTERIX CAT048 for Software Engineers)

In practical systems, these formats often appear at different points in the surveillance processing pipeline, from sensor-level reporting through to multi-sensor fusion systems.

Monoradar vs. Multi-Sensor

CAT-48 is strictly a monoradar category. It carries data from a single surveillance radar system. In this context, the single system may actually be a combination of two co-located sensors of different types, most commonly a primary and secondary radar.  It represents the "raw" observation of a single station at a specific moment in time. Even where a radar site combines PSR and SSR sensors, the resulting CAT-48 data still represents observations from a single physical location.

• Isolation: A CAT-48 message is "blind" to any other sensor. It does not know if another radar is seeing the same aircraft.
• Sensor Dependency: All coordinates and timestamps are relative to the physical location and rotational period of that specific antenna.
• Principal Sources: Primary Surveillance Radar (PSR), Secondary Surveillance Radar (SSR), Mode S.

CAT-62 is a system-level category, intended for transmission of System Tracks generated by a Surveillance Data Processing System (SDPS) or Multi-Radar Tracker (MRT). These tracks are typically generated by surveillance data processing systems that correlate observations from multiple sensors.

• Fusion: The result of merging data from multiple disparate sources (e.g., radar, ADS-B, multilateration).
• Abstraction: It abstracts the complexities of individual sensors into a single, consolidated report.
• Independence: The track exists independently of any single sensor's update rate or failure.

A note on CAT 034 (Radar Service Messages)

CAT-48 messages are usually accompanied by companion messages that provide health and synchronisation information. Category 034 (CAT-34) messages provide this metadata for a specific radar station.

• Antenna Sync: CAT-34 provides North Mark and Sector messages so the receiver knows exactly where the antenna is pointing during its rotation.
• Local Health: Reports sensor-specific errors such as "Receiver Channel A Failure" or "High Clutter Detected."
• Location: CAT-48 provides target positions relative to the sensor.  CAT-34 messages are required to provide the datum, allowing conversion to lat/long coordinates.

 

Data Precision and Item Resolution

A critical difference between the two categories is how they quantify the physical world. CAT-48 uses units tailored for radar physics, while CAT-62 uses units tailored for global navigation and high-speed computation.

Positional Accuracy

• CAT-48 (I048/040 - Polar):
  • Range (Rho): Resolution of 1/256 Nautical Mile (approx. 7.2 metres).
  • Azimuth (Theta): Resolution of 360°/ 216 (0.0055°). This is highly accurate for angular detection but loses precision in physical distance as the target moves farther from the radar.
• CAT-62 (I062/105 - Geodetic):
  • Latitude/Longitude: Resolution of 180° / 225 (approx.. 0.00000536°). This provides a resolution of approximately 0.6 metres at the equator.

It’s important to note that although the CAT-62 standard allows for such fine precision, the actual sensor input data is unlikely to be this accurate.   

Altitude Resolution

• CAT-48 (I048/090 and I048/110): Typically carries Flight Level in increments of ¼ FL (25 feet) or 1 FL (100 feet), depending on whether the radar is receiving Mode S or older Mode C transponders.
• CAT-62 (I062/135): Altitude is often smoothed and can be represented in resolutions as fine as 1 foot (Geometric Altitude), depending on the fusion of ADS-B and Barometric data.

Temporal Precision

• CAT-48 (I048/140): The "Time of Day" is expressed in units of 1/128 seconds.
• CAT-62 (I062/070): The "Time of Track Applicability" is also expressed in 1/128 seconds, but unlike CAT-48, this time is usually a "System Time" used to synchronise all tracks to a single heartbeat, regardless of when the individual sensors actually saw the target.

 

"Resolution vs. Reality" Trap

While the technical specifications of ASTERIX suggest extreme precision, there is a common engineering pitfall regarding how these categories quantify the world. CAT-62 offers a geodetic resolution of approximately 0.6 meters at the equator. However, it is vital to remember that the system-level track is only as accurate as its inputs.

Since CAT-48—the primary source for these tracks—typically has a range resolution of 7.2 meters (1/256 NM), the high precision in a CAT-62 message is often a "fine-grained container" for "coarse-grained data". Engineers must distinguish between resolution (how small the increments are) and accuracy (how close the report is to the aircraft's actual physical position).

CAT-62: Managing the Multi-Sensor Ecosystem

CAT-62 is designed to handle the contradictions and overlaps inherent in using multiple sensors.  As a monoradar report format, CAT-48 does not need to handle this, so it is generally simpler and lighter.

The Fusion Process

This category results from merging data from multiple disparate sources, such as radar, ADS-B and multilateration. To manage this, a correlation engine like SPx Fusion Server is used to:

• Abstract Complexities: It merges individual sensor reports into a single, consolidated system track.
• Ensure Independence: The resulting track exists independently of any single sensor's update rate or failure.

The Track Source List (I062/290)

In a multi-sensor environment, understanding which sensors support a system track is crucial. CAT-62 includes a list of every sensor contributing to the track, providing:

• Traceability: Each entry includes the SAC/SIC (System Area Code/System Identification Code) of the contributing radar or ADS-B station.
• Engineering Value: If a system track starts "jumping," an engineer can look at the Source List to see which specific sensor is providing bad data