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Overview
Introduction
The Eurocontrol Standard Document for Surveillance Data Exchange, Category 240 (henceforth referred to as “CAT-240”) describes the application of ASTERIX to the transmission of rotating radar video.
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This document is applicable to the development of a client application that receives network radar video in ASTERIX CAT-240 format from a Navtech radar system.
Field Representation
The CAT-240 standard defines messaging fields in terms of (groups of) octets; and, in the case of individual bits, field reference numbers. Such definitions are independent of any particular implementation.
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Note, the depiction of multi-byte fields does not imply endianness. See section 2.3 for more details on byte ordering.
Data Transmission
Network Transport
CAT-240 messages are sent as UDP network packets.
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Table 1 - Default IP address and port for CAT-240 data
Supported Message Types
CAT-240 defines two message types:
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Video summary messages may be supported in future releases.
Data Framing & Rates
Video data from a Navtech radar is sent on an azimuth-by-azimuth basis. The data for each azimuth is a sequence of bins (range cells), each representing a power level at a distance from the radar. The first value represents the bin closest to the radar.
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For example, a radar may generate 400 azimuths per rotation, and rotate at 4Hz, giving a message rate (or Packet Rate Frequency, PRF) of 1600 messages/sec
Byte Ordering
All data is sent as Network (that is, Big) endian.
Clients may therefore have to performing byte-swapping on multi-byte words to retrieve data correctly.
Message Structure
Overview
A CAT-240 message is illustrated in Figure 1. It consists of five main elements:
A data block, containing basic message details
The video data header, which defines the context of the video data
The video (FFT) data
A time-of-day field
A special-purpose field containing Navtech-specific extended information
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Message Data Block
The message data block defines basic characteristics of the message and the specific configuration of the message.
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CAT | The message category. Always 240 (0xF0) |
LEN | The overall message length, including this data block |
FSPEC | Feature specifications for this message. |
Feature Specifications
The Feature Specification bit field structure defines which fields are present in the message and, in the case of optional features, which option(s) have been selected.
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For the Navtech implementation of CAT-240 some fields are mandated. Figure 3 shows the default flag settings. Note, in the cases where bit field values have not been specified these fields will be set according to the supplied video data. See later for more details.
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Video Header
The video header provides the context for the FFT data packaged in the message.
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SAC_SIC | System Identification Code / System Area Code |
TYPE | Message type. Always 001 |
MSG_INDEX | Incrementing message counter |
START_AZ | Start angle for data, encoded as 1/360 of a degree. |
END_AZ | End angle in encoded as 1/360 of a degree. |
CELL_DUR | Size of a bin in femtoseconds. |
RES_C | Defines the resolution (number of bits) for each bin; and whether the data is compressed. |
NB_VB | Size of the FFT data, in bytes. |
NB_CELLS | Size of the FFT data, in bins. |
Data Source Identifier
The Data Source Identifier provides a unique identifier for the radar source. The Data Source Identifier is a 16-bit field structured as shown in Figure 5.
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System Area Code | TBD |
System Identifier Code | TBD |
Start & End Azimuth
A Navtech radar outputs its FFT data on discrete azimuths. The number of azimuths sampled per rotation depends on the radar; and is configurable.
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The number of azimuths in a full rotation is not a defined field in CAT-240. Therefore, this value is provided as part of the Special Purpose field. See XXXXX for more details.
Cell Duration
Cell duration defines the size of an FFT bin in femtoseconds.
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If non-contoured data can be assumed, then the range in bins will be the same as the video block cell size.
Resolution
The resolution field defines two properties of the message
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Value | Resolution | Encoding, in bits |
1 | Monobit | 1 |
2 | Low | 2 |
3 | Medium | 4 |
4 | High | 8 |
5 | Very high | 16 |
6 | Ultra high | 32 |
Data Size In Bins
The number of bins in the message is calculated as
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Where resolution in bytes must be computed from the resolution/compression field value.
Video Data
The video data block holds the FFT data. It is structured as shown in Figure 7.
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REP | The number of blocks in the Video data block |
block [n] | A block of FFT data. The size of the block is determined by the configuration in the video message header. |
Video Data Block Size
The framing configuration for an incoming message is specified with two fields:
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For extracting the Time-of-day (and any other subsequent fields) client applications must use REP * block size to calculate the alignment correctly.
Time-Of-Day
Time-of-day is an absolute time stamp, expressed in UTC. The time-of-day is reset each day at midnight; that is, the time-of-day holds no date information.
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For a radar sampling 400 azimuths per rotation, at 4Hz, each azimuth message is generated every 625 microseconds. Time-of-day is too low resolution to use for precise azimuth timing. To expedite this, precise azimuth timing, to nanosecond resolution, is supplied in the Special Purpose field.
Special Purpose Field
The Special Purpose Field allows an application to include custom data that is not specified in the ASTERIX standard. This field includes more detailed information describing the format of the data than is supported by the ASTERIX standard.
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Count | The size of the Special Purpose field, in bytes |
Seconds | Timestamp. UTC seconds since the Unix epoch (01 Jan 1970, 00:00) |
Subseconds | Nanosecond resolution field of the timestamp |
Azimuths per rotation | Total number of azimuths sampled per rotation |
Range in bins | The range of the radar, expressed as a number of bins |
Rotation speed | Current radar rotation speed, in milli-Hz |
Gain | Range scaling factor. Encoded as a IEE 754 binary32 (float) |
Offset | Range offset of the FFT data, in metres. Encoded as a IEE 754 binary32 (float) |
Health | Radar component health status |
Radar Health Status
Radar health status provides information on the current operating status. The field is encoded as a bit field, as shown in Figure 10.
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Value | Definition |
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0 | unhealthy |
1 | warning |
2 | healthy |
3 | Invalid / unknown |
Endianness & Field Layout
Network Endian Encoding
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Bit Field Structure Layout
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Example Video Message
Below is an example video message with representative values for each field. This message is designed to be used to validate client application decoding.
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Rotation rate – 4 Hz
Azimuths per rotation – 400
Radar range in bins – 2856
Radar range in metres – 500m
8-bit (high) resolution, no compression
Message Block
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Video Header
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Special Purpose Field
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Packet Fragmentation
Encoding an azimuth into a single Video Message will result in a UDP network packet that may exceed the size of the MTU (Maximum Transmission Unit) for the network. A typical value for the MTU is 1500 bytes, which is smaller than a typical azimuth FFT message. If the operating system supports Jumbo Frames the MTU may be as high as 9kB. This may be adequate for typical radar returns, but longer-range radar azimuth data may still exceed this.
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