CCSDS TM同步与信道编码标准概览

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"CCSDS TM同步与信道编码" CCSDS（Consultative Committee for Space Data Systems，空间数据系统咨询委员会）是国际上制定航天通信标准的重要组织。该组织发布的技术报告“CCSDS TM Synchronization and Channel Coding”是关于太空数据系统标准的一个重要文档，主要关注的是TM（Telemetry，遥测）数据的同步和信道编码技术。遥测在航天通信中扮演着关键角色，它涉及从航天器收集并传输数据到地面站的过程。同步是确保这些遥测数据能够正确无误地被接收和解析的关键步骤。在TM系统中，同步技术确保了数据流在接收端可以准确地对齐，从而避免由于时序错位导致的数据丢失或错误解读。信道编码则是为了提高数据在受到噪声、干扰或信道衰减等不利条件下的传输可靠性。CCSDS推荐的信道编码标准可能包括前向纠错（Forward Error Correction, FEC）编码，如 Reed-Solomon 或 BCH 编码，以及卷积编码等。这些编码方法能在数据传输过程中添加冗余信息，以便在接收端检测并纠正错误。报告中的“Summary of Concept and Rationale”部分提供了这些技术的基本概念和采用它们的理由。它详细解释了为什么需要特定的同步方法和技术来处理高速、低延迟的航天通信环境，以及为何选择特定的信道编码方案来提高数据的抗干扰能力。该信息报告由CCSDS管理委员会批准，代表了来自CCSDS成员机构的技术专家共识。这些成员机构通常包括全球主要的航天国家的航天局，如美国国家航空航天局（NASA）。报告的发布和维护由CCSDS秘书处负责，确保标准的持续更新和一致性。 “Foreword”部分预示了报告将提供支持CCSDS推荐标准的背景信息和详尽解释，帮助读者理解技术背后的设计原则和考虑因素。对于那些设计、实现或分析航天通信系统的工程师和技术人员来说，这份报告是不可或缺的参考资料。

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TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE

2 OVERVIEW OF CCSDS TELEMETRY SYSTEM

2.1 INTRODUCTION

The purpose of a telemetry system is to reliably and transparently convey measurement

information from a remotely located data generating source to users located in space or on

Earth. Typically, data generators are scientific sensors, science housekeeping sensors,

engineering sensors and other subsystems on-board a spacecraft.

The advent of capable microprocessor based hardware will result in data systems with

demands for greater throughput and a requirement for corresponding increases in spacecraft

autonomy and mission complexity. These facts, along with the current technical and fiscal

environments, create a need for greater telemetering capability and efficiency with reduced

costs.

In the past, most of the telemetry resources used by a science mission have been wholly

contained within a cognizant Project office and, with the exception of the tracking network,

are completely dedicated to that mission. The lack of effective standardization among various

missions forces the ‘multi-mission’ tracking network to implement the lowest level of

telemetry transport service, i.e., bit transport. Higher level data delivery services, oriented

more toward computer-to-computer transfers and typical of modern day commercial and

military networks, had to be custom designed and implemented on a mission-to-mission

basis.

The intent of the CCSDS Telemetry System is not only to ease the transition toward greater

automation within individual space agencies, but also to ensure harmony among the

agencies, thereby resulting in greater cross-support opportunities and services.

The CCSDS Telemetry System is broken down into two major conceptual categories: a ‘TM

Space Data Link Protocol’ concept (references [2] and [7]) and a ‘TM Synchronization and

Channel Coding’ concept (reference [3]).

a) TM Space Data Link Protocol is a concept which facilitates the transfer of space-

acquired data from source to user in a standardized and highly automated manner.

TM Space Data Link Protocol provides a mechanism for implementing common data

structures and protocols which can enhance the development and operation of space

mission systems. TM Space Data Link Protocol addresses the following two

processes:

1) The end-to-end transport of space mission data sets from source application

processes located in space to distributed user application processes located in

space or on Earth.

2) The intermediate transfer of these data sets through space data networks; more

specifically, those elements which contain spacecraft, radio links, tracking

stations and mission control centers as some of their components.

CCSDS 130.1-G-1 Page 2-1 June 2006

TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE

The TM Space Data Link Protocol Recommended Standard contained in

references [2] and [7] is primarily concerned with describing the telemetry formats

which are generated by spacecraft in order to execute their roles in the above processes.

b) TM Synchronization and Channel Coding (reference [3]) is a method by which data

can be sent from a source to a destination by processing it in such a way that distinct

messages are created which are easily distinguishable from one another. This allows

reconstruction of the data with low error probability, thus improving the performance

of the channel. The TM Synchronization and Channel Coding Recommended

Standard contained in reference [3] describes several space TM Synchronization and

Channel Coding schemes. The characteristics of the codes are specified only to the

extent necessary to ensure interoperability and cross-support.

Together, TM Space Data Link Protocol and TM Synchronization and Channel Coding

services provide to the user reliable and transparent delivery of telemetry information.

Figure 2-1 illustrates the CCSDS Telemetry System in terms of a layered service

model. It should be noted that the CCSDS TM Space Data Link Protocol and TM

Synchronization and Channel Coding Recommended Standards only address the five

lower layers of this model.

PROVIDES USERS A METHOD TO INVESTIGATE PHYSICAL

PHENOMENA BY USING THEIR INSTRUMENTS IN SPACE

FOR DATA COLLECTION AND THEIR APPLICATION

PROCESSES FOR ANALYSIS.

PROVIDES TRANSLATION OF PHYSICAL MEASUREMENTS

INTO SETS OF APPLICATION DATA UNITS.

PROVIDES END-TO-END DELIVERY OF APPLICATION

DATA UNITS.

(OPTIONAL) PREPARES LONGER PACKETIZED DATA UNITS

FOR MULTIPLEXING AND TRANSFER THROUGH A SPACE

DATA CHANNEL.

PROVIDES RELIABLE TRANSFER OF PACKETS AND SEGMENTS

IN A COMMON STRUCTURE FOR THEIR TRANSPORT THROUGH

THE SPACECRAFT-TO-GROUND COMMUNICATION LINK.

PROTECTS TRANSFER FRAMES AGAINST ERRORS INDUCED

DURING TRANSMISSION THROUGH THE NOISY PHYSICAL

COMMUNICATIONS CHANNEL.

PROVIDES THE PHYSICAL CONNECTION, VIA RADIO

FREQUENCY SIGNALS, BETWEEN A TRANSMITTING

SPACECRAFT AND THE RECEIVING STATION.

SERVICE PROVIDED BY LAYE

PHYSICAL

MEASUREMENTS

APPLICATION

PROCESS LAYER

SYSTEM MGMT

LAYER

TLM

APPLICATION

DATA

PACKETIZATION

LAYER

PACKET

SEGMENTATION

LAYER

SEGMENT

TRANSFER

LAYER

TRANSFER

FRAME

CODING

LAYER

TLM

BIT STREAM

PHYSICAL

LAYER

PHYSICAL

WAVEFORM

LAYE

Figure 2-1: Layered Telemetry Service Model

CCSDS 130.1-G-1 Page 2-2 June 2006

TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE

2.2 TELEMETRY SYSTEM CONCEPT

2.2.1 GENERAL

The system design technique known as layering was found to be a very useful tool for

transforming the Telemetry System concept into sets of operational and formatting

procedures. The layering approach is patterned after the International Organization for

Standardization’s Open Systems Interconnection layered network model (reference [4]),

which is a seven layer architecture that groups functions logically and provides conventions

for connecting functions at each layer. Layering allows a complex procedure such as the

telemetering of spacecraft data to the users to be decomposed into sets of peer functions

residing in common architectural strata.

Within each layer, the functions exchange data according to established standard rules or

‘protocols’. Each layer draws upon a well defined set of services provided by the layer

below, and provides a similarly well defined set of services to the layer above. As long as

these service interfaces are preserved, the internal operations within a layer are unconstrained

and transparent to other layers. Therefore, an entire layer within a system may be removed

and replaced as dictated by user or technological requirements without destroying the

integrity of the rest of the system. Further, as long as the appropriate interface protocol is

satisfied, a customer (user) can interact with the system/service at any of the component

layers. Layering is therefore a powerful tool for designing structured systems which change

due to the evolution of requirements or technology.

A companion standardization technique that is conceptually simple, yet very robust, is the

encapsulation of data within an envelope or ‘header’. The header contains the identifying

information needed by the layer to provide its service while maintaining the integrity of the

envelope contents.

2.2.2 PACKETIZATION LAYER

Within TM Space Data Link Protocol, spacecraft generated application data are formatted

into end-to-end transportable data units called ‘TM Source Packets’. These data are

encapsulated within a primary header which contains identification, sequence control and

packet length information. A TM Source Packet is the basic data unit telemetered to the user

by the spacecraft and generally contains a meaningful quantity of related measurements from

a particular source.

2.2.3 TRANSFER FRAME LAYER

The TM Transfer Frame is used to reliably transport Source Packets (and Segments) through

the telemetry channel to the receiving telecommunications network. As the heart of the

CCSDS Telemetry System, the TM Transfer Frame protocols offer a range of delivery

service options. An example of such a service option is the multiplexing of TM Transfer

Frames into ‘Virtual Channels’ (VCs).

CCSDS 130.1-G-1 Page 2-3 June 2006

TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE

The TM Transfer Frame is a fixed length unit which was chosen to improve the ability to

synchronize the frame with weak signals such as those found on space-ground links, and for

compatibility with certain block oriented channel coding schemes. The (primary) header

contains frame identification, channel frame count information and frame data field status

information. An attached synchronization marker (ASM) signals the start of the TM Transfer

Frame.

The transfer frame data field may be followed by an optional trailer containing an operational

control field and/or a frame error control field. The first of these fields provides a standard

mechanism for incorporating a small number of real-time functions (e.g., telecommand

verification or spacecraft clock calibration). The error control field provides the capability for

detecting errors which may have been introduced into the frame during the data handling

process.

The delivery of transfer frames requires the services provided by the lower layers (e.g.,

carrier, modulation/detection, and coding/decoding) to accomplish its role.

2.2.4 CHANNEL CODING LAYER

TM Synchronization and Channel Coding is used to protect the transfer frames against

telemetry channel noise-induced errors. Reference [3] describes the CCSDS Recommended

Standard for TM Synchronization and Channel Coding, including specification of a

convolutional code, a Reed-Solomon block-oriented code, a concatenated coding system

consisting of a convolutional inner code and a Reed-Solomon outer code, and of turbo codes.

The basic data units of the CCSDS TM Synchronization and Channel Coding which interface

with the physical layer below are the Channel Symbols output by the channel encoder.

The RF channel physically modulates the channel symbols into RF signal patterns. Within

the error detecting and correcting capability of the channel code chosen, errors which occur

as a result of the physical transmission process may be detected and corrected by the

receiving entity.

Full advantage of all CCSDS Telemetry System services could be realized if a Project

complied with all CCSDS Recommended Standards. Alternatively, Projects can interface

with any layer of the Telemetry System as long as they meet the interface requirements as

specified in the Recommended Standards (references [2], [3], and [5]).

Figure 2-2 illustrates how the various telemetry data structures map into one another. There

is presently no attempt to define the data structures of the top two layers of the telemetry

system; i.e., the Application Process layer and the System Management layer. The Source

Packets are placed into the data field of the Transfer Frame. An attached synchronization

marker is always used, as shown in figure 2-2.

CCSDS 130.1-G-1 Page 2-4 June 2006

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