Nov 15

Overview of the CCSDS Network Protocols

Category: CCSDS, Networking

The CCSDS standards can be intimidating to the uninitiated, this article covers AOS, TC, COP-1, and other protocols used in space communications, how they interact, and where to find the details.

The CCSDS standards committee develops recommendations pertaining to the interoperability of space systems across the globe. The network protocols document how a ground station can communicate with a spacecraft.

This is not intended to tell you everything, but rather to give you an idea of the major protocols and how they are related to each other.

Also, please note that all the information in this document is derived directly from publicly available international standards.

The Space/Ground Link

Typically communications between the ground and space consist of a single link between one ground station and one spacecraft over RF (radio frequency).

These links are often lossy due to environmental factors or spacecraft failures. As a result, the CCSDS space link protocols are designed to account for failure.

In addition, these links are often low bandwidth and assymmetric. As a result of bandwidth limitations, the protocols are designed to be low overhead. More interestingly, the asymmetry of the link means that, unlike most terrestrial protocols, the CCSDS protocol from the ground to the spacecraft is not the same as the protocol used in the other direction.

Commands vs. Telemetry

In the space world, data from the spacecraft to the ground is referred to as telemetry and data from the ground to the spacecraft is referred to as command.

As mentioned before, commands and telemetry are not formatted or transmitted in the same way and the bandwidth allocated to each is typically asymmetric.

Commands

Commands direct the spacecraft to take an action like fire thrusters, load new software, etc. Although every mission is different, commands are typically low bandwidth and sent individually. Additionally, commands must be received once, reliably ,and in sequence by the spacecraft.

Telemetry

Telemetry usually consists primarily of user data. In the case of science missions, this is the science data. Some smaller portion of the telemetry is related to monitor data for the spacecraft. This typically includes data required to determine the health of the spacecraft and resolve any problems it may encounter.

Because telemetry contains the science data it is typically higher rate than command data. Additionally, where the order and content of each command is of great importance, much of the telemetry sent from a spacecraft is repeated regularly. As a result telemetry is typically checked for checksum and sequence errors, but is not automatically retransmitted on failure.

The Standards

Space link overview

Space Packet

What is it?

Space Packets are one of many options for the application layer of space data. Each packet contains,among other things, a unique identifier called the application ID which indicates a destination on the spacecraft (typically a specific application) much like a TCP port.

The format of the data contained in that packet is up to the application.

Where is it?

The format of the space packet is defined in a blue book standard ( officially recommended) CCSDS 132.0-B-1. If you want to see the actual packet layout, skip to section 4, Protocol Specification.

TC Frames

As mentioned previously, the CCSDS protocols are asymmetric. The TC frame defines the format of commands being sent to the spacecraft.

What is it?

TC frames provide the Data Link Layer (Layer 2) of the space protocol from the ground to the spacecraft.

When the ground has command data to send to the spacecraft, it transmits a TC frame. When there is no data to transmit, the link is filled with an idle pattern.

The TC frame contains one or more commands (typically Space Packets) as well as information designed to provide reliable, in-sequence transmission (see COP-1).

Where the Space Packet has an Application ID for routing a particular packet to a particular application, the TC frame identifies the Spacecraft via a SCID (Spacecraft ID) as well as a VCID (Virtual Channel Identifier) which allows the ground to multiplex multiple stream of command frames within the same link.

Where is it?

The TC frame format is defined in CCSDS 232.0-B-1. If you wish to see the actual layout of the frame, skip to section 4, Protocol Specification.

AOS Frames

What is it?

The AOS Frame describes the format of telemetry data being sent from the spacecraft back to the ground.

Unlike commands, telemetry is continuously transmitted even when there is no data available to transmit.

Like TC, AOS specifies a SCID (Spacecraft ID) and a VCID (Virtual Channel ID). Also the same as TC, the SCID indentifies the originating spacecraft and the VCID allows the spacecraft to multiplex telemetry data into a number of separate channels.

Where is it?

The AOS frame format is defined in CCSDS 732.0-B-2. If you want to see the actual layout of the frame skip to section 4, Protocol Specification.

COP-1

What is it?

COP-1 provides a Transport Layer (layer 3) protocol for ensuring the correct transmission of command frames (typically TC frames). Similar to TCP, COP-1 uses sequence numbers, acknowledgments, a sliding window, and retransmissions to ensure that commands are received once and in order by the spacecraft.

The state machine driving the protocol on the spacecraft, or receiving, end is called the FARM. The state machine transmitting the commands is called the FOP.

The FARM sends acknowledgment information to the FOP using the Operational Control Field contained in telemetry frames (typically AOS).

Where can I find it?

A complete description of the state machines for both the FOP and the FARM are in the blue book standard CCSDS 232.1-B-1.

Older Protocols

TM

CCSDS 132.0-B-1 covers an older telemetry frame format called TM. TM is very similar to AOS, but the types of services provided by TM and AOS are somewhat different. (This article covers sending packets over AOS, but it is also possible to send other formats of data, such as bitstreams, raw user-defined frames, etc).

Time Division Multiplex (TDM)

This is an older format for telemetry which is not covered by any active CCSDS standard.

Newer Protocols

CFDP

What is it?

Older spacecraft had custom mission-specific mechanisms for getting customer data from the spacecraft, uploading new software, etc.

As the space industry catches up with the terrestrial software industry, satellites are increasingly using the concept of files to transfer data to and from the spacecraft.

Where is it?

The CFDP protocol is described in CCSDS 727.0-B-4.

IP In Space

There are a number of ways to do this, but nothing appears to be in wide use at this point. I may write an article on this in the future.

The Author

Michael Smit is a software engineer in Seattle, Washington who works for amazon

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