Unified SPUTNIX protocol (USP)

Protocol documentation

Documentation v1.04


The protocol is meant for utilization in Earth-to-Space and Space-to-Earth links in TT&C uses.

The protocol defines the physical and bitstream levels of transmission.


Firstly, the protocol is meant for relatively low speed (1200-115200 baud) half-duplex data links, counting in the needs of small satellites in Low Earth Orbit.

Its implementation is also provided for microcontroller-based devices and integrated transceivers for general usage.


General agreements

Excluding special cases, these agreements are in place:

·         All fields are sent MSB first.

·         For multi-byte fields the bit order is always defined in the documentation.

·         All bit orders, mentioned in the document, are sent left-to-right.


Signal specifications

The protocol isn’t limited to usage in specific bands or frequencies. But, there are recommended configurations, allowing for data links to meet the legal requirements, for usage in amateur bands.

Currently, usage of GMSK – gauss minimum shift keying - modulation is defined and recommended. But, there are no restrictions on usage of other modulations.

If using FSK, 0 is the lower frequency, 1 is the higher frequency.



General frame structure

General frame structure is as follows:


>=32 bits


64 bits

PLS code

64 bits


16 bits*

Data packet


Gray part of the frame is Viterbi-coded, scrambled, and coded with Reed-Solomon.

*bit length shown before coding



A 32-bit preamble consisting of 55555555h is recommended.



The protocol uses a 64-bit 5072F64B2D90B1F5h syncword. Receiver is recommended to count the syncword as correct if there are 13 or less bitflips. A false sync possibility in that case is 9,4*10-7. In a 9600 baud downlink, this will result in an average false sync period of 94 seconds.


Eb/N0 graph for each case is shown in “Protocol energy capabilities” section.


PLS coding

Right after the syncword a PLS code (Physical Layer signaling) is sent. The code is 7 bits long, coded with 64-7 code, and is 64 bits long when coded. Hamming code spacing is 32 bits.

The code is a linear block code with the following matrix:

The original value’s most significant bit is multiplied by the first matrix line, least significant bit – to the last, results are modulo 2 summed.

Resulting value is scrambled by modulo 2 summing (XOR) with 0111000110011101100000111100100101010011010000100010110111111010 as the bit sequence.

Used coding is fully equivalent to the one used in DVB-S2 (section 5.5.2) and CCSDS 131.2-B-1 (section 5.3.3), despite being defined differently.

The PLS code carries FEC codeblock info.

Currently, only one coding is implemented with 2 possible coded block sizes:

PLS code


Data block size, with header, in bytes


˝ Viterbi with 255,223 Reed-Solomon



˝ Viterbi with 255,223 Reed-Solomon*



*When using a data block with a size smaller than Reed-Solomon, the remainder should be 0-filled.

All other values are currently reserved.


Data frame

Next, a data frame with its header is sent, which is coded by the receiver. (Viterbi, Reed-Solomon and scrambling)


Data coding


Viterbi is used as recommended by CCSDS 131/0-B-3, section 3.3.1. But it should be noted, that both syncword and PLS code are not Viterbi-coded.

Code parameters:

Type: Viterbi

Code rate: ˝

Constraint length: 7 bits

Connection vectors: G1=1111001, G2=1011011

G2 is inverted.

Coder scheme is shown below (conforms to CCSDS 131.0-B-3 Fig.3-1):

Squares with a D in them signify a 1-bit delay, sums are all modulo 2. First symbol is set when the switch is in the 1 position.



A scrambler, compliant with CCSDS 131/0-B-3 section 10.4.1 is used.

Scrambling is done by XORing with a polyneme shown below:

h(x) = x8 + x7 + x5 +x3 +1


Reed-Solomon (255,223) code is used as per CCSDS 131/0-B-3, part 4.

Reed Solomon code trimming

If the data block (frame with header) defined by PLS code is less than the data block, trimming is performed by zero-padding in front until Reed-Solomon block is reached, coding, and cropping the additional zeroes before scrambling and Viterbi encoding. On the receiving side the zeroes are added back. This procedure is called “virtual filling” and based on the fact that Reed-Solomon is systematic coding, and appends itself at the end of the data instead of changing the data. Code cropping conforms to section 4.3.7 of CCSDS 131/0-B-3 standard.

175 bytes

48 bytes

32 bytes

Zero pad

Data block

Reed-Solomon control symbols


Gray colored area is the discarded data before scrambling, and recovered after descrambling.

In case the data frame with the header are shorter than the data block, the data block also gets zero-padded, but the padding is not discarded.

Frame structure

Data frame generally is preceded with a 2 byte header, consisting of a type field, which is IEEE 802.3 EtherType.

Length field is generally not provided. If encapsulating other protocols into USP (with EtherType highlighting) that use external framing, i.e. without a length field, adding one might be required. A 2-byte little-endian sequence is recommended for such a field. The length should include all data encapsulated by the protocol, but should not include any USP headers. In other words, all data after the byte length field.

The data block looks like this:


Packet data

16 bits

0-221 bytes

Big endian



If encapsulating the protocol that requires transmitting its length, a following structure is recommended:



Packet data

16 bits

16 bits

0-219 bytes

Big endian

Little endian



Data integrity control

USP does not use a dedicated control sum for integrity control. It relies on Reed-Solomon, which delivers good enough control.

AX.25 packet transfer using USP

AX.25 encapsulation is done similarly, but not identically like the AX.25 BPQ protocol made for such a purpose.

In this case, 08FFh in big endian and FF08h in little endian is used in the EtherType field. It is not official, but is a de-facto standard in already existing implementations.

Before the AX.25 header, a length value is inserted, that carries the AX.25 packet length, including its header length. It should be noted, that the length value is not the same as the AX.25 BPQ implementation for EtherNet, which adds 4 additional bytes to eliminate the confusion and problems, arising from length differences.

HDLC framing is also not used. In other words, the flags and checksum are not sent, and bit stuffing is also not used. The task of determining the packet length is played by the length from the frame header, and the integrity control is carried out by means of the USP.

The final data block looks like this:



AX.25 header

AX.25 Packet data

16 bits

16 bits

15-31 bytes**

0-203 bytes

Big endian

Little endian



*FF08h in little-endian

**in a typical spacecraft telemetry case of using unnumbered frames, the header length is 16 bytes. The max length shown here is applicable only to this case. For different header lengths, the maximum data size should be adjusted.


For HEX-dump packet verification, Wireshark in dummy header mode or with addition of a random 12-byte MAC-address can be used.


Appendix 1. Protocol energy capabilities.

The energy capabilities are shown in the graph below in E­b/N0 form. The graph is shown in 2 variants: with soft decoding (with Viterbi decoding and 13 bitflips) and with hard decoding (up to 7 bitflips in each syncword half).

The contribution of individual components is also shown - the probability of a synchronization error, the probability of incorrect reception of the PLS code and the error probability when decoding the packet.

From the graph below it can be seen that, with the AGWN model, when using the soft decoding method at Eb/N0 of around 2.8, the probability of a good frame decode is 99.9%. (PER <= 0.001)

When decoding with the hard method, the same decode probability is achieved at Eb/N0 of around 4.1.



Appendix 2. Syncword

The syncword is bit-balanced, has a maximum of 5 consecutive zeroes and 5 consecutive ones.

The syncword is optimized in such a way, that it has a good autocorellation by both itself, and its first half – by itself and with the addition of a 0x55 preamble.

A autocorellation function graph for the three use cases in the Hamming distance space is shown below.


Appendix 3. Explanation of the used technical solutions

A review of already existing protocols

Before creating our own protocol, already existing protocols were analyzed. The following protocols were analyzed:

·         CCSDS 131/0-B-3 TM synchronization and channel coding

·         GOMspace NanoCom U482/AX100 UHF transceiver protocol in ASM and ASM+golay modes.

·         AAUSAT-4 protocol

·         AO-40 FEC protocol family and their modifications

All protocols mentioned above use FEC. Below are short descriptions and explanations on why the protocols were not used without modifications.




CCSDS 131/0-B-3 TM synchronization and channel coding


This protocol is the base on which USP is built. The main problem of using this protocol in half-duplex low speed data links is the lack of ability to dynamically change frame length, which leads many protocols to using the channel ineffectively, big round-trips and, as a consequence, slowing down transmissions.

GOMspace NanoCom U482/AX100 UHF transceiver protocol in ASM and ASM+golay modes

This protocol has a length field, which allows it to have smaller packets when needed. But, it has its downsides as well. The protocol uses a pretty small syncword, and a uncoded length field, which limits the energy of the channel to values way too far from FEC abilities.

AO-40 FEC protocol family and their modifications


The protocol is well optimized from the point of signal fades and has good energetic capabilities. The problem is the same as in CCSDS 131/0-B-3 TM: no ability to dynamically change the frame length.















v1.01: First publicly available version.

v1.02: Syncword was changed in order to improve autocorellation of its first half. This improved reception on hardware receivers drastically.

v1.03: Syncword improved even more. Now it is optimized by both full autocorellation, and partial autocorellation – with preamble and without one.

v1.04: Appendixes re-numbered. Solution explanations elaborated.





















Translated by Raov (UB8QBD)