UBX u-blox Binary Protocol

u-blox UBX Packet Decoder

Decode u-blox UBX binary protocol packets from hexadecimal bytes. Inspect sync characters, message class, message ID, payload length, raw payload bytes and the two-byte Fletcher-style UBX checksum.

✓ Sync B5 62 ✓ Class ✓ Message ID ✓ Payload Length ✓ CK_A / CK_B

UBX

Packet Structure Decode

● Ready

u-blox UBX Packet Hex Bytes Enter one complete UBX packet beginning with sync bytes B5 62. Spaces, commas, colons, hyphens and 0x prefixes are accepted. Continuous hexadecimal input is also supported.

UBX packet format: B5 62 are synchronization bytes. The checksum is calculated over CLASS, ID, LENGTH and PAYLOAD bytes only; the two sync bytes and the checksum bytes themselves are excluded.

u-blox UBX Decode Result Decoded

Decoded Message —

Sync Bytes —

Message Class —

Message ID —

Message Name —

Payload Length —

Length Bytes —

Received CK_A —

Received CK_B —

Calculated CK_A —

Calculated CK_B —

Checksum Status —

Total Packet Bytes —

Payload Bytes —

Packet Breakdown —

What Is the u-blox UBX Protocol?

UBX is a binary communication protocol used by many u-blox GNSS receivers. Unlike human-readable NMEA sentences, UBX messages use binary fields and a fixed packet envelope consisting of synchronization bytes, message class, message ID, payload length, payload and checksum.

The protocol is commonly used for navigation output, receiver configuration, acknowledgements, monitoring information, raw GNSS measurements and timing messages.

UBX Packet Structure

Byte 0      0xB5
Byte 1      0x62

Byte 2      CLASS
Byte 3      ID

Byte 4      LENGTH low byte
Byte 5      LENGTH high byte

Bytes 6...  PAYLOAD

Final -2    CK_A
Final -1    CK_B

The payload length is stored as a 16-bit little-endian integer, so the low length byte appears first.

UBX Synchronization Bytes B5 62

Every normal UBX packet begins with hexadecimal bytes B5 62. These bytes allow a parser to locate the start of a binary message stream.

Expected sync:

B5 62

If the first two bytes are different, this calculator reports a synchronization error rather than treating the packet as valid UBX data.

UBX Message Class

The CLASS byte groups related message types. The following are several commonly encountered UBX classes.

Class	Name	Purpose
0x01	NAV	Navigation results
0x02	RXM	Receiver manager / measurement data
0x04	INF	Information messages
0x05	ACK	Acknowledgements
0x06	CFG	Receiver configuration
0x09	UPD	Firmware/update-related messages
0x0A	MON	Monitoring information
0x0D	TIM	Timing messages
0x13	MGA	Multiple GNSS assistance
0x21	LOG	Logging functions
0x27	SEC	Security-related messages

Common UBX NAV Messages

Class / ID	Message
01 01	NAV-POSECEF
01 02	NAV-POSLLH
01 03	NAV-STATUS
01 04	NAV-DOP
01 07	NAV-PVT
01 12	NAV-VELNED
01 20	NAV-TIMEGPS
01 21	NAV-TIMEUTC
01 35	NAV-SAT

UBX NAV-PVT

NAV-PVT is one of the most widely used navigation messages in modern u-blox receivers. Depending on receiver generation and protocol version, its payload contains fields such as GPS time of week, date, UTC time, fix type, number of satellites, longitude, latitude, height, ground speed and heading.

Those payload fields have fixed offsets defined by the applicable u-blox interface specification. This page recognizes the NAV-PVT class and message ID but does not apply a potentially incompatible receiver-generation layout unless that exact payload decoder is explicitly implemented.

UBX ACK-ACK and ACK-NAK

The ACK class reports whether certain configuration messages were accepted or rejected.

ACK-NAK:
Class = 0x05
ID    = 0x00

ACK-ACK:
Class = 0x05
ID    = 0x01

Their payload normally contains the class and ID of the message being acknowledged.

UBX Payload Length

The payload length uses two little-endian bytes.

Length bytes:

02 00

Length =
0x0002
= 2 bytes


Another example:

5C 00

Length =
0x005C
= 92 bytes

The expected full packet length is therefore:

Total packet bytes =

2 sync
+ 1 class
+ 1 ID
+ 2 length
+ payload length
+ 2 checksum

= payload length + 8

How the UBX Checksum Works

UBX uses two eight-bit checksum accumulators named CK_A and CK_B. Calculation begins with both values set to zero.

CK_A = 0
CK_B = 0

For each byte from CLASS through the final PAYLOAD byte:

CK_A = CK_A + byte
CK_B = CK_B + CK_A

Both values are kept modulo 256.

The synchronization bytes B5 62 are not included, and the received checksum bytes are not included in their own calculation.

UBX Checksum Example

Consider the structural portion:

05 01 02 00 06 01

05 = ACK class
01 = ACK-ACK ID
02 00 = 2-byte payload
06 01 = payload

The calculator iterates across those bytes to produce CK_A and CK_B and then compares the result against the final two bytes of the supplied packet.

UBX vs NMEA Messages

A u-blox GNSS receiver may support both UBX binary messages and NMEA text sentences. They are different protocols even when both carry navigation data.

Feature	UBX	NMEA Text
Encoding	Binary	ASCII text
Typical Start	B5 62	$ or !
Message Identification	Class + ID	Sentence identifier
Payload	Binary typed fields	Comma-separated text fields
Integrity	CK_A / CK_B	Sentence checksum

Why UBX Payload Meaning Can Vary

The structural UBX packet format is stable, but the precise payload layout of a particular message can depend on the receiver family and protocol/interface version.

For that reason, a generic structural decoder should not guess field offsets for an unknown message. This calculator always exposes the raw payload and message class/ID even when a safe application-level interpretation is not available.

u-blox UBX Packet Decoder FAQs

What bytes begin a UBX packet?

A normal UBX packet begins with hexadecimal synchronization bytes B5 62.

What does the UBX CLASS byte mean?

CLASS groups messages by function, such as NAV for navigation, CFG for configuration, ACK for acknowledgements and MON for monitoring.

What is UBX-NAV-PVT?

NAV-PVT is a navigation solution message that can contain position, velocity, time and fix information.

What is UBX-ACK-ACK?

ACK-ACK indicates that a supported command or configuration request was acknowledged successfully.

What is UBX-ACK-NAK?

ACK-NAK indicates that a command or configuration request was not acknowledged as successful.

Is the UBX payload length big-endian?

No. The two-byte UBX payload length field is little-endian.

Are B5 and 62 included in the UBX checksum?

No. Checksum calculation begins with the CLASS byte.

Are CK_A and CK_B included in their own calculation?

No. The checksum covers CLASS, ID, LENGTH and PAYLOAD only.

Can this tool detect a corrupted packet?

It can detect length mismatches, invalid synchronization bytes and UBX checksum mismatches.

Can the tool decode every u-blox payload field?

Not generically. Exact payload layouts can depend on message type and u-blox protocol version, so unsupported payloads are preserved as raw bytes.

Decode u-blox UBX Binary Packets

Inspect UBX synchronization bytes, class and message identifiers, little-endian payload length, payload boundaries and CK_A / CK_B checksum validity directly from GNSS packet hexadecimal data.