u-blox ZED-F9P Interface Description - Manual
Advance Information
• Every Frame starts with a 2-byte Preamble consisting of two synchronization characters: 0xB5 0x62.
• A 1-byte Message Class field follows. A Class is a group of messages that are related to each other.
• A 1-byte Message ID field defines the message that is to follow.
• A 2-byte Length field follows. The length is defined as being that of the payload only. It does not include
the Preamble, Message Class, Message ID, Length, or CRC fields. The number format of the length field is a
Little-Endian unsigned 16-bit integer.
• The Payload field contains a variable number of bytes.
• The two 1-byte CK_A and CK_B fields hold a 16-bit checksum whose calculation is defined below. This
concludes the Frame.
5.3 UBX Payload Definition Rules
5.3.1 Structure Packing
Values are placed in an order that structure packing is not a problem. This means that 2-byte values shall start
on offsets which are a multiple of 2; 4-byte values shall start at a multiple of 4; and so on.
5.3.2 Reserved Elements
Some messages contain reserved fields or bits to allow for future expansion. The contents of these elements
should be ignored in output messages and must be set to zero in input messages. Where a message is output
and subsequently returned to the receiver as input message, reserved elements can either be explicitly set to
zero or left with whatever value they were output with.
5.3.3 Undefined Values
The description of some fields provide specific meanings for specific values. For example, the field gnssId
appears in many UBX messages and uses 0 to indicate GPS, 1 for SBAS and so on (see Satellite Numbering for
details); however it is usually stored in a byte with far more possible values than the handful currently defined.
All such undefined values are reserved for future expansion and therefore should not be used.
UBX-18010854 - R04 Advance Information Page 28 of 259
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