Variable Length Packets

Overview

The CCSDS packet primary header contains contains a length field which can vary from packet to packet. Parsing such variable length packets is also supported through the VariableLength. A variable length packet is one containing one and only one variable length field. This packet provides the PacketArray field which can be set to variable length. An example is provided below.

import ccsdspy
from ccsdspy import PacketField, PacketArray

 pkt = ccsdspy.VariableLength([
      PacketField(
           name='SHCOARSE',
           data_type='uint',
           bit_length=32
      ),
      PacketArray(
           name="data",
           data_type="uint",
           bit_length=16,
           array_shape="expand",
      ),
      PacketField(
           name="checksum",
           data_type="uint",
           bit_length=16
      ),
 ])

The efficiency of parsing variable length packets is significantly decreased compared to fixed length packets. Parsing the packets is done in the same way as FixedLength:

result = pkt.load('MyCCSDS.bin')

The result will be a dictionary with the names as the keys. The values are arrays with the PacketArray field providing arrays with variable sizes. It is also possible to get access to the packet primary header. See Getting the CCSDS Header.