flyweb/extract_the_protocol_for_communication_w.md

Flymaster GPS Communication Protocol

Based on the code analysis, the Flymaster GPS device uses a combination of NMEA-style commands and a binary protocol for data transfer. Here's a detailed explanation of the protocol:

1. Basic Communication Parameters

• Baud Rate: 57600 bps (defined as BAUDRATE)
• Flow Control: Uses XON (0x11) and XOFF (0x13) characters for flow control
• Line Termination: Commands end with CR+LF (\r\n)

2. NMEA Command Structure

The Flymaster uses a standard NMEA-style command format for basic commands:

$<command>,<param1>,<param2>,...*<checksum>\r\n

Where:

• Commands start with $
• Fields are separated by commas
• The command ends with * followed by a two-digit hexadecimal checksum
• The checksum is calculated by XORing all characters between $ and * (exclusive)
• The message ends with CR+LF

3. Command Prefixes

• PFM prefix is used for Flymaster-specific commands
• Examples of commands:
  • PFMSNP: Used to get device information
  • PFMDNL: Used to download data (with parameters like "LST" for track list)
  • PFMLST: Used to receive track list data

4. Binary Protocol for Track Download

For downloading track data, a binary packet-based protocol is used:

1. Packet Structure:

   • 2-byte packet ID
   • 1-byte length
   • Variable-length data (as specified by the length byte)
   • 1-byte checksum (XOR of the length and all data bytes)

2. Packet Types (identified by packet ID):

   • 0xa0a0: Flight information
   • 0xa1a1: Key position data (base position)
   • 0xa2a2: Delta position data (incremental changes from base position)
   • 0xa3a3: End of transmission marker

3. Acknowledgment:

   • 0xb1: Positive acknowledgment (continue sending)
   • 0xb3: Negative acknowledgment (error or cancel)

5. Data Structures

The protocol uses specific data structures:

• FM_Flight_Info: Contains flight information
• FM_Key_Position: Contains base position data (latitude, longitude, altitude, etc.)
• FM_Point_Delta: Contains delta values for position updates

6. Communication Flow

Initialization:

1. Set baud rate to 57600
2. Send PFMSNP to get device information
3. Calculate device ID from the response

Track List Download:

1. Send PFMDNL,LST command
2. Receive track list data with PFMLST responses
3. Parse track information (date, start time, duration)

Track Data Download:

1. Send PFMDNL,<timestamp> with the timestamp of the desired track
2. Read binary packets:
   • Process flight info packets (0xa0a0)
   • Process key position packets (0xa1a1)
   • Process delta position packets (0xa2a2)
   • Send acknowledgment (0xb1) after each packet
   • Continue until end marker (0xa3a3) is received

7. Error Handling

• Checksum verification for both NMEA commands and binary packets
• Timeout handling (1 second timeout during download)
• Size verification for data structures
• Error reporting through exceptions

This protocol efficiently combines text-based commands for control operations with a compact binary format for transferring large amounts of track data.