Garmin GPS 17HVS Technical Specifications - Page 31

Ephemeris Download Procedure The following is the sequence of events that occurs when downloading ephemeris. Send a packet containing the command that requests ephemeris data (IOP_DOWN_LOAD_EPH). The packet should look like this: TX Packet: Ephemeris Data Request Byte Description Name HEX Value Delimiter DLE 0x10 Command Data ID IOP_CMND_DATA 0x0A Number of bytes in data SIZE 0x02 Request to D/L ephemeris IOP_DOWN_LOAD_EPH 0x5D Pad to 2 bytes DATA 0x00 Checksum calculation CHKSUM 0x97* Delimiter DLE 0x10 End ETX 0x03 * From now on, checksum calculation will not be shown for every packet example The unit will return an acknowledgement packet that will look like this: RX Packet: Acknowledgement Byte Description Delimiter Acknowledgement ID Number of bytes in data Request to D/L ephemeris Pad Checksum calculation Delimiter End of packet Name DLE IOP_ACK_BYTE SIZE IOP_CMND_DATA DATA CHKSUM DLE ETX HEX Value 0x10 0x06 0x02 0x0A 0x00 ---0x10 0x03 Then, the unit will immediately send a packet communicating how many data packets to expect for the ephemeris download (a maximum of twelve): RX Packet: Number of Data Packets to Expect Byte Description Delimiter Record ID Number of bytes in data Number of records Pad Checksum calculation Delimiter End of packet Name DLE IOP_RECORDS SIZE NUM_SV DATA CHKSUM DLE ETX HEX Value 0x10 0x1B 0x02 0x0C 0x00 ---0x10 0x03 This packet requires acknowledgement, as shown below (note that the data field contains the IOP_RECORDS ID to indicate the acknowledgement of the IOP_RECORDS packet): TX Packet: Acknowledgement Byte Description Delimiter Record ID Number of bytes in data Pad ID of packet being ACK'd Checksum calculation Delimiter End of packet Name DLE IOP_ACK_BYTE SIZE DATA IOP_RECORDS CHKSUM DLE ETX HEX Value 0x10 0x06 0x02 0x00 0x1B ---0x10 0x03 190-00228-21 GPS 16/17 Technical Specifications Page 27 Rev. A