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190-00266-01 GPS 15H & 15L Technical Specifications Rev. D
Page 30
Ephemeris Download Procedure
The following is the sequence of events that occurs when downloading ephemeris data.
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 returns an acknowledgement packet that looks like this:
RX Packet: Acknowledgement
Byte Description Name HEX Value
Delimiter DLE 0x10
Acknowledgement ID IOP_ACK_BYTE 0x06
Number of bytes in data SIZE 0x02
Request to D/L ephemeris IOP_CMND_DATA 0x0A
Pad DATA 0x00
Checksum calculation CHKSUM ----
Delimiter DLE 0x10
End of packet ETX 0x03
Then, the unit immediately sends 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 Name HEX Value
Delimiter DLE 0x10
Record ID IOP_RECORDS 0x1B
Number of bytes in data SIZE 0x02
Number of records NUM_SV 0x0C
Pad DATA 0x00
Checksum calculation CHKSUM ----
Delimiter DLE 0x10
End of packet ETX 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 Name HEX Value
Delimiter DLE 0x10
Record ID IOP_ACK_BYTE 0x06
Number of bytes in data SIZE 0x02
Pad DATA 0x00
ID of packet being ACK’d IOP_RECORDS 0x1B
Checksum calculation CHKSUM ----
Delimiter DLE 0x10
End of packet ETX 0x03