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APPENDIX C: EPHEMERIS DATA DOWNLOAD (PROGRAMMING EXAMPLE)

Synopsis

This section describes, using an example, how to download ephemeris information from a Garmin 15, 16,

17 or 18 family GPS unit with the exception of the GPS 15-W and the GPS 15-F.

Garmin Binary Format Review

To download the ephemeris data, you must first command the unit to output information in Garmin Binary

Format (Garmin mode) instead of the default NMEA output format. To put the unit in Garmin mode,

connect to the unit using a terminal program and send the following NMEA sentence:

$PGRMO,,G*hh<CR><LF>

The checksum *hh is used for parity checking data and generally is not required in normal PC

environments, but is recommended for use in environments containing high electromagnetic noise. When

used, the parity bytes (hh) are the ASCII representation of the exclusive-or (XOR) sum of all the characters

between the "$" and "*" characters, non-inclusive. Sentences may be truncated by <CR><LF> after any

data field and valid fields up to that point will be acted on by the GPS sensor. See Section

Interface.

The unit will stay in Garmin mode until the next power cycle.

Now that unit is in Garmin binary format, transmitted and received packets are structured as follows:

Byte Description

Packet Delimiter

Packet ID (type)

Data Size

Data bytes

Checksum

Packet Delimiter

End of Packet

The DLE (0x10) is a delimiter byte used in conjunction with the ETX byte to determine beginning and

ending of a packet. However, a 0x10 could appear in the data itself; if this occurs, the byte is escaped with

another DLE byte (sometimes referred to as DLE stuffing). In other words, if a DLE occurs in the data,

another DLE is transmitted immediately after to indicate that it is a data byte and it is not being used as a

delimiter. Note that the size byte of the packet does not count the second DLE byte in an escaped DLE pair

in the data field. Since a DLE that is a part of the data will have a second DLE to escape it, a single DLE

followed by an ETX byte means that the end of a packet has been reached.

In order to interpret these packets properly, one must remove the escaped DLE bytes. This can be achieved

using an algorithm similar to the

190-00228-20

Name

Notes

DLE

0x10

Packet type

SIZE

Number of bytes in data portion(not

including escaped DLEs. See below)

DATA

Not to exceed 256 bytes

CHKSUM

2's complement of the arithmetic

sum of all the bytes from the

Packet ID byte to the last DATA

byte(inclusive) not counting

escaped DLEs. See below

DLE

ETX

Sample C Code

fragment on the previous page.

GPS 16/17 Technical Specifications

Page 29

4 Software

0x10

0x03

Rev. C