MDEV-GPS-RM datasheet(8/27 Pages) LINX

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Slow Start Time

The most critical factors in start time are current ephemeris data, signal

strength and sky view. The ephemeris data describes the path of each

satellite as they orbit the earth. This is used to calculate the position of

a satellite at a particular time. This data is only usable for a short period

of time, so if it has been more than a few hours since the last fix or if the

location has significantly changed (a few hundred miles), then the receiver

may need to wait for a new ephemeris transmission before a position can

be calculated. The GPS satellites transmit the ephemeris data every 30

seconds. Transmissions with a low signal strength may not be received

correctly or be corrupted by ambient noise. The view of the sky is important

because the more satellites the receiver can see, the faster the fix and the

more accurate the position will be when the fix is obtained.

If the receiver is in a very poor location, such as inside a building, urban

canyon, or dense foliage, then the time to first fix can be slowed. In very

poor locations with poor signal strength and a limited view of the sky with

outdated ephemeris data, this could be on the order of several minutes.

In the worst cases, the receiver may need to receive almanac data, which

describes the health and course data for every satellite in the constellation.

This data is transmitted every 15 minutes. If a lock is taking a long time, try

to find a location with a better view of the sky and fewer obstructions. Once

locked, it is easier for the receiver to maintain the position fix.

Interfacing with NMEA Messages

Linx modules default to the NMEA protocol. Output messages are sent

from the receiver on the TX line and input messages are sent to the receiver

on the RX line. By default, output messages are sent once every second.

Details of each message are described in the following sections.

The NMEA message format is as follows: <Message-ID + Data Payload +

Checksum + End Sequence>. The serial data structure defaults to

9,600bps, 8 data bits, 1 start bit, 1 stop bit, and no parity. Each message

starts with a $ character and ends with a <CR> <LF>. All fields within

each message are separated by a comma. The checksum follows the *

character and is the last two characters, not including the <CR> <LF>.

It consists of two hex digits representing the exclusive OR (XOR) of all

characters between, but not including, the $ and * characters. When

reading NMEA output messages, if a field has no value assigned to it, the

comma will still be placed following the previous comma. For example,

{,04,,,,,2.0,} shows four empty fields between values 04 and 2.0. When

writing NMEA input messages, all fields are required, none are optional. An

empty field will invalidate the message and it will be ignored.

Reading NMEA output messages:

• Initialize a serial interface to match the serial data structure of the GPS

receiver.

• Read the NMEA data from the TX pin into a receive buffer.

• Separate it into six buffers, one for each message type. Use the

characters ($) and <CR> <LF> as end points for each message.

• For each message, calculate the checksum as mentioned above to

compare with the received checksum.

• Parse the data from each message using commas as field separators.

• Update the application with the parsed field values.

• Clear the receive buffer and be ready for the next set of messages.

Writing NMEA input messages:

• Initialize a serial interface to match the serial data structure of the GPS

receiver.

• Assemble the message to be sent with the calculated checksum.

• Transmit the message to the receiver on the RX line.

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