Re: Gpsd clear fix and merge fix

[Gary E. Miller]

Yo raaj!

On Wed, 22 Apr 2020 11:28:08 +0800
raaj lokanathan <address@hidden> wrote:

> Can you please explain further on CLEAR_IS? What is the purpose of
> clearing the data periodically?

Simple.  All data from a GNSS receiver is for one moment in time,
the fix time.  Even though the data is spread over several messages,
and all messages may not have time stamps.

When a new time is received from the receiver, the data in memory is
for the previous fix time.  Now old and obsolete.

CLEAR_IS is an internal flag, that you should never touch, to clear the
old data.  Then gpsd can collect the data for the new fix time, without
confusion from the old, now expired data.

All data that gpsd gives you for a particular time stamp is what the
GNNS receiver measured at that exact time.  Not old data.

If you do not clear the old data when you start to get the next data
set, then you mix old and new data.

In that case gpsd might, for example, show a lat/lon for time A, but the
altitude from time B.  If you are in an airplane, you may think you are
at an old altitude from time B, when you are instead now at an unknown
altitude waiting for the current altitude.

That is how people die.  We are gpsd do not want anyone to die.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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    "If you can't measure it, you can't improve it." - Lord Kelvin

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