[time-nuts] How best to compute local time from GPS

David Forbes dforbes at dakotacom.net
Tue Mar 25 03:07:22 UTC 2008


O Time Nuts,

I am working feverishly on the software for my new scope clock. This 
is a circuit board that drives a 3" oscilloscope tube with a rather 
good-looking display of numbers and letters, driven by an HC908 MPU 
and a hardware circle-drawing engine. The basic gizmo is shown here:

http://www.cathodecorner.com/sc200c.html

I am writing because I am now coding the GPS receiver interface. I 
realize that GPS time is not the same as either UTC or local time, 
and I am trying to figure out the best way to deal with this fact.

I have provided a user interface to the clock consisting of a rotary 
encoder with built-in pushbutton. This controls a menu stack that 
functions well and is quite extensible.

I am using a little Garmin GPS 18 LVC puck receiver, chosen for its 
reasonable cost and 1PPS output. I designed the DE9 serial port 
connector with extra pins to drive current-limited 5V to the puck and 
to receive 1PPS to the CPU's IRQ line.

I have written code to buffer up and parse the GPRMC sentence that 
gives time and date and location. Now I need to figure out what to do 
with these numbers.

I prefer to have the clock run on GPS or UTC time, then convert to 
local time as it displays the time. This makes the code more logical 
to write, since everything flows from GPS time.

So...

1 Is there a reasonable way to determine the local time zone from the 
GPS sentences?

2. If I have to store the time zone from the user's input, are the 
DST calculations reasonably straightforward these days?

3. What weird time zone operations should it support, such as 15 
minute local offsets or oddball DST dates?

4. In general, is it better to let the user turn DST on and off or 
try to do it automatically? (I live in Arizona, which doesn't worship 
DST, so I have no experience in this matter.)

5. Considering that the source code is being provided with the clock, 
are any of you fine folks interested in getting one of these gadgets 
to see what you can do with it?

I await your reasoned replies.
-- 

--David Forbes, Tucson, AZ
http://www.cathodecorner.com/




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