GPS Time is "On My Side"

Joel Cusick Alaska Support Office GIS Team

<http://web/rgr/rgr2.htm> (Alaska NPS Intranet)

<http://165.83.48.21/rgr/rgr2.htm> (NPS-wide Intranet)

**Background**: Configuring your GPS, digital camera, and computer for
the right time of day is crucial in providing software when you
collected data. Here are the steps in determining the right time zone
for Alaska Users and how to alter Time on Garmin, Camera and Trimble GPS
receivers.

Quiz Question\....

If it is 2:00pm Monday, July 3rd in Anchorage Alaska, select what other
\"times\" this can be expressed in.

A\) 2:00pm Alaska Daylight Savings Time (ADST) B) 1400 Military Time
(ADST)

C\) 10:00pm Greenwich Mean Time (GMT), July 3rd D) 2200 Military Time,
GMT, July 3rd. E) All of the above.

Answer is E!

Going between time zones can be confusing, but with GPS, it\'s always on
your side(Mick Jagger, 1971).

## A. Alaska Time Zone Example -- When to change and the formulas

From April to October (SUMMER)

# GMT - 0800 = Alaska Daylight Savings Time

From October to April (WINTER)

# GMT - 0900 = Alaska Standard Time

In the example above, the local time is simply added into the SUMMER
formula and solved for GMT. I like converting the local time into
Military time first:

1\) Convert 12 hour time to Military 24 hour time:

2:00pm = 1400 Military time(1200+200)

2\) Enter Military time into the SUMMER formula:

GMT - 0800 = 1400

3\) Solving for GMT\...

# GMT = 2200 OR 10:00pm (GMT) July 3rd

### Setting Time on a Garmin

Turn on Garmin and enter setup menu. Access the Time Tab and alter Time
for the time zones above. Depending on the model, Garmin's will either
automatically switch to Daylight savings time, otherwise you must select
the Local Time Zone of either 8 or 9 hours BEHIND UTC.

### Setting the Digital Camera Time

This is critical for using software like GPS PhotoLink. Be sure to check
the Local Time Zone when using a digital camera (usually found under a
menu called Time or Time Settings). Adjust to Local Time zone. Its not
critical you are to the second, but at least within 5 minutes or so.

## D. Trimble Pathfinder Office Users

If you do synchronize your computer and Datalogger with the right time,
you may produce incorrect results when running mission planning software
like QuickPlan or Trimble Planning Software. You can grab the wrong base
files from the internet, if the time zones are set incorrectly. Since
GPS runs on Greenwich Mean Time, it is up to you to configure Pathfinder
to your local time zone using the formulas shown above.

\* First make sure your computer time is accurate and in synch with
Pathfinder - this will lessen confusion. Double click on the time in the
lower right side of your Windows toolbar or go to Startup/Control
Panel/DateTime to ensure accurate time. If the time zone tab is set to
GMT-09:00 Alaska, don\'t panic. Note that the check box for automatic
adjustment for Daylight savings is checked! Your computer will
automatically change times this October.

\* Next, under Options in the main Pathfinder Office menu, select Time
Zone Settings and ensure the time zone is set to \"Alaska Day USA\".
Press OK. This sets your time stamp in the position properties window
and the start and end times of your collected rover files (See last
section below).

\* Ensure your QuickPlan Utility is also set to the correct local time.

Go to QuickPlan and the Options Menu. Select \"Time Zone\". Again make
sure \"Alaska Day USA\" is selected and the time difference from GMT is
\"-08:00\". Press OK. Check your Status box for the correct time when
creating PDOP charts.

\*If you need to go to the web and grab differential files the old
fashioned way, you will need to convert your start and end times of the
rover files to GMT military time, since base files are always stored in
GMT time. Use the formula above to convert from local time. Remember
anytime after 4:00pm in the Summer translates to the next day in GMT!
HINT: The Start and End times shown in the Select Rover dialog box are
set from the Time Zone Settings in Pathfinder, not your computer time.