Product Review
Summary:
Overall, I liked the GPS-200A package.
It provided the components to acquire very accurate time signals in a familiar SMPTE
time code format. Those serious about time keeping and tired of dialing
the U.S. Naval Observatory time (or trying to reliably receive WWV)
will appreciate the GPS-200 ease of use and sensitivity.
Use Satellite Technology to
Synchronize Station clocks,
Computers, Servers and
Automation Systems
by Rich Rarey
WASHINGTON Quick, what time is it?
At W. Clark and Associates, Ltd., timing is everything.
The company has introduced the modestly priced GPS-200 receiver
that allows one to receive the extremely accurate timing signals
from atomic clocks aboard Global Positioning System (GPS) satellites.
The GPS-200 emits SMPTE time code that is accurate to within 10 microseconds.
There are many uses for such accurate timing signals such as radio station automation,
radio station time checks, television station network synchronization,
field recording using multiple DAT sources, and accurate setting
of computer network server clock time. Commercial banks have found the GPS-200 useful
for super-accurate time-stamping purposes.
Priced at $ (*note), the basic GPS-200 package includes power supply
and (preamplified) GPS antenna. For $ (*note), W. Clark and Associates offers the GPS-200,
a TCR-500 SMPTE reader card for DOS based platforms and driver software
that automatically syncs the computer's clock every 30 seconds.
Included is the TCRsync NLM software that locks a Novell Server clock to timecode.
This package gives any radio or TV station the building blocks for detecting and using
the GPS precision. For longer coax runs from the antenna, several preamplified antenna
packages are available that include a pipemount antenna that has 26 dB gain,
and 50 or 100 feet of .25inch coax.
The package I tested included the GPS-200, the TCR-500 PC card, an optional active 26 dB
gain antenna (this model was magnetic mount), 50 feet of .25inch coax and the software.
I opened my PC, inserted the TCR200 card into an empty slot, plugged the GPS-200 cable
into the card's rear connector, then plugged the preamplified antenna into the GPS-200.
Set the clock
After plugging the power supply into the GPS-200, I walked the antenna over to a second
floor window, not really expecting any results. After all, GPS reception depends on
receiving three of 18 satellites in low Earth orbit-in this case, capturing the signal
with an antenna the size of a computer mouse. Imagine my surprise when the GPS-200
locked on to the satellites and began sending Universal Coordinated Time (UTC) timecode
at my computer!
For a permanent installation, W. Clark and Associates president William Clark
recommends mounting the amplified antenna externally using the antenna's pipe mounting
with an unobstructed sky view. The GPS-200 takes a few minutes on first powerup
to get a fix of several GPS satellites. After that, the "fix" information
is internally stored and backed up by a lithium battery.
Even if the battery fails, the GPS-200 will work; it will just have to reestablish the fix.
Other internal jumpers select among 25 or 30 frames per second; nondrop;
and select 0, 5 or 12 VDC for the antenna's preamplifier.
Once installed, the GPS-200 needs no other maintenance.
Conversion
If plain UTC SMPTE is not suitable for your operation, W Clark and Associates makes
the TCI-232 time code interface that has a variety of uses for the radio and TV station.
The TCI-232 interface accepts time code from the GPS-200,
and can regenerate the timecode for your local time zone.
As is, the GPS-200 emits UTC time as sent from the GPS satellites.
The TCI-232 has a number of other useful outputs.
For example, the time information is output using RS-232 or RS-422 and a TTL pulse
or a dry contact closure can be set to fire once a second, minute, hour or day.
Heathkit WWV clock emulation-popular with Grass Valley video switchers is also available.
Although I did not use a TCI-232 for this evaluation,
it certainly will simplify GPS-200 interface tasks.
I loaded the software included in the GPS package into my PC to examine how it worked.
The TCR-500 card in the PC receives SMPTE from the GPS-200,
and can be installed so that it "interrupts" the computer
at the beginning of end of each SMPTE frame.
The Terminate and Stay Resident (TSR) application loaded from your computer's autoexec.bat file
intercepts the interrupts from the TCR-500 card.
Every 30 seconds it forces the PC clock to conform with the GPS-200 time,
which can be offset for your local time zone.
Another DOS application included with the package is a simple display application
that shows the hours, minutes, seconds, frames and date.
Complete, well written assembly language source code of these applications is included,
along with C language source code used in some of the applications.
Using the source code provided, Microsoft Macro Assembler, Visual Basic and some free time,
I converted the low level TCR-500 card software into a Windows DLL,
using Visual Basic to do the display chores.
A caution if you begin to design you own timecode reader:
Microsoft Windows and Visual Basic have a response "latency"
that makes display of frames not meaningful.
It is still okay for showing hours, minutes and seconds.
Although the GPS-200 does all the hard work,
the basic principles of GPS reception and transmission are quite interesting.
The U.S. Defense Department originally designed the GPS satellite system for use
by American and allied military forces to accurately locate themselves anywhere in the world.
GPS works because GPS satellites orbit the Earth in extremely predictable paths.
Coded signals broadcast from the satellites enable the receiver
to get an approximate "range" of distance from a satellite.
When three satellites' ranges are determined, three "spheres of location" intersect,
where you and your receiver are.
Atomic clocks on each satellite provide the time basis to transmit the coded signals
at known times.
An excellent article detailing the history of GPS and how it works was written by
Dr. Thomas Herring and published in the February 1996 Scientific American.
Detailed online descriptions of GPS systems can be seen at
http://wwwhost.cc.utexas.edu/ftp/pub/grg/ gcraft/notes/gps/gps.html.
Written by Peter H. Dana, this browseable site is packed with GPS knowledge.
Overall, I liked the W. Clark and Associates GPS-200 package.
It provided the components to acquire very accurate time signals
in a familiar SMPTE timecode format.
Those serious about time keeping and tired of dialing the U.S. Naval Observatory time
(or trying to reliably receive WWV) will appreciate the GPS-200 ease of use and sensitivity.
When coupled to W. Clark and Associates TCI-232 interface box,
you have built the foundation upon which you can add whatever time display
(within reason) you desire.
For information, contact Masterclock, Inc., 2484 W Clay St. Saint Charles, MO 63301
Telephone 800-940-2248 or fax 626-724-3776
Rich Rarey is the technical director for National Public Radio's "All Things Considered"
and the author of the Public Domain series seen here in Studio Sessions.
*Note - See price list.
