Software for the CNS Clock
and the CNS Clock II
You may download an evaluation copy at
the Tac32 Software Download Page.
Ordering information and prices
The main screen displays the time in BIG DIGITS. You can select from UTC, PC time, Greenwich Mean
Sidereal time, Local Mean Sidereal time, or PC clock error displays. Smaller
windows show detains about GPS time, Local time, Sidereal time, GPS Navigation
data, and GPS satellite status.
You can enable audible time ticks (to assist
you in setting your wrist-watch). Tac32 can automatically set the
PC's internal clock with about 10 to 25 msec accuracy. All the time display
updates and audible ticks happen synchronously with the GPS 1PPS signal
because the PC reads the tick on its DCD line.
Tac32 allows you enter timing offsets and
handles all the arithmetic. It makes corrections for time delays in cables
and the instrumentation. It tells you (with 1 nsec resolution) the actual
epoch of the 1PPS tick and it gives you an estimate of the accuracy of
Tac32 gives you a display of which satellites
are in use and which satellites are above the horizon. This includes a
bar-graph "S-meter" for each of the GPS satellites currently in lock, which
are updated once per second.
Tac32 lets you change operating modes (timing
vs. position, elevation masks, satellite selection criteria, etc) easily.
When running in position-determination mode it will handle all the position
averaging tasks for you.
Configuration selections are saved when
changes are made and restored when Tac32 is restarted.
Tac32 runs on the Windows operating system. Recommended minimum configuration is Pentium 120 with
128 Mb RAM. Tac32 takes about 4Mb RAM to run and does not like being
swapped out of memory because of the real-time nature of its operation.
Tac32's Other Screens
View Satellite Az-El Chart
This chart shows a graphical representation
of the position of the satellite positions on a polar plot to give the
user a feel for the orientation of the satellite constellation. The azimuth
is shown in degrees around the outside of the plot and the elevation is
shown as concentric circles from 90 degrees at the center to zero degrees
at the horizon (the outside ring).
Each satellite is color coded with blue
indicating that the satellite is rising (elevation increasing), red indicating
that the satellite is setting and cyan indicating that the satellite’s
elevation trend is not known.
Data logging is an important feature of Tac32.
New log types are being added to support various special requirements.
The Time, Position, Velocity, Etc. log
contains information about the current PVT status in a comma-delimited
text format that can be imported into a spreadsheet program for analysis.
To support long duration analysis of PVT data where every data point is
not required you may select logging rates from once every second to once
|Raw Data Logs contain
the messages from the GPS receiver preceded by an input or output arrow.
Binary data messages will be interpreted.
Set the PC Clock
You can enable or disable the setting of the
computer’s clock from the CNS Clock.
Selections include Disable, Set
One Time, Every Second, Every Minute, Every Hour,
Every Day, and When PC Clock error exceeds xxx msec.
The time of last sync window shows the
time that the CNS Clock last reset the PC’s clock.
The PC Clock Reset Count window
shows the number of times the PC Clock has been reset.
Set Reference Location
The reference location is the phase center
of your stationary GPS antenna. The more accurately you set the station
reference location, the more accurate your timing results will be.
The location can be set four ways:
From a user-loaded database of reference
locations, identified by name.
Copy a snapshot of the Average Location
to the Reference Location. The average location is Tac32’s version of a
Copy the Current Location to the Reference
Location. If you have no other location available, use this one.
Manual. If you have a better estimate of
the reference location that the other choices above enter you may enter
the data manually.
An altitude calculator is provided for ease in matching the Altitude values. It uses linear
interpolation of the 10-degree Geoid data map published by the US Defense
Tac32 provides the means for adjusting the
1pps time mark offset. There are two reasons for doing this.
First, the delays in signal propagation through the coax cables and hardware
gates must be accounted for so that the leading edge of the 1pps time mark
arrives at its termination at exactly the correct instant. Second, large
offsets in the 1pps time mark are useful in the precise measurement of
event time duration or delay.
For high accuracy timing synchronized to
UTC the Epoch, Offset and Intentional Early Offset controls are normally
set to zero (0) and the Antenna Cable Delay, Measurement Cable Delay and
Internal Receiver Delay controls are set as accurately as possible.
Tac32 supports T-RAIM (Time Receiver Autonomous Integrity Monitor). T-RAIM detects and, if possible,
isolates faulty satellites so they will not be used in the timing calculations.
Selecting Kill 1pps on fail causes the 1 PPS timing mark signal
to be suspended whenever the T-RAIM alarm is active.
Coax Delay Calculator
Tac32 provides a simple means to obtain the
cable delay parameters needed in the Timing Setup dialog box. You
simply choose the length and type of coax. The math is done automatically.
The calculator even allows for two pieces of different types of coax to
be strung in series.
For accurate time keeping, the GPS receiver
needs to be constrained to a fixed position, which should be accurate at
the 10-15 meter level. The position that is used for this is called the
When you begin operation at a new location,
you will probably not know your position accurately. The raw positions
reported by the GPS receiver will wander by up to 300 meters horizontally
and 450 meters vertically because of weak geometry of the satellites, errors
in the broadcast ephemerides, atmospheric propagation errors and especially
because the US Military degrades GPS (known as SA = Selective Availability).
To overcome these errors, it is desirable to average position measurements
for several hours to get an accurate position. When the receiver has been
running in position mode for a while, you can transfer the average position
into the reference position. You may also manually enter coordinates.
This screen provides
information about the progress and statistics of the averaging process,
allows you to make adjustments to that process and shows an estimate of
the current timing accuracy.
GPS Mode Selection
Tac32 gives you full control of the settings
used internally by the GPS receiver to perform its calculations.
This window allows for a large number of
combinations of settings with complex interactions. So what does
the non-expert do? Just click on Navigation Defaults to do your
self survey then click on Timing or DGPS Defaults to enter the precision
timing mode. What could be simpler?
To learn more, Tac32 includes extensive
Tac32 can also activate the GPS receiver's
DGPS output messages.
Tac32 allows full control over which satellites to use and which to ignore. It highlights satellites
that are currently being tracked in green and those that have been used
in cyan. This can be used to develop a picture of the satellite constellation
if left active for at least 12 hours.