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GPS Answers
This page contains answers to frequently asked questions about our
GPS products and about timestamping in general.
For a list of the questions, please return to the main
FAQ Index
page. If you still have questions after reading this page,
email them to info@symres.com
and we'll be glad to answer them for you.
Q: What do abbreviations like GPS, PPS, NMEA mean ?
Like any other technical field, GPS equipment has an array of
acronyms and abbreviations. Some of the more common are:
GPS: The Global Positioning System uses ground based
receivers and a collection of satellites in geosynchronous orbits to
determine the time and location at the receiver position.
PPS: The Pulse Per Second mark is a highly accurate
electrical signal that indicates the beginning of a second as
determined by the GPS satellites. Although it accurately indicates
when the second starts, it gives no indication of WHICH second is
starting. That information is carried in the NMEA messages described
below.
NMEA: The National Marine Electronics Association has
defined a set of standard ASCII messages that marine electronics
equipment should use for communicating. This standard is very popular
and is used even in non marine applications. Many GPS receivers
output a small subset of the possible NMEA messages in RS232 serial
format. The information contained in these messages indicates the
current coarse time to the second and position.
Q: What NMEA messages are available on the PARGPS and what is their format ?
The PARGPS timestamping unit defaults to providing 2 NMEA messages every
second. The most important of these is the GGA message which
typically looks like this:
$GPGGA,163859.0,3609.422,N,11518.713,W,1,03,1.75,00024,M,-024,M,,*62
The first field identifies the message, the next contains the time
shown as hours, minutes, and seconds. The latitude and longitude in
degrees, minutes, and seconds with their quadrants comprise the next
four fields. The following two fields contain a GPS data quality flag
and the number of currently acquired satellites. The rest of the
fields are related to altitude, except for the last one which is a
check sum.
The second NMEA message provided by the PARGPS varies depending on the
model of GPS receiver being used. The Garmin antenna used with the Rev D
PARGPS provides the RMC message, while the Trimble antenna used with the
earlier Rev C PARGPS provides the ZDA message. Both messages include the
date and time and typically look like this:
$GPRMC,204634,A,3608.9137,N,11517.9635,W,000.0,217.1,300508,013.4,E*65
$GPZDA,232225.1,03,06,2003,,*55
For both messages, the second field represents time as hours, minutes,
and seconds all packed together. The third RMC field is a validity
indicator. It is followed by 4 fields containing location information and
2 fields containing speed and course data. The 10th RMC field contains
the date written as DDMMYY. For the ZDA message, the date is recorded as
day, month, and year in fields 3, 4, and 5 respectively.
For a complete description of the supported NMEA messages and their
formats, please see Section 4.2 of the Garmin Users Manual or Appendix E
of the Trimble Ace-III Users Manual that are included in the docs
directory of the PARGPS software.
Q: How does your GPS time stamping work ?
Our systems maintain a high speed counter, and we record two values.
First, the number of counts per second between PPS pulses. This
calibrates the counter in terms of counts per second. Second, we
record the number of counts from a PPS pulse to the next acquired
sample.
With these two numbers the exact time of the acquired sample in
seconds can be computed to within the delta t of the counter.
Furthermore because the counts per second is recomputed every second,
unless you have a huge temperature change over a one second period,
the system tracks very well with temp.
The PAR1CH and PAR4CH systems use a 64 bit counter inside the PC
Pentium. Count values are recorded in response to interrupts occurring
on the PPS signal along with marking the corresponding analog value.
On the USBxCH and PAR8CH, we have a dedicated counter in the FPGA on
board whose count values are saved right along with the analog data
stream without interrupts.
Q: What GPS antennas can I use with your products ?
Our A/D products are designed to work with GPS antennas that provide
both a high precision Pulse Per Second (PPS) signal and industry
standard serial NMEA messages with coarse time and location. We
recommend using the GGA and RMC or ZDA messages.
Our GPS product selector page lists
the GPS antennas and accessories we carry. It also shows the
appropriate A/D for each item. See the PARPGS user manual and the GPS
Time Stamping chapter of the USB4CH user manual for more details.
Q: Can I use the PARGPS by itself to get time without the PARxCH ?
No. The PARGPS PPS signal must be connected to the PARxCH digital IO
connector to work correctly.
Q: How does the PARGPS connect to the PARxCH A/D ?
The PARGPS sends the PPS signal to the PARxCH digital I/O. This is
used for precisely time stamping the acquired data. No analog inputs
on the PARxCH are used by the PARGPS timing module.
The NMEA coarse time and location messages are sent as RS232 strings
to the PC serial port.
Q: Can I use a USB to serial dongle with the PARGPS ?
Sorry, but no. The PARGPS driver talks directly to the serial port
hardware. All the USB to serial software we're aware of works by
intercepting user space calls and would not see the calls our driver
makes.
Add-in serial port cards should work fine as long as they properly
emulate a true motherboard serial port.
Q: Why does the mouse go crazy when the PARGPS is connected ?
If the PARGPS is powered up and sending NMEA messages when a PC
running Windows is turned on, the mouse may appear to go crazy and zip
all around the screen. This happens because Windows automatically
checks the serial ports when it boots to see if a Microsoft
SerialBallPoint Mouse is attached. If Windows happens to see a NMEA
message containing an M, B, or Z character, it mistakenly concludes
the MS mouse is attached and treats every future NMEA message as a
mouse command.
If this happens, you can recover. First, disconnect the PARGPS serial
cable so you have control of the mouse. Then, bring up the device
manager, select the SerialBallPoint mouse, right click, and select
disable. Do not select uninstall or Windows will check for it again
the next time is boots. After the mouse driver is disabled, it is
safe to reconnect the PARGPS serial cable.
Another way of taking care of the problem is to download a free copy
of the COMDisable program provided by Microsoft. This command line
utility program both displays the available serial ports and marks in
the registry that the ports you specify should NOT be checked for a
mouse during bootup. Here is one link for downloading
COMDisable.msi
from Microsoft.
Q: Why is PARGPS diag failing ?
The most common reason PARGPS diag fails is because the Garmin
antenna has not achieved satellite lock and is not outputting a PPS
signal.
There are two diagnostic programs in the PARGPS diags directory, diag
and diagser. We recommend running diagser first since it is simpler.
The PARGPS must be powered up with the antenna and PC serial cable
connected in order for diagser to work. Once diagser is running,
check the NMEA strings it returns. If the third field of the $GPRMC
message is a 'V', it means the GPS information is invalid because
satellite lock has not been achieved. When this field changes to 'A',
the GPS info is acceptable, and you should see the red led on the
front of the PARGPS begin blinking which indicates a PPS signal is
being sent.
Once the red led is blinking, it is ok to run the more extensive
PARGPS diag program. At this point, both the PARxCH and the PARGPS
drivers should be installed and both units should be powered up with
all their cables connected including the 15 pin ribbon cable from the
PARGPS to the PARxCH digital I/O. Also, the PARxCH diag program
should have been run earlier to verify the A/D is working correctly.
Now PARGPS diag should run ok.
Q: What is the typical PARGPS time accuracy ?
The time accuracy for the PAR1CH and PAR4CH is typically 10
microseconds, while for the PAR8CH it is a guaranteed 800 nanoseconds.
Because they rely on PC interrupts, the PAR1CH and PAR4CH have some
time variation due to interrupt latencies. However, this occurs only
if the PC is experiencing heavy network or interrupt activity.
Furthermore, the error is restricted to that one value and is not
cumulative on succeeding values. Running a long term average can help
distinguish and remove any outlier time stamps.
The PAR8CH system maintains an 800 nanosecond clock tick and counter
on the PAR8CH board with no latencies. The PAR8CH always has 800
nanosecond accuracy is not subject to any network or interrupt
latencies.
Q: How can I set the PC time with the PARGPS ?
The command line currtime program and the Visual Basic gpstime program
that report the current GPS time both include an option for setting
the PC from the GPS time. If you have permission to change the PC
time, currtime and gpstime will reset the PC time immediately after
starting and at the start of each new GMT day thereafter.
You can also set the PC time from your own programs using two of the
DLL functions included in the software which comes with the PARGPS.
Call ParGpsGetCurrentTime to get the current GPS time, then call
ParGpsSetPcTime using this newly retrieved GPS time to set the PC
time. Note that ParGpsSetPcTime will not work unless you have
proper file and execution permissions.
Q: Can I use the PARGPS with Win95/98/ME ?
No. Currently our PARGPS software support is only under the
Win2K/XP and Linux operating systems. In the future we may offer
Win98/ME/DOS support if there is enough customer interest.
Q: Can I use the PARGPS in my basement ?
The PARGPS mini-mag antenna we supply must have a sky view "line of
sight" access to at least four satellites to work correctly. That
won't happen if the antenna is located in a basement or inside a
building. However, it is ok to hang the antenna out a window and
have the coax cable coming inside.
The Garmin GPS 18 receiver does not output PPS until it has achieved
satellite lock at least once, so it won't work in the basement at all.
But, the older Trimble Ace-III GPS receiver puts out a steady
pulse-per-second (PPS) signal once per second. Without satellite
lock, the PPS isn't referenced to anything, but is does have a
reasonable one second period. So, you can use that in the basement as
long as you don't need absolute time corresponding to the GPS
satellite clock but are happy with relative time.
Q: What if the antenna cable is too short to reach the window ?
The solution is very easy for the Rev D PARGPS with the Garmin GPS 18x
antenna. Just purchase a DB9 male-female RS232 serial cable and
connect it between the end of the antenna and the PARGPS. We have
successfully received the antenna signals with more than 60 feet of
serial cable between the antenna and the PARGPS.
Unfortunately, the older Rev C PARGPS with the Trimble Ace-III
receiver requires coax for its antenna connection which is
significantly more difficult. We would actually recommend
reorganizing your entire setup to be closer to the window or upgrading
to the newer Rev D PARGPS before trying to extend the coax antenna.
If extension is your only option, you may be able to find suitable
coax parts through a company called Pasternak.
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