GPS FAQ ANSWERS

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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. The Trimble Ace-III GPS receiver outputs 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 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.

For a complete description of the NMEA messages and their formats, please see Appendix E of the Trimble Ace-III Users Manual that is included with the GPS software.

 
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: 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 occuring on the PPS signal along with marking the corresponding analog value.

On the 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 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 outlyer 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 ?

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 other basement situation that works is when you don't need absolute time corresponding to the GPS satellite clock but are happy with relative time. The Trimble Ace-III GPS receiver puts out a reliable pulse-per-second (PPS) signal once per second. Without an antenna signal, the PPS isn't referenced to anything, but does have a reasonable one second period.

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