GPS FAQ

Home Page > FAQ Index > GPS Answers

This page contains answers to frequently asked questions about our GPS products and about timestamping in general

For a list of questions in other categories, return to the main FAQ Index page
If you still have questions after reading this page, Contact Us and we'll be glad to help

 
Q: What do abbreviations like GPS, PPS, NMEA mean ?

Like all electronics GPS has its array of acronyms and abbreviations:

GPS: The Global Positioning System using a collection of orbiting satellites to determine time and location at ground based receivers.

PPS: The Pulse Per Second mark is an electrical signal with its leading edge indicating the beginning of a second to sub-microsecond accuracy as determined by the GPS satellites.  Although it synchronizes when the global second starts, it gives no indication of WHICH second is starting.  That information is carried in the NMEA message strings.

NMEA: The National Marine Electronics Association has defined a set of standard RS232 ASCII message strings used to indicate location and coarse YMDHMS time.  Most GPS receivers output a subset of the possible NMEA strings in RS232 serial format.

YMDHMS: ( year, month, day, hour, minute, seconds ) in various NMEA formats. This is often called "coarse" time, with the seconds only measuring the current second, but no finer.  The PPS signal must be used to determine the exact start of the second tick.  The antenna MUST emit the coarse YMDHMS time between PPS ticks so they can be correlated. Low quality antennas may not and their time may be off by integer seconds.

 
Q: What GPS antennas can I use with your products ?

SR products use GPS antennas with the PPS signal and RS232 NMEA strings. This configuration is common with "hockey puck" antennas from companies like Garmin and Trimble. We recommend configuring your antenna to output the GGA, and RMC or ZDA NMEA string messages.

Our GPS page lists the antennas and accessories we carry.  It also shows the appropriate A/D for each item.  See the USBxCH PDF User Manual for detailed information about using GPS with those products.

 
Q: Can I use GPS in my basement ?

Ideally GPS antennas should have a sky view "line of sight" visibility to at least four satellites. Progress has been made in recent years, relaxing this requirement in some situations. For example, it is likely a GPS puck antenna will work inside a field box. Operation from a basement or inside a building is still optimistic.

For locations with no access to GPS reception, we recommend using TCXO time stamping with products like the USBxCH. If you only need relative time, simply count A/D samples as measured by the TCXO clock.  The PcTime option which all of our products support is also an option. However, this assumes the PC clock is set correctly by other means and is unlikely to be as accurate as GPS.

 
Q: What if the antenna cable is too short to reach the window ?

With hockey puck style antennas, the signals on the GPS cable are generally the PPS tick and RS232 strings, and not high frequency RF from the antenna core itself. Inexpensive standard multiconductor cable can be used to connect the puck to equipment like the USBxCH. Runs of several hundred feet are acceptable if needed to place an antenna in a window or on the roof.

For the USBxCH systems use an inexpensive RS232 DB9MF ( male-female ) extension cables.  Expensive RF coax is NOT required.

 
Q: Why is my GPS giving 19 year old dates ?

Your antenna is not compensating for the "Week Number Roll Over" (WNRO) which occured on the GPS network on April 6, 2019.

At development time the GPS designers decided to store time as a 10-bit week number plus seconds within that week. Of course after 1024 weeks or roughly 19.5 years, the week number overflows and wraps back to zero! This has now happened twice since GPS time started on January 6th of 1980 ...

Luckily only the date is affected. The hours:minutes:seconds part of the time is still correct and unchanged.

Software is expected to deal with the date problem when the wrap occurs. This correction can be performed in the antenna firmware or in the USBxCH post processing programs. Basically it just involves adding 619315200 seconds (the number in 1024 weeks) to the date. Implementing the firmware update method is discussed in the ReadMe.txt file in the Garmin subdirectory of the USBXCH/Docs directory. Details on implementing the USBxCH post processing method are available by running Pak2Bin /i and reading about the CorrectWNRO parameter.

Return to the FAQ Index for links to more answers