Frames, Trajectory Simple Travel Time, and Inital GPS Time of Week in NI GNSS Toolkit

Updated Apr 4, 2023

Reported In


  • GPS Simulation Toolkit


  • NI-RFSG 17.0

Programming Language

  • C

Issue Details

I am trying to understand the functioning of NI GNSS Toolkit using Direct Streaming Generation (On the Fly Parameters) example to generate GPS data from my RFSG. What are frames in a GPS signal ? How does Initial GPS time of week relate to the actual date and time ? What effect does trajectory simple travel time property have on my GPS data ? 


As mentioned in the help files of GNSS toolkit , these parameters are used as follows : 
  • Frames : GPS data is divided into frames with each frame consisting of five subframes which are each 10, 30 bit words long. They are transmitted at 50 bits per second which means that every frame has data corresponding to 30 seconds. Setting the number of frames in your code specifies the amount of time that the code will run and simulate GPS data for. The structure of these frames is as per GPS standard and is well documented online. For example, if the value of number of frames is set to 25, the generated packets have the following structure :
  • Initial GPS time of week : The Ephemeris data on the GPS signal received from satellites is used by the GPS receiver to calculate its precise position. This data set is update and uploaded by NASA as an ephimeris file every two hours for all satellites (which send it every 30 seconds) and can be valid for upto four hours. Follow this link to understand how you can get these updated Ephemeris files from. The Initial Time of week property is used to define the starting point from where ephimeris data will be read by the toolkit from the file and used to generate the GPS signal. Its unit is seconds. The help file explains the same with an example.
  • Trajectory Simple travel time : In simple trajectory mode, this property simply defines the position update rate i.e. the amount of time taken to move from one receiver position to the next in GPS data being generated.

Additional Information

Some of these properties are specific to the standard and more details can be found by reading more about the GPS standard and how it works.