GPS simulator Data

[Matrix85]

Hi,

I am working on the GPS simulator toolkit. My question is, in the GPS simulator toolkit, it is generating the GPS data when we select the Ephemeris and Almanac files from the links provided in the Manual. The file niGPSWritewaveformtofile.vi would generate the GPS data into a external file (.bin file). I have generated just one frame of GPS data using a single satellite which is 1500bits.

I need to use to this GPS data in my application which takes the GPS data as binary digits. But I do not even need the whole frame of 1500bits, rather I need only the 3 subframes consisting of the Ephemeris data and the Almanac data. How can I see what data is generated by the .bin file from the GPS simulator. I am guessing that it is generating the data in ASCII format. Correct me if I am wrong and tell me in what format the data is generated and how to differentiate between each subframe. 

I am looking for a reply as soon as possible and I highly appreciate much needed help.

Thanks,

Sandy 

[dansch]

Hi Sandy,

Thanks for posting to the NI Discussion Forums! I understand that you are working work the GPS toolkit and you are interested in reading from a GPS simulation file. The actual data that is written to the file is in the format that is supplied to the RFSG Write function when generating the simulation data. This data is an array of interleaved IQ data, with each I and Q sample being an I16 data type. 

I am not sure that reading the binary file will necessarily give what you expect, but it can be done using the binary file I/O VIs. This is similar to the way that the niGPS Streaming From File example VI is implemented.

I created a simple example program, attached below, which reads the 1500 bit GPS file and reads a word (30 bits) of data at a time. My VI assumes that there is header information at the beginning of the file which consists of 419,764 bytes. You will need to enter the subframe and word that you wish to read. For example, ephemeris data will be subframes 2 and 3, words 3-10. I am not completely sure what the data beyond the first 512 bytes of the header is, so this implementation may or may not be exactly what you are looking for (you could always use it as a good starting point, though). Hope this helps,

[Matrix85]

Hi Daniel,

Thanks for your help in regards to my question. That is exactly what I was looking for but with some minor changes. I just have one more small question. Your implementation gives me a word within a subframe. But for my implementation I am looking to get the complete 4 subframes consisting of Ephemeris and Almanac data completely. That is 120 bits of data. I tried implementing by making few changes, but I was not able to get it. I removed the word selection part since I do not need it and did some manipulation to get the 4 subframes. 

Could you please help me with that. I highly appreciate your help in this matter. 

One more question is related with those header data included in each subframe. Is there any detailed description of the data generated by the GPS simulator, like say these many bits consists of this information and remaining bits indicates this information etc.. If yes how can I find it out.? 

Thanks once again for your help.

Regards,

Sandy

[John_P1]

Hi Sandy,

It sounds like you are on the right track being able to retrieve a single word from the file.  I believe the subframes are 300 bits long each, so reading 4 complete sub frames would be 1200 bits--would this explain the results you are seeing?  Daniel's code should be a good starting point for acquiring the words that you need, you can just run it for the several words that you are interested in--I'm not immediately sure which words you need.

I'm not personally familiar with the bit-by-bit makeup of a GPS signal, but I found some good information in the GPS Specification.  The GPS Simulator will adhere to the GPS standard, so any information from the specification should apply to the simulated signal as well.

If you'd like, you can post the modified code to this forum so we can take a look to see what exactly might be going on.  You may also want to try posting to the RF Measurement Devices forum, you may find people over there who have more experience dealing with these GPS signals on a lower-level.

-John

[Matrix85]

Hi John,

Thanks for your reply. I was able to get the subframes finally without any problems. I need your help in one more matter which I think I would not be able to implement unless with some help from NI.

The implementation done until now gives me the IQ data which is just a part of the actual GPS data that will be transmitted. In a sense, the navigation data that GPS satellite transmits is in bits consisting of 1500 bits at a bit rate of 50bps. So I want to know how to be able to get the binary bits (1's and 0's) from the .BIN file which the GPS simulator generates. I hope you have understood my question and I would highly appreciate your help in this matter. If I would be able to get that data, I can implement my actual project before the NI week. Waiting eagerly for a positive response.

Thanks,

Sandy

Message Edited by Matrix85 on 06-20-2009 09:26 PM

[Dan_K]

Hello Sandy,

This would involve the decoding of the actual GPS data and is beyond the scope of this toolkit. The GPS Simulation Toolkit is designed to simulate GPS data for transmission to a DUT to test GPS receivers. What you are describing is turning the PXI chassis into a GPS receiver, which is beyond the capabilities of the toolkit.

[Matrix85]

Hi Daniel,

Thanks for your reply. I would try to work on that. 

Regards,

Sandy

[Aaron_W.]

Sandy,

Let us know if you have more questions relating to the GPS toolkit. We are here to help. Thanks!