NMEA GPS Signal Generator

I found this,

signal generator to do this. Anyone know of something like this?

What aspect of the receiver do you want to test? I suppose one could go so far as to deploy an array of controllable pseudolites but that would be overkill if you're just checking the front end, and way overkill if you only need a periodic NMEA source.

uring. I found this,

-147378

vector signal generator to do this. Anyone know of something like this?

We're are building a GPS into one of our instruments. It may be a module or a PCB based one with an antenna. We'll we need to test this thing in manuf acturing and we would like to test the performance of the antenna location in the instrument. GPS doesn't work in the building unless we're near a win dow and varies with what satellites are in the sky. So it is hard to compar e the performance of different devices and even develop a simple pass fail test in manufacturing.

What I would like is a device that sends a fake NMEA string with a range of about a couple feet. I could then create a manufacturing test stand that p uts the DUT at a certain position and passes if it receives the string and fails if it doesn't. I could also test different antennas and antenna posit ions in the device by moving the device closer or farther away from the tra nsmitter and find the minimum distance for communication.

National Instruments and Agilent have test setups but they seem awfully exp ensive. You can get other RF dongles to do Wi-Fi and Bluetooth. I'd think t ransmitting GPS would be simpler than those. I'm surprised there isn't some simple tester available with a satellite code that indicates it is a fake and only for testing.

manufacturing. I found this,

vector signal generator to do this. Anyone know of something like this?

PCB based one with an antenna. We'll we need to test this thing in manufacturing and we would like to test the performance of the antenna location in the instrument. GPS doesn't work in the building unless we're near a window and varies with what satellites are in the sky. So it is hard to compare the performance of different devices and even develop a simple pass fail test in manufacturing.

about a couple feet. I could then create a manufacturing test stand that puts the DUT at a certain position and passes if it receives the string and fails if it doesn't. I could also test different antennas and antenna positions in the device by moving the device closer or farther away from the transmitter and find the minimum distance for communication.

expensive. You can get other RF dongles to do Wi-Fi and Bluetooth. I'd think transmitting GPS would be simpler than those. I'm surprised there isn't some simple tester available with a satellite code that indicates it is a fake and only for testing.

You are under several misapprehensions. The satellites do not send NMEA strings to be picked up by a receiver. The receiver decodes a very complex set of signals from several sats, and computes the local position from that, then outputs the NMEA string ( usually serially ) based on that computed data.

Its very easy to generate NMEA strings, even a very simple script running on a PC terminal program will do that. But that does not help you test a GPS reveiver front-end.

For that you need a pseudolite generator, ie a 'thing' which generates the satellite signals. Available, but very expensive, as you found.

Why not use one of the many active antennas, set up as a repeater, so you mount that on the outside of your building, with a clear view of the sky, and hence a reliable source of the existing sat signals. Feed the signal inside, to your GPS receiver, or even just another antenna acting as a local transmitter to your DUT ?

That's exactly what I setup at one company. Choke ring style GPS antenna on the roof. 30dB amp inside the antenna. About 150ft of LMR-240 coax. I used a power splitter and an AGC controlled amplifier on the test line with a switchable RF attenuator to control the signal level. L1+L2+WAIS patch antenna radiator under the test fixture to minimize leakage to other test stations. I originally installed a GPS bandpass filter to eliminate potential LightSquared interference but that proved to be un-necessary and was removed. The building was fairly well shielded as long as the doors and windows were closed. I also installed a DGPS LF receiver and distribution system on the test line, but since that dramatically increased the test time and few users need DGPS, someone wisely decided that sample testing was good enough.

The key part of the puzzle was the LMR-240 coax and associated amplifiers. Any leakage from the coax or the amps would screw up the test measurements. The switchable RF attenuator produced signal levels down to about -145dBm, which could not be seen by any piece of test equipment available. Typical operating levels at the receiver input were about -135dBm. Watch your shielding carefully. However, if you don't want to deal it, there are shielded boxes that will work.

The GPS location indicated by such a system is the location of the rooftop antenna, which was surveyed to an accuracy where continental drift became part of the reference position calculation. With DGPS active, the largest errors came from the phase errors of the rooftop antenna measured in mm. You could sorta see earthquakes and subsidence when big heavy trucks arrived at the loading dock.

Also, I recommend you get a GPSDO (GPS Disciplined Oscillator) to run the test equipment clocks. Some early head scratching and weirdness was eliminated by syncing everything to a common frequency (and phase).

There was some mumbling about a NEMA-0183 simulator. This can be easily done with a computah and commonly available GPS test programs. etc... Google for "NEMA test generator". Someone else scribbled a test program based on gpsfeed+, which included simulations of radical position excursions, reflection simulation, leap second function, random garbage, loss of signal, and other oddities difficult to test with a live GPS signal.

Good luck.

PCB based one with an antenna. We'll we need to test this thing in manufacturing and we would like to test the performance of the antenna location in the instrument. GPS doesn't work in the building unless we're near a window and varies with what satellites are in the sky. So it is hard to compare the performance of different devices and even develop a simple pass fail test in manufacturing.

about a couple feet. I could then create a manufacturing test stand that puts the DUT at a certain position and passes if it receives the string and fails if it doesn't. I could also test different antennas and antenna positions in the device by moving the device closer or farther away from the transmitter and find the minimum distance for communication.

expensive. You can get other RF dongles to do Wi-Fi and Bluetooth. I'd think transmitting GPS would be simpler than those. I'm surprised there isn't some simple tester available with a satellite code that indicates it is a fake and only for testing.

The GPS satellites do not transmit NMEA-0183 data - that is generated in the GPS receiver based on its calculation of its position and velocity, as determined by the signals received from the GPS satellites.

acturing. I found this,

nas-147378

ive vector signal generator to do this. Anyone know of something like this?

e or a PCB based one with an antenna. We'll we need to test this thing in m anufacturing and we would like to test the performance of the antenna locat ion in the instrument. GPS doesn't work in the building unless we're near a window and varies with what satellites are in the sky. So it is hard to co mpare the performance of different devices and even develop a simple pass f ail test in manufacturing.

e of about a couple feet. I could then create a manufacturing test stand th at puts the DUT at a certain position and passes if it receives the string and fails if it doesn't. I could also test different antennas and antenna p ositions in the device by moving the device closer or farther away from the transmitter and find the minimum distance for communication.

expensive. You can get other RF dongles to do Wi-Fi and Bluetooth. I'd thi nk transmitting GPS would be simpler than those. I'm surprised there isn't some simple tester available with a satellite code that indicates it is a f ake and only for testing.

That explains why I can't find what I'm looking for. I don't know what I'm talking about.

Great Info. I'll definitely look into this.

Thanks

One thing I forgot to mumble. When you use an external antenna, the indicated GPS position is that of the antenna, not the receiver. Repeating the signals through a common path (coax cable) created an odd situation. If connected to a re-radiating indoor antenna, a hand held GPS operating inside the building would always indicate the outside antenna position. Speed indications would always show zero. Direction indications would never indicate. I soon found that there was no effective way to test for any motion related parameters, such as direction and speed. I threw together a scheme using several other outside antennas and switching between them to test direction and speed. The static antenna position indication was also great for testing if the building had any leakage problems. Any GPS signal leaking through the doors and windows would produce an indication of motion.