GPS Delay

max72

I'm using the Parallax GPS unit with the propeller but I found there is a delay in the data.
When I log speed and heading on a display I have a delay of about 3-4 seconds (checked using the GPS on a bike or on a car). On the other hand the GPS unit on my phone is much more responsive. Moreover I guess the propeller chain (serial+data handling+LCD) is not the bottleneck.
I wonder if this delay is common and is related the the GPS unit itself, and in that case which unit should I use if I want to reduce the data delay.

Thanks in advance,
Massimo

Chris Savage

Hello,

If you’re using the GPS Module we sell with the SX co-processor on it you can expect apparent diminished performance when using a high-speed microcontroller such as the Propeller. The reason is that the GPS Module was designed primarily for microcontrollers like the BASIC Stamp Module which do not have the resources available to parse the entire $GPRMC string. So the SX co-processor does this for you and makes the data (such as latitude and longitude) available as separately requested pieces of data. For a BASIC Stamp module this is perfect, but for the Propeller it means that the data is still gotten one piece at a time. The Propeller is more than capable of buffering the entire $GPRMC string and parsing the data much faster so my recommendation would be to use the /RAW mode and handle things that way. I am trying to see if we can make available a cable which would allow Propeller users to tap directly into the GPD Module’s receiver and talk to it at 3.3V. We’ll see what happens. Take care.

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Chris Savage
Parallax Engineering

Rob_W

Hello Chris,

I have the Parallax GPS receiver connected to a Propeller Control Board, my question is if I connect it in the RAW mode via the two pin jumper will the receiver experience the same delay Massimo is experiencing? I see by the schematic it is powered by 5 volts and am wondering if this will make a difference or will it receive and display the $GPRMC string in real time as it is received by the module?
I have tested the receiver being connected to both the Basic Stamp Homework Board and the Propeller Control Board, using my laptop for a display as I drive to and from work (17 miles one way) and it appears they both maintain good reception along my route. I do not have a GPS phone to make any comparison with.

Thank You in advance,
Rob_W

Chris Savage

Remember that using the smart mode to get all the data requires multiple serial commands to be sent and multiple replies for data to be returned. In RAW mode you get all the data in one transaction without having to send anything. Even so the GPS Receiver only sends out one new $GPRMC string per second.

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Chris Savage
Parallax Engineering

max72

Thanks for your answer, but I didn't explain correctly my setup.

In fact I have a modified cable (from a broken unit) and the GPS module is connected directly to the Propeller Protoboard. I'm bypassing the whole SX buffer. I did that because I was wondering if the delay could be related to the SX data handling, even if in RAW mode. I would say it didn't change anything.
I'm using an object from the Obex, and I have a COG for serial data input and a COG for string parsing (in spin).
The main cog gets the data and writes to the LCD.
My guess is the propeller is not slowing or delaying my data, but it could be related to the GPS itself.
I have a similar delay with my DGPS, but in this case I'm asking a centimeter accuracy.

Massimo

Chris Savage

Okay, so we’re down to two possibilities…the GPS only updates once per second. So depending on the object you’re using that may or may not cause delays in your data. I am unfamiliar with the object you’re using so I can’t speak for it. It might be worth checking independently using some test code. Isolate the problem to help identify it.

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Chris Savage
Parallax Engineering

max72

I'll do some testing.
I started with the forum to check is the problem is intrinsic in the unit. The 1s delay is not a problem, but I'm experiencing much longer times.
I'll keep you updated.

Thanks,
Massimo