GPSv2 inaccuracy

After a full cold start I am able to replicate this problem: I am in the Eastern United States, and the gps coordinates for my location (when the gps has full view of the sky and reports 8 satellites found) are correct for latitude but off by 0.665 (decimal) in longitude (at least 60 miles from where I am). The longitude changes as I move the gps to different locations, but it’s always off by this significant amount. I’ve verified my coordinates with multiple other sources (including Google Maps, Apple maps, my mobile phone, etc.) - it’s the rephone gps which is incorrect. I use to convert the decimal values to degrees minute seconds for comparison to other devices.

Any ideas?

Could you please send some displayed value and raw sensor data, preferably binary values read from the GPS?

The GPS library with the rephone only returns decimal values from the gps - I’m not sure what you mean by raw values, unless you mean the NMEA strings which are captured and converted by the library itself. I will instrument the underlying library to print out those strings directly for comparison

Yeah, I meant that :slight_smile:
I think it’s a float conversion problem, since the CPU has no hardware-FPU. Or just the library has a bug.

It looks like the gps NMEA string parsing is done here

The parsing/math in this file is incorrect: … rces/gps.c

When I run my longitude through the algorithm used in that file, it produces the incorrect decimal representation, as the indexes being traversed for the string are using the wrong range (longitude has three digits of degrees, not two, so grabbing the minutes part should start at index 3, not 2).

I cannot find where/how the code from that library is linked into my .vxp file; I need some guidance from that library maintainer on how to make change to it and deploy it to my part. Is this something compiled into the firmware image?

It seems it’s the gps module firmware. As soon as I get to a PC I’ll check everything.

this test code shows the good and bad value being generated; you can confirm by running any constant you use with the website above to convert from that format to the decimal format.

char *longitude = “07218.57692”;

int Data = 0;

// BAD!

for (int i=2; i< 10; i++)
if (i == 5) continue;
Data = Data * 10 + longitude[i] - ‘0’;

Data = Data * 5 / 3;

printf("%d\n", Data);

Data = 0;

// GOOD!

for (int i=3; i<10; i++)
if (i == 5) continue;
Data = Data * 10 + longitude[i] - ‘0’;

Data = Data * 5 / 3;

printf("%d\n", Data);

Arguably, this parsing code shouldn’t live in the firmware, but in the user-space code. I think it would make more sense to have an interface from the firmware which returns the entire NMEA string, and allows user-modifiable code to do the parsing. Plus, there are many existing gps parsing libraries which already exist so they could be leveraged. If this does cause a firmware bump, I’d like to see a follow-on bump which exposes the interface to the gps chip a little more. This could also make it easier to send the gps commands for the different power savings modes more easily.

Thanks for pointing the issue out.

Wondering if there is any possible way to update the GPSv2 firmware without Freescale tool?

I found the GPSv2 base on STM32F030, and the develop IDE is Freescale Kinetis Design Studio.
Maye we can build the project with KDS and flash the hex file with a j-link.

So the next task is to find, if it has a bootloader installed :smiley:

Is there documentation somewhere which shows how to take compiled code from KDS and upload it? Or is this something we need to the rephone development group involved in? Do they monitor these forums?

You just flash the new firmware via KDS. The user manual of the Kinetis SDK is really clear acout flashing and debugging. You can use Segger and OpenOCD too.

I see the new object and hex file posted to the github (where I found the source code). I have a bus pirate and OpenOCD, but I will need a little more direction in terms of how to do so. Additionally, everybody who has received one of these parts so far has the GPS library code in it which is inaccurate and will need to follow some directions for how to update this, so it seems like creating a step-by-step guide is in order.

My gps module is on its way, so I can’t make a tutorial this weekend. After that I can make something, but with segger (I got one with an infineon devboard)

I have ordered the GPS V2 about a week ago im not happy hearing this news, so ill take it that the one that i will receive will be inaccuracy aswell?

Has any contacted SeeedStudio about this issue?

Unfortunately all devices will have the faulty gps parsing code until they are reflashed. Presumably upgraded firmware will be made available at some point. I would like to see the interface changed to provide read/write access to the underlying GPS chipset (the NMEA strings directly) to allow more control. That seems to be a more common method of providing access.

@rhbroberg, and also it’d be nice if it will be some firmware-upgrade interface there, which would not require windows :’(

Is there a work around and till the firmware upgrade?

Anyone? This is delaying my project :frowning: