TinyGPS Fix Determination

Firmware
1

Hi,

I am using the TinyGPS library and it is wonderful, but I would like a way to find out whether or not it has a fix and I can’t seem to find any way to do this.

Any help would be greatly appreciated!

2

I think you will need to read the input from the led status pin (which tells if you have a fix) and interpret from there…

A quick search and poking at the library doesn’t show the possibility via software. Happy to hear if there’s a way to do it though!

3

There is no way to detect a fix unless you’ll have it.
Any GPS is firing right away NMEA-sentences (common, TinyGPS uses it.), whether or not there is a fix.
I had even a chip with a broken antenna, no content, but a bunch of empty sentences. They look like $Gxxx,.
Valid messages, but void content.
The questions boils down to this: When did you get a fix.
First indication is a valid timestamp. Usually you get it with the first satellite in view. Second indicator is the count of satellites, anything lower than 4 does not provide an initial first fix, but you know, it might be coming.

Third is of course a fix with 4 or more satellites in view.
So you have to wait.

Another problem might be, if the antenna looses contact and the position is not updated. You have to check the time-intervall since the last fix.

4

What led do you mean? My GPS -receiver has none.

5

If you look at the 2nd photo, the pads are there but not soldered…

I see that the raw output of the GPS actually tells you whether it’s locked or not

$GPGGA, 161229.487, 3723.24756, N, 12158.34162, W, 1, 07, 1.0, 9.0, M, , , ,0000*18

The value 1 between “W” and “07” is the position fix indicator.

From the datasheet:

Value  Description
 0     Fix not available or invalid
 1     GPS SPS Mode,fix valid
 2     Differential GPS,SPS Mode,fix valid
 3     GPS PPS Mode,fix valid

You will looking at any non “0” value to indicate that there’s a valid fix…

So with this knowledge… you probably need to modify the TinyGPS library to check that field :smiley:

Not sure if it works but the author gave this example:
http://arduiniana.org/libraries/tinygps/

float flat, flon;
unsigned long fix_age; // returns +- latitude/longitude in degrees
gps.f_get_position(&flat, &flon, &fix_age);

if (fix_age == TinyGPS::GPS_INVALID_AGE)
  Serial.println("No fix detected");
else if (fix_age > 5000)
  Serial.println("Warning: possible stale data!");
else
  Serial.println("Data is current.");
6

Thank you, I did not notice that.

The example the author does provide is actually a better suggestion than mine, given his constant “GPS_INVALID_AGE”.
I would not tinker with the library here. NMEA is a beast on its own.

The value of 5 seconds is a point for discussions here, maybe somehow elevate and differentiate it even more. It depends on.

7

The encode(char c) method itself lets you know if it has a fix. My code looks something like this:

            // parse GPS data
            isValidGPS = false;
            if (gps.encode(c))
                isValidGPS = true;
8

Just a side note:

If you can use an expression in an if() statement, you can just use it for a boolean assignment too.

isValdiGPS = gps.encode(c);  // the worst case add a cast like (boolaen)gps.encode(c)

But since this is the declaration of encode()

bool encode(char c); // process one character received from GPS

casting won’t be required

9

There’s an easy to digest write up on getting a fix and what the fix age means here: TinyGPS