Getting GPS to Work on SIM5320A 3G/GPS Module - gps

Question

I have a Adafruit FONA 3G/GPS module (American Version). I have gotten the cellular functionality to work but I am struggling with GPS. I have tried both passive and active antennas.
This is a list of AT commands that are available to the SIM5320A module (pdf). The Adafruit example code uses AT commands that don't run on this SIM module. I send the following sequence:
AT+CGPS=1,1
AT+CGPSINFO
and I am receiving:
+CGPSINFO: ,,,,,,,,
AmpI/AmpQ: 4xx/4xx
What exactly is going wrong? I am connected but am not getting any data out. Also, I understand the mathematical significance of AmpI/AmpQ but what does that mean in terms of connection to GPS network?

Answer 1

First, if you're using the FONATest example from Adafruit, you need to look around line 48. Comment out the line that declares a variable of type Adafruit_FONA and uncomment the line that declares a variable of type Adafruit_FONA_3G.
Also, if you haven't run the FONA_3G setbaud program to set the baud rate to something other than 115,200, you need to run it. (This isn't made very clear by Adafruit, and I thought my board was bad before I found that information.)
Second, unless you bridge the pads labeled bias on the board, you will have to use a passive GPS antenna. The SIMCOM module's GPS is pretty sensitive though. I had a uFL-to-SMA adapter plugged into the board, and stuck a 5-inch jumper wire into the SMA's center pin. The GPS acquired a position within a minute!
I tried the following command: AT+CGPS=1 and then used AT+CGPS? to query the GPS status. It reported +CGPS: 1,1 back to me.
The first few attempts at sending AT+CGPSINFO gave me the same info that you got, i.e. no position.
However, after a few minutes with a wire (or passive antenna), AT+CGPSINFO started returning valid time and position information.
Hope this helps.
UPDATE:
You've asked for code, and I'm happy to oblige. This code is designed to just be a terminal-like relay for serial data between the Arduino (using Tools -> Serial Monitor) and the Fona. So the commands I described above, I just entered by hand. Still, some folks may find it useful.
#include <Adafruit_FONA.h>
#include <SoftwareSerial.h>
#define RX_FROM_FONA 2
#define TX_TO_FONA 3
#define FONA_RST 4
#define FONA_PWR_KEY 5
#define FONA_PWR_STATUS 7
Adafruit_FONA fona = Adafruit_FONA(FONA_RST);
SoftwareSerial fonaSerial = SoftwareSerial(TX_TO_FONA, RX_FROM_FONA);
void setup() {
pinMode(FONA_RST, OUTPUT);
pinMode(FONA_PWR_KEY, OUTPUT);
pinMode(FONA_PWR_STATUS, INPUT_PULLUP);
pinMode(LED_BUILTIN, OUTPUT);
digitalWrite(FONA_RST, HIGH);
digitalWrite(FONA_PWR_KEY, HIGH);
resetFona();
Serial.begin(115200);
fonaSerial.begin(4800);
}
void resetFona() {
digitalWrite(LED_BUILTIN, HIGH);
digitalWrite(FONA_RST, LOW);
delay(200);
digitalWrite(LED_BUILTIN, LOW);
digitalWrite(FONA_RST, HIGH);
delay(1000);
digitalWrite(LED_BUILTIN, HIGH);
digitalWrite(FONA_PWR_KEY, LOW);
delay(5000);
digitalWrite(LED_BUILTIN, LOW);
digitalWrite(FONA_PWR_KEY, HIGH);
delay(100);
}
char buffer[255];
int crIndex = 0;
int bufferIndex = 0;
int foundCR = 0;
void loop() {
if (Serial.available()) {
buffer[bufferIndex++] = Serial.read();
if (buffer[bufferIndex - 1] == 13) {
crIndex = bufferIndex - 1;
foundCR = 1;
}
}
if (bufferIndex > 255) { bufferIndex = 0; }
while (fonaSerial.available()) { Serial.write(fonaSerial.read()); }
if (foundCR > 0) {
for (int i = 0; i <= crIndex; i++) {
fonaSerial.write(buffer[i]);
}
foundCR = 0;
bufferIndex = 0;
crIndex = 0;
}
}

Answer 2

I don't think this is a software problem, I think you have a hardware problem.
I had this same problem. The sensitivity of the SIM5320A is pretty good, but Adafruit hasn't really followed their design guide in implementing the carrier board, and this hurts its sensitivity.
For example, you can read the application notes for the board here. Check out the PDFs on "SMT Module Design" and "Hardware Design."
First thing you'll notice is SIMCOM recommends a LC tuning network to tune your particular antenna to 50-ohm impedance.
Here is the SIMCOM recommended diagram:
And here is the Adafruit design, with no tuning network anywhere to be found:
Now, if you're going to be using this in your own PCB later, you of course are free to make improvements here yourself.
To test your design, you're going to want to know more about what the module is saying, otherwise you're flying blind. You can read the GPS NMEA sequences and decipher them, but there's a lot to it. Another way is to use the GPS demo tool that SIMCOM support offers. To download this, go to:
ftp://simcom.exavault.com/SIM5320A/GPS tool
username=myd
psd=simcommyd
It will show you how many satellites you're connected to, what signal strength they have, etc. You are going to need at least 3 satellites for a lock (technically you should have 4, but some modules will toss out an impossible triangulation solution with just 3, not sure which the SIM5320A has). You want each satellite to have strengths in the 30+ range.
Here is my Before screenshot, you can see I only had one satellite and hence no lock. This was after giving half an hour after cold start to download the almanac and ephemeris tables (will take about 15min on average from cold start). Satellite 21 (with signal of also 21 strength, coincidentally).
What I did to fix my signal gain was add an LNA (Low Noise Amplifier) in series with my passive antenna. I used the BGA524N6BOARDTOBO1 breakout board, which you can get at DigiKey/Mouser/others, and adds 19dB of gain, and an incredibly-low 0.55dB noise figure.
After hooking up my antenna, and waiting some time, I got a lock!
Hope this helps you and others to use this otherwise great module.