GPS Module

An MQTT GPS module, what good could that be in a home automation system? The home isn't moving, of course. GPS tells us where something is right? That isn't how GPS works.

GPS satellites just broadcast the time, and where the other satellites are, and that is it. The GPS receiver calculates the difference in time signals that it receives from the various satellites in view. The GPS receiver can determine a very accurate time for where it is at, along with the altitude, course, speed and position. 

Now the house isn't moving, so the course and speed will be somewhat irrelevant. The altitude can be used to correct a barometer. The time is very useful. 

Many simple inexpensive GPS receivers are available I chose the ublox 7m receiver that I had left over from a quad copter. This module has the antenna built in, along with a compass that I am not using. It is about the same size as the Wemos D1 modules. I stacked them using LED leads. to connect the two data lines to the D1. I needed to cross the power and ground, since those pins are fixed, and matter what order. The data pins can be specified in software. 

Writing another MQTT module was pretty simple. 

I have a JSON message that looks like:

{
  "sensor": "gps",
  "time": [
    2020,
    9,
    7,
    23,
    59,
    57,
    0
  ],
  "Latitude": 44.83777,
  "Longitude": -93.46817,
  "Satellites": 10,
  "HDOP": 84,
  "Altitude": 284.4,
  "Course": 331.19,
  "Speed": 0.38892
}

The time fields are year, month, date, hour, minute, second and milliseconds. This format allows using any parts that are desirable. 

Node red supports a map node. The example map node displays flights from flight radar 24. 

Here is the code:

/*     
 * The code is copyright 2018, Tom Brusehaver 
 * It can be copied using the OSF v3 license. Do what you want with this, but share.
 * 
 * This uses a ublox 7m GPS device at 9600-baud serial hooked up on pins 2(rx) and 0(tx).
 *
 * The board selected in the IDE is WeMos D1 R2 & Mini. Standard everything 
 * 
 */
 
#include <SoftwareSerial.h>

#include <TinyGPS.h>

#include <ESP8266WiFi.h>m
#include <PubSubClient.h>
#include <PubSubClientTools.h>

#include <ArduinoJson.h>

TinyGPS gps;
SoftwareSerial ss(2, 0);

const char* ssid = "raspi-webgui";
const char* password = "xxxxxxxxx";
const char* MQTT_SERVER = "10.3.141.1";

static void smartdelay(unsigned long ms);
static void print_float(float val, float invalid, int len, int prec);
static void print_int(unsigned long val, unsigned long invalid, int len);
static void print_date(TinyGPS &gps);
static void print_str(const char *str, int len);

WiFiClient espClient;
PubSubClient client(MQTT_SERVER, 1883, espClient);
PubSubClientTools mqtt(client);



void setup()
{
  Serial.begin(115200);
  
  ss.begin(9600);

  setup_wifi();
  String s = "";
    // Connect to MQTT
  Serial.print(s+"Connecting to MQTT: "+MQTT_SERVER+" ... ");
  if (client.connect("GPSClient")) {
    Serial.println("connected");
  } else {
    Serial.println(s+"failed, rc="+client.state());
  }
}


void setup_wifi() 
{
  delay(10);
  // We start by connecting to a WiFi network
  Serial.println();
  Serial.print("Connecting to ");
  Serial.println(ssid);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());
}

//////////////////////////////////////////////////////////////////////////
//////////  MQTT Stuff ///////////////////////////////////////////////////

void reconnect()
{
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Attempt to connect
    if (client.connect("GPSClient")) {
      Serial.println("connected");
      // Once connected, publish an announcement...
      client.publish("outTopic", "GPS connected");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void publisher(char *gpsMSG) {
  StaticJsonDocument<300> doc;
  int year;
  byte month, day, hour, minute, second, hundredths;
  unsigned long age;
  float flat, flon;
  //unsigned long age, date, time, chars = 0;
  gps.crack_datetime(&year, &month, &day, &hour, &minute, &second, &hundredths, &age);
  
  doc["sensor"] = "gps";
  JsonArray time = doc.createNestedArray("time");
  time.add(year);
  time.add(month);
  time.add(day);
  time.add(hour);
  time.add(minute);
  time.add(second); 
  time.add(hundredths);

  gps.f_get_position(&flat, &flon, &age);
  doc["Latitude"] = flat;
  doc["Longitude"] = flon;
  doc["Satellites"] = gps.satellites();
  doc["HDOP"] = gps.hdop();
  doc["Altitude"] = gps.f_altitude();
  doc["Course"] = gps.f_course();
  doc["Speed"] = gps.f_speed_kmph();

  // Generate the minified JSON and send it to the Serial port.
  //
  //serializeJson(doc, Serial);
  // The above line prints

  // Start a new line
  Serial.println();

  char buffer[256];
  serializeJson(doc, buffer);
  mqtt.publish("gps/location", buffer);
}

void loop()
{
  float flat, flon;
  unsigned long age, date, time, chars = 0;
  unsigned short sentences = 0, failed = 0;
  static const double LONDON_LAT = 51.508131, LONDON_LON = -0.128002;
  char gpsMsg[300];
  
  gps.stats(&chars, &sentences, &failed);

  publisher(gpsMsg);
  client.loop();
  smartdelay(1000);
}

static void smartdelay(unsigned long ms)
{
  unsigned long start = millis();
  do 
  {
    while (ss.available())
      gps.encode(ss.read());
  } while (millis() - start < ms);
}

static void print_str(const char *str, int len)
{
  int slen = strlen(str);
  for (int i=0; i<len; ++i)
    Serial.print(i<slen ? str[i] : ' ');
  smartdelay(0);
}