GPS position on an Arduino Uno

// GPS position: a NEO-6M on an Uno, the way the tutorials wire it.
//
//   NEO-6M TX  -> pin 4   (the sketch's RX)
//   NEO-6M RX  <- pin 3   (the sketch's TX; the module ignores it here)
//   NEO-6M VCC -> 5V, GND -> GND
//
// The module sends NMEA sentences at 9600 baud, once a second. TinyGPS++
// reads them one character at a time, and the sketch prints the position
// each time a new one arrives.
//
// The serial monitor runs at 115200, as in the TinyGPS++ examples. Mokxi's
// Serial has no send buffer: it waits for each byte to leave. At 9600 a
// printed line takes about 70 ms, the GPS keeps talking meanwhile, and
// SoftwareSerial's 64-byte buffer overflows: the sentence with the satellite
// count and the altitude is the one that gets lost. A real Uno buffers its
// Serial output, so a short line costs far less there; a long print or a long
// delay() can still make it miss bytes.
//
// There is no sky here. Mokxi does not model satellites or a radio. The
// position is a property of the module: click it and change lat and lon in
// the properties panel to move it. Click the module on the canvas to lose the
// fix, and click again to get it back.

#include <SoftwareSerial.h>
#include <TinyGPS++.h>

static const int RXPin = 4, TXPin = 3;
static const uint32_t GPSBaud = 9600;

TinyGPSPlus gps;
SoftwareSerial gpsSerial(RXPin, TXPin);

void setup() {
  Serial.begin(115200);
  gpsSerial.begin(GPSBaud);
  Serial.println("Waiting for the GPS...");
}

void twoDigits(int value) {
  if (value < 10) Serial.print('0');
  Serial.print(value);
}

void showPosition() {
  Serial.print("Lat: ");
  Serial.print(gps.location.lat(), 6);
  Serial.print("  Lng: ");
  Serial.print(gps.location.lng(), 6);
  Serial.print("  Sats: ");
  Serial.print(gps.satellites.value());
  Serial.print("  Alt: ");
  Serial.print(gps.altitude.meters(), 1);
  Serial.print(" m  UTC ");
  twoDigits(gps.time.hour());
  Serial.print(':');
  twoDigits(gps.time.minute());
  Serial.print(':');
  twoDigits(gps.time.second());
  Serial.println();
}

// Every character the module sent since last time, into the parser.
void readGps() {
  while (gpsSerial.available() > 0) {
    gps.encode(gpsSerial.read());
  }
}

unsigned long lastLine = 0;
bool warned = false;

void loop() {
  readGps();
  // The GGA sentence brings the position, the satellites and the altitude
  // together, once a second. (RMC updates the position too, a moment
  // earlier, but carries no altitude.)
  if (gps.location.isUpdated() && gps.altitude.isUpdated()) {
    showPosition();
    lastLine = millis();
  }
  // Sentences but no position: the module has no fix.
  if (gps.charsProcessed() > 10 && millis() - lastLine > 3000) {
    Serial.println("No fix yet: waiting for satellites.");
    lastLine = millis();
  }
  // Nothing at all after five seconds is a wiring fault, not a sky fault.
  if (!warned && millis() > 5000 && gps.charsProcessed() < 10) {
    Serial.println("No data from the GPS: check TX goes to pin 4.");
    warned = true;
  }
}