code

void readMessage(int messageId, char *payload)
{
    double lat_val, lng_val;
    smartDelay(1000); /* Generate precise delay of 1ms */    
    lat_val = gps.location.lat(); /* Get latitude data */
    lng_val = gps.location.lng(); /* Get longtitude data */
    StaticJsonBuffer<MESSAGE_MAX_LEN> jsonBuffer;
    JsonObject &root = jsonBuffer.createObject();
    root["deviceId"] = DEVICE_ID;
    root["messageId"] = messageId;    

    // NAN is not the valid json, change it to NULL
    if (std::isnan(lat_val))
    {
        root["latitude"] = NULL;
    }
    else
    {
        root["latitude"] = lat_val;

    }

    if (std::isnan(lng_val))
    {
        root["longitude"] = NULL;
    }
    else
    {
        root["longitude"] = lng_val;
    }
    root.printTo(payload, MESSAGE_MAX_LEN);
}