Mavros: "No satellites" from "/diagnostics" mavros

I placed my drone in the open area and led of pixhawk is green.
After I run this code
roslaunch mavros px4.launch
and then I run
rostopic echo -n1 /diagnostics
I found that

name: "mavros: GPS"
    message: "No satellites"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Satellites visible"
        value: "0"

But after I connect qgroundcontrol and exit program so I run mavros and check the "/diagnostics" again, I found the GPS show.

level: 0
    name: "mavros: GPS"
    message: "3D fix"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Satellites visible"
        value: "16"
      - 
        key: "Fix type"
        value: "3"

I want to get the value from GPS without having to connect QGC. How can I do this?

MAVROS version and platform

Mavros: 0.25.1
ROS: Kinetic
Ubuntu: 16.04

Autopilot type and version

PX4

Version: lastest

Diagnostics

header: 
  seq: 327
  stamp: 
    secs: 1527908144
    nsecs:  63593259
  frame_id: ''
status: 
  - 
    level: 0
    name: "mavros: FCU connection"
    message: "connected"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Received packets:"
        value: "5340"
      - 
        key: "Dropped packets:"
        value: "0"
      - 
        key: "Buffer overruns:"
        value: "0"
      - 
        key: "Parse errors:"
        value: "0"
      - 
        key: "Rx sequence number:"
        value: "240"
      - 
        key: "Tx sequence number:"
        value: "141"
      - 
        key: "Rx total bytes:"
        value: "132922"
      - 
        key: "Tx total bytes:"
        value: "133365"
      - 
        key: "Rx speed:"
        value: "305.000000"
      - 
        key: "Tx speed:"
        value: "320.000000"
  - 
    level: 2
    name: "mavros: GPS"
    message: "No satellites"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Satellites visible"
        value: "0"
      - 
        key: "Fix type"
        value: "0"
      - 
        key: "EPH (m)"
        value: "Unknown"
      - 
        key: "EPV (m)"
        value: "Unknown"
  - 
    level: 0
    name: "mavros: Heartbeat"
    message: "Normal"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Heartbeats since startup"
        value: "415"
      - 
        key: "Frequency (Hz)"
        value: "0.960080"
      - 
        key: "Vehicle type"
        value: "Quadrotor"
      - 
        key: "Autopilot type"
        value: "ArduPilotMega / ArduCopter"
      - 
        key: "Mode"
        value: "STABILIZE"
      - 
        key: "System status"
        value: "Standby"
  - 
    level: 0
    name: "mavros: System"
    message: "Normal"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Sensor present"
        value: "0x00000000"
      - 
        key: "Sensor enabled"
        value: "0x00000000"
      - 
        key: "Sensor helth"
        value: "0x00000000"
      - 
        key: "CPU Load (%)"
        value: "0.0"
      - 
        key: "Drop rate (%)"
        value: "0.0"
      - 
        key: "Errors comm"
        value: "0"
      - 
        key: "Errors count #1"
        value: "0"
      - 
        key: "Errors count #2"
        value: "0"
      - 
        key: "Errors count #3"
        value: "0"
      - 
        key: "Errors count #4"
        value: "0"
  - 
    level: 2
    name: "mavros: Battery"
    message: "No data"
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Voltage"
        value: "-1.00"
      - 
        key: "Current"
        value: "0.0"
      - 
        key: "Remaining"
        value: "0.0"
  - 
    level: 2
    name: "mavros: Time Sync"
    message: "No events recorded."
    hardware_id: "/dev/ttyUSB0:57600"
    values: 
      - 
        key: "Timesyncs since startup"
        value: "0"
      - 
        key: "Frequency (Hz)"
        value: "0.000000"
      - 
        key: "Last dt (ms)"
        value: "3.333722"
      - 
        key: "Mean dt (ms)"
        value: "0.000000"
      - 
        key: "Last system time (s)"
        value: "916.671332000"
      - 
        key: "Time offset (s)"
        value: "1527907227.282750130"

Check ID

OK. I got messages from 1:1.

---
Received 165 messages, from 1 addresses
sys:comp   list of messages
  1:1     0, 111