My ADSB Receiver Box – GPS Precision Time – Upgrade 1

Here is a first upgrade for my ADSB receiver box – GPS precision time. The MLAT position calculation for aircrafts depends on a precise time. While there is already a time synchronization between MLAT receivers in a specific area it is also possible to add a precise time source to improve the calculation result. A GPS receiver is such source when equipped with a PPS (pulse per second) output. The PPS is a signal with a width of less than one second and a sharply rising or abruptly falling edge that accurately repeats once per second with low jitter. The GPS time and PPS signal  are then used to sync the internal ntp time service and keep a low offset and jitter.

Adafruit GPS Breakout v3 connected to Raspberry Pi

My choice for GPS receiver was the Adafruit GPS v3 breakout board as it was a spare in my boxes. Beside the small size and high channel GPS receiver it has a PPS output and operates at 3.3V-5V input, but 3.3V TTL logic output, perfect for the Raspberry Pi. The additional backup battery come in handy in case of a power cut.

 

GPS board connection to Raspberry Pi 2/3

Adafruit GPS v3 pin Raspberry Pi I/O header pin
VIN Pin 2 = 5V
GND Pin 14 = GND
TX Pin 10 = GPIO 15
RX Pin 8 = GPIO 14
PPS Pin 12 = GPIO 18

 

Enable PPS input and set UART baudrate

To enable the PPS input and setting the correct baudrate for the GPS the Raspberry Pi config file /boot/config.txt must be changed. Add at the end:

# GPS PPS input
dtoverlay=pps-gpio,gpiopin=18
init_uart_baud = 9600

 

Disable serial console on UART0

The GPS is connected to the UART0 serial interface (ttyAMA0) that is usually occupied by the Raspberry Pi serial console. That needs to be disbaled. So we edit /boot/cmdline.txt and change console=ttyAMA0 to console=tty1.

Also the getty instance on ttyAMA0 needs to be disabled:

$ sudo systemctl disable serial-getty@ttyAMA0.service

 

Install GPS service

This service is required to access the GPS and it provides a shared memory the for the ntp service with its own time informations.

$ sudo apt-get install gpsd gpsd-clients python-gps

To successfully run the GPS service, its configuration needs to be changed to access the UART0. So edit /etc/default/gpsd as follows.

# Default settings for the gpsd init script and the hotplug wrapper.
# Start the gpsd daemon automatically at boot time
START_DAEMON="true"
# Use USB hotplugging to add new USB devices automatically to the daemon
USBAUTO="true"
# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
DEVICES="/dev/ttyAMA0"
# Other options you want to pass to gpsd
GPSD_OPTIONS="-n"

Now we reboot and check that PPS and GPS are working.

$ dmesg | grep pps

[    5.957377] pps_core: LinuxPPS API ver. 1 registered
[    5.957515] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[    5.973778] pps pps0: new PPS source pps.-1
[    5.973907] pps pps0: Registered IRQ 184 as PPS source

Installing pps-tools will provide some additional informations. This is a mandatory step in order to get ntp service with PPS working. Compiling ntp has a dependency for timepps.h that comes with pps-tools.

$ sudo apt-get install pps-tools
$ sudo ppstest /dev/pps0

trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1482268186.000006148, sequence: 4647 - clear  0.000000000, sequence: 0>
source 0 - assert 1482268187.000003768, sequence: 4648 - clear  0.000000000, sequence: 0
source 0 - assert 1482268188.000003324, sequence: 4649 - clear  0.000000000, sequence: 0

Checking that GPS service is running:

$ sudo service gpsd status

  gpsd.service - GPS (Global Positioning System) Daemon
  Loaded: loaded (/lib/systemd/system/gpsd.service; static)
  Active: active (running) since Tue 2016-12-20 19:45:08 UTC; 1h 26min ago
  Main PID: 745 (gpsd)
   CGroup: /system.slice/gpsd.service
           └─745 /usr/sbin/gpsd -N -n /dev/ttyAMA0

Dec 20 19:45:08 raspberrypi systemd[1]: Started GPS (Global Positioning System) Daemon.

Also $ cgps -s should output something like:

x    Time:       2016-12-20T21:14:35.000Z   xxPRN:   Elev:  Azim:  SNR:  Used: x
x    Latitude:    xx.xxxxxx N               xx  12    78    291    24      Y   x
x    Longitude:   xx.xxxxxx E               xx  24    67    133    00      Y   x
x    Altitude:   xxx.x m                    xx  25    36    261    29      Y   x
x    Speed:      1.1 kph                    xx  32    33    303    15      Y   x
x    Heading:    71.1 deg (true)            xx  19    32    050    17      Y   x
x    Climb:      0.0 m/min                  xx   6    21    087    00      N   x
x    Status:     3D FIX (5 secs)            xx  17    18    042    00      N   x
x    Longitude Err:   +/- 16 m              xx   2    15    122    00      N   x
x    Latitude Err:    +/- 27 m              xx  14    14    322    21      N   x
x    Altitude Err:    +/- 19 m              xx  15    10    182    19      N   x
x    Course Err:      n/a                   xx  29    04    203    00      N   x
x    Speed Err:       +/- 199 kph           xx  10    01    268    00      N   x
x    Time offset:     0.506                 xx 136    00    000    00      N   x
x    Grid Square:     xxxxxx                xx                                 x

One thought on “My ADSB Receiver Box – GPS Precision Time – Upgrade 1

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