This post describes how I setup a basic ADS-B receiver on a Raspberry PI Zero W using a USB RTL2832U based Software Defined Radio so that you can see any ADS-B equipped aircraft in your local area and present it on your own website.
Lets Encrypt is a new Certificate Authority (CA), run for the public’s benefit by the Internet Security Research Group (ISRG). At the time of writing it’s currently in Beta and is due to go public in December 2015.
Update: Lets Encrypt went into public -beta on December 3 2015. I have updated this article with the minor change needed to work with the live servers.
Now in the default mode, the standard Lets Encrypt client (it’s not the only one) can manage this automatically – however it’s not ideal if you have more than one server.
What I describe here is how to centralize managing certificate registration (& later renewal) on a central machine. When a certificate is then registered or renewed we can then copy the certs to the remote servers.
One of the tasks I want to use a Raspberry PI for is to take over the duties of an existing ITX based linux box running my weather station. Now in theory that should be pretty simple as the current setup uses pywws to connect to the station and as that’s written in python it should work.
Now the Raspberry PI has no onboard Real time clock – which means it needs to use an NTP server to get the time when it starts. Usually you would use the default settings and allow the PI to connect to thenet for it’s time. Now this is fine if you have a working net connection but what if you are not connected to the net? You might be in the field running the PI on batteries.
As the other projects I have lined up for it is to connect my Meade LX200GPS telescope to the local network or to work with my (in prototype) radio telescopes so having an accurate clock is going to be required.
Now the obvious solution here is to use GPS as a time source. GPS works by having a constellation of satellites in orbit and each one carries a highly accurate atomic clock & broadcast both their current position and the time. A GPS receiver then receives these signals and, as long as it has enough satellites and workout where you are by comparing the times from those clocks.
So this article shows how to use A GPS receiver with the Rasperry PI – although these instructions are not specific to the PI.
For this experiment I’m using a USB GPS receiver from Maplin – product code A73KF. I bought this several months ago when they had it on special offer for £19.99 – it usually retails for £29.99.
Now it comes with a CD for Windows machines but we don’t need it – as the majority of GPS receivers I know of use serial & this is no exception. When plugged in it appears as a serial port.
Plug it in and run lsusb
pi@raspberrypi:~$ sudo lsusb Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. Bus 001 Device 004: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp. Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
There the Prolific Technology entry is the GPS appearing as a serial port. If you look in /var/log/syslog you will also notice it will have created the port as /dev/ttyUSB0 as it’s the first serial port.
Using the PI as a GPS Receiver
Now the next step is to get the pi receiving data from the satellites. Now there is a suite of tools available for Linux called gpsd which we’ll install:
pi@raspberrypi:~$ sudo apt-get install gpsd gpsd-clients python-gps
Next we need to start the daemon:
pi@raspberrypi:~$ sudo gpsd /dev/ttyUSB0 -F /var/run/gpsd.sock
Ignore any messages from the console or in the log files, you may see it complaining about IPv6 but you can ignore that.
Viewing whats in the sky & your location
Now GPS doesn’t work indoors – as it needs a clear view of the sky so for this I’ve placed the PI on the window sill. Next I ssh into the pi and run cgps.
pi@raspberrypi:~$ cgps -s
The -s flag is there to tell the command not to write raw data to the screen as well as the processed data.
You should then get the following output:
┌───────────────────────────────────────────┐┌─────────────────────────────────┐ │ Time: 2012-06-18T15:05:10.0Z ││PRN: Elev: Azim: SNR: Used: │ │ Latitude: 51.231848 N ││ 14 43 249 40 Y │ │ Longitude: 0.514014 E ││ 25 75 283 37 Y │ │ Altitude: 132.3 m ││ 2 26 085 31 Y │ │ Speed: 0.0 kph ││ 12 56 070 18 Y │ │ Heading: 0.0 deg (true) ││ 9 19 133 22 Y │ │ Climb: 0.0 m/min ││ 27 09 133 17 Y │ │ Status: 3D FIX (1 secs) ││ 4 17 045 31 Y │ │ GPS Type: ││ 32 05 321 20 Y │ │ Longitude Err: +/- 8 m ││ 29 41 192 18 Y │ │ Latitude Err: +/- 9 m ││ 31 28 304 42 Y │ │ Altitude Err: +/- 27 m ││ │ │ Course Err: n/a ││ │ │ Speed Err: +/- 68 kph ││ │ │ ││ │ │ ││ │ │ ││ │ │ ││ │ │ ││ │ └───────────────────────────────────────────┘└─────────────────────────────────┘
Here you can see it’s receiving from 10 satellites and it has the time and your location. The 3D FIX section tells you it has enough data for a 3D fix on your location (i.e. altitude). The Err lines tell you the error in your position. If you leave it running you should see the Err values change every second or so.
Viewing GPS under X-Windows
Now above I showed how the GPS looks from an SSH connection but you can get a graphical display as well using the xgps client thats also been installed. Now if you have a monitor connected to the pi simply open a terminal and run xgps. However as I’ve not got a monitor against the window I’ve used ssh to connect to it from another machine. To get this to work you need to add -Y to the ssh command.
peter@somehost:~ $ ssh -Y pi@raspberrypi pi@raspberrypi:~$ xgps
You should now get a window like the following open on your local machine – don’t worry if it takes a little while, it might take a second or two:
Setting the computer time using GPS
Now we have a working GPS we can now get the PI to use it for setting the time. To do this we need to configure ntp to use the GPS satellites as a time source. Now you should already have ntp installed but if not then you need to install it:
pi@raspberrypi:~$ sudo apt-get install ntp
Next you need to edit the file: /etc/ntp.conf and add a few lines to it defining the GPS. This can be either before or after the existing lines beginning with server:
# gps ntp server 127.127.28.0 minpoll 4 fudge 127.127.28.0 time1 0.183 refid NMEA server 127.127.28.1 minpoll 4 prefer fudge 127.127.28.1 refid PPS
Now restart ntp:
pi@raspberrypi:~$ sudo service ntp restart
Now if you query the server you should after a while see it synchronize:
pi@raspberrypi:~$ ntpq -p remote refid st t when poll reach delay offset jitter ============================================================================== *ns1.luns.net.uk 220.127.116.11 2 u 54 64 7 65.454 2.666 5.800 +resntp-b-vip.lo 127.151.91.34 3 u 45 64 17 55.704 -5.169 8.482 +bart.nexellent. 18.104.22.168 2 u 17 64 17 76.585 -4.271 57.595 +v01.s01.be.it2g 22.214.171.124 2 u 20 64 37 86.464 -2.374 228.460 xSHM(0) .NMEA. 0 l 11 16 377 0.000 144.714 3.026 SHM(1) .PPS. 0 l - 16 0 0.000 0.000 0.000
A couple of notes:
You might find that ntp doesn’t connect to the gps at first. It appears that it starts gpsd up without the link to the serial port. What I find I have to do is:
pi@raspberrypi:~$ sudo killall gpsd pi@raspberrypi:~$ sudo gpsd /dev/ttyUSB0 -F /var/run/gpsd.sock pi@raspberrypi:~$ sudo service ntp restart
Once I’ve done this then after it gets a fix then it starts working. Sometimes running cgps and waiting for it to get a fix also fixes this.
I don’t know why this happens but it’s an issue I’ve yet to solve.
By default the sshd service is not installed in the debian distrubution used on the Raspberry PI. For most uses this isn’t really necessary but if you are intending to use the pi without a screen then you need some way to get into it over the network.
All you need to do is to install ssh & tell debian to run it on boot:
sudo apt-get install ssh sudo update-rc.d ssh defaults
Now, as long as you know the IP address (or you give it a static one) then you can now ssh into it.
Finally after waiting for months my Raspberry PI finally arrived:
When you open the box & get it out you then realize how tiny the thing is:
Now when I ordered this I also ordered a PSU (normal micro-USB) and a 4Gb SD card. The other required items like HDMI & USB keyboard/mouse I already had, so the next thing was to download Debian & install it on the sd card.
I followed the instructions on elinux.org as I couldn’t get RasPiWrite to run but then it was a simple task. It will take a while to copy to the card.
Once done plug everything in and start it up. At first you’ll see it boot & it’s pretty quick. Once you get the login prompt login and type startx. You’ll then get the desktop.
There’s a simple browser provided which works to a point – some things don’t work but it’s early days.
There’s only one thing that’s not right is that the screen doesn’t fit the monitor – but then I blame the monitor than the PI – it’s a cheap TV/DVD from Asda which I know has an odd resolution (1400×900) & the PI is detecting it with a higher resolution, but other than that, initial use shows it’s working pretty well.