CoSQM - a color hack to the SQM-LE for Light At Night Sensing

Software package for CoSQM v2

Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

1.  Introduction

The aim of this project is to develop a system capable of remotely controlling a light pollution monitoring system consisting in a 5 slot filters wheel aiming to add multispectral detection capabilities to the well known Sky Quality Meter (SQM-LE). We chose the SQM as the light sensor to allow a full backward compatibility to the impressive historical SQM database. Four of the slots are filled with color filers (Red, Green, Blue, Yellow). The fift slot is empty and therefore allow the standard SQM measurement. The SQM-LE is communicating with a raspberry pi 4b (RPI).

The linux distribution used for this system is Raspberry Pi OS (32-bit). We chose a development philosophy based on a complete control from the Linux command line. The system can be controlled simply with a low speed remote connection provided by a SSH client installed on the user workstation. The RPI connected to the system act as a server. Also to promote the implementation of measuring network, the data acquired by the CoSQM is stored on a web server installed on the RPI. Thanks to that web server, the data from multiple CoSQM units all around the world can be downloaded by a central server. Locally, the web server can be accessed with any portable device through a proprietary wifi network. SSH can also be accessed through this wifi network.

1.1  How to access the CoSQM

From internet:

ssh -p 2022 sand@router_public_ip


inside a private network

ssh sand@servers_local_ip

From the internet, port 2022 is used instead of standard port 22 to reduce risk of attacks by hackers. A password and user name is required. The standard username is sand. The system administrator should be able to provide you the password for sand. On the local network, we typically reserve the IP for the raspberry pi while using or for the SQM-LE.

2.  Installing and configuring subsystems

2.1  Basic system installations and dependancies

  • Download and install the Raspberry Pi Imager ( (v1.4 as for this guide)
  • Open the Pi Imager and choose the Raspbian OS and your micro-SD card and push on \"WRITE\" button.
  • Connect the GPS dongle, the backup flash drive, the keyboard, the mouse and the micro HDMI cable.
  • Put the microSD card on the RPI and power it on (connect a USB-C cable with min 4 A.).
  • Set your country, time zone etc to whatever you want.
  • Skip the wifi config step
  • Update software
  • Restart the computer
  • Set the correct time, time zone and date
    • time zone have to be GMT
    sudo dpkg-reconfigure tzdata
    • Select None of the above
    • Select GMT
    • If required, set time and date (this should be required)
    sudo date -s '2017-02-03 01:35:55'
    • Adapt the date and time to your situation.
  • Create the sand account
sudo useradd sand
sudo passwd sand
sudo mkdir /home/sand
sudo cp /home/pi/.bashrc /home/sand/
sudo chown -R sand /home/sand
sudo chgrp -R sand /home/sand
sudo sh -c 'echo "sand  ALL=(ALL:ALL) ALL" >> /etc/sudoers'
  • Use the raspberry configuration tool to desactivate the Auto login
sudo raspi-config
  • Select 1 System Options
  • Select S5 Boot / Auto Login
  • Select B3 Desktop GUI, requiring user to login
  • Select Finish
  • Accept to reboot the raspberry
  • Login as sand
  • Open a terminal window
  • Install the following packages
sudo apt update && sudo apt-get upgrade
sudo apt install gfortran
sudo ln -s `which gfortran` /usr/local/bin/g77
sudo apt install vim
sudo apt install vim-common
sudo apt install apache2
sudo apt install gpsd
sudo apt install gpsd-clients
sudo apt install lm-sensors
sudo apt install imagemagick
sudo apt install ntp
sudo apt install lm-sensors
sudo apt install openssh-client
sudo apt install openssh-server
sudo apt install imagemagick
sudo apt install netpbm
sudo apt install git
sudo apt install python-serial
sudo apt install python-dev python-rpi.gpio
sudo apt install python3-gpiozero
  • Create the data backup directory
sudo mkdir /home/sand/backup

2.2  Mount the usb stick on /home/sand/backup

The usb stick will be the support to backup all the data. The backup will be done at 9 AM UT. That will be managed by the crontab.

  • Create the data backup directory
mkdir /home/sand/backup
  • Plug the usb stick in one of the RPI usb port. Type the following command:

You should see the usb drive on the sda1 drive

  • Edit fstab to mount the usb stick at /home/sand/backup
umount /dev/sda1
sudo su
echo "/dev/sda1 /home/sand/backup vfat rw,user,exec,umask=000 0 0" >> /etc/fstab
sudo reboot


  • We assume that the file system on your usb stick is FAT. Please be sure to format the usb stick accordingly before the installation.
  • Be sure to replace /dev/sda1 with your actual device.

2.3  SSH

SSH server allows any remote SSH client to connect to the server to remotely control system programs from the command line or to download (upstream / downstream) data.

  • We must first be sure that the SSH server start automatically when the RPI boots.
sudo raspi-config
  • Select 5 Interfacing Options
    • SSH
      • Yes (to enable ssh)
  • ok
  • Finish
  • Set up your personal firewall to allow the SSH and http connections to pass through the firewall.
sudo iptables -I INPUT 1 -p tcp --dport 22 -j ACCEPT
sudo iptables -I INPUT 1 -p tcp --dport 80 -j ACCEPT
sudo iptables-save

2.4  Activating the pi camera module

  • Use the raspberry configuration tool to activate the raspberry pi camera
sudo raspi-config
  • Select 5 Interfacing Options Configure connections to peripher
  • P1 Camera Enable/Disable connection to the
    • Yes (to enable camera interface)
  • ok
  • finish
  • reboot

Add sand user to the video group

sudo usermod -a -G video sand

2.5  Installing CoSQM applications

Latest available package release on github repository

  • To download CoSQM apps:
mkdir git
cd git
git clone
  • Copy apps to system directory:
sudo cp -f /home/sand/git/cosqm/* /usr/local/bin/
sudo chmod a+x /usr/local/bin/*
  • Copy local configuration file
cp -f /home/sand/git/cosqm/localconfig /home/sand

Automatic startup of observe-sqm-stepper.bash code

cd /home/sand/git/cosqm
chmod a+x *
sudo cp * /usr/local/bin
sudo cp observe.service /etc/systemd/system
sudo systemctl enable observe.service
sudo systemctl start observe.service

2.6  Apache server

We are using the Apache2 web server. This server allows browsing the database and look at the log file.

To activate the web server, we must:

  • activate userdir module
sudo a2enmod userdir
sudo service apache2 restart
  • Create the data directory and modify the permissions by doing:
sudo mkdir /var/www/html/data
sudo chmod a+rx /var/www/html/data
  • Edit the apache2 configuration file
sudo vim /etc/apache2/apache2.conf
  • Then type < i > to enter insert mode and add the following lines to the file
# added for cosqm
<Directory /var/www/html/data>
Options Indexes FollowSymLinks MultiViews
AllowOverride None
Require all granted
  • Then exit and save by typing: <escape> :wq
  • Restart the webserver
sudo a2enmod userdir
sudo service apache2 restart

2.7  crontab

The crond daemon is scheduling repetitive tasks on linux systems. Users who have the right to use crond can schedule their own tasks to crond. We are using crond to schedule a camera picture acquisition every 15 min.

Add program Camera.bash to the root crond schedule

This scripts include an automatic integration time algorithm so that the sky must be visible no matter the level of radiance.

  • Edit crontab
sudo su
crontab -e
  • chose editor 3. /usr/bin/vim.basic
  • type < i > and add the following content:
*/15 * * * * /usr/local/bin/Camera.bash
00 12 * * * /sbin/shutdown -r now
00 09 * * * /usr/bin/rsync -a /var/www/html/data/* /home/sand/backup
  • Save and quit by typing: <escape> :wq

N.B. the time for daily shutdown above was set to local noon (which is 00 12 in Greenwich). Please adapt to your time zone so that it will be executed around noon local time.

  • Then logout from the root

2.8  USB GPS Stratux Vk-162 Remote Mount USB GPS (U-blox)

The GPS code in no more compatible with the new version of gpsd. So that we recommend not to use the GPS module until that bug will be corrected. The latitude/longitude of your site can simply be written ii the /home/sand/localconfig file

The GPS is controlled by the gpsd server. The GPS startup may easily take 45 seconds (time to connect to available satellites). Sometimes, the GPS does not succeed to connect to satellites. Most of the time this occur when satellites are masked by obstacles like mountains or buildings. The observe-sqm-stepper.bash is able to extract latitude and longitude from the GPS. To be sure that the gpsd server will work properly, do the following steps.

  • Edit the gpsd configuration
sudo vim /etc/default/gpsd
  • type < i > and set the following parameters
# Default settings for the gpsd init script and the hotplug wrapper.
# Start the gpsd daemon automatically at boot time
# Use USB hotplugging to add new USB devices automatically to the daemon
# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group
# Other options you want to pass to gpsd
GPSD_OPTIONS="-F /var/run/gpsd.sock -D 5 -N -n"
  • Save and quit by typing: <escape> :wq

2.9  Shutdown/reboot button

  • Enter the following commands in the terminal
cd /home/sand/git/cosqm
chmod a+x
sudo cp /usr/local/bin
sudo cp shutdown_button.service /etc/systemd/system
sudo systemctl enable shutdown_button.service
sudo systemctl start shutdown_button.service

2.10  Real time clock

We are using the DS3231 RTC module to keep the date and time after powering off the system. In CoSQMv2, the RTC module is included with the UPS hat.

  • Update the RPI
sudo apt-get update
sudo apt-get upgrade

Configure the I2C interface

sudo raspi-config

This command will bring up the configuration tool; this tool is an easy way to make a variety of changes to your Raspberry Pi’s configuration. Today, however, we will only by exploring how to enable the I2C interface.

Use the arrow keys to go down and select “5 Interfacing Options“. Once this option has been selected, you can press Enter.

  1. On the next screen, you will want to use the arrow keys to select “P5 I2C“, press Enter once highlighted to choose this option.
  2. You will now be asked if you want to enable the “ARM I2C Interface“, select Yes with your arrow keys and press Enter to proceed.
  3. Once the raspi-config tool makes the needed changes, the following text should appear on the screen: “The ARM I2C interface is enabled“.

We must restart the Raspberry Pi. To do this first get back to the terminal by pressing Enter and then ESC.

  • Type the following command into the terminal on your Raspberry Pi to restart it.
sudo reboot
  • Run the following command on your Raspberry Pi to install python-smbus and i2c-tools:
sudo apt-get install python-smbus i2c-tools
  • With those tools now installed run the following command on your Raspberry Pi to detect that you have correctly wired up your RTC device.
sudo i2cdetect -y 1

If you have successfully wired up your RTC circuit, you should see the ID #68 appear. This id is the address of the DS3231 RTC Chips. Once we have the Kernel driver up and running the tool will start to display UU instead, this is an indication that it is working as intended.

Setting up the Raspberry Pi RTC Time

With I2C successfully setup and verified that we could see our RTC circuit then we can begin the process of configuring the Raspberry Pi to use our RTC Chip for its time.

  1. To do this, we will first have to modify the Raspberry Pi’s boot configuration file so that the correct Kernel driver for our RTC circuit will be successfully loaded in.
  • Run the following command on your Raspberry PI to begin editing the /boot/config.txt file.
sudo vim /boot/config.txt
  • Within this file, you will want to add the following line to the bottom of the file.

Once you have added the correct line for your device to the bottom of the file you can save and quit out of it by pressing Esc, then :wq and then Enter.

With that change made we need to restart the Raspberry Pi, so it loads in the latest configuration changes.

  • Run the following command on your Raspberry Pi to restart it.
sudo reboot
  • Once your Raspberry Pi has finished restarting we can now run the following command, this is so we can make sure that the kernel drivers for the RTC Chip are loaded in.
sudo i2cdetect -y 1

You should see a wall of text appear, if UU appears instead of 68 then we have successfully loaded in the Kernel driver for our RTC circuit.

Now that we have successfully got the kernel driver activated for the RTC Chip and we know it’s communicating with the Raspberry Pi, we need to remove the fake hwclock package. This package acts as a placeholder for the real hardware clock when you don’t have one.

  • Type the following two commands into the terminal on your Raspberry Pi to remove the fake-hwclock package. We also remove hwclock from any startup scripts as we will no longer need this.
sudo apt-get -y remove fake-hwclock
sudo update-rc.d -f fake-hwclock remove

Now that we have disabled the fake-hwclock package we can proceed with getting the original hardware clock script that is included in Raspbian up and running again by commenting out a section of code.

  • Run the following command to begin editing the original RTC script.
sudo vim /lib/udev/hwclock-set

If you upgrade the operating system, you may need to redo the following step.

  • Find and comment out the following three lines by placing # in front of it as we have done below.
    • Find
    if [ -e /run/systemd/system ] ; then
    exit 0
    • Replace With
    #if [ -e /run/systemd/system ] ; then
    # exit 0
  • Once you have made the change, save the file by pressing Esc then :wq then Enter.

Syncing time from the Pi to the RTC module

Now that we have our RTC module all hooked up and Raspbian and the Raspberry Pi configured correctly we need to synchronize the time with our RTC Module. The reason for this is that the time provided by a new RTC module will be incorrect.

  • You can read the time directly from the RTC module by running the following command if you try it now you will notice it is currently way off our current real-time.
sudo hwclock -D -r
  • Now before we go ahead and sync the correct time from our Raspberry Pi to our RTC module, we need to run the following command to make sure the time on the Raspberry Pi is in fact correct. If the time is not right, make sure that you are connected to a Wi-Fi or Ethernet connection.
  • If the time displayed by the date command is correct, we can go ahead and run the following command on your Raspberry Pi. This command will write the time from the Raspberry Pi to the RTC Module.
sudo hwclock -w
  • Now if you read the time directly from the RTC module again, you will notice that it has been changed to the same time as what your Raspberry Pi was set at. You should never have to rerun the previous command if you keep a battery in your RTC module.
sudo hwclock -r

You should hopefully now have a fully operational RTC module that is actively keeping your Raspberry Pi’s time correct even when it loses power or loses an internet connection.

2.11  Set up the wifi access point


  • In order to work as an access point, the Raspberry Pi needs to have the hostapd access point software package installed:
sudo apt install hostapd
  • Enable the wireless access point service and set it to start when your Raspberry Pi boots:
sudo systemctl unmask hostapd
sudo systemctl enable hostapd
  • In order to provide network management services (DNS, DHCP) to wireless clients, the Raspberry Pi needs to have the dnsmasq software package installed:
sudo apt install dnsmasq
  • Finally, install netfilter-persistent and its plugin iptables-persistent. This utility helps by saving firewall rules and restoring them when the Raspberry Pi boots:
sudo DEBIAN_FRONTEND=noninteractive apt install -y netfilter-persistent iptables-persistent

Set up the network router

The Raspberry Pi will run and manage a standalone wireless network. It will also route between the wireless and Ethernet network, providing internet access to wireless clients when the wired network interface is connected to the web.

Define the wireless interface IP configuration

The Raspberry Pi runs a DHCP server for the wireless network; this requires static IP configuration for the wireless interface (wlan0) in the Raspberry Pi. The Raspberry Pi also acts as the router on the wireless network. We give the router the first IP address in the network:

  • To configure the static IP address of the WiFi interface, edit the configuration file for dhcpcd with:
sudo vim /etc/dhcpcd.conf
  • Go to the end of the file and add the following:
interface wlan0
static ip_address=
nohook wpa_supplicant

Enable routing and IP masquerading

This section configures the Raspberry Pi to let wireless clients access computers on the main (Ethernet) network, and from there, to access the internet (if the wired interface is connected to the web). NOTE: If you prefer to block wireless clients from accessing the Ethernet network and the internet, skip this section.

  • To enable routing, i.e. to allow traffic to flow from one network to the other in the Raspberry Pi, create a file using the following command, with the contents below:
sudo vim /etc/sysctl.d/routed-ap.conf
  • File content:
# Enable IPv4 routing

Enabling routing will allow hosts from network to reach the LAN and the main router towards the internet. In order to allow traffic between clients on this foreign wireless network and the internet without changing the configuration of the main router, the Raspberry Pi can substitute the IP address of wireless clients with its own IP address on the LAN using a "masquerade" firewall rule.

The main router will see all outgoing traffic from wireless clients as coming from the Raspberry Pi, allowing communication with the internet. The Raspberry Pi will receive all incoming traffic, substitute the IP addresses back, and forward traffic to the original wireless client.

  • This process is configured by adding a single firewall rule in the Raspberry Pi:
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
  • Now save the current firewall rules for IPv4 (including the rule above) and IPv6 to be loaded at boot by the netfilter-persistent service:
sudo netfilter-persistent save

Filtering rules are saved to the directory /etc/iptables/. If in the future you change the configuration of your firewall, make sure to save the configuration before rebooting.

Configure the DHCP and DNS services for the wireless network

The DHCP and DNS services are provided by dnsmasq. The default configuration file serves as a template for all possible configuration options, whereas we only need a few. It is easier to start from an empty file.

  • Rename the default configuration file and edit a new one:
sudo mv /etc/dnsmasq.conf /etc/dnsmasq.conf.orig
sudo vim /etc/dnsmasq.conf
  • Add the following to the file and save it:
interface=wlan0 # Listening interface
# Pool of IP addresses served via DHCP
domain=wlan # Local wireless DNS domain
# Alias for this router

The Raspberry Pi will deliver IP addresses between and, with a lease time of 24 hours, to wireless DHCP clients.

There are many more options for dnsmasq; see the default configuration file (/etc/dnsmasq.conf) or the online documentation for details.

Ensure wireless operation

Countries around the world regulate the use of telecommunication radio frequency bands to ensure interference-free operation. The Linux OS helps users comply with these rules by allowing applications to be configured with a two-letter "WiFi country code", e.g. US for a computer used in the United States.

In the Raspberry Pi OS, 5 GHz wireless networking is disabled until a WiFi country code has been configured by the user, usually as part of the initial installation process.

  • To ensure WiFi radio is not blocked on your Raspberry Pi, execute the following command:
sudo rfkill unblock wlan

This setting will be automatically restored at boot time. We will define an appropriate country code in the access point software configuration, next.

Configure the access point

  • Create the hostapd configuration file, located at /etc/hostapd/hostapd.conf, to add the various parameters for your wireless network.
sudo vim /etc/hostapd/hostapd.conf
  • Add the information below to the configuration file. This configuration assumes we are using channel 7, with a network name of cosqm, and a passphrase cosqmwifi. Note that the name and password should not have quotes around them. The passphrase should be between 8 and 64 characters in length.

Note the line country_code=CA: it configures the computer to use the correct wireless frequencies in Canada. Adapt this line and specify the two-letter ISO code of your country. See Wikipedia for a list of two-letter ISO 3166-1 country codes.

To use the 5 GHz band, you can change the operations mode from hw_mode=g to hw_mode=a. Possible values for hw_mode are:

a = IEEE 802.11a (5 GHz)
b = IEEE 802.11b (2.4 GHz)
g = IEEE 802.11g (2.4 GHz)
ad = IEEE 802.11ad (60 GHz)

Run your new wireless access point

  • Now restart your Raspberry Pi and verify that the wireless access point becomes automatically available.
sudo systemctl reboot

Once your Raspberry Pi has restarted, search for wireless networks with your wireless client. The network SSID cosqm should now be present, and it should be accessible with the password cosqmwifi.

If SSH is enabled on the Raspberry Pi, it should be possible to connect to it from your wireless client as follows, assuming the pi account is present: ssh sand@ or ssh sand@cosqm

2.12  Configuring the wired IP address of the SQM

The SQM-LE does not come shipped with a fixed IP address, so it may be necessary to fix that address before installing the unit into such a direct connection system without DHCP server software running.

  • Install the unit into a DHCP system where the IP addresses are assigned to connected devices.
  • Determine the SQM-LE IP address by querying the router or using the “Lantronix Device Installer” in Windows. In Linux, you may also use “nmap” to discover connected device IP addresses.
  • Use a browser to go to the SQM-LE IP address as in the following example: http://192.168.1.nnn
  • There is no default username and password, just press OK.
  • The built in Lantronix XPort Device Server Configuration Manager will appear.
  • Select NETWORK from the left side.
  • Select the radiobutton associated with “Use the following IP configuration:”, and enter the IP address that you would like the unit to occupy in your direct connection system. This IP Address should match the one indicated in the /home/sand/localconfig file. It should actually be
IP Address:
Subnet Mask:
Default Gateway: field may be left blank.
  • Press “OK” at the bottom then “Apply Settings” on the menu of the left side of the page and the unit will applythe settings and reboot in about 15 seconds. Since you may have changed the IP address, the web browser will notrespond unless you browse to the new address.
  • The unit is now ready for connection into a non-DHCP system or a direct connection system with a crossover cable

2.13  Configure the network parameters of the raspberry pi

  • Edit the file /etc/dhcpcd.conf
sudo vim /etc/dhcpcd.conf
  • Uncomment the section related to the static ip configurations but set ip address to i.e.
# Example static IP configuration:
interface eth0
static ip_address=
static ip6_address=fd51:42f8:caae:d92e::ff/64
static routers=
static domain_name_servers= fd51:42f8:caae:d92e::1

2.14  UPS hat

The UPS is a power battery backup. It will protect the system against power fluctuations and will allow the system to continue its ongoing tasks during short power failure.

  • To enable the ups do the following commands
cd /home/sand/git/cosqm
chmod a+x *
sudo cp * /usr/local/bin
sudo cp ups.service /etc/systemd/system
sudo systemctl enable ups.service
sudo systemctl start ups.service

2.15  System reboot

Now all the relevant software are installed, in order to activate all newly added functionalities, restart the computer.

3.  Optional stuff

3.1  D-LINK router configuration

The router is useful when CoSQM is connected to the Internet and that the appropriate ports are open to take control of the instrument remotely. If the system is not connected to the internet, a simple network switch or hub is sufficient.

  • Router local IP:
  • Disable DHCP server and attribute Fixed IP as follow:
    • = raspberry pi
    • = SQM
    • 192.168.0.other = any other computer connected temporarily or permanently to the router
  • Create a virtual server to redirect port 2022 of the router toward SSH port 22 of
  • Do the same for port 2080 to allow web access redirection (port 80)
  • Create virtual server from port 2180 toward port 80 (webcam server). This redirection will allow a user to see the webcam image from the web even if the computer is off.
GlossyBlue theme adapté par David Gilbert et Martin Aubé
Powered by PmWiki