ntripcaster-docker

BKG's Open Source NTRIP Caster Running Under Docker

This repository contains the BKG NTRIP caster set up to run under docker.

NTRIP was invented by the German organisation Bundesamt für Kartographieund Geodäsie (BKG) - in English, the Federal Agency for Cartography and Geodesy. It's used to transmit corrections from a GNSS base station to a GNSS rover over the Internet. (GNSS is the general term for what we often call GPS.)

To summarise: A GNSS receiver receives signals from GNSS satellites such as GPS and uses them to calculate its position. The signals are distorted by obstacles such as the ionosphere, which causes errors in the calculated position. The base station and the rover are both GNSS receivers. The base station is fixed in one spot and knows its position very precisely. Its observations of the satellite signals can be used to work out the distortion of the signals and thus correct the errors. A rover within a few kilometres of a base station can use these corrections to calculate its position accurate to 2cm. The NTRIP protocol provides the means for the base station to send data to the rover via the Internet. It does this via an intermediate server called an NTRIP caster.

This technology has been around for about twenty years, but it used to be expensive. Recently base stations, rovers and servers have become much cheaper and you can buy a complete system for less than $1,000. All you need to get it working is some free software, including this caster.

I should also say at the beginning that you don't necessarily need your own NTRIP caster. There are a number already available. Use your favourite seach engine to find them.

If you have your own network of Windows machines and you know how to configure them, you may be able to get the caster working on one of those. Here I assume that you are going to set it up on a server on the Internet with a proper domain name. Nowadays, that's fairly cheap, and it's one less machine to manage. Your caster must run on a server machine on the public Internet with a published domain name. I run mine on a Digital Ocean Droplet costing $5 per month. (Other providers of virtual private servers are available.)

BKG's caster software is free and open source. It implements NTRIP Version 1. NTRIP is now at Version 2. BKG offers a version 2 caster, but it's not free or open source. If you are going to use the free caster, you need to check that your base station and rover can use a Version 1 service.

Docker provides a ready-made predictable environment in which you can build and run software. The steps to build and run a docker application are the same regardless of your operating system.

Until docker came along, installing software like this caster could be a nightmarish mess, because the environment provided by every computer was different and unpredictable - Windows 7, Windows 10, Ubuntu Linux, Red Hat Linux or whatever, and usually with different optional software installed.

When docker runs an application, it creates a stripped-down Linux environment within whichever operating system your computer is actually running. This irons out most of the gotchas and makes success much more likely.

Each docker application is encapsulated in its own environment, which also reduces problems caused by hidden interactions. Two applications can only communicate through well understood interfaces.

This does mean that to put a solution together you may have to learn a few new skills, which is a cost, but most people find that the benefits of using docker far outweigh this. In particular, you need to understand how to use a UNIX command window. You can learn the basics of that using my dockerised learn package.

This document explains what NTRIP is, how to set up a suitable environment for your caster, how to build and run it and how to manage it once it's running.

If you do need to run your own caster, read on.

Configuration Files

Once you've downloaded the caster (see how below) and before you install it, you need to set up a couple of configuration files. They are called sourcetable.dat and ntripcaster.conf. The distribution includes an example of each file.

BKG's original build and installation instructions are here They include some manual steps, to be followed once the software is built. Docker uses completely automated builds, so I've reworked the process so that you start by producing a couple of configuration files and the rest of the process is automatic.

There's a copy of BKG's documentation here. It may make more sense if you look at the example configuration files while you are reading it.

The important part is this:

"Go to the configuration directory and rename "sourcetable.dat.dist" and "ntripcaster.conf.dist" to "sourcetable.dat" and "ntripcaster.conf". Edit both files according to your needs. For details about "sourcetable.dat" see file "NtripSourcetable.doc". In the configuration file "ntripcaster.conf" you have to specify the name of the machine the server is running on (no IP adress!!) and you can adapt other settings, like the listening ports, the server limits and the access control."

sourcetable.dat

This file lives in the directory conf (ntripcaster/ntripcaster/conf) . You need to edit it to suit your needs.

BKG's instructions say: "Whatever the content of your "sourcetable.dat" finally might be, it is recommended to include the following line in that configuration file: CAS;rtcm-ntrip.org;2101;NtripInfoCaster;BKG;0;DEU;50.12;8.69;http://www.rtcm-ntrip.org/home".

That line is already in the example source table file so you can just leave it.

Apart from that first line, sourcetable.dat defines your mountpoints. You need one for each base station. I have one base station. It's on the roof of my shed in Leatherhead in the UK. I call my mountpoint "uk_leatherhead".

Each line of sourcetable.dat is a list of fields separate by semicolons. Mine looks like this:

CAS;rtcm-ntrip.org;2101;NtripInfoCaster;BKG;0;DEU;50.12;8.69;http://www.rtcm-ntrip.org/home
STR;uk_leatherhead;Leatherhead;RTCM 3.0;;;;;GBR;51.29;-0.32;1;0;sNTRIP;none;N;N;0;;

Field 1 of the second line is "STR" which says that it's defining a mountpoint.

Field 2 "uk_leatherhead" is the name of my mountpoint.

Field 3 "Leatherhead" is the nearest town to my base station.

RTCM 3.0 means that the NTRIP connection is carrying RTCM version 3 messages, which is what my base station produces.

GBR is the three-letter code for the UK. You can find the two and three letter code for your country here.

"51.29;-0.32" gives the longitude and latitude of my base station. If you don't have that information, you can use "0.00;0.00".

ntripcaster.conf

This file also lives in the directory conf (ntripcaster/ntripcaster/conf). You need to edit it to suit your needs.

The file defines all sorts of things, including the user names and password used to access the system.

All base stations use the same password. There's no facility to specify a user name. (My base station software insists on supplying one, but for this caster ignores it, so it doesn't matter what user name I use.)

There is a set of usernames and passords for each mountpoint which a rover needs to supply when it connects.

The lines you need to change are scattered through the file:

rp_email [email protected]       # substitute your email address
server_url http://caster.ifag.de   # substitute your domain name

encoder_password sesam01           # Password for base stations.  Choose something more secure.

server_name igs.ifag.de             # substitute your domain name

The last few lines of the file specify the user names and passwords for the rovers. You need one line for each of the mountpoints you specified in sourcetable.dat. In this file, the mountpoint names must start with "/" (but not in sourcetable.dat). For example:

/uk_leatherhead:user1:password1,user2:password2

creates user names "user1" and "user2" which a rover can use to connect to that mountpoint.

You can create a public mountpoint with no user name or password. Any rover can connect to that mountpoint:

/uk_leatherhead

The mountpoint names are case sensitive - "MY_MOUNTPOINT", "My_Mountpoint" and "my_mountpoint" are all different names. When you configure your rover, the case has to match.

Quick Instructions For Building and Running the Caster

These instructions are for readers who are familiar with concepts such as docker, remote management of computers, domain names, virtual private servers and so on. If you are not one of those people, continue to the next section.

The caster must run on a server machine on the public Internet. (I run mine on a Digital Ocean Droplet.) The server must have an Internet domain name, so you need to buy one of those and assign it to your server. (If you don't understand all of that, read the more detailed explanation below.) When your configure your caster you have to put the same name in your configuration file.

I'm going to assume that you control your server via a sudo user rather than logging in as root, so I'm going to use the sudo command wherever necessary.

Log in to your server via a command window and install git and docker:

sudo apt update
sudo apt install git docker.io

Installing docker ceates a UNIX group called "docker". To run the docker command, you need your user (the one you are logged in as on the server) to be in that group. In this example, the user is called "gps":

sudo usermod -aG docker gps

The Docker service needs to be set up to run whenever your server machine starts up:

sudo systemctl start docker
    
sudo systemctl enable docker

Download the caster:

git clone git://github.com/goblimey/ntripcaster.git

That creates a directory called ntripcaster. Inside that is another directory with the same name plus a couple of other files. Inside the lower ntripcaster directory is the conf directory, where you create your configuration files:

cd ntripcaster/ntripcaster/conf
cp ntripcaster.conf.dist ntripcaster.conf
cp sourcetable.dat.dist sourcetable.dat

Edit your configuration files sourcetable.dat and ntripcaster.conf as explained above.

Move back to the top level of the project and build your docker image:

cd ../..
docker build . -t ntripcaster

The build will take a little while and at the end you should see something like this:

Step 11/11 : CMD /usr/local/ntripcaster/bin/ntripcaster
 ---> Running in a13e5bbd3545
Removing intermediate container a13e5bbd3545
 ---> fc4f331c1db4
Successfully built fc4f331c1db4
Successfully tagged ntripcaster:latest

Running the Caster

Onc you've built the caster, run it like so:

docker run -p2101:2101 ntripcaster >/dev/null 2>&1 &

The caster runs on port 2101. The -p option publishes that port, which makes it available to the outside world. In this case, it's accesible via port 2101.

">/dev/null" connects the docker command's standard output channel to a special file that just discards anything written to it. "2>&1" connects the standard error channel to whatever the standard output channel is connected to. That means that the docker image will run quietly, without sending anything to the console. The "&" at the end of the command runs it in the background, so you get another prompt and you can issue more commands. The caster will survive you ending the ssh session that you used to start it. It will run until something goes wrong and it dies, or until it's forcibly shut down.

To view the running caster:

docker ps

That produces a list of running docker containers, something like this:

CONTAINER ID  IMAGE        COMMAND                 CREATED        STATUS        PORTS                  NAMES
fb90a0a44e14  ntripcaster  "/bin/sh -c /usr/loc…"  16 minutes ago Up 16 minutes 0.0.0.0:2101->2101/tcp great_sammet

So docker is running one container. Its container ID is fb90a0a44e14.

The container is running the image ntripcaster which we built earlier.

You need to know that container ID for various control purposes, as we wil see later.

For a quick check that the caster is working, if you have curl installed on your server, you can use it to fetch the caster's home page:

curl --http0.9 http://localhost:2101/

(That's http, NOT https.)

That request should produce a copy of the source table. This shows that the caster is running and that it's found its configuration files.

Back on your local computer, if it's got curl installed you can run a similar test to check that things are working across the Internet:

curl http://my.domain.name:2101/

That should produce the same result as before. If the first test worked and this one doesn't, the most likely explanation is that you haven't arranged with your VPS provider to open up port 2101 to tcp traffic.

Stopping the Caster

The caster will run until you stop it or reboot the server machine.

To stop the container you need to find its container ID using docker ps as shown earlier. Given that, stop it like so:

docker kill {container_id}

For example:

docker kill fb90a0a44e14

Start the caster again as before:

docker run -p2101:2101 ntripcaster >/dev/null 2>&1 &

The Log File

The caster creates a log file as it runs, which you can use to debug problems. To see the log you have to know the ID of the container, in this example "fb90a0a44e14".

(By the way, BKG's original installation instructions are slightly misleading. they say that to run the caster you should change directory to /usr/local/ntripcaster/bin and run the program from there. It will then pick up the configuration files from /usr/local/ntripcaster/conf and write a log file in /usr/local/ntripcaster/logs. Not quite. When the server starts up it creates a log file in the current drectory, whatever that is. So the docker image changes directory to /usr/local/ntripcaster/logs and runs the caster software from there, and the log ends up in the right place.)

You can track what's written to the log using the tail command. The -f option makes tail run forever, displaying new lines as they arrive:

docker exec -it fb90a0a44e14 tail -f /usr/local/ntripcaster/logs/ntripcaster.log

[29/Aug/2019:16:55:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:16:56:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:16:57:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:16:58:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:16:59:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:17:00:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:17:01:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:17:02:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:17:03:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0
[29/Aug/2019:17:04:26] [1:Calendar Thread] Bandwidth:0.000000KB/s Sources:0 Clients:0

If you connect a base station, the source value will increase by one. If you connect a rover, the client value will increase by one.

Use ctrl/c to stop the tail command.

NTRIP Basics

There are a lot of acronyms in this field. I'll start by unpicking some of them.

A Global Navigation Satellite System (GNSS) is a network of satellites that allows a receiver on the Earth to find its position accurately. The first and best-known was the Global Positioning System (GPS), originally created by the American military for missile guidance. GNSS systems now include the European Galileo, the Russian GLONASS and the Chinese Baidou. Each has its own network of satellites (known as a "constellation"). Many satellite navigation receivers are capable of picking up signals from all these systems and making use of any of them.

A moving receiver (for example a hand-held device, or in a car, a boat or an aircraft) that can see enough satellites can find its position reasonably accurately, typically to within three or four metres.

If the receiver knows that it's in a fixed position it can do better by repeatedly finding its position and averaging the results. Most receiver have some kind of "fixed" mode, where they assume that they are stationary. The result depends on the device and on how long you leave it taking averages. You can achieve maybe 1m accuracy this way.

Dual-band GNSS Receivers are now available that can analyse two signals coming from the same satellite on different frequencies to get an even more acurate position - typically within half a metre.

RTCM

The Radio Technical Commission for Maritime Services (RTCM) produced a standard protocol for sending satellite observation data over a radio link. so a base station from one manufacturer can send observations to a rover from another manufactuer. This is called the RTCM protocol. It's currently at version 3.

RTCM over radio is used for all sorts of purposes including the control of drones. The drone contains a GNSS rover which sends position information to its flight controller. The base station sits on the ground in a fixed position, sending GNSS data to the rover, which it uses to find its position more accurately. The two are connected via Long Range (LoRa) radio, which works over a few kilometres. The operator can pre-program the drone to follow a path around a site, avoiding obstacles.

This setup probably works well when the base station is in a prominent position and the rover is high in the air, so the two have line of site between them. If the rover is closer to the ground, for example in a hand-held tracker, obstacles such as buildings, trees and hedges can interfere with the radio connection and make it very flaky.

NTRIP

To provide a better alternative to RTCM over a radio link, the Bundesamt für Kartographieund Geodäsie (BKG) defined the Network Transport of Rtcm via Internet Protocol (NTRIP) which replaces the radio link with an HTTP connection over the Internet.

This requires the base station and the rover to be connected to the Internet and to be able to communicate with each other. To achieve that, one of the devices involved must have a published Internet location, ie a fixed IP address and a domain name. That's the purpose of the caster. A caster takes NTRIP traffic from one or more base stations and routes it to one or more rovers. Having the intermediate caster means that neither the base station nor the rover need any fancy kind of Internet connection.

This allows a complete system to be built for less than $1,000.

The rover is typically connected to the mobile (cell phone) Internet, perhaps using an Internet modem or connected by WiFi to a mobile phone. It won't have a published domain name and its IP address will change very frequently. My rover is an Emlid M+ connected to my iPad over WiFi to get an Internet connection. The Reach costs about $300 including antenna. It's designed to work within a drone. As a hand-held rover it's a bit of a lashup, but it works quite well.

The base station can connect to the Internet using a conventional home broadband system. My base station is in my garden shed with its antenna on the roof. It's essentially a special bit of circuitry bought off the shelf, connected to a Raspbery Pi running some free software, which you can also find in another of my repositories. Total cost, about $700.

This caster software runs on a virtual private server in one of Digital Ocean's data centres, which costs me $5 per month, plus $13 per year for the domain name.

This is how it all works:

The base station sends data encoded in NTRIP format to a nearby rover via a caster. It sends its position and its recent readings ("observations") of signals from the satellites that it can see. Knowing the base station's position, it's possible to work out what the signals ought to look like, and therefore what errors the distortion has introduced. If the moving rover is close to the base station, it sees the same satellites and suffers the same distortions, so it can use the base station's observations to correct the errors in its own observations.

If the rover is within about 10 Km of the base station and the base station knows its position perfectly, the rover can find its position within 2 cm. Up to 20 Kilometres away it can produce 4 cm accuracy, and so on.

All this assumes that base station knows its position very accurately. If its notion of its position is wrong, the rover's calculated position will be wrong by the same amount. Imagine you have rover close to a base station and and you move it around a site, measuring the positions of various features and draw a map. Each position on the map should be accurate to 2 cm. However, if the base station's notion of its position is half a metre to the North of where it really is, your whole map will be shifted by half a metre to the North.

So getting everything to work properly mainly involves figuring out the position of the base station accurately.

Getting a Domain and Server

To run the caster, you need a server with a well-known name. You can achieve that by buying an Internet domain. Strictly you don't buy a domain. You rent it from a domain registrar, so you have to pay regularly to keep it going.

You can obtain a domain from various domain registrars such as namecheap or ionos. There are many others. I mention those two because I know that their initial registration fees and their subsequent renewal fees are both reasonable. When choosing a registrar, always check the renewal fees. Beware of introductory offers that cost you a lot more later.

Once you have your domain name, you need a computer on the Internet that answers to it. You can rent a Virtual Private Server (VPS) rather than running your own machine. You may be able to find a VPS supplier that can also handle your domain registration.

Amazon Web Services is one of the best-known VPS suppliers, but they can be expensive. Digital Ocean offer a VPS called a droplet that you can rent for $5 per month. In the UK, Mythic Beasts offer a VPS for a comparable price.

You don't need much computer power or network bandwidth to run an NTRIP caster, which is why you can use these cheap servers.

When you rent the VPS you can choose what operating system it runs. These instruction assume that you are running Linux rather than Microsoft Windows on your VPS. It's more secure and more reliable.

Once you've hired your domain and your VPS, you need to configure the VPS to answer to the domain name. How to do that varies according to the two suppliers. Your VPS supplier's tech support people should be able to explain how to do it, particilarly if you rent your domain from one of the better-known domain registrars such as Ionos or namecheap.

Each network service on a computer runs on a numbered port, for example, a web server will usually run its http service on port 80 and its https service on port 443. The NTRIP caster runs on port 2101. For security reasons many VPS suppliers stop access to ports by default. You have to ensure that port 2101 is open for tcp access. You may need to ask the tech support people how to do this.

Connecting to Your VPS

Once your VPS is set up and responding to your domain name, you need to connect to it from whatever computer you normally use. These instructions assume that you are running MS Windows on your local machine (because most people do) and that your VPS is running the Linux operation system (because that's also what most people do). If you are running Windows on your VPS, the procedure to connect will be similar but different. You need to consult your VPS supplier about that. The docker commands will be the same.

There are various ways to connect to your VPS. The ssh command is probably the most common. Your Windows machine can't do that out of the box, you need to install some software. My suggestion is git for windows. Once you've installed that, go to your start menu and run Git Bash. That starts a command window and you can run ssh in that.

To connect to your VPS you need your user name and your domain name. If your user name is "user" and your domain is "my.domain.name", connect like so:

ssh [email protected]

When you set up your VPS you may have been asked to create a public/private key pair. They are files in the .ssh directory in your home directory on your local machine. If the machine you are connecting from has your private key installed and the machine you are connecting to has your public key installed, you don't need a password.

If you didn't create keys, you will be asked for your password when you connect to your VPS. That means you can connect to it from any computer, but so can anybody else who can guess your password. If you created a key pair, your VPS should be set up so that it's only possible to connect from a computer that holds a copy of the private key. (So now would be a very good time to make a backup copy of your key pair on a memory stick.)

Logging in with a key pair means that you don't have to remember yet another password. That's not just convenient, it's also much more secure. Your VPS supplier should have arranged that it's not possible for anybody to log in over the network using a password.

To see why, once you are connected to your VPS, try this:

tail -100 /var/log/auth.log

It shows the recent log of attempted logins. If you haven't tried this before, the result is quite scary. This is what I got:

Aug 29 09:31:31 audolatry sshd[24565]: Received disconnect from 122.195.200.148 port 14902:11:  [preauth]
Aug 29 09:31:31 audolatry sshd[24565]: Disconnected from authenticating user root 122.195.200.148 port 14902 [preauth]
Aug 29 09:31:36 audolatry sshd[24567]: Received disconnect from 222.186.15.101 port 58740:11:  [preauth]
Aug 29 09:31:36 audolatry sshd[24567]: Disconnected from authenticating user root 222.186.15.101 port 58740 [preauth]
Aug 29 09:31:37 audolatry sshd[24569]: Received disconnect from 222.186.42.117 port 51976:11:  [preauth]
Aug 29 09:31:37 audolatry sshd[24569]: Disconnected from authenticating user root 222.186.42.117 port 51976 [preauth]
Aug 29 09:32:06 audolatry sshd[24571]: Invfrom the git bash windowalid user rabbitmq from 180.240.229.254 port 40846
Aug 29 09:32:07 audolatry sshd[24571]: Received disconnect from 180.240.229.254 port 40846:11: Bye Bye [preauth]
Aug 29 09:32:07 audolatry sshd[24571]: Disconnected from invalid user rabbitmq 180.240.229.254 port 40846 [preauth]
Aug 29 09:35:30 audolatry sshd[24576]: Received disconnect from 122.195.200.148 port 42495:11:  [preauth]
Aug 29 09:35:30 audolatry sshd[24576]: Disconnected from authenticating user root 122.195.200.148 port 42495 [preauth]
Aug 29 09:38:31 audolatry sshd[24578]: Received disconnect from 36.156.24.43 port 51260:11:  [preauth]
Aug 29 09:38:31 audolatry sshd[24578]: Disconnected from authenticating user root 36.156.24.43 port 51260 [preauth]
Aug 29 09:47:41 audolatry sshd[24582]: Received disconnect from 183.131.82.99 port 17573:11:  [preauth]
Aug 29 09:47:41 audolatry sshd[24582]: Disconnected from authenticating user root 183.131.82.99 port 17573 [preauth]
Aug 29 09:52:41 audolatry sshd[24586]: Received disconnect from 222.186.30.111 port 48306:11:  [preauth]
Aug 29 09:52:41 audolatry sshd[24586]: Disconnected from authenticating user root 222.186.30.111 port 48306 [preauth]
Aug 29 09:55:04 audolatry sshd[24589]: Received disconnect from 183.131.82.99 port 45178:11:  [preauth]
Aug 29 09:55:04 audolatry sshd[24589]: Disconnected from authenticating user root 183.131.82.99 port 45178 [preauth]
Aug 29 09:57:22 audolatry sshd[24592]: Invalid user fsc from 80.211.171.195 port 35560
Aug 29 09:57:22 audolatry sshd[24592]: Received disconnect from 80.211.171.195 port 35560:11: Bye Bye [preauth]
Aug 29 09:57:22 audolatry sshd[24592]: Disconnected from invalid user fsc 80.211.171.195 port 35560 [preauth]
Aug 29 09:57:54 audolatry sshd[24594]: Accepted publickey for root from xxx.xxx.xxx.xxx port 50986 ssh2: RSA SHA256:FheBvetqJo0CCYC3ghFpdnvVJwzXYEXxwwUavFgugXs
Aug 29 09:57:54 audolatry sshd[24594]: pam_unix(sshd:session): session opened for user root by (uid=0)
Aug 29 09:57:54 audolatry systemd-logind[814]: New session 2383 of user root.
Aug 29 09:59:45 audolatry sshd[24722]: Received disconnect from 36.156.24.79 port 47652:11:  [preauth]
Aug 29 09:59:45 audolatry sshd[24722]: Disconnected from authenticating user root 36.156.24.79 port 47652 [preauth]
Aug 29 10:01:11 audolatry sshd[24732]: Received disconnect from 222.186.15.110 port 50693:11:  [preauth]
Aug 29 10:01:11 audolatry sshd[24732]: Disconnected from authenticating user root 222.186.15.110 port 50693 [preauth]
Aug 29 10:03:48 audolatry sshd[28383]: Connection closed by authenticating user root 69.16.201.246 port 56496 [preauth]
Aug 29 10:06:24 audolatry sshd[28387]: Received disconnect from 222.186.30.111 port 24240:11:  [preauth]
Aug 29 10:06:24 audolatry sshd[28387]: Disconnected from authenticating user root 222.186.30.111 port 24240 [preauth]
Aug 29 10:06:24 audolatry sshd[28389]: Received disconnect from 49.88.112.80 port 23981:11:  [preauth]
Aug 29 10:06:24 audolatry sshd[28389]: Disconnected from authenticating user root 49.88.112.80 port 23981 [preauth]
Aug 29 10:08:50 audolatry sshd[28392]: Received disconnect from 183.131.82.99 port 64115:11:  [preauth]
Aug 29 10:08:50 audolatry sshd[28392]: Disconnected from authenticating user root 183.131.82.99 port 64115 [preauth]
Aug 29 10:12:54 audolatry sshd[28397]: error: maximum authentication attempts exceeded for root from 202.104.174.163 port 40946 ssh2 [preauth]
Aug 29 10:12:54 audolatry sshd[28397]: Disconnecting authenticating user root 202.104.174.163 port 40946: Too many authentication failures [preauth]
Aug 29 10:16:22 audolatry sshd[28401]: Connection closed by authenticating user root 78.97.92.249 port 54430 [preauth]
Aug 29 10:17:01 audolatry CRON[28403]: pam_unix(cron:session): session opened for user root by (uid=0)
Aug 29 10:17:01 audolatry CRON[28403]: pam_unix(cron:session): session closed for user root
Aug 29 10:20:08 audolatry sshd[28407]: Received disconnect from 122.195.200.148 port 10224:11:  [preauth]
Aug 29 10:20:08 audolatry sshd[28407]: Disconnected from authenticating user root 122.195.200.148 port 10224 [preauth]
Aug 29 10:21:35 audolatry sshd[28410]: Connection closed by authenticating user git 78.97.92.249 port 41622 [preauth]
Aug 29 10:22:29 audolatry sshd[28412]: Connection reset by 49.88.112.85 port 13845 [preauth]

The line that says "Accepted publickey for root" is me connecting using my key. The rest show other people all round the world trying every few seconds to connect to my VPS by guessing user names and passwords. Ths will have started as soon as my domain was created and announced. One tried connecting as the user root, another tried as the user rabbtmq, another as fsc, and so on. Those user names are standard and exist on lots of servers. The hackers run software that guesses a password, tries to connect, guesses another password, tries to connect and so on. You can see which IP address they are coming in from, but they are probably using somebody else's computer that they've already compromised.

If you allow connection by user name and password, it's just a question of time before somebody makes a correct guess and gets in. That would be bad. They can use your VPS for all sorts of nefarious purposes, for which you could be blamed.

Prevent this by configuring your VPS to refuse logins over the network using a password. Consult your VPS supplier about how to do that. Keep a safe copy of your keys, otherwise you could lock yourself out as well as the hackers.

That's the security sermon over. Now let's build an NTRIP caster.

More on Installing the Caster

There's a user called root that has special privileges. You need them to install things. Your VPS supplier may set things up so that you log in using another user that doesn't have those privileges, but can get them.

If so, you get the extra privileges by putting "sudo" at the start of any command. Forcing you to use sudo is safer because those privileges also allow you to make disastrous mistakes. Having to start each dangerous command with "sudo" is a reminder to be careful. Using docker makes things even safer, because it automates all of the dangerous operations.

You don't need to add "sudo" to the start of each command if you are logged in as the root user, because you already have the right privileges, but it does no harm. So you can copy and paste shown earlier into your git bash window whichever user you are logged in as, for example:

sudo apt install docker.io

Concerning pasting, you can't use the usual Windows shortcut ctrl/v to paste within the ssh window. Right click and a small menu appears with a paste option.)

To fetch the caster project, you did this:

git clone git://github.com/goblimey/ntripcaster.git

which created a directory called ntripcaster

Then you need to edit your configuration files. If you are not familiar with Linux, use the editor nano, for example:

nano sourcetable.dat

While you are in nano, move around the file using the arrow keys. The mouse doesn't work. When you have finished editing the file, use ctrl/o to write your changes and ctrl/x to exit.

Once you've created those two configuration files, the instructions tell you to use docker to build your caster. The Dockerfile in the top level directory of your project looks something like this:

FROM ubuntu:18.04 as builder

COPY ntripcaster /ntripcaster

WORKDIR /ntripcaster

RUN apt-get update && apt-get install build-essential --assume-yes

RUN ./configure

RUN make install

# The builder image is dumped and a fresh image is used
# just with the built binary, config and logs made from 'make install'
FROM ubuntu:18.04
COPY --from=builder /usr/local/ntripcaster/ /usr/local/ntripcaster/

EXPOSE 2101
WORKDIR /usr/local/ntripcaster/logs
CMD /usr/local/ntripcaster/bin/ntripcaster

The directives in the Dockerfile automate a build and deploy process which is similar to the steps described by BKG's original installation instructions. Tit runs in two stages. The first stage installs the software needed to build the executable program, and then builds it. That's done just once when you run the the "docker build" command. That material is then all cleared away. When you run the server using "docker run" that invokes just the second stage, which takes the built executable program and runs it.

The first FROM directive defines which version of Linux docker will run the build stage, in this case Ubuntu 18.4. Check the website for Linux distribution you are using and choose the latest stable version. For Ubuntu, that's this page. Choose the LTS version.

The top level directory of your project contains the Dockerfile and a directory ntripcaster. The COPY copies the contents of that directory into a workspace.

WORKDIR sets the current directory to that workspace.

The version of Linux that docker creates is very minimal and it doesn't contain any software build tools. The first RUN installs what's needed. The second one uses them to configure the caster for this Linux environment and the third one uses the make tool to build and install the caster.

The second FROM directive starts the second stage. Again it specifies which version of Ubuntu to use.

the COPY directive fetches the executable program produced by the build stage.

When the caster runs, it accepts network connections on port 2101. The EXPOSE allows the rest of the system to access that port.

The next directive WORKDIR sets the working directory when the docker image is run. When the program starts running it creates a log file in the current directory. The workdir sets that to /usr/local/ntripcaster/logs.

The CMD directive runs a command when the docker image is started. In this case the container will run the caster with the WORKDIR as the current directory. The container runs until that program terminates. Normally it runs indefinitely.

To build your docker image, you move to the top level directory of the project and run:

docker build .

Note the "." which means "the current directory". Docker looks in the given directory for a file called Dockerfile and obeys the directives in it.

You can run the image like so:

docker run -p2101:2101 ntripcaster

The -p connects port 2101 of the docker image to port 2101 of the mchine on which you are running.

The "docker run" ties up your git bash window. You can start another and use ssh to connect to your server machine as before.

Killing the docker container stops the service:

docker kill {container_id}

When the container stops, the Linux image that was running and any files in it are destroyed, The advantage of that is that you don't have to do any tidying yourself. The disadvantage is that if something goes wrong and the caster crashes, the container dies and all evidence vanishes with it.

You can set up the docker image so that things like the server log file survive. Read the docker manual to find out how.

When you use docker, there's also all sorts of tidying you have to do - removing old image files and containers. Again, you need to read the manual,

Whenever you change anything in the project, you need to run the docker build again. That will produce a new image.

When you started the docker image earlier, it tied up your git bash window. To avoid that, start the image using this magic:

docker run ntripserver >/dev/null 2>&1 &

">/dev/null" connects the command's standard output channel to a special file that just discards anything written to it.

"2>&1" connects the standard error channel to whatever the standard output channel is connected to. That means that the docker image will run quietly.

The "&" at the end of the command runs it in the background, so you can issue more commands.in that window.

The caster will run until something goes wrong and it dies, or until it's shut down.

If you run "docker ps" again, you will see that the container ID is different. Running a docker image creates a new container.

The docker container is a complete separate Linux environment and if you know its container ID you can run commands in it. For example, if the container id is f473f0749fd0, this will run the ps command in the container, which shows you what programs are running there:

docker exec -tt f473f0749fd0 ps -aef

UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 16:35 ?        00:00:00 /bin/sh -c ./ntripcaster
root         6     1  0 16:35 ?        00:00:00 ./ntripcaster
root         9     0 22 16:50 pts/0    00:00:00 ps -aef

The running programs include the ps that you are running to see this output.

More potential confusion:

docker ps

is a docker command that lists the running containers.

 ps -aef

is a linux command that lists the running programs. The two commands do similar jobs and one is named after the other.

You can configure your server machine to run a command whenever it starts. Unfortunately, how to do that depends on what version of Linux you are running. (Not to be confused with the version that you specified in the Dockerfile. That's what runnin within the container.)

I'm running Ubuntu on my VPS, and a Google search led me to https://askubuntu.com/questions/814/how-to-run-scripts-on-start-up. The relevent part is "The upstart system will execute all scripts from which it finds a configuration in directory /etc/init. These scripts will run during system startup (or in response to certain events, e.g., a shutdown request) and so are the place to run commands that do not interact with the user; all servers are started using this mechanism."

Configuring Your Base Station and Rover

Configuring your base station and your rover depends on what you are using. Read the manuals. You will need to supply the information that you set up in the ntripserver.conf configuration file. Both will need the server URL, port (2101), and mountpoint name. The base station should use the encoder password (and any user name). The rover will need the user name and password for the mountpoint that it's going to use.

When any of your devices connect to the caster, you should see some activity in the caster's log on your VPS.

Tweaks to The Original Source Code From BKG

The caster is free software originally written in C by BKG and distributed by them. I would have preferred to use that as my starting point, but when the time came, I couldn't find it. Instead I used a copy on github: https://github.com/nunojpg/ntripcaster.

There are a huge number of ready-made projects out there written in C and C++ that you can download, build and run. The procedure tends to be:

./configure
make install

If you can automate the whole process, including any configuration tweaks, you can use docker to build the project and run the result. This section may be useful if you want to do that.

The software is built using the make command. There is a file called makefile in each directory that tells make what to do.

The configure command runs another command automake which creates each makefile from a template called makefile.in and the settings in makefile.am.

To automate the manual configuration step, I edited Makefile.am and Makefile.in in the conf directory. The etc_DATA setting controls the files that are copied from that directory when the software is installed. The original contains:

etc_DATA = ntripcaster.conf.dist sourcetable.dat.dist

so only those two files are copied.

In my version that becomes

etc_DATA = ntripcaster.conf.dist sourcetable.dat.dist ntripcaster.conf sourcetable.dat

Now when docker runs "make install", it copies four files from the conf directory rather than two.

So you just have to create ntripcaster.conf and sourcetable.dat and then run docker. It runs configure, which runs automake to create the makefiles, then it runs make to build and install the software.

Actually, what's supposed to happen is that the template Makefile.in is edited by automake based on the settings in Makefile.am, so it should only be necessary to change makefile.am. I think what's happened is that somebody has run configure and then committed the result, so I've inherited a Makefile.in that is not the original version. Ah well, that's the fun world of open-source software for you.

Commercial NTRIP services

A number of GNSS devices can send and receive NTRIP corrections, notable manufacturers include Emlid; U-Blox; Trimble and Leica.

There are also a number of sources of NTRIP correction data. Trimble and Leica both provide these across the world for their own devices and others, but their services are expensive (hundreds of dollars per month). For somebody like a sureveyor working in the Oil and Gas sector, this makes a lot of sense: wherever you are in the world, your rover will receive some sort of correction data, making it more accurate than it would be otherwise. In places like Western Europe, it wlll be accurate to within a few centimetres.

For a surveyor working for a small local government authority, or a field archaeologist, those services may be too expensive, and running their own base station is more feasible. Even better, they might be able to use somebody else's base station and only need buy a $300 rover.

There are free NTRIP services such as the International GNSS Service (IGS). and rtk2go.com, provided by snip.com, a company that makes and sells commercial NTRIP software. The free services are very limited, with just a few base stations scattered over large distances. For example, my nearest IGS mountpoint is at Herstmonceaux. If I lived in Brighton or Eastbourne, that would be great, but I don't. I'm about 60 Km away. The corrections are useful at that distance, but running my own base station is better.

The rtk2go.com NTRIP service allows you to connect your own base station and share its corrections with other people. Unfortunately, a lot of the base station owners (in the UK at least) seem to switch them off when they are not using them. All the ones near me are only on occasionally. Also, the service is only free now while it's in beta test. The owners say that they plan to charge for it eventually.

NTRIP on a Budget

In the past, GNSS devices that could produce NTRIP corrections were expensive, but recently that's changed.

Emlid sell a ready-made device the Reach RS+ for $800. You can buy that, connect it to a caster, and it will provide corrections to your rover. Emlid's Rover, the Reach M+, costs about $300 including antenna. Without corrections, it's accurate to about 4 m, with them, to 2 cm.

In 2019, U-Blox launched the ZED-F9P, a dual-band chip which can be used in a base station or rover. As a rover it can find its position to within half a metre witout a correction source. With suitable corrections, to 2cm.

Sparkfun sell a version of the U-Blox chip mounted on a circuit board. It can be connected to a Raspberry Pi to produce an complete base station. No electronics knowledge or soldering needed.