Abstract
The idea behind controlling systems remotely is quite comprehensible with the techniques previously described to control systems locally. As all devices are represented by individual autonomous cells, which offer a dedicated RPC interface as connection to the Internet, they can be used from anywhere. This means it is quite easy to take a local GUI client for an autonomous coordination cell and use it miles away on a separate computer to control the cell remotely (see the illustration in Fig. 5.1). As long as the systems detect and react to errors autonomously within the specified time, it results in a risk only slightly greater than for local control.
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Notes
- 1.
Each autonomous control cell could be seen as a single agent. But as their main focus is not on coordination tasks, and as they are very specialized, the autonomous coordination cell can be seen as the agent for the observing system.
- 2.
Mobile agents are usually used to solve huge calculations, where the agents migrate over the network to the place with the best conditions for a dedicated task, taking the algorithms and the data used with them.
- 3.
A similar strategy is used at the Liverpool robotic telescope to share observation times between the astronomical institutes.
- 4.
The red shifts of the quasars indicate cosmological distances, and their emission must be powered by mass flows into massive black holes (Plag and Pearlman 2009, l.c. page 259).
- 5.
During the correlation process, the different signals are shifted along the time axis until they are «congruent» and offer a maximum correlation amplitude.
- 6.
The geodetic, geometric reference point is defined by the axes intersection of the azimuth and elevation axis. But physically the reference point is in the feed-horn where the radio signals are focused and converted to electrical signals transmitted on a wave guide. Both points are in relation to each other.
- 7.
A «cold-head» is an expander using a compression and expansion space, and a displacer to cool down to a few Kelvin in a cryocooler system.
- 8.
The standard equipment is the Mark-III rack with the IF distributor, the 16 video or baseband converters (usually 14 are used for geodetic experiments), and the formatter. «The IF distributor provides two totally independent channels (IFI and IF2) for distribution of IF signals to the video converters. Each channel includes a switch for selecting one of two IF inputs, a step attenuator, a power divider [or filters] for splitting the IF into two different frequency bands (upper and lower sub-channels [, upper sub-channels from 216 to 504 Megahertz and lower sub-channels from 96 to 224 Megahertz (Kilger 1990, l.c. page 48)]), bandpass filters, and eight-way power dividers to send the IF signals to the converters». ((Field System Team 1997a) l.c. page MKIII-8) The old Mark-IV formatters are nowadays upgraded to VLBI Standard Interface (VSI) formatters. In modern systems, such as VGOS compatible systems, the whole rack is replaced by a Digital Back End (DBE) or Digital Baseband Converter (DBBC), solving the same task digitally on an FPGA.
- 9.
For astronomical observations, a 2-bit sampling can be selected.
- 10.
The standard equipment are Mark 5 systems of different versions like Mark 5A, Mark 5B, or Mark 5C, which use a stack of eight hard drives to record the data with high data rates of one Gigabit per second and more.
- 11.
Increasing network capacities, new technologies, and higher transfer rates make the direct transmission of large data sets through the Internet possible.
- 12.
DiFX means digital correlation where the Fourier transformation (symbolized by the «F») is done before the cross-correlation, symbolized by the «X». Software correlators replaced the hardware correlators, using hardware boards with integrated circuits to process the correlation, as nowadays FPGAs can be used to set up programmable correlation arrays.
- 13.
In laser ranging systems almost every component is individual and must be controlled individually.
- 14.
Special functions in the control system, like the NASA Field System, support a pointing test, e.g., «fivept» in the Field System.
- 15.
1 microsecond = 2.5 picosecond one-way delay.
- 16.
Currently a vertical position accuracy of about three millimeters is possible, because of the limiting factors of the per-observation delay measurement error (Niell et al. 2005, l.c. page 4).
- 17.
FILA10G is a double port 10 gigabit per second VLBI-Ethernet interface, formatting raw VLBI for the recording on high-speed recording systems.
- 18.
At this point, special thanks should be extended to Ed Himwich from the Goddard Space Flight Center, NASA, NVI Inc. for his support, explanations, openness for new developments, and directness in responses during the development of new remote control components for the NASA Field System.
- 19.
The formatter is the digitizer of system.
- 20.
The original NASA Field System was written for the Hewlett-Packard Real-Time Executive operating system, which required a special structure and design of the control system. To enable a further use, it was ported to the UNIX System V, which offered similar structures for shared memory, IPC, program scheduling, semaphores, suspending and resuming, etc., and which could be converted easily to more modern Linux operating systems such as Debian. It was only necessary to emulate some of the original Hewlett-Packard Real-Time Executive features (Field System Team 1997b, l.c. page ARCH-26 ff.).
- 21.
With autonomous systems, it might take too long if system states are only checked each 20 seconds. Therefore, it is necessary to implement additional, independent monitoring tools or to extend the existing ones.
- 22.
The VEX2 format is currently under specification. Information can be found under 7 https://safe.nrao.edu/wiki/bin/view/VLBA/Vex2community.
- 23.
For example, «schedule=i14167wz,#1», where «i14167wz» is the experiment name and «#1» is the line number at which the processing should start.
- 24.
This cast can easily be hidden if a new type definition is realized for the pointer, e.g., «typedef void * ACUDescriptor;».
- 25.
«Hot pluggable» means that hardware or software can be exchanged without influencing the rest of the system during operations.
- 26.
«Cold standby» means that hardware or software can be exchanged after the equipment is shut down and off.
- 27.
But with a real web browser and standard HTML, the software implementations, like the automatic safety device, become much more difficult because additional, stateless layers are integrated, so that the server cannot monitor its clients so well.
- 28.
«Hot» spares can be replaced during observations without any losses. «Cold» spares require the processing to stop while the replacement is performed.
- 29.
Unlike the control hierarchy described, where always the next higher level requests data (data pulling) and sends orders, it is also possible to stream data to centralized servers (data pushing). This technique is usually used for the forwarding of monitoring data (see also . Fig. 5.12), but under certain circumstances it can also be used to return feedback to the telescopes, which can be used for control decisions.
- 30.
The reason for the use of Secure Copy (SCP) instead of the standard RPC communication is that also legacy systems without «e-RemoteCtrl» can support that service. They just have to send status files with the specified structure to the service, using the requested key file for the authentication. But legacy systems are not allowed to add the «[eRC]» tag to the status text.
- 31.
The output of the content of a directory with the command «ls -al» prints such flag settings as well as the file mode which defines the file type.
- 32.
A different mechanism is the stream cipher, which encrypts data bit- or byte-wise. An example is the ARCFOUR (RC4) cipher (Barrett et al. 2005, l.c. page 88).
- 33.
The size of the secret key defines the security level reached for the encryption.
- 34.
The password information is split into three files under Linux: «/etc/password» with the general user information, «/etc/group» with the information about the group assignments, and «/etc/shadow» with the encrypted, «shadowed» password (Kofler 2010, l.c. page 504).
- 35.
Such a «setuid» or «setgid» permission flag can be set as superuser with the commands «chown root <executable>» and «chmod 4555 <executable>», where <executable>» is the name of the program file. The flags for the file rights are then «r-sr-xr-x».
- 36.
A very well-known example of reading private data with this method is the «Heartbleed» bug in the Open SSL implementation, which was a security leak in several SSH implementations.
- 37.
Originally «bash» comes from «bourne again shell», as it was a follow-on of the original Bourne shell on Unix systems, which was besides Korn and C shell the main commando input terminal.
- 38.
with «man rbash».
- 39.
«/» is the path delimiter.
- 40.
It is only important to be careful with programs, like the editor «vi», which makes it possible to return to a local shell within the program run, as it might be possible to open a free «bash».
- 41.
«iptables» in firewalls are used as a form of ACLs.
- 42.
Two policy approaches are conceivable: everything is allowed what is not prohibited, or everything is prohibited what is not explicitly permitted. The used policy depends on the network owner. But companies usually use the second policy type, so that explicit permissions must be granted for each service. This increases the controlled security (see (Gerloni et al. 2004, l.c. page 177 f.).
- 43.
Active rules of filter tables can be printed with the command «iptables –list».
- 44.
The example uses the long variant of command parameters, which intuitively describe its meaning. As the command «iptables» is very complex, only a short overview can be given here.
- 45.
Therefore, it is also known as public-key algorithm.
- 46.
The public key of the sender can be automatically transferred to the receiver during the establishment of the communication to encrypt the responding answer messages.
- 47.
A «primitive root», also called a «primitive generator» r for a dedicated prime number p, is given if the result of r n modulo p goes through all numbers from 1 to (p-1) for the entered numbers n = 1 ... (p − 1). For example: 3 is a primitive root of the prime number 5 (Morgan 2003).
- 48.
They must fulfill additional, dedicated rules.
- 49.
A session is a communication process which starts when one of the communication partners initiates the communication and ends if one of them closes the communication channels. After a session is closed, a new one must be initiated for a following communication.
- 50.
SSH uses «zlib» for its data compression (Barrett et al. 2005, l.c. page 91).
- 51.
There are also other authentication methods available under SSH , for example, host-based or keyboard-interactive, but these are not so relevant here.
- 52.
That more modern versions of Open SSL also have such potential risks was shown in the case of the well-known Heartbleed virus (see 7 Sect. 5.5.1).
- 53.
«127.0.0.1» addresses the local machine of the SSH server, where the tunnel arrives. It is even possible to address another machine in the same network of the server machine. The SSH tunnel would then be used to forward communication streams to this other machine.
- 54.
«-p 22» defines that the SSH daemon on the remote server machine listens on port 22, which is the standard SSH port.
- 55.
It cannot be avoided completely, as the checksum algorithms usually do not map uniquely between an arbitrarily long message block to a smaller, fixed checksum. This can lead to ambiguities.
- 56.
«RIPE» stands for «RACE Integrity Primitives Evaluation», where «RACE» are the «Research and development in Advanced Communication tEchnologies», a project of the European Union.
- 57.
The password is also hosted as an encrypted version in the configuration file to validate the user identity.
- 58.
In the case of remote control safety, supervisors can be treated as superusers.
- 59.
After a longer network blackout, only a supervisor or superuser and the former permitted remote operator can continue the control.
- 60.
The X Window System is the most widely used graphical display system under Linux, which is based on client-server architectures. It uses the X network protocol for the communication.
- 61.
Special thanks to Thomas Klügel for the underlying station map of the observatory Wettzell.
- 62.
VLAN setups or suitable Domain Name Service (DNS) entries allow the use of internal addresses also in the external network. But this might be a security risk, as all protected network addresses from the internal network are directly usable and visible in the potentially dangerous, external network.
- 63.
For example, firewalls are also used for the load balancing to different servers which offer the same service.
- 64.
«-p 22000» defines that the dedicated firewall port which is forwarded to the target machine with «iptables» is port number 22000.
- 65.
«Host» in this context means the SSH server.
- 66.
A real proxy firewall actually separates the two communications from the client to the firewall and from the firewall to the server and is not only a simple forwarding of the same data stream.
- 67.
During a spoofing attack, an offender sends IP packages with a faked, internal address from the external network or simulates a DNS service to redirect communications (DNS Spoofing) (Gerloni et al. 2004, l.c. page 203).
- 68.
While the SSH tunneling method described is an application-specific assurance of the data security, VPN is based on an extension to the IP standard with security mechanisms. This new IPSec standard implements a possibility for authentication with an IP Authentication Header and an encrypted communication with the IP Encapsulating Security Payload functionality on the lower protocol layers. This can be used to create different virtual network topologies over the WAN , like a Gateway-Gateway-VPN between two locations of a company (Gerloni et al. 2004, l.c. page 284 ff.), so that computers in both networks seem to be part of only one common network.
- 69.
A multi-homed host has several physical interfaces with several, different networks behind, which are shielded against each other.
- 70.
For example, with a «demilitarized zone» which is used as an additional network layer between the Internet and the LAN.
- 71.
Originally a terminal or console was the physical interface to a command shell. Hardware offered access to enter commands and to receive answers in the same serial text style as earlier with the teletype writers. Therefore, the usual acronym for a terminal is TeleTYpe (TTY). Nowadays, terminals are only software emulations of such former terminals within a graphical desktop on a screen.
- 72.
«stdin» has the file descriptor ID «0».
- 73.
In the case of monitor and control of external applications, this technique is used in combination with «e-RemoteCtrl» in the form of an «e-shell», which offers access to inputs and outputs in the form of Sun RPC servers.
- 74.
The address is only an artificial one, and the link to a potentially existing computer with this address might be just accidental.
- 75.
Steganography is a technique to hide secret information cleverly within a harmless carrier data set.
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Neidhardt, A.N.J. (2017). Controlling a VLBI System Remotely. In: Applied Computer Science for GGOS Observatories. Springer Textbooks in Earth Sciences, Geography and Environment. Springer, Cham. https://doi.org/10.1007/978-3-319-40139-3_5
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