Wednesday, July 30, 2008

Strapdown systems

Lightweight digital computers permit the system to eliminate the gimbals, creating "strapdown" systems, so called because their sensors are simply strapped to the vehicle. This reduces the cost, eliminates gimbal lock, removes the need for some calibrations, and increases the reliability by eliminating some of the moving parts. Angular rate sensors called "rate gyros" measure how the angular velocity of the vehicle changes.

A strapdown system has a dynamic measurement range several hundred times that required by a gimbaled system. That is, it must integrate the vehicle's attitude changes in pitch, roll and yaw, as well as gross movements. Gimballed systems could usually do well with update rates of 50 to 60 updates per second. However, strapdown systems normally update about 2000 times per second. The higher rate is needed to keep the maximum angular measurement within a practical range for real rate gyros: about 4 milliradians. Most rate gyros are now laser interferometers.

The data updating algorithms ("direction cosines" or "quaternions") involved are too complex to be accurately performed except by digital electronics. However, digital computers are now so inexpensive and fast that rate gyro systems can now be practically used and mass-produced. The Apollo lunar module used a strapdown system in its backup Abort Guidance System (AGS).

Strapdown systems are nowadays commonly used in commercial and tactical applications (arcraft, missiles, etc). However they are still not widespread in applications where superb accuracy is required (like submarine navigation or strategic ICBM guidance).

Wednesday, July 23, 2008

A 'Transaction Processing SystemTo be considered a transaction processing system the computer must pass the ACID test.

From a technical perspective, a Transaction Processing System (or Transaction Processing Monitor) monitors transaction programs, a special kind of programs. The essence of a transaction program is that it manages data that must be left in a consistent state. E.g. if an electronic payment is made, the amount must be either both withdrawn from one account and added to the other, or none at all. In case of a failure preventing transaction completion, the partially executed transaction must be 'rolled back' by the TPS. While this type of integrity must be provided also for batch transaction processing, it is particularly important for online processing: if e.g. an airline seat reservation system is accessed by multiple operators, after an empty seat inquiry, the seat reservation data must be locked until the reservation is made, otherwise another user may get the impression a seat is still free while it is actually being booked at the time. Without proper transaction monitoring, double bookings may occur. Other transaction monitor functions include deadlock detection and resolution (deadlocks may be inevitable in certain cases of cross-dependence on data), and transaction logging (in 'journals') for 'forward recovery' in case of massive failures.

Transaction Processing is not limited to application programs. The 'journaled file system' provided with IBMs AIX Unix operating system employs similar techniques to maintain file system integrity, including a journal.

Wednesday, July 16, 2008

Media ecology

In 1977, Marshall McLuhan said that media ecology "means arranging various media to help each other so they won't cancel each other out, to buttress one medium with another. You might say, for example, that radio is a bigger help to literacy than television, but television might be a very wonderful aid to teaching languages. And so you can do some things on some media that you cannot do on others. And, therefore, if you watch the whole field, you can prevent this waste that comes by one canceling the other out."

Inspired by McLuhan, Neil Postman founded the Program in Media Ecology at New York University in 1971. He described it as

Media ecology looks into the matter of how media of communication affect human perception, understanding, feeling, and value; and how our interaction with media facilitates or impedes our chances of survival. The word ecology implies the study of environments: their structure, content, and impact on people.

Thursday, July 10, 2008

Object database

In an object database (also object oriented database), information is represented in the form of objects as used in object-oriented programming. When database capabilities are combined with object programming language capabilities, the result is an object database management system (ODBMS). An ODBMS makes database objects appear as programming language objects in one or more object programming languages. An ODBMS extends the programming language with transparently persistent data, concurrency control, data recovery, associative queries, and other capabilities.

Some object-oriented databases are designed to work well with object-oriented programming languages such as Python, Java, C#, Visual Basic .NET, C++ and Smalltalk. Others have their own programming languages. ODBMSs use exactly the same model as object-oriented programming languages.

Wednesday, July 02, 2008

E-mail spam

E-mail spam, also known as "bulk e-mail" or "junk e-mail," is a subset of spam that involves nearly identical messages sent to numerous recipients by e-mail. A common synonym for spam is unsolicited bulk e-mail (UBE). Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. "UCE" refers specifically to "unsolicited commercial e-mail."

E-mail spam slowly but exponentially grew for several decades to several billion messages a day. Spam has frustrated, confused, and annoyed e-mail users. Laws against spam have been sporadically implemented, with some being opt-out and others requiring opt in e-mail. The total volume of spam (over 100 billion emails per day as of April 2008) has leveled off slightly in recent years, and is no longer growing exponentially. The amount received by most e-mail users has decreased, mostly because of better filtering. About 80% of all spam is sent by fewer than 200 spammers. Botnets, networks of virus-infected computers, are used to send about 80% of spam. The cost of spam is borne mostly by the recipient, so it is a form of postage due advertising.