Site Map - skip to main content - dyslexic font - mobile - text - print

Hacker Public Radio

Your ideas, projects, opinions - podcasted.

New episodes Monday through Friday.

Please support our Patrons

Our hosting is kindly provided by Josh from We would appreciate it if you could donate to help reduce his costs in funding the hosting. He is also accepting bitcoins to 1KsxJr9HtsdaUeU7yaV9bk9bQi21UPBtUq
Please also consider supporting the who are now hosting our media files. Right now, a generous supporter will match your contributions 3-to-1. So your $5 donation results in $20 for the Internet Archive.

In-Depth Series

April Fools Shows

How to Point a Satellite Dish - Ken Fallon | 2016-04-01

This show is dedicated to Procrastination, the avoidance of doing a task which needs to be accomplished.

I've been trying to record this particular show for ages but I can never seem to finish it. I find the topic just too interesting. When I start then I get distracted by some other aspect. Every time I try to record it Murphy gets in the way, with lost recordings and broken cards etc. This is the email that prompted this show.

-------- Forwarded Message --------
Subject: TWAT - Satellite communications
Date: Tue, 13 Dec 2005 12:00:54 +0100

Hi Droops,

I heard your call for content and I'd like to send you some shows. I
don't have a lot of spare time with work and a young family so I can't
do a regular show but I can send you a series on a topic. I was
thinking of doing a series on Satellite Communications.

So after eleven years, I set the deadline of episode 2000 to force myself to finish this show.

Let's start.

What are orbits ?

In physics, an orbit is the gravitationally curved path of an object about a point in space, for example the orbit of a planet about a star or a natural satellite around a planet. Orbits of planets are typically elliptical, and the central mass being orbited is at a focal point of the ellipse.

Newton's cannonball was a thought experiment Isaac Newton used to hypothesize that the force of gravity was universal, and it was the key force for planetary motion. It appeared in his book A Treatise of the System of the World.

  • No orbit
  • Suborbital trajectories, trajectory intersects the atmosphere so that it does not complete one orbital revolution.
  • Orbital trajectories (or simply "orbits")
  • Open (or escape) trajectories

It is worth noting that orbital rockets are launched vertically at first to lift the rocket above the atmosphere (which causes frictional drag), and then slowly pitch over and finish firing the rocket engine parallel to the atmosphere to achieve orbit speed.

Once in orbit, their speed keeps them in orbit above the atmosphere. If e.g., an elliptical orbit dips into dense air, the object will lose speed and re-enter (i.e. fall). Occasionally a space craft will intentionally intercept the atmosphere, in an act commonly referred to as an aerobraking maneuver.

Types of orbits

There are many ways to classify orbits

The choice of which orbit to use is based on the intended purpose of the satellite.

  • Centric classifications: Based on what they orbit
  • Altitude classifications: Based on how high they are
  • Inclination classifications: Based on the angle of rotation with respect to the Equator.
  • Eccentricity classifications: Based on their path
  • Synchronicity classifications: Based on how often they rotate

Low Earth orbit (LEO)

0 to 2,000 km (0–1,240 miles).

  • 0 km / mi - Sea Level.
  • 37.6 km / 23.4 mi - Self Propelled Jet Aircraft Flight Ceiling (Record Set in 1977).
  • 215 km / 133.6 mi - Sputnik-1 The first artificial satellite of earth.
  • 340 km / 211.3 mi - International Space Station.
  • 390 km / 242.3 mi - Former Russian Space Station MIR.
  • 595 km / 369.7 mi - Hubble Space Telescope.
  • 600 - 800 km / 372.8 - 497.1 mi - Sun-synchronous Satellites.

These satellites orbit the Earth in near exact polar orbits north to south. They cross the equator multiple times per day and each time they are at the same anglewith respect to the sun. Satellites on these types of orbits are particularly useful for capturing images of the Earth’s surface or images of the sun

Medium Earth orbit (MEO)

Geocentric orbits ranging in altitude from 2,000 km (1,240 miles) to just below geosynchronous orbit at 35,786 kilometers (22,236 mi).

GPS (Global Positioning System) Satellites reside here. These Satellites are on a Semi-synchronous Orbit (SSO) meaning that they orbit the earth in exactly 12 hours (twice per day)

Geosynchronous orbit (GSO) and Geostationary orbit (GEO)

Orbits around Earth matching Earth's sidereal rotation period. 42,164 km (26,199 mi). Sidereal time is a "time scale that is based on the Earth's rate of rotation measured relative to the fixed stars" rather than the Sun.

Geosynchronous satellites orbit the Earth at the same rate that the Earth rotates. Thus they remain stationary over a single line of longitude. A geostationary satellite will remain in a fixed location as observed from the surface of the earth, allowing a satellite dish to be alligned to it.


High Earth orbit

Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,240 miles).

Polar orbits

They are often used for earth-mapping, earth observation, capturing the earth as time passes from one point, reconnaissance satellites, as well as for some weather satellites. The Iridium satellite constellation also uses a polar orbit to provide telecommunications services. The disadvantage to this orbit is that no one spot on the Earth's surface can be sensed continuously from a satellite in a polar orbit.

Molniya orbit

Orbita was a system that consisted of 3 highly elliptical Molniya satellites, Moscow-based ground uplink facilities and about 20 downlink stations, located in cities and towns of remote regions of Siberia and Far East. Each station had a 12-meter receiving parabolic antenna and transmitters for re-broadcasting TV signal to local householders.

A perfectly scaled diagram showing the orbital altitudes of several significant satellites of earth. all planets and orbital distances are drawn to scale and the altitude data was collected from many Wikipedia articles and various other sites.

Atmospheric electromagnetic opacity

Atmospheric electromagnetic opacity


Sputnik 1

Sputnik 1 was the first artificial Earth satellite. The Soviet Union launched it into an elliptical low Earth orbit on October 4, 1957. It was a 58 cm (23 in) diameter polished metal sphere, with four external radio antennae to broadcast radio pulses. It was visible all around the Earth and its radio pulses were detectable. This surprise success precipitated the American Sputnik crisis and triggered the Space Race, a part of the larger Cold War. The launch ushered in new political, military, technological, and scientific developments.

ProtoStar II Mission Overview


ASTRA 2G is the third spacecraft of a three satellite investment programme (ASTRA 2E, 2F and 2G) that SES contracted with Airbus Defence and Space in order to provide replacement as well as incremental satellite capacity in the orbital arc of 28.2/28.5 degrees East.

ASTRA 2G carries 62 Ku-band transponders as well as 4 Ka-band transponders. The different beams provide coverage over the UK and Ireland, Europe and West Africa.

The footprint over Ireland and the UK

Components of a Communications Satelites

  • Rocket motors
  • Fuel tanks
  • Solar panels
  • Batteries
  • Computer
  • Antennas and transceivers/transponders

The word "transponder" is derived from the words "transmitter" and "responder."

A communications satellite's transponder is the series of interconnected units that form a communications channel between the receiving and the transmitting antennas. It is mainly used in satellite communication to transfer the received signals.

A transponder is typically composed of

  • An input band limiting device (a band pass filter)
  • An input low-noise amplifier (LNA), designed to amplify the (normally very weak, because of the large distances involved) signals received from the earth station
  • A frequency translator (normally composed of an oscillator and a frequency mixer) used to convert the frequency of the received signal to the frequency required for the transmitted signal
  • An output band pass filter
  • A power amplifier (this can be a traveling-wave tube or a solid state amplifier)

Boeing commercial communications satellites geosynchronous orbit

boeing commercial communications satellites geosynchronous orbit

Finding Astra 28.2E

This is one of the many sites that will give you a birds eye view of where you need to point your dish.

Terms needed when pointing a dish

The azimuth is the angle formed between a reference direction (North) and a line from the observer to a point of interest projected on the same plane as the reference direction orthogonal to the zenith

screen shot of the dispointer page

The Dish, on Kens Roof.

Ken Sat Dish

Reading the elevation from the dish assembley.


Freesat is broadcast from the same satellites (Astra 28.2E and Eurobird 1) as Sky Digital.

This is a list of all of the free-to-air channels that are currently available via satellite from SES Astra satellites (Astra 2E/2F/2G) located at 28.2 °E.

Here is a link to a page on how to get mythtv working with FreeSat.

Credit Card PIN breach - Various Hosts | 2015-04-01

On September the 10th, 2012 an anonymous malicious hacker released 10,000 pin codes onto the site paste bin dot com. How the attacker gained access to the codes is not known, but it is thought that it may be linked to a breach that occurred at the end of March 2012 to the Credit card processor Global Payments. That attack exposed 1.5 million consumers financial data. These codes have been confirmed by security experts to be legitimate and in wide spread use even today. Despite this exposure been "common knowledge" among the security community, major banks and credit card companies have yet to issue any statement on the breach.

Tired of waiting for action by big business, we bring you a list of the codes so you can check for yourself if your data is compromised.

OSI layer 3 - Various Hosts | 2014-04-01

In today's show we continue our look at The OSI model for network communications, with examples of Layer 3 been given with particular focus on Geography diverse Host addressing.

From Wikipedia:

In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.


The network layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.

Functions of the network layer include:

  • Connection model: connectionless communication
For example, IP is connectionless, in that a datagram can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other, higher layers of the OSI model.
  • Host addressing
Every host in the network must have a unique address that determines where it is. This address is normally assigned from a hierarchical system. For example, you can be "Fred Murphy" to people in your house, "Fred Murphy, 1 Main Street" to Dubliners, or "Fred Murphy, 1 Main Street, Dublin" to people in Ireland, or "Fred Murphy, 1 Main Street, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addresses.
  • Message forwarding
Since many networks are partitioned into subnetworks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers, to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.

Within the service layering semantics of the OSI network architecture, the network layer responds to service requests from the transport layer and issues service requests to the data link layer.

Digital Data Transfer - Ken Fallon | 2013-04-01

In this the first in a series exploring The Open Systems Interconnection (OSI) model (ISO/IEC 7498-1)

OSI model
From Wikipedia, the free encyclopedia

The Open Systems Interconnection (OSI) model (ISO/IEC 7498-1) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. A layer serves the layer above it and is served by the layer below it.

For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.

In today's show Ken starts off with a practical example of Layer One, the The Physical Layer, Although we are limited to audio for the purposes of the show, the same techniques could and are used across the light spectrum.