The IEC international technical standard for AIS allows two operating modes for AtoN AIS.

These are;
• RATDMA [Random Access Time Division Multiple Access]
• FATDMA [Fixed Access Time Division Multiple Access].

RATDMA An AtoN AIS unit operating in RATDMA mode uses its receiver to listen to both AIS frequencies for about one minute, and makes and stores a map of all the AIS "slots" [or message spaces] on the VHF data link [VDL]. It then looks for two free adjacent slots in which to send its [2-slot long] AtoN message 21 or meteorological and hydrological message 8.

This mode is ideal for many applications because the AtoN or weather/tide AIS unit can be placed at any location, and requires no reservation of slots by a base station. It can be used whether base stations exist in the area or not. Its drawback is that in order to make the "slot map" the unit must turn its receiver on for at least one minute before transmitting, and this is the main power consuming factor with an RATDMA AtoN or weather AIS unit.

FATDMA An AtoN AIS unit operating in FATDMA mode will transmit in a pair of slots which are reserved by an AIS base station. Ships receive a message from the base station, indicating that certain slots are reserved. The ship AIS transponders store this reserved slot information in their slot maps, and do not transmit in these slots. The FATDMA AtoN or weather unit is programmed to transmit in two consecutive slots of those reserved by the base station. It is possible to "re-cycle" slots by having a number of AtoN units use the same pair of slots, but use them sequentially.

FATDMA allows greatly reduced power drain for an AtoN AIS unit, because no receiving period to build a slot map is needed.

What is Carrier Sense TDMA? Can I use it for AtoN AIS?

Carrier Sense [CSTDMA] Carrier Sense technology is being developed for "Class-B" ship transponders. Class B transponders will be suitable for use on non-SOLAS vessels, fishing boats, pleasure craft, etc. Class B CSTDMA transponders will send messages which are 1-slot long.
Carrier Sense is not suitable for AtoN AIS, or for broadcasting meteorological and hydrological messages. The reason is as follows.

The Carrier Sense concept does not employ a "slot map" and so a CSTDMA unit cannot know in advance what slots are occupied and what slots might be free. It also does not rely on a base station reserving slots. Instead the CS unit listens at the start of a slot to see if a transmission is starting [e.g. from a nearby ship]. If it determines that the slot is not being used, it quickly powers up its transmitter and sends a 1-slot message and then powers down.

If this technique were applied to AtoN or weather messages, which are two slots long, the CS unit could only determine if the first of the two slots were free. However the 2-slot message would of course run on into the second slot, which may have been already reserved by a nearby ship or base station. Thus there is a great possibility, especially in busy waters, that both the ship message and the CS 2-slot message will clash and neither will be readable.

Love him or hate him, he sure hits the nail on the head with this!

To anyone with kids of any age, here's some advice. Bill Gates recently gave a speech at a High School about 11 things they did not and will not learn in school. He talks about how feel-good, politically correct teachings created a generation of kids with no concept of reality and how this concept set them up for failure in the real world.

Rule 1: Life is not fair -- get used to it!

Rule 2: The world won't care about your self-esteem. The world will expect you to accomplish something BEFORE you feel good about yourself.

Rule 3: You will NOT make $60,000 a year right out of high school. You won't be a vice-president with a car phone until you earn both.

Rule 4: If you think your teacher is tough, wait till you get a boss.

Rule 5: Flipping burgers is not beneath your dignity. Your Grandparents had a different word for burger flipping -- they called it opportunity.

Rule 6: If you mess up, it's not your parents' fault, so don't whine about your mistakes, learn from them.

Rule 7: Before you were born, your parents weren't as boring as they are now. They got that way from paying your bills, cleaning your clothes and listening to you talk about how cool you thought you are. So before you save the rain forest from the parasites of your parent's generation, try delousing the closet in your own room.

Rule 8: Your school may have done away with winners and losers, but life HAS NOT. In some schools they have abolished failing grades and they'll give you as MANY TIMES as you want to get the right answer. This doesn't bear the slightest resemblance to ANYTHING in real life.

Rule 9: Life is not divided into semesters. You don't get summers off and very few employers are interested in helping you FIND YOURSELF. Do that on your own time.

Rule 10: Television is NOT real life. In real life people actually have to leave the coffee shop and go to jobs.

Rule 11: Be nice to nerds. Chances are you'll end up working for one.

From Wikipedia, the free encyclopedia

Earth as seen from Apollo 17

Modern geologists and geophysicists consider the age of the Earth to be around 4.54 billion years (4.54 × 10⁹ years ± 1%).^[1][2] This age has been determined byradiometric age dating of meteorite material and is consistent with the ages of the oldest-known terrestrial and lunar samples.

Following the scientific revolution and the development of radiometric age dating, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.^[3] The oldest such minerals analysed to date – small crystals of zircon from the Jack Hills of Western Australia – are at least 4.404 billion years old.^[4][5][6] Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system cannot be much older than those rocks. Ca-Al-rich inclusions (inclusions rich in calcium and aluminium) – the oldest known solid constituents within meteorites that are formed within the solar system – are 4.567 billion years old,^[7][8] giving an age for the solar system and an upper limit for the age of Earth. It is hypothesised that the accretion of Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Because the exact accretion time of Earth is not yet known, and the predictions from different accretion models range from a few millions up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. The Acasta Gneiss of Northern Canada may be the oldest known exposed crustal rock.^[9]

Scientists have just confirmed the existence of Dark Matter. This mysterious Dark Matter is invisible however it provides the bulk of gravity that holds galaxies (not the regular matter that forms stars and planets). This invisible Dark Matter has weird collision properties (collisionless). Simply put: we cannot see Dark Matter nor collide with it but we can detect its gravity. In this image, two huge clusters of invisible Dark Matter (in blue) clashed with each other.

With each cluster having a mass of 10,000 galaxies (each cluster a quadrillion times the mass of the Sun, 10¹⁵)!!! (See: Universe Today). However upon impact, instead of disintegrating into smaller pieces, they passed through each other unscaved! This means that they did not collide with each other either! They simply passed through each other!!!

In order to explain why this invisible Dark Matter neither collided with each other, nor did it collide with us but still we detected its gravity, physicists and cosmologists are working on theories with extra dimensions:

"This means that the world at very small scales is not made up of particles living in three spacial dimensions, but perhaps strings and branes living in nine dimensions. String theories are the most active area of research in physics today..."

Gravity, Black Holes, and the Very Early Universe; T.Chow, Springer 2008 (p. 108)

Traditionally scientists thought that we live in a four dimensional universe: Time and the usual three spacial dimensions of length, width and height (x,y,z). However today there is evidence for six extra spacial dimensions. So we cannot see nor collide with this Dark Matter because it is mass in the remaining six extra dimensions.

According to the Quran, we cannot see nor collide with Jinn but they have weight:

[Quran 55.31] We [Allah] will settle your affair, both you of weight (man and jinn)

The Jinn have weight means that we can detect their gravity and they can detect our gravity. So according to the Quran we cannot see the Jinn nor collide with them but we can detect their gravity.

Just as the Jinn is of different type that we cannot see nor collide with but we can detect their gravity, there are six other Heavens that we cannot see nor collide with either but we can detect their gravity, superimposed above the visible one:

[Quran 41.12] So [Allah] decreed them as seven heavens (one above the other) in two days and revealed to each heaven its orders. And We [Allah] adorned the lowest heaven with lights, and protection. Such is the decree of the Exalted; the Knowledgeable.

According to the Quran, only the lowest heaven has visible light. This means that this Dark Matter exists in the six Heavens superimposed above the lowest one. Also according to the Quran, each of these remaining six Heavens is of a different type and each has its own planets like Earth:

[Quran 65.12] Allah is the one who created seven Heavens and from Earth like them (of corresponding type); [Allah’s] command descends among them so that you may know that Allah is capable of anything and that Allah knows everything.

Earth is not a unique planet in Islam. Other planets like Earth do exist throughout the other six Heavens. It is just that we cannot see them nor collide with them but we can detect their gravity.

General relativity predicted gravitational lensing, that is, the gravitational field generated by a galaxy causes the light passing through it to bend (change direction). Dark Matter does not emit any light but scientists can still map its location by using gravitational lensing, that is, by detecting where light is bending in places where it shouldn't. In the above image and video light gets bent by invisible dark matter in places where it shouldn't: Moslems say that this is how God challenges nonbelievers to detect the remaining six superimposed Heavens:

[Quran 67.3-4] [Allah] is the one who created seven superimposed Heavens. You do not see variations in the formations of the Compassionate, so redirect your sight, do you see any creation? Then redirect your sight again, your vision returns to you in defeat and regret.

Today they looked again and found those creations indeed.