GPS antenna location | YBW Forum

If you talk to Garmin support they will tell you it works perfectly well indoors. We have three and they are all installed under the cockpit coaming, in the lazarette or at the chart table.

Quite possibly not in a steel boat though.

Just pick a convenient place on deck, which will depend on your particular boat. On the pushpit is quite common but I don't really like this as they can be vulnerable to snagging with mooring lines etc. Mine happens to be flat on the deck (it's like an inch-thick hockey puck) just aft of the mast and slightly to one side.

High up (which it sounds like you have in mind talking about VHF antennas?) is not good for them, as the rolling of the boat means they're moving from side to side instead of being able to settle on one position.

Pete

3mm of fibreglass can have no effect, or lots, the devil is in the detail as we say.
3 mm of grp with a good film of salt water running over it can be in a different league.

GPS is a digital system. It works up to a certain level of signal degradation then fails catastrophically.

Personally, I think the pushpit is a good compromise.

I am not a scientist so I can only speak from experience.

Satellite signals come from above, so who knows if a steel hull makes and difference at all. Is the OP's boat steel coach roof as well?

Based on Garmin advice our aerials are under GRP and a whole load more than 3mm probably more like 10mm.

We have never had a problem, either in rough seas, torrential rain, or even with a couple of inches of snow.

Surely the pragmatic thing to do is to temporarily install the aerial somewhere convenient near the plotter or whatever else it is serving. If it works fine then fix it permanently. If it doesn't find a better place.

Why drill holes in your boat and run long cables if there is no need?

I am not a scientist so I can only speak from experience.

Satellite signals come from above, so who knows if a steel hull makes and difference at all. Is the OP's boat steel coach roof as well?

Based on Garmin advice our aerials are under GRP and a whole load more than 3mm probably more like 10mm.

We have never had a problem, either in rough seas, torrential rain, or even with a couple of inches of snow.

Surely the pragmatic thing to do is to temporarily install the aerial somewhere convenient near the plotter or whatever else it is serving. If it works fine then fix it permanently. If it doesn't find a better place.

Why drill holes in your boat and run long cables if there is no need?

The trouble with this approach is that you never know if you have tested with the worst level of atmospheric attenuation you might reasonably expect.
Also, the GPS satellites are currently putting out signals well above the stated minimum. As the satellites age, or for other reasons, that could reduce significantly.

We do get people on here with unexplained GPS outages.
In my view, I can envisage situations where anything that delays GPS position after say a power interruption might be a very bad idea.

Last week we were in our motorhome on the Igoumenitsa - Ancona ferry. Sitting in the van aboard the ship waiting to disembark I turned on the Tomtom gps and was astonished when it gave our position after a short wait. We were on deck five, about midships, so six or seven steel decks above us.

That is the trouble with these discussions, the modern receivers are amazingly good up to a point. The GPS in my will sometimes work deep within buildings, sometimes not.
I used to use a tomtom in a Mondeo with a heated screen. 99.9% fine, but just occasionally it would lose the plot under a few trees. Driving in central London it struggled more when surrounded by tall buildings, compared to doing the same journey in another car. It used to think I was going sideways down the wrong side of the road at some points.
GPS is RF at about 1.575GHz, that's about 20cm wavelength, so will go through doorways, companionways etc.
So maybe any tests need to be done with the washboards in and people slinging salt water around in a merry fashion.

Seems easier just to mount it outdoors and know you've done a proper job. GPS reception works best with line-of-sight to the satelites. The antenna can see through GRP and most plastics unless they are coated with a metallic film which will degrade the signal or block it completely.

The GPS first recieves an almanac from the satelites, this tells it where all the satelites are and which ones it should be able to see - sometimes this takes a few seconds to aquire if the system has been off for a long time.

When the individual satelite signals reach the ground they are bounced around by buildings, metal surfaces etc. The effect of this is that they still provide a fix, just not as accurate as one where it is direct line-of-sight. A bit like laser range-finding on an object while standing in a hall of mirrors.

Car navigation systems also deploy map-matching on top of the GPS position (i.e. they assume you are on a road) which with reflected signals can lead to your position jumping around or being false. A gyro records your path and then roads in the vacinity are checked for a match so sometimes the vehicle is placed on a road which is not exactly where the GPS position is.

The advice to trial the system with the antenna in as good a position as you can find (not below a metal deck or in the shadow of metallic objects) is sound. The plotter should have a screen displaying the no. of satelites and the signal strength and fix accuracy. Use this to find a suitable location. Our antenna is directly below the coaming at the rear of the cockpit seats and provides a good fix accuracy and sees almost all of the 'visible' satelites with good signal strength.

I wouldn't be too concerned about the effect of rolling and pitching unless there is a gyro included in the GPS reciever, the GPS signals travel at light speed - in cars it works fine up to 320kmh (I haven't managed to test them any faster )

You also need to take into account the possibility of lines and so on getting caught up on the aerial, which was occasionally a problem when we had ours on the pushpit. They also make very tempting hand-holds for the ignorant on other craft coming alongside.

Agree - I don't really know why this is such a common place when there are so many downsides and no real benefits. Flat to the deck in a place un-walked-upon is much better.

I do mean flat though - I was once on a boat with an old system whose antenna looked like a golf ball on top of a tee, but scaled up to about four or five inches in diameter. It was on the coachroof above the chart table, so just in front of the spray hood. A better shape for catching lines or being kicked would be hard to come up with on purpose. How we didn't rip it off its stalk just in the one week I have no idea.

Pete

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To receive and distribute and authenticated UTC time source there are currently two types of NTP server, the GPS NTP server and the radio referenced NTP server. While both these systems distribute UTC in identical ways the way they receive the timing information differs.

A GPS NTP time server is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.  Each GPS satellite transmits in two frequencies L2 for the military use and L1 for use by civilians transmitted at MHz, Low-cost GPS antennas and receivers are now widely available.

The radio signal transmitted by the satellite can pass through windows but can be blocked by buildings so the ideal location for a GPS antenna is on a rooftop with a good view of the sky. The more satellites it can receive from the better the signal. However, roof-mounted antennas can be prone to lighting strikes or other voltage surges so a suppressor is highly recommend being installed inline on the GPS cable.

The cable between the GPS antenna and receiver is also critical. The maximum distance that a cable can run is normally only 20-30 metres but a high quality coax cable combined with a GPS amplifier placed in-line to boost the gain of the antenna can allow in excess of 100 metre cable runs. This can provide difficulties in installation in larger buildings if the server is too far from the antenna.

An alternative solution is to use a radio referenced NTP time server. These rely on a number of national time and frequency radio transmissions that that broadcast UTC time. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in the USA (WWVB) and in France, Germany and Japan.

A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. It should always be mounted horizontally at a right angle toward the transmission for optimum signal strength. Data is sent in pulses, 60 a second. These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

Methods of keeping track of time have altered throughout history with ever increasing accuracy has being the catalyst for change.

Most methods of timekeeping have traditionally been based on the movement of the Earth around the Sun. For millennia, a day has been divided into 24 equal parts that have become known as hours. Basing our timescales on the rotation of the Earth has been adequate for most of our historical needs, however as technology advances, the need for an ever increasingly accurate timescale has been evident.

The problem with the traditional methods became apparent when the first truly accurate timepieces – the atomic clock was developed in the ’s. Because these timepieces  was based on the frequency of atoms and were accurate to within a second every million years it was soon discovered that our day, that we had always presumed as being precisely 24 hours, altered from day to day.

The affects of the Moon’s gravity on our oceans causes the Earth to slow and speed up during its rotation – some days are longer than 24 hours whilst others are shorter. Whilst this minute differences in the length of a day have made little difference to our daily lives it this inaccuracy has implications for many of our modern technologies such as satellite communication and global positioning.

A timescale has been developed to deal with the inaccuracies in the Earth’s spin – Coordinated Universal Time (UTC). It is based on the traditional 24-hour Earth rotation known as Greenwich Meantime (GMT) but accounts for the inaccuracies in the earth’s spin by having so-called ‘Leap Seconds’ added (or subtracted).

As UTC is based on the time told by atomic clocks it is incredibly accurate and therefore has been adopted as the World’s civilian timescale and is used by business and commerce all over the globe.

Most computer networks can be synchronised to UTC by using a dedicated NTP time server.

A GPS time server is really a communication device. Its purpose is to receive a timing signal and then distribute it amongst all devices on a network. Time server s are often called different things from network time server, GPS time server, radio time server and NTP server.

Most time servers use the protocol NTP (Network Time Protocol). NTP is one of the Internet’s oldest protocols and is used by the majority of machines that use a time server. NTP is often installed, in a basic form, in most operating systems.

A GPS time server, as the names suggests, receives a timing signal from the GPS network. GPS satellites are really nothing more than orbiting clocks. Onboard each GPS satellite is an atomic clock. The ultra-precise time from this clock is what is transmitted from the satellite (along with the satellite’s position).

A satellite navigation system works by receiving the time signal from three or more satellites and by working out the position of the satellites and how long the signals took to arrive, it can triangulate a position.

A GPS time server needs even less information and only one satellite is required in order to receive a timing reference. A GPS time server’s antenna will receive a timing signal from one of the 33 orbiting satellites via line of sight, so the best place to fix the antenna is the roof.

Most dedicated GPS NTP time servers require a good 48 hours to locate and get a steady fix on a satellite but once they have it is rare for communication to be lost.

The time relayed by GPS satellites is known as GPS time and although it differs to the official global timescale UTC (Coordinated Universal Time) as they are both based on atomic time (TAI) GPS time is easily converted by NTP.

A GPS time server is often referred to as a stratum 1 NTP device, a stratum 2 device is a machine that receives the time from the GPS time server. Stratum 2 and stratum 3 devices can also be used as a time servers and in this way a single GPS time server can operate as a timing source for an unlimited amount of computers and devices as long as the hierarchy of NTP is followed.

A time server is a common office tool but what is it for?

We are all used to having a different time from the rest of the world. When America is waking up, Honk Kong is going to bed which is why the world is divided into time zones. Even in the same time-zone there can still be differences. In mainland Europe for instance most countries are an hour ahead of the UK because of Britain’s seasonal clock changing.

However, when it comes to global communication, having different times all over the world can cause problem particularly if you have to conduct time sensitive transactions such as buying or selling shares.

For this purpose it was clear by the early ’s that a global timescale was required. It was introduced on 1 January and was called UTC – Coordinated Universal Time. UTC is kept by atomic clock but is based on Greenwich Meantime (GMT – often called UT1) which is itself a timescale based on the rotation of the Earth. Unfortunately the Earth varies in its spin so UTC accounts for this by adding a second once or twice a year (Leap Second).

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Whilst controversial to many, leap seconds are needed by astronomers and other institutions to prevent the day from drifting otherwise it would be impossible to work out the position of the stars in the night sky.

UTC is now used all over the world. Not only is it the official global timescale but is used by hundreds of thousands of computer networks all over the world.

Computer networks use a network time server to synchronise all devices on a network to UTC. Most time servers use the protocol NTP (Network Time Protocol) to distribute time.

NTP time servers receive the time from atomic clocks by either long-wave radio transmissions from national physics laboratories or from the GPS network (Global Positioning System). GPS satellites all carry an onboard atomic clock that beams the time back to Earth. Whilst this time signal is not strictly speaking UTC (it is known as GPS time) because of the accuracy of the transmission it is easily converted to UTC by a GPS NTP server.

Atomic clocks are used for thousands of applications all over the world. From controlling satellites to even synchronising a computer network using a NTP server, atomic clocks have changed the way we control and govern time.

In terms of accuracy an atomic clock is unrivalled. Digital quartz clocks may keep accurate time for a week, not losing more than a second but an atomic clock can keep time for millions of years without drifting as much.

Atomic clocks work on the principle of quantum leaps, a branch of quantum mechanics which states that an electron; a negatively charged particle, will orbit a nucleus of an atom (the centre) in a certain plain or level. When it absorbs or releases enough energy, in the form of electromagnetic radiation, the electron will jump to a different plane – the quantum leap.

By measuring the frequency of the electromagnetic radiation corresponding to the transition between the two levels, the passage of time can be recorded. Caesium atoms (caesium 133) are preferred for timing as they have 9,192,631,770 cycles of radiation in every second. Because the energy levels of the caesium atom (the quantum standards) are always the same and is such a high number, the caesium atomic clock is incredibly precise.

The most common form of atomic clock used in the world today is the caesium fountain. In this type of clock a cloud of atoms is projected up into a microwave chamber and allowed to fall down under gravity. Laser beams slow these atoms down and the transition between the atom’s energy levels are measured.

The next generation of atomic clocks are being developed use ion traps rather than a fountain. Ions are positively charged atoms which can be trapped by a magnetic field. Other elements such as strontium are being used in these next generation clocks and it is estimated that the potential accuracy of a strontium ion trap clock could be times that of the current atomic clocks.

Atomic clocks are utilised by all sorts of technologies; satellite communication, the Global Positioning System and even Internet trading is reliant on atomic clocks. Most computers synchronise indirectly to an atomic clock by using a NTP server. These devices receive the time from an atomic clock and distribute around their networks ensuring precise time on all devices.

Atomic clocks are the pinnacle of time keeping devices. Modern atomic clocks can keep time to such accuracy that in 100,000,000 years (100 million) they do not lose even a second in time. Because of this high level of accuracy, atomic clocks are the basis for the world’s timescale.

To allow global communication and time sensitive transactions such as the buying of stacks and shares a global timescale, based on the time told by atomic clocks, was developed in . This timescale, Coordinated Universal Time (UTC) is governed and controlled by the International Bureau of weights and Measures (BIPM) who use a constellation of over 230 atomic clocks from 65 laboratories all over the world to ensure high levels of accuracy.

Atomic clocks are based on the fundamental properties of the atom, known as quantum mechanics.  Quantum mechanics suggest that an electron (negatively charged particle) that orbits an atom’s nucleus can exist in different levels or orbit planes depending if they absorb or release the correct amount of energy. Once an electron has absorbed or released enough energy in can ‘jump’ to another level, this is known as a quantum jump.

The frequency between these two energy states is what is used to keep time. Most atomic clocks are based on the caesium atom which has 9,192,631,770 periods of radiation corresponding to the transition between the two levels. Because of the accuracy of caesium clocks the BIPM now considers a second to be defined as 9,192,631,770 cycles of the caesium atom.

Atomic clocks are used in thousands of different applications where precise timing is essential. Satellite communication, air traffic control, internet trading and GPs all require atomic clocks to keep time. Atomic clocks can also be used as a method of synchronising computer networks.

A computer network using a NTP time server can use either a radio transmission or the signals broadcast by GPS satellites (Global Positioning System) as a timing source. The NTP program (or daemon) will then ensure all devices on that network will be synchronised to the time as told by the atomic clock.

By using a NTP server synchronised to an atomic clock, a computer network can run the identical coordinated universal time as other networks allowing time sensitive transactions to be conducted from across the globe.

NTP servers are used by computer networks as a timing reference for synchronisation. An NTP server is really a communication device that receives the time from an atomic clock and distributes it. NTP servers that receive a direct atomic clock time are known as stratum 1 NTP servers.

A stratum 0 device is an atomic clock itself. These are highly expensive and delicate pieces of machinery and are only to be found in large scale physics laboratories. Unfortunately there are many rules governing who can access a stratum 1 server because of bandwidth considerations. Most stratum 1 NTP servers are set-up by universities or other non-profit organisations and so have to restrict who accesses them.

Fortunately stratum 2 time servers can offer decent enough accuracy as a timing source and any device receiving a time signal can itself be used as a time reference (a device receiving time from a stratum 2 device is a stratum 3 server. Devices that receive time from a stratum 3 server are stratum 4 devices, and so-on).

Ntp.org, is the official home of the NTP pool project and by far the best place to go to find a public NTP server. There are two lists of public servers available in the pool; primary servers, which displays the stratum 1 servers (most of which are closed access) and secondary which are all stratum 2 servers.

When using a public NTP server is important to abide by the access rules as failure to do so can cause the server to become clogged with traffic and if the problems persist possibly discontinued as most public NTP servers are set-up as acts of generosity.

There are some important points to remember when using a timing source from over the Internet. First, Internet timing sources can’t be authenticated. Authentication is an in-built security measure utilised by NTP but unavailable over the net. Secondly, to use an Internet timing source requires an open port in your firewall. A hole in a firewall can be used by malicious users and can leave a system vulnerable to attack.

For those requiring a secure timing source or when accuracy is highly important, a dedicated NTP server that receives a timing signal from either long wave radio transmissions or the GPs network.

NTP (Network Time Protocol) is the most widely used time synchronisation protocol on the Internet. The reason for its success is that is both flexible and highly accurate (as well as being free). NTP is also arranged into a hierarchical structure allowing thousands of machines to be able to receive a timing signal from just one NTP server.

Obviously, if a thousand machines on a network all attempted to receive a timing signal from the NTP server at the same time the network would become bottlenecked and the NTP server would be rendered useless.

For this reason, the NTP stratum tree exists. At the top of the tree is the NTP time server which is a stratum 1 device (a stratum 0 device being the atomic clock that the server receives its time from). Below the NTP server, several servers or computers receive timing information from the stratum 1 device. These trusted devices become stratum 2 servers, which in turn distribute their timing information to another layer of computers or servers. These then become stratum 3 devices which in turn can distribute timing information to lower strata (stratum 4, stratum 5 etc).

In all NTP can support up to nine stratum levels although the further away from the original stratum 1 device they are the less accurate the synchronisation. For an example of how a NTP hierarchy is setup please see this stratum tree

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