GPS and RADAR ANTENNAE

alisdair4

alisdair4

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Isle of Bute
midnightdrifter.net
I'm about to replace the elderly GPS on MD with a Garmin GPS 152. The existing )old) antennae is on the top rail of the pushpit (aka taffrail). MD has a goalpost with radar radome (and, now, thanks to forumites a Rutland 913 attached. Question is, why would you put the GPS antenna lower down than it has to be. Does Radar interefere with GPS reception? If not, woulld the sensible thing be to mount the new GPS antenna as high as possible?
 
No - you don't want to mount the antenna as high as possible - the swing it will get as the boat heels over will give false readings. As long as it is out of the way on the pushpit then that is fine.
 
Mount your GPS antenna low and out of the firing line of the radar, look in your radar manual for the vertical beamwidth and work out where the cone will intercept with your chosen mounting point (and maybe you or your crew!).
 
Lower is better, not because of 'heel errors' ( that's compasses and magnetism ) but because of 'multipath errors'....

....where one or more space segment transmitted signals can arrive at the antenna by both a direct and an reflected ( off the water ) route. Does it matter for a metre or so? Yes, it gives rise to erroneous precision time-signal measurement, erroneous answers in the use of the position computation algorithm, and erroneous outputs of calculated position that can be relatively large.

Just don't mount the GPS antenna where someone will sit on it, and blugger up the shielded lead connection rather permanently.

But if you use a carpenter's pencil to plot ( whazat? ) the output position, and a very old Admiralty Chart still in fathoms, who's gonna notice?

/forums/images/graemlins/wink.gif
 
While it might be stating the obvious GPS gets it's signals from satelllites so it does not need to be high. A "goalpost" on the stern at a max of say up to 5 or 6 metres above WL is OK but better lower down for the reasons already stated. The Radar however, is "line of sight" so mount as high as possible. In practice however the crosstrees or thereabout is high enough as the average yottie set isn't powerful enough to see further. Usually mounting Radar on a rear arch or goalposts restricts it's range, especially in rough conditions. Mounting higher also reduces sea clutter a bit at close range when its rough.
 
Urban myth...

[ QUOTE ]
Mounting higher also reduces sea clutter a bit at close range when its rough.

[/ QUOTE ]Sorry, that's an urban myth. Putting the radome higher actually makes sea clutter worse.
 
Re: Urban myth...

Mmmm and if you talk with Furuno in Aberdeen they also say that all other things been equal the clearer sight of all round sky the better ie on a boat higher is good and velocity etc from rolling not a problem. I look to the berth behind me where I see HMS Explorer and her antenna way way up above the flying bridge on a goal post structure.
 
Re: Urban myth...

It's a geometric thing, basically. Sea clutter is caused by reflections from the sloping faces of waves. The higher the radar scanner, the more likely it is to pick up reflections from waves, especially close to the boat. You'll often see people saying that higher scanners result in less sea clutter, but the opposite is true. Commercial/naval ships have scanners mounted high to get range and to avoid reflections from structures on deck. For a leisure boat, a scanner on a stern pole is plenty high enough for adequate range and will result in less sea clutter effect.
 
Re: Urban myth...

Thanks all - looks like this issue is more complex than I thought. Fortunately, the radar is where it is. I have suffered in the past from people sitting on, or putting towels over, the pushpit-mounted GPS antenna. From what I have gleaned, though, a metre increase in height for the GPS antenna is not likely to make a big difference to the output position.
 
Re: Urban myth...

Absolutely correct. We have an NCI watch station on the cliff top on the mouth of the River Dart and the height causes us problems with sea clutter.
And that is with a 6 foot open scanner, it was much worse before we upgraded from a 2 foot radome.
 
Re: Urban myth...

[ QUOTE ]
Why?

[/ QUOTE ]

You need to research something called the "Brewster Angle". This angle is about 7 degrees for seawater. RF radiation incident with the sea surface below this angle only reflects away from the source, thus for a low elevation source, sea clutter reflection only occurs very close to the source and probably mostly within a radar's blanking zone. As source height increases, the range at which the radiation incedence on the sea surface is greater than 7 degrees also increases thus sea-clutter reception area increases correspondingly. At least that is what I remember from being professionally involved in radio propagation at sea 35+ years ago!
 
Re: Urban myth...

Lot of theory on here but not so much practicality. Ever worked out what the thickness of a pencil line is on a typical chart? Means that in real life the difference in accuracy due to the height of the gps antenna is irrelevant - if you need accuracy to withing a few metres you are invariably in a situation where you are using your eyes. If you're out of eyeball range, then accuracy to 10 metres or so is enough. You cant rely on chart accuracy to be less than that, and its sometimes as much as 100 metres.

So put the aerial where the poor old electronics are well protected and not in the line of fire of the radar. Again in theory it should be OK in the line of fire, but why take the risk?
 
Re: Urban myth...

With the greatest possible respect under the circumstances - bolleaux! /forums/images/graemlins/grin.gif

The problem with GPS-frequency 'multipath' signals is not that they produce a single resolution which is a few centimetres out ( and still within the 2RDMS scatter of position expectation of, now, around 6-20 metres ) but that random 'jumps' of resolved position occur which are both unpredictable and unquantifiable.

There's lots on the web about this - over 240,000 for 'gps multipath' - and you might care to have a browse..... I'll leave it to you to cut 'n paste....

[ QUOTE ]
<span style="color:blue"> http://www.esrl.noaa.gov/psd/psd4/wave/gps/ </span>

“The GPS signal can also arrive to the receiver from nearby reflecting surfaces. This phenomenon is known as multipath. Normally, multipath signals interfere with the signals received directly from the satellites, reducing positioning accuracy.”


<span style="color:blue"> http://www.ias.ac.in/currsci/jan252006/207.pdf </span>

"...multipath is more commonly considered to be the reflections due to surfaces surrounding the antenna and can cause a range of errors as high as 15 cm for the L1 carrier and of the order of 15–20 m for the pseudoranges2. By proper siting and antenna selection, the net
impact of multipath is less than 1 m under most circumstances."


<span style="color:blue"> http://gpsinformation.net/multipath.htm </span>

"....Multipath can cause longer term "stable" errors or it can cause your position to wander at varying rates (even thousands of miles per hour if your GPS could follow such speeds). Sometimes GPS wanderings caused by multipath can cause your GPS to "jump" from one position to another as the multipath signal "comes and goes" and causes your GPS to jump from using one group of erroneous signals to another."


[/ QUOTE ]


/forums/images/graemlins/smirk.gif
 
Re: Urban myth...

Thats interesting, and it sounds as if in theory at least what I said originally may possibly be wrong. If so I am happy to be corrected. In practice though I had my scanner on an arch about 4 metres above sea level and got limited range. Remounted on the mast at about 12 metres high I get much better range and the sea clutter is certainly no worse than it was. I actually thought that it was better, but this is a subjective judgement and may be wrong. I agree that the main reason for mounting a scanner high on a large ship is to clear superstructure but surely the vertical beamwidth (typically about 20-25deg) means that objects close up (like waves) are less likely to show a strong return with a higher scanner. As waves are actually not very good at reflecting microwaves sea clutter is usually only a problem at very close range anyway. If however you go to the extremes mentioned and put your scanner on a clifftop, you probably wont see very much close in because the main pulse is going too high to get a return anyway and any target, being looked down on against a background of sea wont return a well discerned echo anyway. The way I see it thats a simple problem of geometry not sea clutter. I would have thought that ideally you need your scanner high enough to see as far as the power you have will allow, but not so high that it is looking down on the targets against a background of seawater!
 
Re: Urban myth...

[ QUOTE ]
With the greatest possible respect under the circumstances - bolleaux! /forums/images/graemlins/grin.gif



[/ QUOTE ]
Now Bilbo, you dont mean the "greatest possible respect , do you? /forums/images/graemlins/grin.gif But if you had read what I said a bit more carefully you would have seen that it largely agreed with your "But if you use a carpenter's pencil to plot ( whazat? ) the output position, and a very old Admiralty Chart still in fathoms, who's gonna notice?"

Because the 2m or whatever accuracy of the GPS is spurious for most normal sailing uses - its well less than the accuracy of most charts or the thickness of the pencil line (or the size of a pixel if you are all techno) - and when location beccomes critical to that degree, you can almost always see where you are / need to be.

OK there is the potential to get momentarily inaccurate information from reflected / refracted signals but is this a real life problem for yotties or an academic one - I've certainly never has such an issue in what must now be 15 years of GPS use? Whats more, what little I can remember of Uni physics ( I did my Masters when electronics = pentodes and triodes etc /forums/images/graemlins/frown.gif) I would take some convincing that a metre or two of height difference would make any serious difference to reflected signals from the water other than to reduce their strength.

But the proof of the pudding is in the eating. So has anybody on here really experienced locations way out or supersonic speed readings from their GPS? To ensure reliability, answer only from those who can program their own VCR at home, please. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
 
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