Installation Location of Standard Horizon GPS

[Ed Steele]

My brother-in-law has just bought a Moody 31 and is considering adding a Standard Horizon CP-300i GPS/Chart plotter. The 300i version has a built in internal antenna. The unit is also available with an external antenna BUT you then have to run cables through bulkheads and deal with mounting the external GPS antenna somewhere where it wont be damaged. He wants to install the 300i (internal antenna version) inside the cabin at the nav station. I have heard that the new generation GPS units are very good at picking up satellite signals even indoors. Has anyone tried this? Any comments?
Ed

[markpj23]

The best I've been able to do is get a signal near the top of the companionway stairs. All systems and boat configurations are different... so I think the only answer is to test it out. If the internal antenna unit also has the ability to connect an external one you'd have some insurance I guess.

[Pblais]

I have heard that the new generation GPS units are very good at picking up satellite signals even indoors.

Very good? Not really. GPS is line of sight reception and that has not changed since it was invented. It's not like your car radio. Yes, you may read some signal indoors but if it gets bounced off the walls a few times the accuracy won't be as good as it should. The multiple signals are used to measure the differences in time so bouncing the signal around is a bad idea. You only need 4 good signals to get a decent fix but they also have to be in a configuration that lends itself to triangulation. So on any day maybe you have 8 to 9 of which 5 or 6 are good. Down below you won't get that many. It means the location just winks out from time to time, but worse it takes about 10 seconds to reestablish the signal lock when you lose it. It's not unlike trying to use a GPS in a downtown urban area.

The external antenna would be best if the unit is located inside below. I've not see a problem under the canvas in the cockpit however so there is some degree of flexibility but not enough for down below accuracy. You only need the antenna where it has a decent view in most directions. Off the aft corner of the Bimini is a great position but you can get more flexible if it is away from obstructions.

[merlin]

I think you'll have to test it in different locations. New model GPS units have a chipset called sirf which is much more sensitive to receiving signals. I have use handheld units both with and without the sirf chip. I can tell you the sirf enabled unit far outperformed in terms of being able to get a reading indoors (even in a closed garage).

I think the external antenna is ideal, but maybe some testing will tell you how well it work for you

[Pblais]

The ability of sirf to perform well in poor signal conditions is tied to the ability to add external acceleration and heading sensors to augment the GPS. Since you won't have any of those aboard it won't matter. The Sirf chipset is a good signal processor and quite powerful but it can't make a signal better than it is. It can't receive a GPS signal where another receiver can not. A better antenna is still what you need in the case of a poor signal. GPS is the same as it was many years ago and it can't be made "better". There are newer correction ssytems like WAAS that add accuracy to a good signal but it's not any better.

I did work in this area some years ago where we integrated a ring laser gyro to a gps receiver as well as differential correction and post processed computations. It worked amazingly well but the problem is the cost of a ring laser gyro is about $100,000 if you can get a deal.

sirf is being used in cars since GPS reception in the city stinks. You lose lock so often and it takes too long to get it back that you lose it before then. The newer car systems will go a few minutes using the sensors and not get you lost in the mean time. We were going more than 45 minutes keeping exceptionally high accuracy better than you can get with GPS today. I don't see any of this technology being used on the water. It's not like having tall buildings or trees in the way. A clear view of the sky is all you need and you certainly get that on the water.

[fairbank56]

Originally Posted by Pblais

It can't receive a GPS signal where another receiver can not. A better antenna is still what you need in the case of a poor signal. GPS is the same as it was many years ago and it can't be made "better". There are newer correction ssytems like WAAS that add accuracy to a good signal but it's not any better.

I think you need to do a little more research on this. I agree with Merlin. The new chipsets are far more sensitive and will pick up signals where a receiver with an older technology will not. Iv'e seen this firsthand myself with two handheld units side by side under heavy foilage. Here's a site with some more info and if you google "sirf and gps" you'll find lots more.
Globalsat BT338 Review

Eric

[delmarrey]

I have a S. H. GPS and the antenna was sitting on a port bunk that's above the solon. It was there for two years before I finally mounted it on a stern rail. It did just fine shooting thru the deck................._/)

[Latitude9.5]

i've got a new standard horizon gps, the big bastard and i flush mounted the antenna to the port side above the salon, never had one issue with it.

[Pblais]

I think you need to do a little more research on this. I agree with Merlin. The new chip sets are far more sensitive and will pick up signals where a receiver with an older technology will not.

I don't have a version III but do have a version II sirf Pocket PC by Mio. It was cheap at under $200 with a color display and could use the Fugawai software I already had. I do like it so don't get me wrong about what I have to say about sirf.

The big deal is the Time to First Fix being faster. It's done at the expense of accuracy. If you leave it off for more than a few days it loses all the ephemeris data that is used by all GPS's to establish a position so in that way it does not really do anything brand new. It just guesses faster. Perhaps a minor illusion at best. I actually find of my three on board GPS's it reads velocity and altitude the poorest. Those are the hardest readings to compute. With position on the water just how could you know? It does seem to give a result inside the house but only after you fire it up outside. It is basically guessing. My old Garmin 128 with the external antenna gives better accuracy but takes about 30 seconds to start up It has less non volatile RAM so it starts from the ground up most of the time unless you leave it on all the time (you could). My Raychart 400 does about the same as Pocket PC with the sirf chip but has a brighter display and uses vector Navonics chips. It clearly is the best looking albeit small display. The Garmin 128 as a display is basically a joke as it only displays way points and tracks but no chart.

In all the sirf reviews they say "enhanced sensitivity" but the reality is that it isn't more sensitive. They are just reprinting the manufacturers hype verbatim. It can continue to display a indoors value but not accurately. A GPS signal coming in low over the horizon is distorted and can not be "enhanced" with any sensitivity because it has lost it's timing through attenuation. It's the time encoded that is what determines the position and you can't make that out of nothing. The fact that is is a poorer signal means it actually took longer to get to you than the receiver can know. It gives the illusion that it is farther away from you than it really is and that assumes you decode the signal totally accurate.

What is most interesting is the new in vehicle navigation systems are what is driving the market so all the hype is geared towards the problems of "GPS in urban canyons". It's been the big topic for 20 years. They now have added what auto pilots aboard ship have always had - heading and acceleration sensors. They have done it because there is a limit to what you can do with a GPS signal. There is no such thing as "enhanced sensitivity". It's all marketing BS.

[fairbank56]

Originally Posted by Pblais

In all the sirf reviews they say "enhanced sensitivity" but the reality is that it isn't more sensitive.

So, what about the fact that I'm under dense foilage with two receivers and the one with the sirfIII shows more satellites with stronger signal levels and gets a good fix while the other one has fewer satellites showing with much weaker levels and cannot get a fix? These are both garmin units with the helix type antenna.

Eric

[Pblais]

These are both Garmin units with the helix type antenna.

There can be a difference in receivers. It's not to say that one can't be better. I would question if the antennas are really the same. There also is multi path filtering possible as well. That would hep a lot in the city but not very much on the water.

How each displays the signal "quality" can be totally different. It's not fair to say that one shows a stronger signal so it must be better because it says so. Most GPS units would internally ignore some satellite signals even though it shows them on the sky view. You would have to assume they determine what is better in the same way and measure using the same scale. Why would they have to?

What happens when you compare the location or compute velocity and heading? You may little difference or you may not.Sirf would smooth out the lumps to make you feel better. What happens if you just park the two side by side and look at the differences? If they both agreed would it make any difference to you? if they were different how could you know which was accurate? perhaps more important if the difference was small would it even matter?

[fairbank56]

Originally Posted by Pblais

It's not fair to say that one shows a stronger signal so it must be better because it says so.

It's not just one example. There's a whole lot of users out there that are amazed at how well the receivers with the sirfIII perform over the older ones. Sensitivity being the main issue.

Eric

[fairbank56]

A friend here at work just walked in to show off his new Garmin 76CSx GPS. I could not believe how well it worked in my shop. My shop is an interior room, no exterior walls, has a drop ceiling with multiple 2x4 fluorescent light fixtures, tons of conduit, ductwork, and sprinkler pipes above the drop ceiling, and a roof that is a foot thick. The exterior walls of the building are aluminum panels with no windows. This thing picked up 6 satellites in less than 30 seconds. Signal levels were good and the fix was accurate. I was holding an older Garmin 76 in my other hand. I won't even find one satellite no matter how long you leave it on and yes, I took it outside and left it on for 30 minutes to verify that it is working fine and the data is up to date. The 76CSx unit has the SiRFstarIII chip and is advertised as a high-sensitivity receiver. This technology clearly has better sensitivity and the previously made statement about this technology, "It can't receive a GPS signal where another receiver can not" is incorrect.

Eric