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Internal VS External GPS Antenna

Old 11-19-2016, 02:50 PM
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Default Internal VS External GPS Antenna

I am contemplating installing a GS25 to my NSS-12 evo1 system. My current fix and position is very poor. Please see picture. Does anybody know what level of improvement would be obtained by an external antenna. Thanks for your help.

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Last edited by nfairbank; 12-03-2016 at 01:24 PM. Reason: Add Pic
Old 11-19-2016, 02:57 PM
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See this thread here: http://www.thehulltruth.com/marine-e...s-antenna.html
Old 11-19-2016, 03:00 PM
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You will get a huge improvement, the antenna and receiver in GS25 (which is the same as in NSS Evo2) is much more sensitive than in first generation NSS. In addition having free sky view for the antenna will give much better reception. If you don't already have a heading compass GS25 will give you this as well.
Old 11-19-2016, 03:38 PM
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Thanks iFishMD for the link, it was interesting reading.

Abbor, thanks, for me my EPE of 44.3 feet is way too much when looking for a small ledge that can be only 8 or 10 feet long. I will purchase a GS25.
Old 11-19-2016, 08:00 PM
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similar experience with Garmin I get 7' accuracy with the external antenna around 30' with the built in one.
Old 11-19-2016, 08:51 PM
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Nothing to wonder about, you want the GS25 and it will improve accuracy.
Old 11-20-2016, 12:29 AM
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Originally Posted by nfairbank View Post
Thanks iFishMD for the link, it was interesting reading.

Abbor, thanks, for me my EPE of 44.3 feet is way too much when looking for a small ledge that can be only 8 or 10 feet long. I will purchase a GS25.

Have you enabled WAAS? Push Configure at the satellite screen you posted to enable it. All Lowrance and Simrad units come with WAAS disabled and it will be enabled again after factory reset (which is a good thing to do when updating the software).

Also for GS25 you will have to enable WAAS. Also remember to select GS25 as source for GPS data when you have added it.
Old 11-20-2016, 01:45 AM
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The issue is typically obstructed signal due to internal antenna issues, enabling WAAS does not make obstructed signal issues go away. One should really fix the actual issue re internal GPS antenna as WAAS (in a marine situation) really relies on a clear unobstructed sky view.
Old 12-03-2016, 01:15 PM
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Picture submitted for comparison. Today, I installed my GS25. It was not in an optimum area, but still locks well.

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Last edited by nfairbank; 12-03-2016 at 01:26 PM.
Old 12-03-2016, 01:31 PM
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Thank you for posting results
Old 12-03-2016, 01:36 PM
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Originally Posted by nfairbank View Post
Picture submitted for comparison. Today, I installed my GS25. It was not in an optimum area, but still locks well.
Plus you have heading info if you didnt have a heading sensor to begin with
Old 12-03-2016, 07:30 PM
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Originally Posted by Kerry View Post
The issue is typically obstructed signal due to internal antenna issues, enabling WAAS does not make obstructed signal issues go away. One should really fix the actual issue re internal GPS antenna as WAAS (in a marine situation) really relies on a clear unobstructed sky view.
not true my antenna is inside the console mounted to the ceiling just behind my 7616's, works great. External antennas are just much better than the built-in ones. As long as the GPS antenna is only looking through fiberglass they work just fine. Mine is receiving through the fiberglass of the console, the two layers of fiberglass and coring in the T-top, the radar and all the pipework, outriggers ect, and it works fine.
Old 12-04-2016, 02:46 AM
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Originally Posted by pas View Post
not true my antenna is inside the console mounted to the ceiling just behind my 7616's, works great. External antennas are just much better than the built-in ones. As long as the GPS antenna is only looking through fiberglass they work just fine. Mine is receiving through the fiberglass of the console, the two layers of fiberglass and coring in the T-top, the radar and all the pipework, outriggers ect, and it works fine.
The OPs photo and comments do not advise the construction materials of the boat, or the mounting of his unit. You refer to your own fiberglass boat and are correct that the signal penetrates fiberglass.
Kerry's comments are very true also especially on aluminum boats. Flush mounted units at best, have the antenna shielded and at some random angle or other to the sky.

I have witnessed the signal strength and loss of satellites on flush mounted units when I grab the handrail on aluminum boats because the human body when 'grounded' by the handrail has resistance and capacitance that absorbs / blocks / conducts the signals when physically blocking the antennas view of satellites
Old 12-04-2016, 04:48 AM
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There are also high sensitivity receivers which help. I have external antennas for my boat chartplotter receivers, but my phone and tablet get the same resolution while sitting on the shelf in my boat which has a enclosed cabin. It depends on your situation. My cell phone gets good GPS fixes in commercial airplanes which are pretty effective Faraday cages.
Old 12-04-2016, 05:20 AM
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For clarification, my boat is a 35" Scarab. It is of fiberglass construction and the antennae is mounted directly underneath the dash of the helm. Nothing is between the antennae and the fiberglass. The helm sits beneath a canvas T-Top with a substantial aluminum structure holding electronics box, radar and radio mount.

I did attach the antennae and tried several positions. The best position was on top of the electronics box. I decided to sacrifice best results location for convenience as my T-Top is mounted on my gunnel, not on the sole of the boat. These are the results with nothing but canvas between the between the antennae and the sky.

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Old 12-04-2016, 05:55 AM
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Originally Posted by ssobol2533@aol.com View Post
My cell phone gets good GPS fixes in commercial airplanes which are pretty effective Faraday cages.
A passenger plane is not a good Faraday cage for GPS frequencies due to the large holes (windows).
Old 12-04-2016, 06:03 AM
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The reduced estimated position error (EPE) is related to the horizontal dilution of position (HDOP) and use of augmentation.

In your first screen shot, there is no augmentation (no WAAS), and the HDOP is 1.4. In your best screen shot, there is augmentation (WAAS) and the HDOP is 0.5. Both contribute to improved accuracy.

HDOP is a function of the particular arrangement of the satellites in view at your location at that time. You cannot control that.

At your latitude (27-N) you should have no problem seeing a WAAS satellite over the equator. It should be quite high in elevation. Some receivers require particular configuration in order to use augmentation. You may not have enabled augmentation in the first receiver.

Also the Carrier-to-Noise Density Ratio (C/No) (mistakenly shown as SNR) is not very good in the first screen shot, only averaging 26 dB-Hz. In the best screen shot the C/No is much better , averaging 42 dB-Hz. This means the second receiver is either getting stronger signals or its radio receiver has a better design.

The new receiver is not particularly more accurate, it is just receiving more satellites, with stronger signals, and is using augmentation.

Last edited by jhebert; 12-04-2016 at 06:14 AM.
Old 12-04-2016, 08:44 AM
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Originally Posted by jhebert View Post
The reduced estimated position error (EPE) is related to the horizontal dilution of position (HDOP) and use of augmentation.

In your first screen shot, there is no augmentation (no WAAS), and the HDOP is 1.4. In your best screen shot, there is augmentation (WAAS) and the HDOP is 0.5. Both contribute to improved accuracy.

HDOP is a function of the particular arrangement of the satellites in view at your location at that time. You cannot control that.

At your latitude (27-N) you should have no problem seeing a WAAS satellite over the equator. It should be quite high in elevation. Some receivers require particular configuration in order to use augmentation. You may not have enabled augmentation in the first receiver.

Also the Carrier-to-Noise Density Ratio (C/No) (mistakenly shown as SNR) is not very good in the first screen shot, only averaging 26 dB-Hz. In the best screen shot the C/No is much better , averaging 42 dB-Hz. This means the second receiver is either getting stronger signals or its radio receiver has a better design.

The new receiver is not particularly more accurate, it is just receiving more satellites, with stronger signals, and is using augmentation.
Are you saying that even though the EPE has decreased, I may not be any closer to the target location?
Old 12-04-2016, 02:01 PM
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Originally Posted by pas View Post
not true my antenna is inside the console mounted to the ceiling just behind my 7616's, works great. External antennas are just much better than the built-in ones. As long as the GPS antenna is only looking through fiberglass they work just fine. Mine is receiving through the fiberglass of the console, the two layers of fiberglass and coring in the T-top, the radar and all the pipework, outriggers ect, and it works fine.
There are many many variations involved with using internal GPS units and without looking at each install individually the generalization is that no internal will ever perform equiv to an external it is simply not possible. Just because you have the "impression" that your setup works great may be misleading, maybe it does ... some of the time but I wouldn't be expecting it to all of the time.

Users need to understand what the failing of an internal can be and accept the possibilities, great or not.
Old 12-04-2016, 02:13 PM
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Originally Posted by nfairbank View Post
Are you saying that even though the EPE has decreased, I may not be any closer to the target location?
EPE is telling you what the unit thinks the Estimated Position Error is based on values computed / derived basically from the satellites, signal strength, satellite geometry etc. There is no standard for deriving EPE and most manufacturers use their own tricked up version but based on some standard variables that are standard. The lower the EPE is, then typically the more confidence can be placed on the position and the better the position should be.

"Target location" can be quite a few different things and when referring to GPS accuracy then the only way to determine accuracy is compare the position to a known position of truth. Typically waypoints sourced by most means or saved waypoints are not absolute positions of truth, EPE does not really indicate the accuracy (as such) of your coordinates.

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