Are all GPS antenna's created equal?

[Privilege]

This is a wild shot in the dark but I thought I'd ask anyway. I recently purchased a Vesper marine XP 8000 AIS which included the GPS antenna. I already have a GPS antenna installed for my Sirius satellite radio. Installing the antenna was a real PITA- it runs through the hard top, inside the supports and across a number of inaccessible bulkheads before coming out by my navigation station. The two antennas look the same, what are the chances that they are exactly the same thing?

[smac999]

the sirus is not a gps antenna. even if it was, you'd still need both. as the vepser requires it's own.

the vesper one is a coax antenna. different then most gps antennas today which are notmraly receivers, not antennas.

if you helm is fiberglass I would just mount it under the helm. it does not need to be on the roof. you can test the signal in the vesper phone app

[RaymondR]

Interesting question.

I have not had anything to do with external GPS antennas since my Magellan handheld days. The external antenna for the Magellan 4000D appeared to have a small RF amplifier built into it to amplify the signal received by the antenna section and sent RF down the coax to the GPS unit. Do they still do it this way or do they put complete GPS receiver/processor chips in the external antenna unit and pass NMEA or some other coded digital data instead.

[Neptune's Gear]

No chance. They are different.

[AedanC]

Our Vesper XB6000 AIS works fine using the inbuilt antenna below decks (fibreglass boat). Try the antenna below and see how you get on first.

[CarinaPDX]

I think your specific question has been answered: the Sirius antenna is an antenna for receiving satellite broadcasts of music, news, weather,etc and not a GPS receiver. In any event you should use the GPS that came with the AIS as they are approved to work together, and have the AIS put out GPS position on an NMEA interface for other uses.

Again as posted earlier there is a world of difference between a GPS antenna and a GPS receiver. But it is a bit more complicated. GPS antennas usually, but not always, contain a low noise amplifier (LNA) which is powered by a DC voltage on the coax, with the AC signal coming back down. The DC power could be 0V (no amplifier), 3.3V, 5V, 12V, or some other voltage. Also the amplifier gain can vary quite a bit, from unity (no amp) to 20dB or more. There are also some bandwidth considerations. The mirror of this is that the GPS module also varies to match the antenna. In short it would just be luck if a random antenna would match with a GPS module.

GPS receivers with internal antennas are a little more generic but still require some knowledge. Of course there are differences in performance (sensitivity, speed of acquisition) which are constantly improving. Accuracy is also improving, but at a slow pace - these days propagation is the largest source of error. The interface speed is one source of problems: NMEA 0183 calls for either 4800 or 34,800 baud - my newest GPS was set to 9600 (which my Furuno MFD doesn't support). In the past most GPS modules output in standard NMEA 0183 sentences by default (note that Garmin did not), but often there is an alternative language available for configuration and binary output (SiRF and uBlox for example). Most GPS modules output in 3.3V-5V signals, and should be differential to meet the RS-422 spec used by NMEA, but others use 12V RS-232 without differential signals.

The bottom line is that two "GPS" boxes are probably not interchangeable, at least before configuring.

Greg

[jpstevens]

I think that AedanC is correct. Dave Miller, Milltech Marine, an acknowledged expert, says that you must use the vesper antenna, but it will function below deck

[Brian.D]

I am going to take a SWAG and guess that the antenna you have for the Vesper is actually a GPS receiver/antenna. The reason being...

1. There would need to be a coax in the cable from the antenna to the below decks unit. That cable would induce losses.
2. It is easier to move baseband data over a cable than to move RF over a cable.

As others have mentioned, your external antenna should work fine below decks as long as there is no metal or any moisture in the deck. Moisture absorbs very high frequencies.

[Cpt Pat]

Originally Posted by AedanC

Our Vesper XB6000 AIS works fine using the inbuilt antenna below decks (fibreglass boat). Try the antenna below and see how you get on first.

I also have my GPS antenna below deck in my fiberglass boat. It works fine.

My receiver combines GPS and GLONASS (Russian GPS). I never have fewer than 11 satellites in view, and as many as 21 sats. If your receiver supports displaying the horizontal dilution of precession (HDOP), than any HDOP less than 1.0 means you're doing great.

When you combine two or more satellite constellations for a positional solution, technically you have a global navigation satellite system (GNSS) receiver. New receivers that combine GPS and the European Galileo constellation are now becoming available.

The US has promised to never shut down or de-precision civilian GPS. Neither the Russians nor the Europeans trust that promise, which is why they've spent billions to put up their own constellations. There have also been instances of jamming and spoofing GPS in areas that are less than friendly to US interests. I have a GPS/GLONASS/Both switch so I can select either constellation separately. Relying on just one constellation, or a combined solution from both when one is disrupted, may be unwise. I plan to upgrade to a GPS/GLONASS/Galileo receiver soon.

[Siberian Sea]

Many people seem to believe that mounting your GPS antenna high upon the masthead will give it better reception. But unlike your VHF it’s not line of sight depending. It only needs a clear view of the sky. Matter fact some units require you to enter the correction for meters above the earth or sea mounting or your exact location might be off just a bit. Perhaps the best location for the antenna is just on the stern rail with clear vision of the sky and a few meters from sea level. Oh, you Sirius/XM antenna is not a GPS antenna so don’t try that. In the event your sailing away from the USA your Sirius receiver will no longer get a signal much east of Bermuda or South of Trinidad. Works great all through the Caribbean and on the West Coast all of Alaska. Just like the GPS it’s satellites are not orbiting but rather in fixed position and orbit with the earth. GPS has satellites fixed so all locations are covered and Sirius has only satellites for coverage of North America. The key to maximize your Sirius coverage is to forget the small cheap antenna that came with your unit and go to a Truck Stop and but the larger more heavy duty antenna that the truckers place on their Semi Trucks. Its much more sensitive and the connecting cable is more heavy to cover signal loss. Expect to pay between 75 and 100 dollars for this antenna but it’s worth it.

[RaymondR]

Originally Posted by Siberian Sea

Many people seem to believe that mounting your GPS antenna high upon the masthead will give it better reception. But unlike your VHF it’s not line of sight depending. It only needs a clear view of the sky. Matter fact some units require you to enter the correction for meters above the earth or sea mounting or your exact location might be off just a bit. Perhaps the best location for the antenna is just on the stern rail with clear vision of the sky and a few meters from sea level. Oh, you Sirius/XM antenna is not a GPS antenna so don’t try that. In the event your sailing away from the USA your Sirius receiver will no longer get a signal much east of Bermuda or South of Trinidad. Works great all through the Caribbean and on the West Coast all of Alaska. Just like the GPS it’s satellites are not orbiting but rather in fixed position and orbit with the earth. GPS has satellites fixed so all locations are covered and Sirius has only satellites for coverage of North America. The key to maximize your Sirius coverage is to forget the small cheap antenna that came with your unit and go to a Truck Stop and but the larger more heavy duty antenna that the truckers place on their Semi Trucks. Its much more sensitive and the connecting cable is more heavy to cover signal loss. Expect to pay between 75 and 100 dollars for this antenna but it’s worth it.

Being a bit pedantic here but;

All satellites orbit otherwise they would fall back to earth. Geostationary satellites orbit over the same geographical position because they orbit once per day so whilst they appear stationary from the earths surface they are still whizzing around earth. In contrast, the GPS satellites are in relatively low earth orbit and consequently orbit more times than once per day and there are about 24 of them to give full earth surface coverage of which I think your GPS receiver requires three to calculate a position and more if elevation is required.

The first satellite navigation system, which I think was called the Transit System, was put up by the US navy to allow the missile submarines to check their position to correct for drift in the inertial positioning systems they used to fix the firing position for the missiles. An antenna was mounted on a periscope and they could come up to the periscope depth to receive the position signals after dark without being detected. It took a small room full of instrumentation and it is incredible that we now have GPS systems in our cheap mobile phones. It's an ill wind which blows no good.

In the trustworthiness stakes I am still going to retain the U.S.A. as above the Europeans, Russians or Chinese on my list. A perverse U.S. Congress and the difficulty of keeping anything secret for long has a lot going for it.

[Siberian Sea]

Well I did not know the exact term “geostationary” but that’s what I meant. If your position on the planet is stationary then you always have the same view of the same gps satellite. It’s orbiting with the earth not around. Early Sat Nav you needed to wait for the satellite to pass before you could get a fix. Today your always covered.

[Jim Cate]

FWIW: I've found that the internal GPS receiver in our Vesper Vision AIS is quite adequate. It is permanently mounted under our dodger, but I tried it below decks and it worked there too (no attempt to quantify signal strength differential). Have you tried this on your AIS?

Jim

[Wotname]

Originally Posted by Siberian Sea

Well I did not know the exact term “geostationary” but that’s what I meant. If your position on the planet is stationary then you always have the same view of the same gps satellite. It’s orbiting with the earth not around. Early Sat Nav you needed to wait for the satellite to pass before you could get a fix. Today your always covered.

Err... no, this is totally incorrect!
It is the exact opposite, if you are stationary, the GPS satellites will be hoving into and out of view all the time.

A simple google search of the the GPS constellation of satellites will confirm this or look at the the visible satellites (and signal strength) on your GPS unit to confirm what satellites are visible at any one moment. Better GPS receivers will also show the the azimuth and elevation of each satellite in real time.

Information upthread about the differences between GPS antennas (and receiver/antennas) are mostly correct .

[Cpt Pat]

Originally Posted by RaymondR

Being a bit pedantic here but;

All satellites orbit otherwise they would fall back to earth. Geostationary satellites orbit over the same geographical position because they orbit once per day so whilst they appear stationary from the earths surface they are still whizzing around earth. In contrast, the GPS satellites are in relatively low earth orbit and consequently orbit more times than once per day and there are about 24 of them to give full earth surface coverage of which I think your GPS receiver requires three to calculate a position and more if elevation is required.

The first satellite navigation system, which I think was called the Transit System, was put up by the US navy to allow the missile submarines to check their position to correct for drift in the inertial positioning systems they used to fix the firing position for the missiles. An antenna was mounted on a periscope and they could come up to the periscope depth to receive the position signals after dark without being detected. It took a small room full of instrumentation and it is incredible that we now have GPS systems in our cheap mobile phones. It's an ill wind which blows no good.

In the trustworthiness stakes I am still going to retain the U.S.A. as above the Europeans, Russians or Chinese on my list. A perverse U.S. Congress and the difficulty of keeping anything secret for long has a lot going for it.

All right-on Raymond! Thanks for the post. I don't expect sailors to be experts in orbital mechanics, but considering our dependence on GPS, we should at least understand the basics of how it works. Yes, the GPS satellites are constantly changing positions. Fundamentally, they are just very accurate clocks and the process for using them is just a very accurate form of celestial navigation with man made stars - except the clocks used aren't on our boats, the clocks are in the satellites.

You can watch them move on a map here: https://in-the-sky.org/satmap_worldmap.php.

It's amazing how little some people know about GPS. I had a recent conversation with a very experienced engineer who was riding in my car. He asked why I was using a standalone GPS on my dashboard instead of a smartphone app (that depends on cellular service). I told him my GPS worked in the mountains without cellular service and also that I didn't like leaving breadcrumb trails on my travels, and he challenged that my own GPS receiver was transmitting my position back to the satellites. When I disagreed, saying GPS is a receive-only technology, he challenged: "then how do you get the moving map on the display?" "Ah, the maps are derived from a static database stored in my receiver..."

I've run into several people (all smartphone addicts) who believe their GPS receiver transmits its position to the GPS satellites, and then it receives map customized for them in response back from the GPS satellites. Just imagine the bandwidth that would require!

Worse: I recently retired from NASA. I had to explain to executives there how a GPS receiver doesn't transmit anything. (Granted -- there weren't any scientists in the conversation.)

I respect your opinion about using only GPS. But if tensions rise, it's unlikely the Russians will jam their own satellites. But they are jamming and spoofing ours NOW. Here's one example of spoofing that has the Big Ships worried:

https://rntfnd.org/2019/05/08/gps-sp...ip-technology/

Because the signal received from GPS satellites has less than the kinetic energy of a snowflake striking the ground, it is very vulnerable to jamming and spoofing from much stronger malicious transmitters on the ground. You can buy jammers on ebay. GPS is fragile.

As with most things in reliability: diversity is good.

Maybe it's seen as over the top, but I still have a sextant and a current Nautical Almanac on board. Just in case...