8 vs 12 sattelites - what's the difference

[Guest nicuss]

Given the same features otherwise, what's the difference performance-wise between a unit with 8 sat tracking and one with 12? Are 12 significantly better under tree cover? Or?

[Guest Kerry]

In reality there is very little difference and some of the better (older?) 8 channel receivers will out perform some of the "newer" 12 channel, significant No.

 

The main thing to consider is the number of satellites actually available which on average is around 7 to 9. 12 channel receivers can still only use the satellites that are available. The are times when there will be 12 (even more) but that percentage is low when compared to the rest of the time.

 

Again another one of these issues where the lowest elevation satellites are the worst as far as atmospherics goes. Most recreational receivers don't have capabilities to mask these sometime problem sats out of the solution. Really by default an 8 channel does that automatically (to a point) as they will use the higher quality satellites before the crap ones.

 

Cheers, Kerry.

[Guest bob_renner]

One other item to keep in mind is the fact that some of the earlier GPSRs that would track 8 satellites did it with a multiplexed receiver. They did not have 8 parallel channels for the 8 satellites, but rather a fewer number (perhaps 1 to 3) that were time shared across the 8 satellites. The Garmin GPS 38 was like this. As a result, it was slower to acquire lock and easier to lose lock. I believe all of the newer 12 channel receivers are true 12 parallel channels. Each channel can independantly track a separate satellite.

 

Bob

[Guest mcb]

Another problem with the older 8 channel multiplexing units is that since the range to each satellite is not calculate for the same instant you introduce another source of error into the calculations. Most 8 channel unit will be less accurate even if they are locked onto the exact same satellites as a parallel unit. They are also just older and technology has improve a lot since they were built and the newer units take less power to operate and have more sensitive receivers.

 

mcb

[Guest CharlieP]

in tree cover is the ability of the antenna and receiver to pull in sat signals weakened by the wooden overcast, especially the ones nearer to the horizon which must penetrate tree trunks and not just a few wimpy leaves. The ability to receive signals through overhead pine needles is another strong plus here in Georgia.

 

FWIW,

CharlieP

[Guest mcb]

That would be true if you were comparing an 8 channel parallel to a 12 channel parallel. But I don't know of any 8 channel parallel units (not saying they never made them). Most unit that track 8 satellites are multiplex units and I think you will find that if you have a 6 satellite lock with an 8 channel multiplex unit and compare that to a modern 12 channel parallel unit the 12 channel will have a noticeable smaller EPE.

 

Some of the 8 channel multiplex unit I know of would be:

Garmin 38

Garmin II (not the II+)

Garmin 40

Magellan Meridian/Meridian XL (the first Meridian GPS (95-97) not the new line)

Magellan Trailblazer/Trailblazer XL

 

I sure there are other GPS units out there.

 

The other thing is the 8-channel units are older technology and will be less sensitive then it modern counter part. So under tree canopy a modern 12 channel unit even with a patch antenna will probable get better reception than these older units.

 

I have a GPSMAP 76, eTrex basic and an old Magellan Trailblazer XL. The GPSMAP 76 is a 12 channel parallel unit with quadrifilar antenna and is by far the best unit I have. Giving me the best accuracy even with WAAS off. The eTrex is a close second only loosing accuracy when in heavy tree cover and similar situations; this is due to lose in number of satellites it is talking to. The eTrex of course is a 12 channel parallel unit with a less sensitive patch antenna. The poor Magellan Trailblazer is an old 8 channel multiplex unit and although it has a quadrifilar type antenna if you made me chose between the eTrex and the Trailblazer I would probable go with the eTrex, although I like the features on the Trailblazer better. Even with the patch antenna the eTrex is usually more accurate.

 

After all that long winded spouting,the best handheld GPS units out there will have 12 channel parallel receivers.

 

Later

mcb

[Guest Kerry]

A couple of other consdierations when discussing this 12 v 10 v 9 v 8 v 5 type channel stuff is the max number of satellites that differential will transmit corrections for which is 9 (as far as I'm aware). Also those satellites must be in view at BOTH the reference and users position.

 

Similar with WADGPS (WAAS etc) can only apply corrections for common satellites. WAAS receivers now really become a 10 channel but when one takes into account reference site settings that's about all there would be absolute max. Once there's a few more WAAS geo satellites (if and when that happens?) one might start to see more 14 or even 16 channel receivers? (some already exist). Now not that the end result will be any different but 16 sure "sounds" better than 14 which sounds better than 12 whic sounds better than 10 etc but is it all in the mind?

 

Cheers, Kerry.

[Guest CharlieP]

quote:

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Originally posted by mcb:

But I don't know of any 8 channel parallel units (not saying they never made them).

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FWIW, I have one, an Apelco/Raytheon GPS11, which has 8 parallel channels. It will track more than 8 sats, but will only receive 8 at a time.