Spotting: Time to brush up on your skills.

[billvon 3,008]

>>Different grades of receivers can vary a lot.

>Do you have evidence or a reference for that?

We have similar experience with GPS receivers. A good receiver usually translates to greater # of channels received, and therefore less PDOP. Ironically, some OEM receiver modules (like one Garmin one we tested) are far more sensitive than the GPS receiver used in even Bendix/King avionics.

[skydivegirl 0]

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Different grades of receivers can vary a lot.

Do you have evidence or a reference for that? The signals are digital and the precision timing is done by cesium clocks in the satellites, so it's hard to see how there could be any difference as long as the signal is received at all.

To learn more about how GPS works and timing errors associated with it, here's a helpful site.

In short, yes satellites use atomic clocks but the receivers don't... and there's your error.

Pink Mafia Sis #26

[kallend 2,029]

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But the electronics in the reciever can vary a lot. Same as pc's - sure they can all run windows, but the speed and reliability can vary from machine to machine.

What does that have to do with the accuracy of the answer you get? Does a cheapo PC come up with less accurate answers than a top-of- the-line PC when running an Excel spreadsheet?

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,029]

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Different grades of receivers can vary a lot.

Do you have evidence or a reference for that? The signals are digital and the precision timing is done by cesium clocks in the satellites, so it's hard to see how there could be any difference as long as the signal is received at all.

To learn more about how GPS works and timing errors associated with it, here's a helpful site.

In short, yes satellites use atomic clocks but the receivers don't... and there's your error.

Apparently not. To quote from the web site you referenced, on the local "clock errors" page, using a standard correction algorithm:

"And from then on its clock is synced to universal time. Of course this correction process would have to be repeated constantly to make sure the receiver's clocks stay synced. But with it, even the lowliest GPS receiver turns into an atomic-accuracy clock."

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The only sure way to survive a canopy collision is not to have one.

[parachutesj 0]

on one off the largest dropzones on earth:
I've been the first to go...green light...out... shit! Where the hell is the f**** DZ...
Me and my buddy landed about 3 miles off the DZ. The pilot said later he accidentially hit the green light :-(
Never ever jump without spotting!

Steffen

[Samurai136 0]

Additionally, the signal is encrypted on transmission. Civilian receivers are limited in the number of significant digits of decryption. If the encryption is altered, thus the number of significant digits decrypted, a receiver's accucracy can also be altered beyond synchronization errors.

Ken

"Buttons aren't toys." - Trillian
Ken

[indyz 1]

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I use a handheld Garmin GPS III Pilot. When I first got it before SA was turned off I had an EPE of 60 feet. Not bad. Then SA was turned off and the EPE went down to like 20 feet. Now it was up to 45 feet. I'll tell you, if I was always able to fly within 45 feet of course I'd be a shuttle pilot.

Best bet is to try again later, or try from a different location. My Garmin eTrex and eTrex Legend (both 12 channel receivers, one with WAAS) report different EPE depending on time of day, weather, skyview, etc. Even side-by-side with WAAS turned off on the Legend, the units will frequently report different EPE. It may have something to do with the quality of the reciever, but it appears that the Legend is just the basic eTrex with a couple of extra buttons and better software.

[robskydiv 0]

Terrain Associaion is so helpful in Land Navigation (Orienteering). I beleve that it is also very useful in spotting also.

[skydivegirl 0]

Well then I guess people are pretty silly to spend tens of thousands of dollars on survey grade GPS receivers when you could just buy $300 receivers from Wal-Mart.

Pink Mafia Sis #26

[kallend 2,029]

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Additionally, the signal is encrypted on transmission. Civilian receivers are limited in the number of significant digits of decryption. If the encryption is altered, thus the number of significant digits decrypted, a receiver's accucracy can also be altered beyond synchronization errors.

Ken

But does that depend on the "quality" of the receiver? I'm not sure you followed the thread.

I was under the impression that civilian and military signals differed in the type of modulation used.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,029]

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Well then I guess people are pretty silly to spend tens of thousands of dollars on survey grade GPS receivers when you could just buy $300 receivers from Wal-Mart.

Is that with WAAS?

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The only sure way to survive a canopy collision is not to have one.

[Hooknswoop 19]

If the receiver can receive more than one satellite at a time, it's accuracy goes up. The "multiplex" GPS receivers are more accurate and update faster because they can receive multiple signals at once, instead of bouncing from satellite to satellite.

WAAS (Wide Area Augmentation System) capability also increases accuracy.

The military GPS I have used, could accept a "fill" that filtered out the erroneous signals from the satellites. It had a mode, which if you weren't moving, could define the position of the unit to 10 cm. I used that function when placing the panel and Cole-man lantern the Apaches used to bore-sight their lasers. They knew their position from their GPS, and I radioed the coordinates of the panel. Simple math told them the distance between them and the panel. Then they would lock on the panel, using the heat source of the lantern, and "lase" it. The laser would tell them the distance, which they would adjust based on the GPS calculations.

Aviation units are expensive because of the databases, maps, capabilities, features, and information stored in them, especially IFR units, which have to be updated occasionally to be legal to use for IFR navigation.

Not all GPS receivers are built equal.

Hook

[indyz 1]

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But does that depend on the "quality" of the receiver? I'm not sure you followed the thread.

I was under the impression that civilian and military signals differed in the type of modulation used.

Garmin's "about" page has a pretty good description of the various things that can cause accuracy loss in GPS receivers (down at the bottom of the page). They mention that civilian users use a UHF signal designated "L1," and that a second L2 channel exists (for the military? It would appear that even Google doesn't know).

[kallend 2,029]

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But does that depend on the "quality" of the receiver? I'm not sure you followed the thread.

I was under the impression that civilian and military signals differed in the type of modulation used.

Garmin's "about" page has a pretty good description of the various things that can cause accuracy loss in GPS receivers (down at the bottom of the page). They mention that civilian users use a UHF signal designated "L1," and that a second L2 channel exists (for the military? It would appear that even Google doesn't know).

I'm sure that is correct - I was on a PhD qualifying exam committee for a student working on WAAS reliability assessment and he described the difference between civilian and military sytems at one point. Pity I wasn't paying more attention to the details, but the gist was that the military used different signals altogether.

While we're about it, let's not forget the difference between accuracy and precision. Adding more channels apparently increases precision, but does it necessarily increase accuracy?

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The only sure way to survive a canopy collision is not to have one.

[LouDiamond 1]

Used my Garmin V yesterday for my birdman jumps and was able to get accuracy within 12ft,9ft and 3ft on different occassions. I checked the grids coords against a 1:50,000 map and they were pretty much balls on accurate. I don't believe there is any spoofing going on right now.

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[kallend 2,029]

I had a talk today with my colleague who's doing research on precision GPS. He said:

There are 2 signals, L1 and L2. L1 is civilian and military, L2 is military only. L2 is modulated with an encryption code that only the military has access to.

The system was designed so that an unambiguous position code (ranging signal) is transmitted on both L1 and L2, and the receivers decode this - 4 channels gives a position if the satellite geometry is correct.

The standard deviation in position is the product of a satellite geometry term (called DOP, dilution of precision) and the standard deviation of the ranging signal. The more channels you have, the smaller sigma becomes. Since there are 24 satellites, only 12 are in view at any time, so there's no point in having more than 12 channels.

Older receivers typically may have only 4 or 6 channels, and use a software algorithm based on the satellite almanac to decide which satellites to listen to to get the best DOP and the most accurate position.

He said the hardware chips are now so cheap that even the cheapest GPS receivers have 12 channels, so that source of error has disappeared. Typically they have 3 chips with 4 channels per chip.

An alternative method of ranging was developed after the system was designed, based on carrier wave phase detection rather than the ranging code itself. This is more accurate than the coded signal and is not subject to being perturbed by the military. Consequently there is no point any more in the military degrading the civilian system. This can be used on L1 and L2 with a suitable receiver. These receivers are much more expensive, and also take a long time to get an unabiguous lock on the carrier phase, maybe ten minutes or more. These are the receivers used typically for geodetic surveys where the time to lock on doesn't matter much. Once locked on, the position can be measured to a few cm accuracy. This technique is no use for fast moving objects at present, although research is being done on algorithms to disambiguate the phase information more quickly.

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The only sure way to survive a canopy collision is not to have one.