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HelicopterMain Discussion › 2.4GHz, here I come!
11-26-2007 10:37 PM  9 years agoPost 41
bigdog714

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Rochester, MN U.S.A.

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I am in asphalt and there are GPS systems for all types of gradeing and paveing, the only draw back is big brother only allows contact with three satellites at a time limiting the accuracy of the system, thier scared we are going to start blowing things up, to be more specific, "them", and I think it can be done on a cloudy day.

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11-27-2007 12:33 AM  9 years agoPost 42
stickyfox

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Rochester, NY - US

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Beg to differ. GPS can be used as the PRIMARY navigation system for aircraft in instrument conditions. That means under and in heavy cloud cover. The signals have no problems penetrating the clouds.
FASST uses FHSS (frequency-hopping), a system that basically occupies all the channels all the time.
The Futaba FASST system does not use ALL the channels ALL the time. It hops every 2 milliseconds. Almost all of the channels are NOT being used at any given point in time. If it arrives on a frequency occupied by another piece of equipment (Futaba's or someone else's), it won't be on the frequency long enough to cause a problem.
To clarify a couple things:

Aircraft navigation systems have different design constraints from handheld units, like the one I used when I was in the 7th Cavalry. This was a fully-functional military system running on battery power (kind of like an RC model), and I assure you, they crap out under heavy rain and clouds, trees, and some kinds of military smoke screens. None of these are problems aircraft typically have to deal with. The only primary navigation system approved for a Cav trooper comes on paper and doesn't use power! Dish network, the upper UHF channels, and X/K band satellite TV does the same thing (altho not nearly as much as the cable company wants you to think!) I just cited GPS as an example that most people with a handheld Garmin can appreciate. Yes, there are ways that technology can overcome the directionality of microwaves. Perhaps a better example is how FM radio can reach you in a parking garage, while your GPS navigator is useless.

And what I meant about FASST was, there is no answer to the question, "what channel are you on?" Basically... the key word is basically. If you're flying, you're using all of them. You will be using the same channel as someone else, but only occasionally. DSSS, on the other hand, does use discrete channels, and once they're all used up, they're all used up. Unlike FHSS, DSSS can tolerate two users on the same frequency, because to each one, the other just looks like noise. But there is a point (which depends on the nature of the traffic) beyond which the signal becomes so garbled that you need a supercomputer to decode it. I believe the FCC rules state that SS devices on the ISM bands need to move over if someone's already on channel, so it's an academic matter anyway.
If it takes 20ms to update all the channels and collect all the data, what information is the xmitter sending when it is hopping every 2 ms?
The transmitter chops your channel numbers into a bunch of little pieces, and encodes them so that the receiver can tell if they're garbled. It adds a few extra bits to identify the transmitter and help the receiver stay in sync, and sends the pieces one at a time. There is plenty of bandwidth to spare, so the system sends redundant data. If everything's working fine, the receiver will get multiple copies of the data every 20 ms, but it's designed to work under the worst conditions without causing a crash.

-fox

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11-28-2007 04:17 AM  9 years agoPost 43
d23

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Oakhurst, CA

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GPS

I am in asphalt and there are GPS systems for all types of gradeing and paveing, the only draw back is big brother only allows contact with three satellites at a time limiting the accuracy of the system, thier scared we are going to start blowing things up, to be more specific, "them", and I think it can be done on a cloudy day.
my Garmin locks me into 12 Sat's solid - Lat., Lon., Alt. within 6' is typical

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11-28-2007 12:27 PM  9 years agoPost 44
bigdog714

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Rochester, MN U.S.A.

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my Garmin locks me into 12 Sat's solid - Lat., Lon., Alt. within 6' is typical
We get to use Government satellites, with three our accuracy is within a 1/4 inch, the Garmen will get you within about 150 feet, not the same.

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11-28-2007 01:26 PM  9 years agoPost 45
jester4

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Brampton, Ontario

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We get to use Government satellites, with three our accuracy is within a 1/4 inch, the Garmen will get you within about 150 feet, not the same.
Is that for real? That's impressive.....

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11-28-2007 05:12 PM  9 years agoPost 46
stickyfox

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Rochester, NY - US

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There are two mechanisms the GPS system uses to limit civilian-grade performance. One of them is encryption. The satellites transmit data in two forms, a low-res version and a hi-res version. Only the low-res version is sent in the clear for civilian applications. The hi-res version requires a: a special decoding chip, and b: the decryption key used to decode the data. These features are found mostly on military systems.

The high-resolution code instantly gives you your position within less than a meter. I'm not sure about 1/4", but computers get better and better all the time.

With the low-res code only, you can still get very precise positioning information; perhaps just as precise as the hi-res code. The only difference is it takes a very long time. Most handhelds have "surveying mode" or something similar, which averages position over time. The longer you leave the unit in place, the more accurate the position data becomes. If you have an hour to wait for it, you can get down to the same degree of accuracy that the military units give you in real time.

Theoretically, the low-res code gets you down to 3m, and the hi-res down to 30 cm. But mechanisms like "surveying mode," keeping track of error over time, and measuring atmospheric conditions by comparing the GPS signal with other signals on different frequencies can all improve accuracy. Typically, consumer-grade receivers have cheap electronics that account for errors much greater than 3m, so the more advanced precision features are found only on the really expensive units.

The second mechanism is called selective availability (SA), and it is basically a "fudge factor" introduced by the satellites to make it slightly unreliable. It's not a very well-understood technology. The intent is that people won't buy $99 handheld navigators and strap them to homemade bombs. What it does is, it moves everything a few hundred meters in some random direction. The error is relatively uniform over a wide area (say the size of the US), and over a given time period (like a day). This doesn't mean that your positioning is less precise, it just means the position you get is incorrect. Like the hi-res precision code above, SA can be decrypted out of the original signal if you have the right friends.

Civilian paving, landscaping, and surveying systems can still work around SA because it is assumed to be relatively stable over short periods. If you know you're at point A, the relative distance and bearing to point B is unchanged by the fact that both points A and B are offset by the same error factor.

Incidentally, SA can be turned on and off in different regions, and it's been disabled here for years, so SA currently has nothing to do with the difference between civilian and military system performance.

The number of satellites used also influences how well the receiver performs. You need two satellites to find your position on the surface of the earth, three if you're in the air (assuming your GPS knows the local altitude). Adding more satellites makes the figure more precise. The number of satellites used by a receiver isn't regulated by anyone; it's just a matter of how sophisticated the machine is. And of course, an expensive set with three satellites and a good clock can beat a cheapo set with a less accurate clock tracking any number of satellites.

Not especially relevant to the topic but I think it's terribly interesting stuff, maybe you guys do too..

-fox

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11-28-2007 06:04 PM  9 years agoPost 47
jester4

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Brampton, Ontario

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That's a good read, thanks for posting

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11-28-2007 06:09 PM  9 years agoPost 48
JKos

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Redondo Beach, CA

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Fox,
You forgot to mention differential GPS which is typically used for construction and surveying. Here's a quick read on it for those interested: http://www.esri.com/news/arcuser/01...ential1of2.html

Also, the FASST system is hybrid SS as it does use DSSS and hops. This can be verified via the FCC test report. For the testing they actually locked on certain frequencies so the test equipment could capture the signal characteristics.

> Incidentally, SA can be turned on and off in different regions, and
> it's been disabled here for years

A handy comparison of SA on and off is that with SA on, you know your location to within about a football field of your real geographic location. With SA off, you know it to within about a tennis court.

> We get to use Government satellites, with three our accuracy is
> within a 1/4 inch, the Garmen will get you within about 150 feet,
> not the same.

That sounds a bit fishy.

- John

RR rules!

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11-28-2007 06:49 PM  9 years agoPost 49
tadawson

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Lewisville, TX

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Regarding the accuracy, I don't know if you are speaking of repeatability or of the ability to hit lat/long pairs with a degree of accuracy. I know that the landmark I set for my house on my Garmin at least five years ago repeats within 5 feet every time . . . . . so there is apparently a stellar repeatability with this unit, but as I said, not sure the accuracy of the lat/long, since I have no real way of knowing what it should be . . . Oh, and the unit is a Garmin GPS-IIIplus, if anyone cares . . . .

Oh, and I just looked, and Garmin is stating accuracy on the GPS-V (the unit that replaced mine) to be "better than 15 meters" with no enhancements, and three meters using WAAS . . . . it will also track 12 satellites simultaneously, so methinks someone here has some bad info regarding limitations . . .

- Tim

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11-28-2007 11:17 PM  9 years agoPost 50
bigdog714

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Rochester, MN U.S.A.

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I live in Ohio and the dept. of roads requires GPS systems when working on the turnpike, the engineering is loaded into the computer and sent to the machines via GPS, and yes it is within a 1/4inch, ODOT is very stringent on accuracy and ride-ability, or you dont get paid, not only do they want to control material yield, they want the road in exactly the right place.
The most accurate system is the station system, one main station where you plug your laptop with all the prints, and satellite recievers placed at specific stations in a series, like a chain, that projects a 3D image that is followed by the sensors on the equipment. This is the system that the federal highway dept is trying to develope for hands free freeway travel, just plug in your destination and sit back and relax

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11-29-2007 06:34 PM  9 years agoPost 51
mac24

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Rotterdam - Netherlands

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Bigdog, could they develop a handsfree system for my heli's too

Just kidding of course. This is one of the most interesting threads here.

Darn, didn't know a heli could make such a big crater

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11-29-2007 11:32 PM  9 years agoPost 52
BC Don

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Calgary, AB Canada

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Ummmm, need FOUR satellite signals to get a 3D position (Elevation plus lat plus long) and you need Three to get a 2D position (lat plus long).

To visualize this, think of each satellite putting out a "globe". Now, your receiver can be anwhere on the outside of that globe in 3D space. Now add in a second satellite and your receiver can be anywhere where the two "globes" intersect. This is a circle. Now add in a third satellite and you will end up with two points where your receiver is, you need the 4th to identify your elevation.

Now if you don't agree with the above then read the manual that comes with your GPS, it doesn't explain why but it does say that you need 3 satellites for a 3D Postion and 4 for a 3D Position.

And yes, you do end up locking onto a dozen of those suckers on a good day.

Got Money? Send it to me, I'm a Heli Addict.

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11-30-2007 02:27 AM  9 years agoPost 53
bigdog714

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Rochester, MN U.S.A.

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the problem is your too literal, wether it was 3 or 4 satellites wasnt the point, it has been a couple of years since I used the system, the point was a system exists that achieves accuracy to a 1/4 inch, and the system was limited to satellite access based on its deadly accuracy, and your 12 satellite Garmen cant tell you if your in the driveway or in the garage, so there is an obvious differance in technology between the two GPS systems

God I wish I could spell

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11-30-2007 02:40 AM  9 years agoPost 54
stickyfox

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Rochester, NY - US

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"Need" is a subjective term depending on how you define your receiver.
To visualize this, think of each satellite putting out a "globe". Now, your receiver can be anywhere on the outside of that globe in 3D space. Now add in a second satellite and your receiver can be anywhere where the two "globes" intersect. This is a circle. Now add in a third satellite and you will end up with two points where your receiver is, you need the 4th to identify your elevation.
The intersection of two spheres (two satellites) gives you a circle. The third sphere is the surface of the Earth. The receiver is programmed (some are anyway) with local elevation and the exact positions of the satellites relative to the surface of the earth is being broadcast along with the time signals. The manual for my receiver (a Rockwell) does not explain this, but it does indicate the minimum number of satellites for a fix on the Earth's surface (two). It also specifies that you need to have a gross estimate for your location if you don't have three or more satellites, presumably to rule out that second point; and that there can be times when two satellites are not enough (such as, I imagine, when the intersection of a circle and a sphere is the original circle).

You can also measure the differences in arrival times between satellites. This will give you a set of hyperboloids to intersect, with a whole different set of intersection curves.

Bigdog: I knew it was called something like that, and it had to do with "where am I now?" versus "where was I a minute ago?" I just couldn't remember the name.

-fox

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11-30-2007 04:28 AM  9 years agoPost 55
JKos

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Redondo Beach, CA

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> You can also measure the differences in arrival times between
> satellites.

That is exactly how GPS works. In simplistic terms, that's how you "know" where the spheres intersect. The GPS rx knows where each satellite was located when it sent the signal (the satellites are actually moving very quickly in Low Earth Orbits (LEO)) via the almanac which the GPS rx can collect from the GPS signals (the data rate is very low at 50 bits/second so it can take a little while). Keeping the almanac up to date is why it takes longer to lock onto satellites if you haven't used a GPS unit for a while. The almanac is only good for so long before it becomes too inaccurate.

By knowing where two satellites are that sent signals at exactly the same time and the time difference of arrival of the signals (and you know which one arrived first), you can determine the circle on which you must be located. Add to that more time differences from other satellites and you can better and better define your exact location.

There are even sexier methods which go beyond time difference of arrival and analyze and compare the phase differences of the carrier signals as well.

- John

RR rules!

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11-30-2007 04:37 AM  9 years agoPost 56
bigdog714

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Rochester, MN U.S.A.

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Yah, what he said, I knew I wasn't completely crazy.
[quote]Bigdog, could they develop a handsfree system for my heli's too
The station system could work in theory by projecting a 3D object like a sphere as fox described or a cube or any number of geometric shapes and ploting a course within the peramiters of the object, I am not sure of the elevation limitations, and the size of the receiver may be of some concern, but it would be cool to see how many helis you can fly in a confined space, I have yet to see anyone fly more than one heli at a time.

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11-30-2007 05:05 AM  9 years agoPost 57
oldboldpilot

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Southern California

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flyboy0413,

To get back to the original post

The Spectrum folks are happy with their two Rx, and I am very happy with my XPS one Rx, and I'll bet the Futaba folk will be happy with that 2.4 system.

Aside from not having to declare a specific frequency, any other 2.4 advantages?

Yes.

Electrical noise from things such as metal-to-metal and arcing ranges up to 300 Mhz. This certainly covers 72 Mhz.

Does 2.4 solve all possible interferences?

No.

If you have noise from your ESC or t/r servo getting into your Rx, "so solly."

Servo wire shorted due to fraying against carbon frame? "so solly."

Noise from your regulator? "so solly."

These are noise sources which go directly to your Rx, transmitter frequency be damned.

Disadvantages, or at least things to get right with 2.4: Rx in the clear of conductive surfaces by two inches minimum; wires within 6 inches immobile; Rx ideally 6 inches above ground when on the ground; ground range check done to the letters. If it doesn't pass ground range check, you shouldn't fly until you figure out how to move or restrain things so that your system does..

I have found that 2.4 on airplane models is really easy - i.e., ground range check far exceeds minimums given that you install Rx per guidelines.

On my electric heli (Logo 14 carbon 50-sized), Rx in the clear by virtue of a platform behind the canopy, ground range check was just above minimums on two azimuths (above minimums everywhere else) - and there have been no issues at all flying out as far as I can see her.

Helis are Man's Defiance of the Laws of Nature - OCHC

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11-30-2007 06:40 AM  9 years agoPost 58
tadawson

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Lewisville, TX

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and your 12 satellite Garmen cant tell you if your in the driveway or in the garage
Well, unless your driveway or garage is less that 5 feet in size, mine sure as hell can . . . as I said earlier, it can bring me back to the driveway with a position accuracy of typically 3 to 5 feet, on a waypoint set over 5 years ago - IE no drift over time! Not 1/4", but a heck of a lot better than you are claiming!

- Tim

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11-30-2007 12:23 PM  9 years agoPost 59
Ed1955

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Pocono Mountains, PA USA

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Mac24 "could they develop a handsfree system for my heli's too"

From what I've read, the Yamaha RMax UAV (Unmanned Aerial Vehicle) Helicopter with the optional GPS system can be programed to start itself, fly over a vast farm and inventory the entire crop growth and transmit digital photos back to the home base on the owners PC. Software then is capable of calculating crop yield all from the base station desk. I believe that heli and system is well over $100,000 so the way I look at it is for $100K, I can crash one hell of alot of T-Rex 600's learning to fly like the pros. lol
Respectfully,
Ed

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