Independent tests prove lack of frequency hopping with XPS

[aviti]

	Quote: Originally Posted by exeter_acres
	Seems the gist of this thread seems to say lots of people are getting shot down (I believe any loss of frequency is a "shoot down")

Shot down by other people turning on to your frequency. Is 2.4 new? Yes. Are there risks with new technology? Yes. Are we talking about 2.4 or XPS? Lumping these together isn't exactly fair. I've been flying on 2.4 for several months now and have been happy so far. To me it is worth it not having to worry about some moron turning on my frequency. If you don't agree, then stay with 72. But there is at least one reason to switch.

[KrisW]

	Quote: Originally Posted by aviti
	Shot down by other people turning on to your frequency. Is 2.4 new? Yes. Are there risks with new technology? Yes. Are we talking about 2.4 or XPS? Lumping these together isn't exactly fair. I've been flying on 2.4 for several months now and have been happy so far. To me it is worth it not having to worry about some moron turning on my frequency. If you don't agree, then stay with 72. But there is at least one reason to switch.

Then there's the dual receiver binding problem. I've always run dual receivers in my big stuff. . just in case.

[exeter_acres]

what "issue"

[Kiwi]

OK gentlemen before this one goes off the rails how about we stick to what Xjet started here and that is the evident fact that XPS does not channel hop as advertised and the fact that guys are and have and likely will loose aircraft until its fixed or disappears.

Its not about anyone having to prove why 2.4 is better than 72, 50, 40, 35 or 36 Mhz as all those frequencies are used in some part of the world every day on RC airplanes.

Kris, let this thread run as the guy desired please, dont turn it into a personal thing.

[jonkoppisch]

I switched so that I wouldn't get shot down... and I got shot down

[3ddd]

well i for one am still waiting on xjet to post just that.post count on pure bs=76,post count of proof xps is dangerous=0,we will keep waiting.beeb flying xps in three planes all year and not one misshap,not even a slow response time

[XJet]

	Quote: Originally Posted by Kiwi
	So Xjet what Mr Drew wrote here and these are his own words is totally BS.?? (Quote Jim Drew on RCU) I am not sure what answers you are looking for. The bottom line is that we don't have a problem with the system, and I don't expect to ever see a problem. We are the ONLY 2.4GHz system that has the ability to monitor the available frequencies in the 2.4GHz range and change as necessary, in real time.

JD forgot to add "under ideal circumstances" to that.

Remember that JD also said:

	Quote:
	If the noise becomes so high that it saturates the channel, the hopping counter is reset and the system will not switch until there is a decrase in the noise level

and

	Quote:
	The frquency hop requires a rise in the noise floor over a number of frames. A sudden saturation will reset the noise coutner and not switch at all

In checking back over the earlier claims made on the XPS website I see he's been *very* careful with his use of words, claiming only that the system would frequency hop when it detected potential interference. In fact, the exact words were:

	Quote:
	Only changes frequencies when potential interference is encountered

It doesn't claim that *actual* interference will produce a frequency hop.

The reason for this (and what JD is now saying) is that if there's a gradual rise in noise (ie: an interfering signal of slowly increasing strength) on the frequency XPS is using, it will do a quick scan and jump to another (quieter) frequency.

Now that sounds fine -- and indeed it may well happen (something I'm going to be testing shortly) -- but the reality is that (despite what JD claims) interfering signals don't always (or even often) introduce themselves by slowly ramping up from nothing -- this allowing XPS the time to change to another frequency.

As I've stated before, the big problem with single-frequency non-agile systems is that 2.4GHz is very much a "line of sight" system. When you're down at ground-level in the pits, there's a good chance that some very strong interfering 2.4GHz signals are completely blocked by what's called ground-clutter. Trees, buildings, vehicles and even people all act like a very effective RF screen that means when XPS does its initial frequency scan at start-up time, it may be totally unaware that a powerful 2.4GHz transmitter is operating just a mile away. The signal from that 2.4GHz interference source is simply soaked up by the ground clutter between it and your model.

To see what I mean -- get down to "plane level" (ie: lay down on the runway" and look around. You can't see much else that might be at ground level beyond the confines of the flying field can you? That's the view your RC gear gets when it sets about looking for a "clean" channel to use.

After locating what it thinks is a clear channel (one out of the 12 available), XPS will then start providing a link between transmitter and receiver.

But, what happens when you take off?

Suddenly it's like climbing to the top of a tall building. You can see things in the distance or just past the runway fence, that you couldn't see before while laying on the runway.

At the same time you're flying away from the transmitter (ie: its signal is getting weaker) strong interfering signals are suddenly becoming "visible" to the receiver.

What if one of the things you can now see is a very strong source of 2.4GHz signal that just happens to be right smack dab on the channel your XPS system is using?

If you're lucky, the strength of the signal from that source will rise slowly enough to trigger XPS's alarm bells, allow it to scan the other 11 channels it can use, advise the transmitter it wishes to change, then perform the frequency change -- a process that does take time and if interrupted, will result in a failed hop attempt.

If you're not lucky, the receiver experiences a very sudden increase in the noise level, something which constantly resets the noise counter which JD admits is a problem. It then then unable to change frequencies so goes into failsafe -- and your plane becomes a very expensive shovel.

Another scenario is that your XPS system is flying quite happily, receiving a steady stream of data from the transmitter and sending back very short "ACK" signals from its 60mW onboard transmitter. A short (or not so short) distance away, someone turns on another 2.4GHz device with 600mW or more of power. It may be a video transmitter, it may be some other data link.

Suddenly the signal from XPS's own 10dB weaker 60mW transmitter on the ground is lost, overwhelmed by the much stronger signal from a transmitter that may have the benefit of greater transmitter power, a directional antenna, lower path-loss and extra visibility arising from a matching polarization.

In an instant, the XPS receiver sees this as a massive increase in the noise level - sufficient to throw it into this counter-reset mode from which it can't hop frequencies. Lockout time.

A satellite receiver and/or diversity antennas would help mitigate this somewhat by avoiding the situation where the transmitter and receiver are cross-polarized or the receiver's null-point is oriented directly towards the transmitter. The single-antenna setup makes XPS doubly vulnerable.

	Quote:
	We don't conflict with any other 2.4GHz systems, and they don't conflict with us.

I think that ought to read: we dont' conflict with any other 2.4GHz RC systems"

There are plenty of other systems out there that will conflict with XPS (video senders for a start).

	Quote:
	Our systems are special, one of a kind radio configurations and no two radio modules have the same ID. It is physically impossible to have a conflict unless two identical ID's were available. We can't even get MaxStream to do that for us for testing the theory, so we know that it will not happen

This is irrelevant to the frequency hopping issue -- it simply refers to the fact that each module is bound to the other such that the data packets carry a unique identifier. That'll stop one XPS system from responding to the transmission from another (because if the numbers don't match, the data is ignored) but it won't stop a strong interfering signal from causing a lockout. The best analogy is that a JR PCM receiver won't respond to a Futaba PCM transmitter -- but they can still shoot each other down if they're on the same frequency because lockout will occur.

	Quote:
	Anything is prone to a problem. 2.4GHz will not transmit through large sections of earth (hills, etc.). So, as long as you are flying LOS (line of sight) you will never encounter and issue with the radio link that is related to our system or any other 2.4GHz system available.

This is the ground-clutter I was referring to earlier. The fact that ground clutter obscures what may be powerful nearby sources of interference is exactly what makes a single-frequency non-agile system so vulnerable.

	Quote:
	Even 2.4GHz cameras are not a problem as we can move around their frequency, even with huge drift that occurs in the cheap overseas versions. Our system is extremely well thought out for safety reasons.

There are plenty of posts on the Net which dispute this. In my testing, XPS can operate if turned on in the presence of an already operating video transmitter but if you turn the transmitter on after the XPS system, control is sometimes immediately lost and failsafe results.

	Quote:
	People have witnessed and flown the system first hand. We did a range check with the 40% gas plane, and it was so high that we lost it in the camera and could no longer hear the engine running (and it was!) That is some distance. The highest recorded flight to date was 4947 feet, but I believe we have gone higher than that, just not recorded the flights with the altimeter.

This is a total red herring, range has never been a problem with 2.4GHz systems and has nothing to do with the frequency-hopping issue that's being debated.

	Quote:
	If you care to argue, I have no reason to reply.

That kind of sums things up right now. JD won't argue the facts or provide substantiating proof for his claims.

Some will believe as an act of faith, others will be more objective and recall the many other claims from XPS that have since been debunked.

[XJet]

	Quote: Originally Posted by DesertHucker
	PS XJet did you get the XPS i sent?

Yep, turned up just before Christmas.

I'll be back to working on this first-thing in the New Year.

[sweetpea]

As posted on RCgroups

"Originally Posted by JimDrew
As stated from the beginning, the frequency hopping is predictive and based on the noise level increase over a period of time. If there is a sudden burst of noise, it does not makes sense to change the frequency as we have found in nearly every case that a sudden burst does not occur on a single frequency, but on the entire band (as well as above and below 2.405GHz and 2.485GHz). "

Xjet quote:

"JD won't argue the facts or provide substantiating proof for his claims."


Well to date neither have you! But I'm waiting, along with others for you to produce it.

I'm very interested in Xjet's test of the gradual increase of noise......But I still want to see the data for both.