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Effect of Rotors on GPS Reception

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Effect of Rotors on GPS Reception

Old 3rd Jan 2004, 03:35
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Effect of Rotors on GPS Reception

CAA Paper

This report describes the results of a series of experiments undertaken in October 2002 on behalf of the UK Civil Aviation Authority (CAA) to investigate the effect of helicopter rotor blades upon the reception of GPS signals.
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Old 3rd Jan 2004, 04:01
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Thumbs up Rotor flicker or magnetic interference

I had mentioned on several occasions about magnetic interference on the VOR on the Bell 214. The blades were not earthed to the airframe and as a result developed a torroidal shaped magnetic field with the center of the field at the mast. Bell said it was the flicker of the rotor that interfered with the reception of the VOR and suggested that the pilots reduce rotor speed so as to eliminate the interference. The magnetic field magnetized the mast to the point that the flux level of the mast was 18 times the allowable magnetic level for the mast. Those pilots that tried Bells suggestion said the helicopter was about to stall due to low rotor speed. They just had to endure the ineffectiveness of the VOR. Being in Iran it really did not matter because the helicopters did not venture far from the base. I don’t know if the problem still exists but now the 214s are equipped with GPS and I wonder if they have any problems flying using GPS as a navigation aid.

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Old 3rd Jan 2004, 04:33
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There is also a huge difference in antenna performance and between different receivers, which may also colour the results of their tests.

The simple suction-cup antenna of the portable Garmin GPS 196 cannot see through the thick front window glass of a UH-1H or the heated glass of an S-76, but gets perfect reception when we put it in (of all places!) the chin bubble. This set will now lock onto the constellation well before the built-in Garmin or the UNS-1K.

Two blades or four blades don't seem to affect it.
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Old 3rd Jan 2004, 05:30
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Several years ago, the company I was working for had to come up with a special mount for a WAAS enabled GPS system. The mount put the antenna on the top of the vertical stabilizer. It just wouldn't get a lock anywhere else. WAAS is coming down the pipe here in North America to greatly improve the accuracy of GPS. Think precision GPS approaches, GPS guided aerial spraying, etc.
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Old 3rd Jan 2004, 06:24
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Thumbs up

I've used my Garmin 196 in a R22 Mariner, and it seems pretty accurate. The GPS and built in HSI is really helpful when offshore and you don't have any land reference to fly towards (magnetic compass in the Robbie SUCKS). Anyway, the Robbie has small blades and the bubble is pretty thin, so reception is quite good.

When closer to home, I've cross-referenced GPS position to small islands on the map; it is dead on target (even without getting WAAS signals). Robinson has external GPS antennae mounts for their panel mounted Garmin 430's (mounted on tail section). I wonder if it makes reception that much better? (I've been putting my 196 on top of the instrument panel with Velcro).
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Old 3rd Jan 2004, 10:20
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The report is most impressive, but somewhat disconcerting. Inspite of the facts it shows that no accuracies were ever affected - none - the laundry list of "interferences" is exactly enough to justify the pay of the folks who did the work.

The bottom line is that if all those recommendations are implimented, the cost, weight and testing requirements for gps receivers would all rise, due in no small part to the fact that nothing was found. Nothing.
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Old 3rd Jan 2004, 13:58
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This is what your exam fees and licence money are paying for folks - someone has doubtless got promotion or a pay rise for this stunningly useless piece of work. Now I am sure there will be some questions on 'rotor interference' in the Nav exams
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Old 3rd Jan 2004, 16:34
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This is a very worrying paper, primarily because I would guess its authors and their recommendations will carry some weight with some beurocrat somewhere. As has been said someone is going to take these recommendations seriously!

The trouble is this is a crap report. It is the sort of report that is presented to medical journals frequently but thankfully are mostly refused publication after peer review exposes its flaws.

What this study has done is:

1. Think of a potential problem
Don't examine the problem but think of a theoretic potential for a problem.

2. Do a study which DOESN'T examine the problem but examines a theoretic point several stages removed from the problem.

3. Find some (anything will do) effect using non-real world testing equipment.

4. Extrapolate the findings back toward the problem. (But ignore the fact that no problem existed in the first place.)


This study should have been preceeded by a study examining whether GPS gave wrong readings while navigating - Its easy enough - just fly around and log when you do and don't end up where you should. If there was no problem then stop doing the rest of the study!

IF there was a problem then go on to try to find the cause of the errors. But these blokes themselves state there was no evidence of problems - then go on to make recommendations for major changes.

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Old 3rd Jan 2004, 17:45
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RobboRider,
Your post very nicely states the concern!

I have worked several flight tests of GPS systems, and it seems that the rotor modulation treatment within the receivers works very well. Generally, there is more issue with banking and its influence on the antenna's ability to see satillites on the horizon. There are times when the gps will drop below proper satellite geometry when in a bank, yet be very healthy in level flight.

It seems that the way gps is being handled depends on whose government built it. The idea expressed in the CAA paper that it is unsafe to depend on it is extreme, to say the least, as is the idea that RAIM is not conservative enough. Curiouser and curiouser.
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Old 3rd Jan 2004, 18:45
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The idea expressed in the CAA paper that it is unsafe to depend on it is extreme.
And that, dear Nick, neatly sums up CAA thinking and is the ethos that leads to the ridiculously restrictive regulations under which we have to operate.
Don't knock it too much. We're grateful for small mercies here. At least they let us fly, with strong misgivings that they shouldn't really allow such a dangerous activity to take place at all.
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Old 3rd Jan 2004, 23:49
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As far as I know, the GPS antenna beeing located under the main rotor doesn't seem to be too much affected ; on our S365's, the antenna is normally located on top of the tail fin, but on one of them it's just on the cockpit roof. We never noticed any difference on a normal use with the same equipment (trimble 2100 series).
Besides this, I have seen of a full loss of signal on a GPS antenna located on the tail boom of a AS332, but the reason was a thin layer of carbon from exhaust fumes...
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Old 3rd Jan 2004, 23:59
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When we first installed the AG-NAV GPS on our Bell 47 we had to move the antennae back mid way on tail boom to get a signal. Not a big deal except in our business accuracy counts. We have our light bar set on 1 foot increments to guide us across the field while spraying. Our swath width is usually 35 feet, so, in a cross wind the further back the antennae is the more "off" a pilot can get. It's not enough to see unless you fly back over the pass you just made and have a ditch or ? for a visual reference. Two feet one way then two feet the other and you've got four feet off which can under some situations cause streaking in the field. We compromised and found a sweet spot that didn't hurt reception or accuracy (at least that we can tell).

Barry
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Old 5th Jan 2004, 04:10
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I have flight inspected a considerable number of GPS IFR procedures for helicopters, and rotor interference has never been an issue (on BK-117s, S-76s, B222s). We all know GPS works in practice, now we have to get it to work in theory? This is a "make-up work" project ... haven't they got anything better to do in the UK?
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Old 5th Jan 2004, 07:31
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I use the litte Garmin Pilot III using it's own little antenna frequently in 206's, 205's and 350's and get no problems with reception at all and that's even when I have to stick it in my lap! The only problem is losing sats temporarily due to bank angles as previously said.
Best GPS I've had in years.
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Old 5th Jan 2004, 23:52
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The only time I've had a problem with a GPS that could be construed as a rotor interference problem was in the initial certification of an FMS on a large helicopter (no names, as the problem got solved). There was a minor issue with the GPS engine in the FMS not working with that rotor speed, but both manufacturers (airframe and FMS) knew about the problem, and had decided to try it anyway. Didn't work, so changing the GPS card did solve the problem nicely.
Later in the same program, had an loss of signal when transmitting on known VHF frequencies - the avionics engineer / technician was going out of his mind trying to figure out why one comm would give the problem and not the other (you could literally watch the satellites drop off when the 'bad' comm was used).
Eventually, swapped comm boxes and both worked perfectly - the problem was traced to bad connections on the 'bad' comm box- corrosion on the terminals, evidently.
So, just as always, the mechanical connections to electronics are going to be the worst problem, not the electronics themselves.
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Old 6th Jan 2004, 00:01
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Like most others, I was so surprised by this blue sky research that I was moved to ask the question why?

It transpires that this is the last of four studies that were conducted as a result of the flight trials of DGPS guidance for helicopter approaches to offshore platforms - reported in CAA Paper 2000/5 - which, in turn, was instigated in response to the findings of the Helicopter Human Factors Working Group, reported in CAA paper 87007 (formed as a result of the HARP report - CAP 491).

The General Foreword of CAA Paper 2003/2 (which contains the first three studies) contained the statement “Perhaps the most significant finding of all of the work reported in this paper is the poor satellite tracking performance observed during the flight trials, and identified in the third study. As a consequence, further experimental work addressing the effects of helicopter rotors on GPS reception has been commissioned by CAA and has recently been completed. This work will be published in a separate CAA paper”.

It transpires that the foreword (reproduced below) had been omitted from the referenced report: it will have been added by the time you read this post.

When reading the latest report it is important to note that it is not accuracy but availability of satellites that is the main issue: at present the constellation contains 27 satellites when only 24 are required; it would also appear that most, if not all, of these are operating at above specification. A reduction to the minimum number of satellites, a reduction of power to specification and a loss of signal due to tail rotor interference, could result in an unacceptable reduction of performance if GPS is the sole method of navigation or the only available approach aid.

Effect of Helicopter Rotors on GPS Reception

Foreword


The research reported in this paper was funded by the Safety Regulation Group of the UK Civil Aviation Authority, and was performed by Cranfield Aerospace Ltd and the CAA Institute of Satellite Navigation at the University of Leeds. The work comprises a study of the effect of helicopter rotors on GPS reception which was highlighted as an issue for investigation during the flight trials of DGPS guidance for helicopter approaches to offshore platforms reported in CAA Paper 2000/5. Further impetus was added by the results of the third of the three follow-on trial studies reported in CAA Paper 2003/2 which identified a significant difference between the number of satellites tracked and those which were expected to have been tracked; the evidence available indicated the most likely cause to be the tail rotor (the GPS antenna had been mounted on the tail fin on the trials aircraft).

The CAA accepts the results of the study which will be taken into account in overseeing the use of GPS in helicopter operations, for which the most significant use is currently offshore enroute navigation. The GPS antennas on all helicopters currently operating in the North Sea region are understood to be located on the tail boom under the main rotor. The results of this study suggest that such installations may be expected to be less susceptible to the rotor effects identified and, based on the favourable in-service experience to date, this appears to be the case. However, little or no objective data for in-service operations exists, and it is noted that most of this experience is based on a GPS space segment with more satellites and a higher signal strength than that guaranteed by the system provider (US DoD).

In view of the foregoing, and in line with the recommendations contained within this paper, CAA proposes to pursue two courses of action:

(i) Establish a requirement and a means of compliance for confirming the acceptability of GPS antenna installations on helicopters. It is anticipated that this will take the form of a ground test procedure for use by helicopter operators and/or helicopter manufacturers in validating both existing and new antenna installations.

(ii) Establish a practical means of monitoring GPS performance in order to detect any significant degradation. This may involve monitoring of the GPS space segment at a fixed location and/or monitoring of overall performance on board helicopters during routine operations.
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Old 6th Jan 2004, 03:42
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CAA proposes to pursue two courses of action
Does this mean they don't do this already for aircraft with IFR certified GPS ?
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Old 6th Jan 2004, 08:14
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I sure hope the CAA would not be so lame as to act on such a limited report...... I’ll admit it’s a pretty report. But I’d say their conclusions are HIGHLY questionable.... One phenomenon, one helicopter, one blade construction, only a few receiver designs... and after all that data , which really only shows the effect on the signal – NOT THE PERFORMANCE, they go ahead and draw sweeping conclusions.

By their own admission, one receiver showed no appreciable change in accuracies, and the other – they couldn’t get access to the data to see the real effect. And then, based on this, they essentially recommend dropping the navigation provisions of TSO-c129 certified systems for helicopter operations?!?!? Give me a break!!!!

The report admits this is not a likely a case of inaccurate data from your GPS... but mostly an availability issue: That in some cases, you might not get the predicted RAIM accuracy when you actually get there. And because of that, it’s unsafe !?!?!

Even this loss of availability is a conclusion – not supported by data in itself. And even they say that this loss of perfomance (which is not really quantified) depends not only on the rotor masking, but also on the receiver design (and I'm sure it depends on even more than that.)

Well, if we’re going to do that, let’s not just pick on helicopters, and let's not just pick onthe rotor masking phenomenon. Let’s look at all phenomenon that can potentially effect GPS reception.... There’s weather phenomenon, installation variations, multi-tail configurations, airframe in certain pitch-up maneuvers, antenna cable length, number splitters, location of bends in the cable, corrosion in the cable and connectors over time, ice accumulation on the antenna. Lot's of reasons can affect the GPS reception performance. At least in a helicopter, the obstructions tend to move. In a fixed wing, in certain constellations and maneuvers, you can find yourself blocked from a satellite. They live with their problems - we live with ours.

Let’s remember what predicted RAIM is.... it’s whether the satellite constellation visible at the time over the location will have the geometry to support the accuracy for the phase of flight (either enroute, terminal, or approach) We’re talking accuracy requirements in the hundreds of meters – and the effect cited in the report was what??? 2m? Even this was for which geometries of satellites??? Also to my knowledge, predicted RAIM is established for the worst case accuracy when the US-DOD has SA cranked up to maximum. There’s lots of slop built in. Even so, that says a lot for what the report didn’t examine. And that’s why we depend on the real RAIM determined by the receiver, and not just the predicted.

You might as well say that since weather doesn’t always match what’s predicted, flight in general is unsafe and all air traffic should be stopped.
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Old 8th Jan 2004, 04:40
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Effect of CAA Interference on GPS Navigation

'Case not proven', at worst, and more like 'Hooray for GPS, max. 2m error irrespective of aerial position'.

Our experience is that the top of the tail fin is one of the least practical sites for the GPS aerial, because the inevitable vibration shakes the plug off from underneath the aerial. It's a devil of a job to fish the plug back up from the bottom of the inside of the fin.

Interesting to read (Lu on 2nd January) of 'rotor flicker' causing problems with more traditional nav aids. Although we did not experience the same problems on our Bell 214STs (in the desert) that Lu describes, we did have a similar phenomenon on an A109. In this case the only way to eliminate the ILS Glideslope fluctuations (or reduce them to a flyable level) was to beep the rotor back to 98% Nr on finals.
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Old 8th Jan 2004, 20:19
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I have been flying Bell 214STs with GPS since 1997, and have flown one of them halfway around the world, without a single incident of lost signal, and this was one of the older version that only received 9 satellites, instead of 12, which is standard. We routinely fly IFR using only GPS, without any problems. I think any report has to have real world testing not conjecture.
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