RAIM failure on GPS approach
Craig Prouse
gotten pretty good with the basic features of the box, and although my
instructor is more familiar with the Apollo and Garmin boxes, she's picking
up the King unit pretty quickly.
For all intents and purposes the unit seems to work perfectly, except we are
surprised by how frequently we get RAIM failures just as we reach the FAF.
I'd say about 25-30% of our approaches result in a RAIM failure, including
in at least one case after we had performed a RAIM prediction and RAIM was
predicted to be available. Is this typical?
I seem to be tracking plenty of satellites, and my altitude encoder was
recently checked. Seems like satellite geometry, but is it really that
unreliable? Anybody have info on what percentage of the time RAIM is
available at a particular point on the earth's surface?
Craig
Larry Fransson
Craig Prouse <cra...@apple.com> wrote:
> I'd say about 25-30% of our approaches result in a RAIM failure, including
> in at least one case after we had performed a RAIM prediction and RAIM was
> predicted to be available. Is this typical?
Something is screwy. In over 1000 hours of flying all over the country
with GPS, I've had RAIM warnings only a very few times.
Bill Zaleski
approaches. This was with a Northstar M3. Sometimes there are
planned outages
On Thu, 16 Aug 2001 22:59:38 -0700, Craig Prouse <cra...@apple.com>
wrote:
macho...@nosoup4u.com
yeah, but the unit does some kind of special RAIM check at the FAF, I
believe, so if you are not doing a lot of approahes, you might not
have the same experience.
I have experienced the same thing - RAIM failure at the FAF in a box
seeming to be functioniung perfectly up to that point.
gwengler
course of three years. I remember perhaps *ONE* RAIM failure. There
seems to be something wrong with yours...
Gerd
C-GMAL
Walker Mangum
I had a KLN89B, and now have a KLN94. When I switch my KX155
COM to certain frequencies, I get a RAIM failure. This happened
with both GPS units. Switching to another COM frequency caused
the RAIM failure to go away.
The one frequency that I specifically recall having a problem
with is 119.7, but there are others as well. My problem also
seems to be specific to the KX155. If I call up the same
COM frequency on my KY197A, I have no problems.
--
Walker Mangum
Houston, TX
ArtP
>recently checked. Seems like satellite geometry, but is it really that
>unreliable? Anybody have info on what percentage of the time RAIM is
>available at a particular point on the earth's surface?
I have an Airmap 100 hand held with a 12 channel receiver. It almost
always works, but there are some areas where it consistently starts
flashing indicating it can't get even 3 satellites. So before I blamed
the unit I would try approaches at a different airport a distance away
from where it fails. If it still fails it is probably the unit. If it
doesn't it is probably the location. Although it could be the
installation (antenna blocked in certain plane attitudes).
JerryK
430.
jerry
"Craig Prouse" <cra...@apple.com> wrote in message
news:B7A200DA.20E30%cra...@apple.com...
Ron Natalie
Walker Mangum wrote:
> The one frequency that I specifically recall having a problem
> with is 119.7, but there are others as well. My problem also
> seems to be specific to the KX155. If I call up the same
> COM frequency on my KY197A, I have no problems.
I believe that the docs for the GPS even mention the frequencies
that cause the problem. It's a known problem especially with
the KX155's. I think there are some filters that help alleviate
the problem.
C J Campbell
install an approach certified GPS. The KX155 is poorly shielded and is not
intended for use with approach certified GPSs. At least this is what Dan at
The Avionics Shop at Tacoma Narrows told me when I had him install a KLN94.
"Walker Mangum" <man...@swen.unitedspacealliance.com> wrote in message
news:3B7D163D...@swen.unitedspacealliance.com...
C J Campbell
there a steep turn can sometimes block enough satellites to cause a RAIM
error. Keep those turns standard rate, guys.
"ArtP" <ArtNo...@his.com> wrote in message
news:ne6qntsgnvbpnfpkh...@4ax.com...
BARR DOUG
The further away, the better.
I have NEVER had a RAIM failure on my KLN90B. (Done numerous GPS approaches
too).
In article <lLaf7.28530$vW2.12...@news1.sttln1.wa.home.com>,
Jon Parmet
> gotten pretty good with the basic features of the box, and although my
> instructor is more familiar with the Apollo and Garmin boxes, she's picking
> up the King unit pretty quickly.
>
> For all intents and purposes the unit seems to work perfectly, except we are
> surprised by how frequently we get RAIM failures just as we reach the FAF.
If you would provide the lat/lon, I can run it through the prediction
software we have here. This software uses the same algorithm as that
specified for TSO C129A. Different receivers play different games, so
if we predict an outage, your receiver might vary a minute or so here
and there. If the duration is small enough, we might predict an outage
where your reciever doesn't. But overall, it'll give you a sense as to
whether there's anything happening there or not.
> I'd say about 25-30% of our approaches result in a RAIM failure, including
> in at least one case after we had performed a RAIM prediction and RAIM was
> predicted to be available. Is this typical?
The constellation is maintained by 2SOPS and they will optimize it
according to whatever procedures/objectives they have. Since there
isn't 100% availability of RAIM of 100% of the earth, there are always
going to be some areas that will tend to have poorer availability.
Before 2SOPS moved some birds around, portions of the Pacific SW
(Australia) had RAIM holes on a daily basis. Portions of Europe also.
Recent optimizations have resulted in improvements in those areas,
while other areas (I've noticed CYOW on a regular basis) are now
showing RAIM holes.
One thing to note is that since the orbits are 11:58 in duration,
you'll tend to notice that when there is a RAIM hole at a particular
location, if it's there the next day it'll be shifted by 4 minutes.
The above assumes a healthy constellation, i.e. no GPS SV outages.
Sometimes an SV Outage will have no effect on your location. Perhaps
it'll be OTS for 6 hours, and during those 6 it's on the other side of
the earth relative to your location. Perhaps there are enough other
birds well in view with good enough geometry such that that bird being
unhealthy is of no consequence.
Here is some information from one of my collegaues, an internationally
recognized expert in GPS Integrity:
"Here are some availability results based on a global average for a
nominal 24-SV constellation. They are for NPA (TSO C129A assuming
baro aiding). Of course at the equator the availability is going to
be 100% and the
availability at the mid-latitudes will be lower.
The avg. availability of RAIM (SA-On) is 99.93% with a max. outage
duration of 25 min. The avg. availability of RAIM with SA off is
99.999% with a max. outage duration of 5 min."
One thing to note is, that although SA has in fact been turned off, a
lot of the receivers out there aren't 'taking credit' for it. Since
they were released on the market before this occured, they are using
certain values (sigma ~= 33m) which were to be applied for the SA-on
condition. So, believe it or not, you'll see the newer $100 handhelds
actually providing better *accuracy* than the expensive certified
units. However, accuracy doesn't mean much if you don't know when you
can trust that it's correct (integrity). Being able to reduce that
sigma value, though, would improve the availability since the accuracy
of the measurements does in fact drive the integrity calculations (and
thus, the availability of the integrity).
> I seem to be tracking plenty of satellites, and my altitude encoder was
> recently checked. Seems like satellite geometry, but is it really that
> unreliable? Anybody have info on what percentage of the time RAIM is
> available at a particular point on the earth's surface?
It's very difficult to make a statement about any particular point due
to the above mentioned things (SV outages, constellation). This is
what the prediction is for. For example, There are what we call 'edge
of coverage' cases, perhaps due to a setting/rising bird. You'll see 1
minute fluctuations on either the start/end times. Another case of
this is where, one day you might have a 5 minute duration of
unavailability and the next day you might only have a 4 minutes. In
terms of civilian requirements, 5 minutes is considered an outage,
while 4 isn't.
> Craig
Hope some of that helped.
Regards,
Jon
BARR DOUG
location and compare it's satellites to the ones in your IFR unit when the
outage occurs. This way you will know if it is satellites or unit (antennae)
problems.
Ross Richardson
with a Apollo 2001 VFR GPS and contacted them. They confirmed the
problem. I always had problems when I contacted APP on 121.0, and the
unit would go blank until I quit transmitting. there are other freqs.
Also I just read the the KX155 do have a problem, thus the KX155As. In
the IRF installation the installer is supposed to verify all the
frequencies before signing it off. I have the KLN89B and not experienced
a RAIM alert. I have't tried that many yet, still learning.
Ross
Dan Luke
> I'd say about 25-30% of our approaches result in a RAIM failure, including
> in at least one case after we had performed a RAIM prediction and RAIM was
> predicted to be available. Is this typical?
No, it's not typical.
Does it happen at (a) just one particular FAF or (b) several different ones?
(a) may mean there is some external interference source in the FAF area.
(b) may mean there is a problem with your GPS or there is some internal
interference source in the arplane.
--
Dan
N9387D at BFM
George Sconyers
different planes over the last 2 & 1/2 years and must have done at least 200
approaches during that time and have never seen a single RAIM failure during
an approach. Only once enroute over Illinois at 10.5 did I see RAIM failure
indications on both units for about 2 minutes last summer. Both
installations are Piper factory installations and the two GPS antenna are
next to each other on the top of the plane and are about 5 feet from the com
antenna and about 12+ feet from the nav (VOR antenna).
George Sconyers
PP-ASEL-IA
Piper Saratoga II TC N6124M
"Craig Prouse" <cra...@apple.com> wrote in message
news:B7A200DA.20E30%cra...@apple.com...
Piper News Reader
news:dIaf7.28513$vW2.12...@news1.sttln1.wa.home.com...
> This is why King recommends replacing the KX155 with the KX155a when you
> install an approach certified GPS. The KX155 is poorly shielded and is not
> intended for use with approach certified GPSs. At least this is what Dan at
> The Avionics Shop at Tacoma Narrows told me when I had him install a KLN94.
>
simple filters usually fix the problem. you are Dan's good friend now for buying 155a's
instead of the filters ;-)
Phil T
-----------------------------
N9312P PA24-260C
www.AirLifeLine.org
We Make A Difference
C J Campbell
"Piper News Reader" <ph...@DELETEieee.org> wrote in message
news:3b7d...@sys13.hou.wt.net...
Kevin Mac
In addition to what others have said about the KX155 radios, several
years ago Tom @ Avionics West said he discovered a DME frequency that
interfered with the GPS. You might try different frequencies and see
if it helps.
I've had RAIM problems in the past *on of the first KLN-90B
installations), and one thing they asked me to check was: when the
RAIM error occurs, go to the Satellite page and see how many
satellites are begin used for the calculation. I think there's an
asterisk next to each. And look at the Signal to Noise Ratio column.
Numbers in the mid 40's are good. If they're lower than that, then you
may have antenna problems (or noise from KX155's or DME).
Others have suggested that certain areas or locations might be worse
than others. Since the satellites are always moving, I don't see how
this is possible. But there are experts out there who might be able to
explain how this is so.
- Kevin
Mike Beede
<gwen...@parkprop.com> wrote:
Probably 20 approaches with a Garman 430 and I've seen three or
four RAIM failures near the FAF. Another poster said that there are
"planned outages" of the GPS system. This is news to me, unless what
he meant was that there are times when the satellite geometry is
unfavorable. Are there really planned outages?
Mike
Craig Prouse
wrote on 8/18/01 7:04 AM:
I read a PIREP that GPS was unavailable between Reno and Santa Maria last
Friday, which approximately corresponds to GPS interference tests running at
Nellis and China Lake. NOTAMs regarding that activity indicate that it has
been nearly continuous for the last week or more.
My RAIM failures have all been outside the NOTAMed airspace, but perhaps for
approach purposes I was "close enough."
Craig
Craig Prouse
c7...@bellsouth.net wrote on 8/17/01 5:18 PM:
Yeah, sure doesn't sound like it, huh?
Sometimes it happens, sometimes it doesn't. It's not consistent by location
or by the radio frequency I'm using.
I've really just started with GPS approaches, so I haven't flown enough of
them to have a really good statistical sample. Because I'm flying missed
approaches anyway for training purposes, the RAIM failure hasn't gotten
inconvenient yet. I'd just like to get a handle on this before I need to
fly one for real, and it's one of those problems where if I take it to an
avionics shop, they'll probably just charge me $75 for the privilege of
laughing at me. "Let me get this straight, it usually works, but sometimes
not and you don't have any idea what the failure cases have in common? What
do you want me to do about it?"
In a year and a half, I've had one RAIM failure while enroute, and that
resolved after just a minute or so. That doesn't really surprise me. It's
the approach RAIM that seems to be on a bit of a hair trigger, now that I've
developed an interest in GPS approaches.
Craig
Craig Prouse
> news:ne6qntsgnvbpnfpkh...@4ax.com...
>> I have an Airmap 100 hand held with a 12 channel receiver. It almost
>> always works, but there are some areas where it consistently starts
>> flashing indicating it can't get even 3 satellites. So before I blamed
>> the unit I would try approaches at a different airport a distance away
>> from where it fails. If it still fails it is probably the unit. If it
>> doesn't it is probably the location. Although it could be the
>> installation (antenna blocked in certain plane attitudes).
in article lLaf7.28530$vW2.12...@news1.sttln1.wa.home.com, C J Campbell
at christopherc...@hotmail.com wrote on 8/17/01 8:29 AM:
> I have heard this, too. The antenna is usually mounted on top, but even
> there a steep turn can sometimes block enough satellites to cause a RAIM
> error. Keep those turns standard rate, guys.
So far all of my GPS approaches have been within about 60 nm of home, but
Petaluma, Oakdale, and Watsonville make a good size footprint until such
time as I get out of Northern California.
Generally at and just prior to the FAF on a GPS approach, where the GPS
makes its final RAIM calculations, the aircraft attitude is straight and
level. The antenna on my C182 is right over my head.
Craig
Craig Prouse
man...@swen.unitedspacealliance.com wrote on 8/17/01 6:03 AM:
That's an interesting idea and something I hadn't considered. In my case,
though, I am in a new Cessna, so it's got KX 155As and KLN 89B factory
installed. Still, anything is possible.
I need to go back through my logbook and count how many GPS approaches I've
attempted. I know I've had failures at Watsonville, Palo Alto, Livermore,
and Stockton. But I've also had successful approaches at Palo Alto,
Watsonville, Oakdale, and Petaluma. So the results are not consistent at
any one airport, or for any particular approach frequency. The only thing
that's really consistent is that I've been flying at the same times of day
in the same geographical area.
Thanks,
Craig
Craig Prouse
If it's easy enough for you to run the numbers through your software, what
I'd be interested to know (by way of a practical example) is what are the
predicted periods of RAIM unavailability during the period 8/20 - 8/24 at
the following locations:
KPAO Lat/Long: 37-27-40.030N / 122-06-54.160W
KWVI Lat/Long: 36-56-08.628N / 121-47-22.624W
KLVK Lat/Long: 37-41-36.238N / 121-49-13.267W
KSCK Lat/Long: 37-53-39.361N / 121-14-19.033W
Craig
Piper News Reader
> Others have suggested that certain areas or locations might be worse
> than others. Since the satellites are always moving, I don't see how
> this is possible. But there are experts out there who might be able to
> explain how this is so.
>
> - Kevin
All the comments about frequencies, antennas, etc are right on, but I think a
real RAIM failure (the anticipated failure that RAIM checks) is caused by how
the satellites line up from your position. RAIM prediction estimates where the
satellites will be over any point at the ETA. If you get one behind the other (well
the idea of one behind the other) and there is a minimum set in view, you will get
a RAIM failure.
Some areas are possibly more prone to real RAIM failure than others. Then if
you live near China Lake or similar all bets are off ;-)
Jon Parmet
More than likely, what he was referring to were planned (i.e. Scheduled)
outages on an individual Satellite, which may or may not affect RAIM
availability at a specific location.
The user community is generally informaed of scheduled via a Notice
Advisory to Navstar User (NANU). The information pertaining to the
outage is also sent to USNOF (usually at least 72 hours in advance) and
a Domestic format NOTAM with International crossover is created. You can
find out more info in the FRP as well as at locations like:
http://www.navcen.uscg.gov/gps/default.htm
http://www.schriever.af.mil (2SOPS) which is currently down at hte
moment.
Regards,
Jon
Jon Parmet
Note that the following 2 Satellites are OTS:
PRN Start End
19 010316 01:44 Until Further Notice
17 010821 14:00 010822 02:00
Running it with a 5.0 degrees mask angle, there were no outages >= 5
minutes. I'm not sure of the specs on the receiver you have; but some of
the older ones used a higher angle, so I also ran it with 7.5 and
produced the following:
200108200322 200108200327 KPAO
200108210318 200108210323 KPAO
200108220314 200108220319 KPAO
200108230310 200108230315 KPAO
200108200321 200108200326 KWVI
200108210317 200108210322 KWVI
200108220313 200108220318 KWVI
200108230309 200108230314 KWVI
200108200322 200108200328 KLVK
200108210318 200108210324 KLVK
200108220314 200108220320 KLVK
200108230310 200108230316 KLVK
200108200322 200108200328 KSCK
200108210318 200108210324 KSCK
200108220314 200108220320 KSCK
200108230309 200108230316 KSCK
Some comments:
This is a good example of what I meant.
Note that tomorrow there's a period of time during which 2 birds are OTS
concurrently (assuming they don't NOTAMC either/both). 2SOPS is
generally good about scheduling no more more than 2 out at the same
time, although PRN 19 has is on her last
Yet it's not at all intuitive how satellite-specific outages affect
airport-specific outages. One might think that the situation should
worsen (additional outages, an increase in duration of any existing
outages, or combinations of both). In this case, though, PRN 17 had no
effect on your locations. More than likely it's either not visible to
those locations when it's OTS, or there are enough other satellites
visible such that it's not having enough of an effect on geometry to
cause the HAL (Horizontal Alert Limit) for NPA to trigger.
Traditional navaid outages are quite intuitively mapped in the head.
When one goes OTS, the affected area will be some well known distance
(perhaps a radius) near the navaid's location. With GPS, however, the
affected areas move (both in position and/or time) since the satellites
are in constant motion.
One thing you can notice above is how KLVK and KSCK match up exactly,
since they're close enough to each other. And they're both experiencing
the outage for 1 minute longer than KPAO, most likely due to 'setting'
bird.
I believe we're GMT-5 on the East Coast during daylight's savings time,
so 0320-ish GMT could 1920 PST yes? I'd be curious to see how, if at
all, your receiver matches up (Planning any flights after dinner this
week? ;) If you do manage to actually encounter it (they're of such a
short duration), a really good test would to be fly back around after
going Missed and see if availability returns after the predicted end
time.
As you saw in the numbers I quoted from her in a previous post, RAIM
availability (for NPA) is quite good. My colleague thinks that, if
nothing else, this gives you another reason (interference, for example)
to search to explain the problems.
Regards,
Jon
Newps
Peter wrote:
> I don't know what RAIM is (other than a link to an altimeter which the
> GPS uses for cross-checking its calculation) but may I ask what effect
> the situation you describe would have on a normal handheld GPS?
>
> Peter.
> --
> Return address is invalid to help stop junk mail.
> E-mail replies to zX...@digiYserve.com but remove the X and the Y.
> Please do NOT copy usenet posts to email - it is NOT necessary.
C J Campbell
determine the accuracy of the information they receive from satellites. They
use extra lines of position to check the consistency of the information they
receive. RAIM requires a minimum of five lines of position in good geometry,
one of which may be the pressure altitude received from the altimeter, or it
can just use five satellites. Tolerances are tighter during the approach
phase than they are during the enroute and terminal phases, so you are most
likely to lose RAIM during an approach.
Some handhelds also are capable of a RAIM-like over-determined position
solution, most notably the Garmin GPSMap 295, but they must use
over-sampling of satellites instead of pressure altitude. Several models
designed for hikers and mountain climbers have built-in pressure altimeters
as well as the capability to check satellites against each other. Some
IFR-certified GPS units will also predict whether RAIM will be available
when you reach your destination.
If a GPS does not have RAIM available it's position may still be accurate;
there is just no way of checking it. So it flashes a warning and the
position should be cross-checked with other nav aids every few minutes. I
would guess that the next generation of GPS units will do this cross
checking automatically.
If satellites are out of service they are not available for either
navigation or integrity monitoring. A GPS can probably find enough other
satellites to continue its navigation functions, but an IFR-certified GPS
might not be able to find enough satellites for integrity monitoring. So a
handheld would not report any problem -- even if it uses oversampling of
many satellites it is not going to warn you if it can see only four
satellites. But an IFR-certified GPS is going to warn you that RAIM is not
available.
"Peter" <nob...@somewhere-in-the-uk.com> wrote in message
news:6n55otgtmu3ol7cfu...@4ax.com...
Julian Scarfe
Worth a play.
in article bk77otg9ocgbdcpl3...@4ax.com, Peter at
nob...@somewhere-in-the-uk.com wrote on 8/22/01 13:30:
> Therefore it would appear that, aside from the need for an encoding
> altimeter, the RAIM function is just extra software. I therefore
> wonder why an IFR-certified GPS costs so much more.
I'd guess that this is the sort of electronics product where the cost is
calculated as:
(Development Cost + Certification Cost)/(No. of Units sold) + epsilon
where epsilon is not very significant.
Julian Scarfe
Craig Prouse
"Jon Parmet" <j...@parmetpc.volpe.dot.gov> wrote in message
news:3B817794...@parmetpc.volpe.dot.gov...
> I believe we're GMT-5 on the East Coast during daylight's savings time,
> so 0320-ish GMT could 1920 PST yes?
EDT = GMT - 4
PDT = GMT - 7
0320Z = 2020 PDT (8:20 PM)
> I'd be curious to see how, if at
> all, your receiver matches up (Planning any flights after dinner this
> week? ;) If you do manage to actually encounter it (they're of such a
> short duration), a really good test would to be fly back around after
> going Missed and see if availability returns after the predicted end
> time.
Unfortunately, I won't be doing any flying that late in the day. I have
shot a couple more approaches into KWVI and KPAO this week, however, and had
no RAIM problems.
Next time (if? when?) I have to miss due to a RAIM failure, I'll request
another GPS approach.
> As you saw in the numbers I quoted from her in a previous post, RAIM
> availability (for NPA) is quite good. My colleague thinks that, if
> nothing else, this gives you another reason (interference, for example)
> to search to explain the problems.
I imagine that there are a number of possibilities other than bad hardware.
As previously noted, the military may have been conducting interference
tests a couple hundred NM SE. If I was on vectors to final at the time (I
don't recall) I might have been maneuvering near the FAF while the box was
predicting RAIM, and that could affect satellite visibility or the algorithm
used by the GPS to arm the approach ACTV mode.
Craig
C J Campbell
approach-certifiable GPS, use the same Honeywell GPS PC card. It is only an
8 channel receiver, not even as good as the 12 channel receiver in the
Garmin handhelds. I haven't torn mine apart to actually look, but the manual
description makes it sound like you could rip it out and stick it in the PC
card slot on your laptop or handheld device. The only difference between the
KLN 89 and the KLN 94 is the color display and some software. Oh, the KLN 94
has a couple more buttons and it has a couple of additional outputs on the
back that allow it, if you are willing to wade through several layers of
menus (perhaps while trying to control the airplane in moderate turbulence
in hard IFR conditions, no doubt), to download the frequencies appropriate
for your next waypoint into your COMM radios, provided they are compatible
and your database is current (the avionics shop guy was far more impressed
with this feature than I was). There is even less difference between the KLN
89 and the approach-certifiable KLN 89b. Here the difference is some
software and the letter "b" painted on the front.
A RAIM GPS will still give you a fix with the same number of satellites as
the non-RAIM GPS, but it will give you a warning that RAIM is not available.
IOW it will tell you that the fix might not be accurate, while a non-RAIM
GPS doesn't bother to tell you that. Loss of RAIM does not indicate any loss
of accuracy at all in and of itself. It is just telling you that a backup
safety feature that warns you of the remote possibility that you are using
bad data is currently unavailable.
If you applied the same logic to product labeling, a package of Wonder Bread
with RAIM would tell you that the bread might be contaminated with mercury
(well, it might -- you just never know about these things), while the
non-RAIM Wonder Bread will omit any mention of mercury. Neither label will
guarantee that the bread is free of mercury. Then the manufacturers of
Wonder Bread would triple the price of the RAIM version to cover the cost of
extra labelling. And everyone would want the RAIM version because it is
"safer." And the manufacturers of whole wheat would go out of business
entirely because they were not as concerned about your well-being as the
makers of Wonder Bread. :-)
Seriously, RAIM is provided to give GPS a failure mode, similar to the "NAV"
flag on a VOR. Whether this mode is actually necessary or whether it is
effective is a subject of constant debate on this ng.
And if I seem a little cynical about all the hand-wringing over every
minuscule risk, sorry, I went to the old school where I was taught that it
was better to swallow an occasional gnat instead of a whole camel. To put it
in perspective: if RAIM were not mandatory, would most people buy it? Sure,
some would. Even I probably would want it. But how many people are not going
to get GPS because they can't afford RAIM -- and then die flying an NDB
approach? How can anyone say that a handheld GPS without RAIM is not
accurate enough to fly an NDB approach, while the typical ADF with its
unstable needle and reception problems is? GPS is not nearly as subject to
shoreline effect, ionospheric disturbance, line of sight problems, static
interference, or any of a host of other problems that ADF is subject to.
Plus, GPS works when the NDB (or even VOR or localizer!) is OTS, which is
far more often than the number of times you can't pick up enough satellites.
Plus, GPS actually lets you know when you aren't picking up enough
satellites, while the ADF needle just keeps pointing straight ahead when the
station goes off air without warning. And last, GPS receivers are generally
much less temperamental than ADF receivers. It is my opinion that mandatory
RAIM kills people unnecessarily, and that is before we even begin to talk
about people killed flying an unnecessary missed approach because they got a
false RAIM warning -- and let's face it, RAIM is flaky. The vast majority of
RAIM warnings, possibly all of them, are false alarms.
Requiring RAIM is like requiring RNAV for IFR certified VOR installations.
Worse, it is like requiring that you be able to pick up at least four VORs
in order to be allowed to use your RNAV for IFR.
I could go on, but I don't want to vent and possibly offend some techie who
can't see the forest for the trees. So I will hold back and not say my full
opinion because I would not want offend some damned murderer at the FAA who
thinks he's God's gift to aviation because he found that there might be some
small problem and doesn't care how many people he kills to solve it.
"Peter" <nob...@somewhere-in-the-uk.com> wrote in message
news:bk77otg9ocgbdcpl3...@4ax.com...
>
> Thank you for the explanation. Is the following summary accurate:
> Whereas a non-RAIM GPS will give you a 2D fix with 3 satellites (or a
> 3D fix with 4 I think; my Skymap 2 needs 5 normally for 3D), a RAIM
> GPS needs at least 5, plus it uses additional software to cross-check.
Tarver Engineering
<snip>
> Seriously, RAIM is provided to give GPS a failure mode, similar to the
"NAV"
> flag on a VOR. Whether this mode is actually necessary or whether it is
> effective is a subject of constant debate on this ng.
RAIM improves the probabilities numbers in equations. As to whether the
model is accurate, that could be a matter of debate. I trust a "better
than" evaluation as sufficient proof.
John
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Sharp Family
I too have dual Garmin 430s and a Garmin 155 in my panel and have flown all
over the USA, the Caribbean, and Canada and have never seen a single RAIM
failure during any type of approach. I always perform a RAIM calculation
sometime before and during the route or just before the approach to ensure I
will have proper coverage.
I did have an integrity light come on once from the lower unit at 7k for
about 10 seconds while climbing through a real DARK cloud over the central
valley while enroute to Tahoe from RHV but before I could check the RAIM
coverage the light went out. There are a few areas (China Lake etc) that
NOTAM GPS outage for jamming. Always look at the FDC notams before each
planned enroute flight while using GPS.
Craig you might want to have the system checked.
I could go up with you sometime to do a preliminary inspection.
Randy
in article 9lkd20$5g4$1...@slb4.atl.mindspring.net, George Sconyers at
gsc...@ix.netcom.com wrote on 8/17/01 5:29 PM:
Jon Parmet
> [snip]
> Note that the following 2 Satellites are OTS:
>
> PRN Start End
> 19 010316 01:44 Until Further Notice
> 17 010821 14:00 010822 02:00
>
>
> Running it with a 5.0 degrees mask angle, there were no outages >= 5
> minutes. I'm not sure of the specs on the receiver you have; but some of
> the older ones used a higher angle, so I also ran it with 7.5 and
> produced the following:
>
>
> 200108200322 200108200327 KPAO
> 200108210318 200108210323 KPAO
> 200108220314 200108220319 KPAO
> 200108230310 200108230315 KPAO
>
> 200108200321 200108200326 KWVI
> 200108210317 200108210322 KWVI
> 200108220313 200108220318 KWVI
> 200108230309 200108230314 KWVI
>
> 200108200322 200108200328 KLVK
> 200108210318 200108210324 KLVK
> 200108220314 200108220320 KLVK
> 200108230310 200108230316 KLVK
>
> 200108200322 200108200328 KSCK
> 200108210318 200108210324 KSCK
> 200108220314 200108220320 KSCK
> 200108230309 200108230316 KSCK
>
Little different story today with 3 birds concurrently OTS. There's
some outages that you'd have a chance of seeing.
Regards,
Jon
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RAIM Prediction Period From: 30/08/2001 15:59 UTC
To: 01/09/2001 15:58 UTC
PRN# 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, 17
18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
Number of GPS Satellites in Constellation: 29
Satellite(s) Scheduled Out of Service:
PRN Start End
19 010316 01:44 Until Further Notice
26 010905 01:00 010905 13:00
6 010831 06:00 010831 18:00
30 With Immediate Effect Until Further Notice
Number of Operational SVs: 26
NPA RAIM OUTAGES (Based on 5.0 Degrees Mask Angle):
Outages >= 5 Minutes in Duration Reported
ID Start End Duration (min)
KPAO 010831 08:27 010831 08:32 5
KPAO 010831 09:19 010831 10:06 47
KPAO 010831 14:30 010831 14:41 11
KPAO 010901 08:23 010901 08:28 5
KWVI 010831 09:20 010831 10:10 50
KWVI 010831 14:30 010831 14:40 10
KLVK 010831 08:27 010831 08:33 6
KLVK 010831 09:20 010831 10:06 46
KLVK 010831 14:30 010831 14:42 12
KLVK 010901 08:23 010901 08:28 5
KSCK 010831 09:22 010831 10:07 45
KSCK 010831 14:30 010831 14:42 12
CYEG 010831 08:56 010831 09:07 11
CYEG 010901 08:57 010901 09:03 6
CYEV 010831 09:42 010831 09:52 10
CYFB 010831 10:50 010831 10:59 9
CYFB 010831 11:08 010831 11:21 13
CYOW 010830 21:04 010830 21:25 21
CYOW 010831 10:44 010831 11:26 42
CYOW 010831 21:00 010831 21:21 21
CYOW 010901 10:41 010901 11:02 21
CYVQ 010831 09:31 010831 09:41 10
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
karl l gruber
quality of your GPS receiver and it's installation.
First generation IFR GPS receivers had an eight channel series receiver.
These units easily lose lock, especially if the aircraft is maneuvering,
such as doing holding patterns or procedure turns. The Garmin GPS155 is
one of these units.
Garmin's next IFR receiver was the GPS155XL. The 155XL is kind of a
first and a half generation. This receiver has better software logic, a
crude moving map and introduced the far superior 12 channel parallel
receiver. Garmin had MANY warranty claims on the straight 155 and ended
up sending 155XLs to the loudest complainers (including me!)
Most second generation units have the FAR better 12 channel parallel
receiver. Not only do these newer receivers hold lock better but they
lock on much faster. I've never seen a raim message on a GPS155XL or
GNS430.
KArl
Jon Parmet
> Raim doesn't only depend on satellite coverage. It also depends on the
> quality of your GPS receiver and it's installation.
Indeed.
When validating our prediction against receivers, we found variations
across receivers than couldn't be explained only by slight differences
in input (mainly the GPS almanac). All other things being equal -
satellite outages, almanac, location lat/lon - generally the
differences are on the order of +/- 1 minute on either side (start/end
time) of the outage, but it clearly shows that manufacturers are using
slightly different algorithms. The TSO doesn't specify beyond a
certain point _how_ the algorithm must be implemented, only a level of
performance which much be demonstrated by the receiver in order to
pass certification.
> First generation IFR GPS receivers had an eight channel series receiver.
> These units easily lose lock, especially if the aircraft is maneuvering,
> such as doing holding patterns or procedure turns. The Garmin GPS155 is
> one of these units.
>
> Garmin's next IFR receiver was the GPS155XL. The 155XL is kind of a
> first and a half generation. This receiver has better software logic, a
> crude moving map and introduced the far superior 12 channel parallel
> receiver. Garmin had MANY warranty claims on the straight 155 and ended
> up sending 155XLs to the loudest complainers (including me!)
Nice!
Can't remember if it was that model or not, but I recall some 'issues'
they had a few years back with mask angles. In an attempt to
demonstrate superior availability, phenomenon like multipath
apparently weren't considered when they tweaked the angle used in the
receiver's algorithm to 0 degrees. oops ;)
Regards,
Jon