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MJT
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 Posted: Fri Dec 23, 2005 3:19 am Post subject: GPSes and Aviation - How accurate are they? |
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In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
--
Women get minks the same way minks get minks.
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A User
Guest
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| Posted: Fri Dec 23, 2005 5:02 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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On Fri, 23 Dec 2005 14:19:27 +1100, MJT <mjpt57 (AT) gmail (DOT) com> wrote:
| Quote: | In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
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Speed on a GPS receiver is measured by doppler shift, not based on
coordinates. The frequency is accurately fixed.
I believe most civilian GPS receviers have a max speed and altitude
limitation built into the hardware, as well as SA.
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Keith
Guest
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| Posted: Fri Dec 23, 2005 5:32 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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A User wrote:
| Quote: | On Fri, 23 Dec 2005 14:19:27 +1100, MJT <mjpt57 (AT) gmail (DOT) com> wrote:
In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
|
Since the speed is measured by comparing succeeding positions this is not
true, but the indicated speed does lag during acceleration or deceleration
due to the filtering of the reading. On my unit, it takes 10-20 seconds to
read zero after a sudden stop. On some units you can set the averaging time
to either have a smooth reading or a short lag.
There used to be a random position offset applied to the signal called "SA",
but that has been turned off for some years now.
| Quote: | "...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
Speed on a GPS receiver is measured by doppler shift, not based on
coordinates. The frequency is accurately fixed.
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I don't think so. The majority of the doppler detected would be due to the
satellite's movement not the ground station's.
| Quote: | I believe most civilian GPS receviers have a max speed and altitude
limitation built into the hardware, as well as SA.
|
That may have been true once, but my Garmin GPSMap 76CS reads perfectly
believeable speeds (in the 600 mph range) when held up to the window in an
airliner. SA has been turned off for some years now, in the US with WAAS,
you can get readings good to 2-3 metres depending on your view of the sky
and the current positions of the satellites in view. Since we do not have
WAAS in Australia, 5 metres is a good result.
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Brian May
Guest
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| Posted: Fri Dec 23, 2005 5:35 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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| Quote: | "MJT" == MJT <mjpt57 (AT) gmail (DOT) com> writes:
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MJT> In another forum, talking about something non-aviation
MJT> related, the accuracy of GPSes came up. Mainly about
MJT> measuring speed. One fellow who says that he's a pilot said
MJT> that they aren't accurate, that the US military who runs the
MJT> GPS system says that there is an off-set so terrorists and
MJT> hostile forces can't use them to guide missiles accurately.
MJT> He also said this below. How true is it?
MJT> "...the standard US military GPS is a 'ruggedised' Magellan
MJT> unit based on the 315 civilian model. The degradation is not
MJT> vast anyway in position terms, around 1-3 meters, it is the
MJT> response lag that makes it difficult to use for missile
MJT> guidance. A side effect of this is that GPS is not available
MJT> as a primary approach aid for civilian aircraft, which still
MJT> rely largely on VOR and ADF technology developed in the
MJT> 1940's..".
I don't believe this is true (maybe in the past it was though). GPS
already be used for non-precision IFR approaches at defined airports,
and standards are being developed to allow for precision IFR
approaches using GPS.
One thing to consider regarding speed though - it is an average
calculation only, so may not respond as fast as a speed indicator
(especially noticeable in cars). This is not a restriction by the
military though, but a constraint caused by the fact the GPS
calculations give position data only, not speed data. Speed data must
be estimated using the position and time data. At least that is my
understanding.
--
Brian May <bam (AT) snoopy (DOT) apana.org.au>
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Brian May
Guest
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| Posted: Fri Dec 23, 2005 5:47 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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| Quote: | "A" == A User <other-news (AT) usa (DOT) net> writes:
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A> I believe most civilian GPS receviers have a max speed and altitude
A> limitation built into the hardware, as well as SA.
I don't think so - having seen extremely high GPS speed readouts
(Garmin Bluetooth GPS, I think I got similar results from my Garmin
296 though) when driving on mountain roads and accuracy isn't always
very good - No, we definitely were not driving at right angles across
the main road at 3500km/h (or something like that) for several
seconds, thank you. Besides, I don't think the car can accelerate
instantly from 60km/h to 3500km/h and back down to 60km/h either.
On another car trip, my brothers Pocket PC placed him "transporting"
from Victoria (north of Melbourne) to North of Indonesia (maybe China,
not sure of the exact location now) with seconds, and back again - I
am not quite sure what happened here... He does claim he is Doctor Who
though  .
Conclusion: don't trust GPS directions when driving on mountain roads
. Seems to be good elsewhere though.
--
Brian May <bam (AT) snoopy (DOT) apana.org.au>
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brad
Guest
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| Posted: Fri Dec 23, 2005 8:18 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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TSO143a GPS's update their position and velocity data 5 times per
second using a 15 channel receiver and doppler is used for velocity.
This makes the information far more immediate. Older GPS receivers were
only single channel (GPS100), 3 channel (Trimble TNL2000 and many
others) and more recently 12 channel (GNS430 for example).
MJT's quoted statements above are well out of date as GPS Precision
Approach is available in the USA and GPS will be adopted as Sole Means
Navigation in Australia in the new year.
THe information about "...the standard US military GPS is a
'ruggedised' Magellan
unit based on the 315 civilian model." seems totally bass
ackwards. The civilian units were a spinoff of the research and
product development for the military units.
The Trimble Transpak for example, was a spinoff of the Trimble Trimpak
military unit and was immensely popular on the first Gulf War. There
were a number of differences between them, as with all military units,
which operate on 1224MHz at 10 times the bit rate compared to the
civilian 1575MHz units.
It was this higher bit rate which was originally supposed to give the
military the advantage of higher accuracy, but then better algorithms,
carrier measurement (doppler) , higher channel capacity units and
finally differential reception which made civilian units as accurate as
the military, and completely defeating the purpose of SA.
Brad.
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MC
Guest
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| Posted: Fri Dec 23, 2005 8:58 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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MJT wrote:
| Quote: | In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
|
Well, the US DOD can fiddle with data accuracy in particular areas.
Differential GPS negates a lot of the inherent inaccuracy, but in
a hostile area a transmitter would be a prime target.
| Quote: | He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
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Civilian grade GPSs are customised by their manufacturers for particular
applications, eg. maritime and hiking units have their position and
velocity data heavily smoothed, thereby making them unsuitable for
aviation purposes.
Civilian grade units generally only export their data once per second
and in one second an aircraft can travel quite a distance (laterally
and vertically)
GPS is inherently less accurate vertically as compared to horizontally
which makes them very unsuitable in aircraft for low-visibility
approaches.
And finally, it's hard to turn-off all the ground-based VOR and NDB
aids, which is why the Russians built their own GPS system and the
EU is currently building its' own.
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Bruiser
Guest
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| Posted: Fri Dec 23, 2005 9:04 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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"MC" <MC (AT) nonexistent (DOT) place> wrote
| Quote: | MJT wrote:
In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
Well, the US DOD can fiddle with data accuracy in particular areas.
Differential GPS negates a lot of the inherent inaccuracy, but in
a hostile area a transmitter would be a prime target.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
Civilian grade GPSs are customised by their manufacturers for particular
applications, eg. maritime and hiking units have their position and
velocity data heavily smoothed, thereby making them unsuitable for
aviation purposes.
Civilian grade units generally only export their data once per second
and in one second an aircraft can travel quite a distance (laterally
and vertically)
GPS is inherently less accurate vertically as compared to horizontally
which makes them very unsuitable in aircraft for low-visibility
approaches.
|
I have worked a lot with differential GPS. Aircraft often don't have
differential GPS, but it is accurate to a centimetre or better.
| Quote: | And finally, it's hard to turn-off all the ground-based VOR and NDB
aids, which is why the Russians built their own GPS system and the
EU is currently building its' own.
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Bruiser
Guest
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| Posted: Fri Dec 23, 2005 9:06 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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"Bruiser" <no (AT) remove (DOT) com> wrote
| Quote: |
"MC" <MC (AT) nonexistent (DOT) place> wrote in message
news:doge70$gn0$1 (AT) news-01 (DOT) bur.connect.com.au...
MJT wrote:
In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
Well, the US DOD can fiddle with data accuracy in particular areas.
Differential GPS negates a lot of the inherent inaccuracy, but in
a hostile area a transmitter would be a prime target.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
Civilian grade GPSs are customised by their manufacturers for particular
applications, eg. maritime and hiking units have their position and
velocity data heavily smoothed, thereby making them unsuitable for
aviation purposes.
Civilian grade units generally only export their data once per second
and in one second an aircraft can travel quite a distance (laterally
and vertically)
GPS is inherently less accurate vertically as compared to horizontally
which makes them very unsuitable in aircraft for low-visibility
approaches.
I have worked a lot with differential GPS. Aircraft often don't have
differential GPS, but it is accurate to a centimetre or better.
|
Sorry I meant in altitude
| Quote: |
And finally, it's hard to turn-off all the ground-based VOR and NDB
aids, which is why the Russians built their own GPS system and the
EU is currently building its' own.
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brad
Guest
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| Posted: Fri Dec 23, 2005 9:02 pm Post subject: Re: GPSes and Aviation - How accurate are they? |
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" And finally, it's hard to turn-off all the ground-based VOR and NDB
aids,"
Air services is about to do just that. We have about 94 VOR's in this
country and 250 NDB's, but by 2010 they would like that number to be 7
and 50. They are counting on GPS to fill all the gaps, hence the move
to Sole Means next year.
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A User
Guest
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| Posted: Sat Dec 24, 2005 12:13 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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On Fri, 23 Dec 2005 05:32:40 GMT, Keith <keith (AT) nowhere (DOT) com.au> wrote:
| Quote: | A User wrote:
On Fri, 23 Dec 2005 14:19:27 +1100, MJT <mjpt57 (AT) gmail (DOT) com> wrote:
In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
Since the speed is measured by comparing succeeding positions this is not
true, but the indicated speed does lag during acceleration or deceleration
due to the filtering of the reading. On my unit, it takes 10-20 seconds to
read zero after a sudden stop. On some units you can set the averaging time
to either have a smooth reading or a short lag.
|
Total BS. Look up the technical documents. It's Doppler.
| Quote: |
There used to be a random position offset applied to the signal called "SA",
but that has been turned off for some years now.
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
Speed on a GPS receiver is measured by doppler shift, not based on
coordinates. The frequency is accurately fixed.
I don't think so. The majority of the doppler detected would be due to the
satellite's movement not the ground station's.
That is already in the calculation. |
Have you done a GPS course for Aviation use? It's in the syllablus.
| Quote: | I believe most civilian GPS receviers have a max speed and altitude
limitation built into the hardware, as well as SA.
That may have been true once, but my Garmin GPSMap 76CS reads perfectly
believeable speeds (in the 600 mph range) when held up to the window in an
airliner. SA has been turned off for some years now, in the US with WAAS,
you can get readings good to 2-3 metres depending on your view of the sky
and the current positions of the satellites in view. Since we do not have
WAAS in Australia, 5 metres is a good result.
|
Still true, read the docs. 650 knots and 50,000 feet come to mind.
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brad
Guest
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| Posted: Sat Dec 24, 2005 1:08 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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The TSO only requires/guarantees performance to 50,000ft (well, DO-160C
does anyway).
Aviation GPS's are capable of 999kts at 6g acceleration. There should
be no noticeable lag in those speed calculations!
Note: I keep quoting Aviation approved GPS's as they are the only ones
that matter for aviation. Portable civilian 'outdoor' units do not
comply in any way, portable aviation units are also not approved for
navigation.
(Figures quoted for Garmin GNS430/A)
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matt weber
Guest
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| Posted: Sat Dec 24, 2005 3:11 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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On Fri, 23 Dec 2005 14:19:27 +1100, MJT <mjpt57 (AT) gmail (DOT) com> wrote:
| Quote: | In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
|
Now that SA has been turned off, there isn't much difference in
accuracy between military and civilian units. If you are in an area
where WAAS is available, Civilian with WAAS beats Military without.
There are in fact now a number of Civilian airports that have GPS
approved approaches, but you usually have to have RNP that is
adequately for the task.
Baring a local DGPS beacon or a pair of GPS's making a carrier phase
measurement, 3 meter accuracy is about the best you can hope for.
Even with WAAS enabled 3 meter represented roughly the uncertainty in
the permeability/permitivity of the popogation media over the path,
length. It is about 1 part in 10 million. Without WAAS it about 10-15
meters.
Early Garmin units were designed not to work if the speed exceeded
100mph or so to discourage use in aircraft, and current US law
prevents Civilian units from operating a speeds in excess of 1000 mph,
so they cannot be used in Ballistic missles. (But should do just fine
in your home made cruise missile)
Realistically, there are no weapons systems with CEP's better than
about 3 meters anyway. If 1000kg of TNT falls on you, does it really
make any difference whether it is on your head, or 3 meters away?
The early military units are actually versions of the Magellan Navpro
1000, which was a Multiplexed receiver. Mux'd receivers have a variety
of issues, but no matter what you do with them, even the Military
version, about 25 meters is as good as it gets. With SA and civilian
signals, it was much worse, however Mux'd receivers go out of service
fairly quickly in favor of multi-channel true parallel receivers.
How accurate the speed measurement is, relates to how it is measured,
and how good the sat geometry is. There are two ways to get it. You
can get from dS/dT, or you can extract it from a Kalman Filter. At
speeds above about 40m/s it doesn't make much difference with SA
turned off. With SA turned on, the short term positional errors played
havoc with dS/dT at low speeds. If you are moving at 10 m/s, and SA
produces a 100 meter CEP, the error is very large over short
baselines.
At higher speeds, and with SA turned off, the positional errors become
small compared to the total distance, at low speeds the doppler
measurement is probably preferable.
bottom line is that these days, it is rare for a GPS position to have
a CEP worse than 15 meters without WAAS, and about 5 meters with WAAS
available and in use. Most GPS's incorporate a smoothing function into
the speed measurement, so the instanteous number may not be all that
great, but over a period of a few seconds, it tends to be quite
accurate as long as you aren't accelerating or decelerating rapidly.
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Bob O'Rilley
Guest
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| Posted: Sat Dec 24, 2005 5:34 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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"MJT" <mjpt57 (AT) gmail (DOT) com> wrote
| Quote: | In another forum, talking about something non-aviation related, the
accuracy of GPSes came up. Mainly about measuring speed. One fellow who
says that he's a pilot said that they aren't accurate, that the US
military who runs the GPS system says that there is an off-set so
terrorists and hostile forces can't use them to guide missiles
accurately.
He also said this below. How true is it?
"...the standard US military GPS is a 'ruggedised' Magellan unit based
on the 315 civilian model. The degradation is not vast anyway in
position terms, around 1-3 meters, it is the response lag that makes it
difficult to use for missile guidance. A side effect of this is that GPS
is not available as a primary approach aid for civilian aircraft, which
still rely largely on VOR and ADF technology developed in the 1940's..".
--
Women get minks the same way minks get minks.
|
They are way more accurate than your 'watch and compass'... The main
inaccuracy is the idiot who doesn't know how to use one effectively and
claims, "Dead Reckoning" is the only way to go. I nav by GPS with DR and
Pilotage etc as Back-up. Can't go wrong :)
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budgie
Guest
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| Posted: Sat Dec 24, 2005 5:36 am Post subject: Re: GPSes and Aviation - How accurate are they? |
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On Fri, 23 Dec 2005 20:11:40 -0700, matt weber <mattheww50 (AT) cox (DOT) net> wrote:
(snip)
| Quote: | Realistically, there are no weapons systems with CEP's better than
about 3 meters anyway. If 1000kg of TNT falls on you, does it really
make any difference whether it is on your head, or 3 meters away?
|
Back in the seventies there emerged an expression for this:
"In nuclear warfare, there is no such thing as a near miss"
(snip rest)
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