View Full Version : GPS Accuracy - the Unknown

16-03-2004, 09:08 AM
Something for those that rely (too much over reliance) on GPS as a primary or sole means of navigation and/or scream around in confined waterways in the dark or at times have simply felt a little lost #:-X

GPS Accuracy - the Unknown (http://www.cqnet.com.au/~user/aitken/gps/gps%20the%20other%205%20percent.htm)

Quite interesting this little event due to the sudden change in positioning characterics, the average errors, the maximum errors of such a wide area and the time span/s it lasted for apparently undetected by the monitor stations.

Cheers, Kerry.

16-03-2004, 09:16 AM
Hi Kerry

I have a Garmin GPS that shows the accuracy level based on the number of satellites, etc. It can get as low as 5 meters...I assume that figure would be accurate?

16-03-2004, 10:21 AM
The are many errors induced into a single 'fix'. The more samples taken the better the accuracy.

Many of the GPS receivers today have a 'wake trail', which if displayed, gives the operator some confidence in position. This 'wake trail' is a series of dots displayed on the lcd or other receiver display which connected together shows a reasonable accurate 'trail'. If you receive a gross error the 'dot' will be way out of sequence and can effectively be ignored. If you have a continious error, there is little you can do with the data, but, the onus is always on you to use other means of verification of your position.


16-03-2004, 10:51 AM
i have a old garmin 38 that is 8 years old and until 2 months ago it always put me in the same spot or very close to when i came back onto the marks i must admit fishing in the last 2 months has been harder cause it doesnt work as good
i have ordered the newer version of it which is the 72 i think anyway if this is any better i will be more than happy (it is a 12 channel the 38 was a 8 channel)


16-03-2004, 11:26 AM
one thig to remember with the on going fight against terror is that the milatary both aust and usa (and the rest of the worlds) uses alot of saterlites so on one day you may get as many at 6+ to position you giveing you very acurate reading as close as 2 or 3 m but on other depending of what is going on in the world you may get as least as 2 giving you if my remeorie is right a reading as far out as 150m

16-03-2004, 11:44 AM
as long as mine gets me close to my favorite lumps and bumps i will be happy! ;D

16-03-2004, 03:47 PM
The "accuracy estimate" many receivers display is nothing more than that, an Estimated Position Error as really the receiver doesn't know where it is anyway. Depending on the circumstances the estimated error might be 5 metres but the actual position could be hundreds of metres out.

.... the onus is always on you to use other means of verification of your position ...

That's about the strength of things but how many actually use other means of verification?

As a comment on this thought that the U.S military still induce errors, not so and there's been absolutely no indication that any terror activity, the Afghan war or the war in Iraq has seen a need to re-introduce Selective Availability (SA). Simply didn't happen and this is also U.S Gov policy not to re-activate selective Availability.

These days there are other methods used by the military to deny GPS as and when required but this has no affect on the rest of the world.

Cheers, Kerry.

16-03-2004, 04:19 PM

Very interesting topic. I have a Lowrance LMS-350A and added a differential GPS aerial to get the accuracy that bit better. It worked just fine around Brisbane area and added a bit ---gave accuracy - however off Fraser Is starts to wobble and difficult to track back up where you have been on a drift. Now does the same off Brisbane. Does anyone know why? Is it the fact that you need to re-initialize because of the various distances travelled or do you have to put the frequency of the Differential Beacon. Don't know what the frequency is off Brisbane let alone Fraser( Gladstone). Can anyone help.

16-03-2004, 04:21 PM
well i do beg to differ kerry! and i'm coming from very relable sorces that the public to have access limited when ever the miatary need that saterlite but you may be right after all out of thousands of saterlits orbiting the milatary only ever uses a small handful

16-03-2004, 04:40 PM

Many years ago. the US & UK Submarines used SatNav with a highly accurate multidoppler signal able to place the submarine with great accuracy. The commmerical version had a accuracy of about 100M from an unecoded doppler pair.

GPS users until recent times had a degraded signal from each emmiter. This 'degradation' has been removed.

It is my understanding that the various smart tools still use 'other' signals and encoded information. The signals we use may be effected by many local sources including rain, antenna movement across the receive arc, land mass and other 'local' factors.

If you are interested in some light reading start with the following link:


Good luck


17-03-2004, 04:21 AM
Basserman, In affect the US Military could close the whole system down in a matter of seconds but there is so much reliance on the system for such a wide range of applications that this simply isn't going to happen, this side of a full scale conflict.

Since May 1, 200 when Selective Availability was set to zero.

Selective Availability Off (http://www.cqnet.com.au/~user/aitken/gps/sa_off.htm)

there has been no re-activation of SA either after 9/11 or any other conflict. "Set to zero" is not terminated (as such) just set to zero, as SA is still effectively in place and is still installed on every GPS Sat and will be installed on every future GPS sat but set to zero. SA could be re-activated in a matter of minutes but since May 1, 200 this has not occured.

Many thought/expected this would occur but it simply did not and the following outlines the actual policy and all the data supports this http://www.igeb.gov/sa.shtml
U.S. Policy Statement Regarding Civil GPS Availability
March 21, 2003
The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace.
The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without disrupting or degrading civilian uses outside the theater of operations.

We believe we can ensure that GPS continues to be available as an invaluable global utility at all time, while at the same time, protecting U.S. and coalition security requirements.

September 17, 2001
GPS Selective Availability (SA) has not been used since its deactivation by the President on May 1, 2000. At that time, the United States Government stated that it has no intent to ever use SA again. There has been no change in this policy.
It is important to note that, pursuant to both the United States GPS Policy and the Federal Radionavigation Plan, the Global Positioning System and all other federally provided radionavigation systems are responsive to the National Command Authorities.

For the most current information on the operational status of GPS and GPS-related services, please refer to the U.S. Coast Guard Navigation Center, which is responsible for disseminating operational GPS information to the civilian user community.

From data available just after 9/11 (within about 5 minutes actually) the system actually became very stable, no maintainence so as to provide maximum availability.

Also GPS only has provision for 32 satellites max and these sats are specifically dedicated GPS satellites. In recent years there's normally been around 28-29 in orbit and available.

Your reliable source needs to be brought up to date a little I feel.

Cheers, Kerry.

17-03-2004, 04:47 AM
DogDoctor, I can't recall if the Lowrance differential can be setup for automatic tuning or not. The StarLink MRB-2A will but not sure if the Lowrance differential thinks it is a StarLink or a MagnaVox.

Anyway Brisbane (Bribie island) AMSA differential is 294kHZ, Gladstone 313kHZ and all AMSA beacons are 200bps.

Whatever works the best is the way to go but around Fraser Island one might imagine Gladstone? would probably be the best signal considering it romps out about 320 watts compared to Brisbane's 180 watts.

After travelling more than a few hundred miles with a receiver turned off it certainly has to be re-initialized (nothing really to do specifically with dGPS) but have no idea about the sudden wobbles. Perhaps try checking/re-setting the differential configuration (Freq, Baud rate etc) manually?

Off Fraser both Brisbane & Gladstone could be received and even off Brisbane Gladstone will still transmit that far but would be bordering on the edge of reception depending on ones location, so if still set to Gladstone then reception could be intermittent, which could cause the position to wobble in and out of dGPS/GPS accuracy.

Cheers, Kerry.

17-03-2004, 05:01 AM
Another instance of a "glitch". Vessel is travelling on a bearing of around 220 degrees (from top right hand corner to lower left). About centre screen the vessel suddenly jumps/steps about half a mile right & backwards (but of course not physically possible) then the GPS position slowly shows the vessel gradually coming back onto the actual track. Without a plotter this "event" might have went unnoticed?


Cheers, Kerry.

17-03-2004, 06:35 AM
well kerry you proved me right and my sorce was right! the milatary can still affect the SA as you sayed it is just set at 0 at the moment but who knows what around the next cornor! they can and may still ristrice gsp satrlites
not once in my prev post did i say they were doing it right now i was saying that they could
sorry if you misunderstood me

p.s. i don't know if they ever will again and i dout you do to but they have done in the past and still could if they need to

17-03-2004, 08:10 AM
.... but who knows what around the next cornor! .... i was saying that they could .... sorry if you misunderstood me

They could, and yes the facility is still there BUT they won't and the reasons for this is something which hasn't been mentioned, which is Selective Deniability.

Selective Deniability is something that allowed SA to be removed in the first place and the mechanics is rather boring.

What they did in the past was policy, that's changed and with Russia's Glonass now coming back up to strength and soon to be launched Galileo (both of which don't have anything like SA) turning SA back on for any reason really doesn't achieve much unless the other systems are also taken out of service.

Not to far around the corner when the new civil GPS freq are put into orbit your (new) garden variety handheld GPS will give something in the accuracy order of 1.5m @ 95% standalone.

Cheers, Kerry.

17-03-2004, 08:49 AM
geez that would be good
i have used the malatary gps units (bloody as big as a computer) and their accuaracy is unreal only ever used them in australia but every time we were within 1m of where we wanted to and that was check the old fasion way and satrack but still i would never go wondering around with out a map even if they did get with in 1.5m as you said it is always up to us the check after all you never know what could happen to computer they always seem to have a glich in them here or there :)

17-03-2004, 09:46 AM
Kerry, good to see you haven't lost your grasp of GPS technology ... so answer me this if you will ...

What technology are surveyors with GPS-like backpacks using and is it "survey accurate"?

17-03-2004, 10:42 AM
.... What technology are surveyors with GPS-like backpacks using and is it "survey accurate"?

Accurate as required for the survey. If the survey only required metre or sub metre accuracy then they might be using GIS type units and possibly some form of differential service. This service could be provided via satellite (such as OmniStar - http://www.omnistar.com/), which requires a black box receiver and is a subscription service (the more accuracy the more the service will cost) or via the sub-carrier of some FM radio stations such as JJJ or via cell phone or radio or they could just be recording data and then processing this latter to achieve the required accuracy.

If the survey requires accuracy that can be achieved via the AMSA differential system then that can be defined as survey accuracy as well.

Going up a rung they could be using dual freq receivers in either real-time (via some form of radio link either being transmitted from another base receiver setup close by (generally within 10km of a base) or corrections being transmitted by some third party system or they could also be recording data for post processing latter. This type of real-time / post processing system will achieve accuracies in the cm range.

Above this again is really another level of survey accuracy that takes much more time but is not limited by distance etc so survey accuracy is a variable type of term.

Cheers, Kerry.

17-03-2004, 04:51 PM
Just a short note to say thanks

17-03-2004, 06:13 PM
Kerry - What make/model GPS do you have in the cat ??

18-03-2004, 09:01 AM
Kerry - What make/model GPS do you have in the cat ??

The oldest one (and one probably used the most 'cause i can see it) is a Globalnav the original 5 channel version and all and by all rights should have had issues with internal batteries & memory a few years ago but still going strong, still does the basics as good as anything comparable today, especially in a marine environment and the big plus it has BIG numbers and keypads #[smiley=2thumbsup.gif]

Anybody who claims that 12 channel receivers (just because they have 12 channels) are more accurate than some of these older units are probably in sales #;D. #

There's also a GPS48 handheld on external antenna interfaced to a Furuno and a GPS45 as a backup to the 48 as mounting brackets, antenna's and cables are identical between the 48 & 45. The 48/45 also continuously records track.

There's also a separate backup for the GlobalNav and just about everything can interface to anything else if and when required.

All getting rather old at the present especially considering the whole setup is actually superceded, some several times over as far as the 45 is concerned but they still do what they are supposed to do. Considering the original cost of the GlobalNav (this is a model before the GlobalNav II) things have come a long way but in essence the actual Global Positioning System itself hasn't really changed much in over 20 years and probably the biggest change for many was discontinuing Selective Availability.

Cheers, Kerry.