How to get better accuracy

[Guest supertech]

d by the MIL but has an old computation of the geod [the average true shape of the earth]

If you're not in the CONUS, find out which geod model is right for your area. [map datum] This will help both lon/lat as well as elevation. Still, elevation is a guess at best.

 

NOTE: Surveying GPS units will be more accurate and may use other methods to derive an accuracy of 2cm +/-.5 cm.. Not bad eh? Only - they cost thousands of USDs!!!

[Guest Kerry]

Wasn?t but for what it's worth I?ll make some comments anyway, just as general interest.

 

As for clouds: a clear sky relates more to lack of obstructions and clouds don?t fall into that category. Clouds really aren?t all that much of a concern as much as the atmospherics (ionosphere, troposphere including sun spot activity) well above any cloud horizon. Much of the atmospherics can be handled with dual frequency. GPS is basically an ?all weather? system but day the clouds are big enough, black enough, ugly enough and nasty enough to cause some real concernable issues by far the last thing most users will have on their mind is GPS.

 

I?m curious now, does anybody have any locations (anywhere, world wide) in recent times of real real bad storm/cloud activity as these types of GPS issues can/could/might be able to be checked?

 

Differential GPS: as for accuracy the sky?s the limits controlled by the number of zeros on the cheque ($$). The sub metre/decimeter level differential has been available for years but again only from service providers as a subscription service ($$) with additional $$ for the decoders and receivers. Not the type of accuracy the general user would pay for or want to pay $$ for as there?s no free-to-air service that I?m aware with that type of accuracy capability.

 

As for 4 sats in a big enough circle? Really the ultimate strength is a receiver being at the centroid of a tetrahedron (a satellite effectively located at each corner) but that?s not possible when limited to a hemisphere. Just being in a big circle is really not enough (unless purely 2D) without some elevation relativity. Really if all satellites were all low on the horizon they would be in the biggest circle possible but the accuracy would be totally RS. Also the lower the satellites (hence making a ?bigger circle?) the more atmospheric affect there will be which generally makes the position solution less accurate. Rule of thumb is don?t use satellites below 5 degrees (preferably 10 degrees) but not many recreational receivers allow this capability although some manufacturers do have fixed elevation cut-offs.

 

GPS Satellites have a general 12 hour orbit, speed is around/approx 4000m/sec (~14,000km/hour or 9000mph). All things being equal religiously pass overhead twice per day (all be it 4 minutes different each day).

 

As for the Datum (Ellipsoid) issue all compatible datum?s be they NAD83, WGS84 or any based on GRS80 are for all practical purposes identical and purely a ?mathematical? determination of an ?earth best fit?. A ?geoid model? and ?map datum? is basically different things. The initial spheroid height (GPS height) although generally of less accuracy than a horizontal determination (in GPS capable terms) is really a separate issue to determination of sea level heights based on geoid models. In simple terms Geoid models really have no affect on GPS accuracy as such as they are something that are secondary in determining sea level based heights relative to spheroid heights. That apart from transformation parameters (distortion grid issues), which come after the fact as well.

 

Cheers, Kerry.

[Guest Iron Chef]

quote:

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Originally posted by Kerry:

I?m curious now, does anybody have any locations (anywhere, world wide) in recent times of real real bad storm/cloud activity as these types of GPS issues can/could/might be able to be checked?

---

 

We had a pretty hairy electrical storm blow through about a week and a half ago. GPS accuracy was nuts (in the bad way). The GPS had no idea where we were at and it never did get a lock.

 

All the lightening and foul weather really messed up the signal. If the weather is bad enough to mess with AM/FM radio station transmission then it's a bad day to use a GPSR since the signal power is substantially lower.

 

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-Iron Chef

_ _ _ _ _ _ _ _ _ _ _ _ _

agefive.com/geocache/ ~ Fe-26

Lets Drive Fast and Eat Cheese!

[Guest Kerry]

quote:

---

Originally posted by Iron Chef:

We had a pretty hairy electrical storm blow through about a week and a half ago. GPS accuracy was nuts (in the bad way). The GPS had no idea where we were at and it never did get a lock.

 

All the lightening and foul weather really messed up the signal. If the weather is bad enough to mess with AM/FM radio station transmission then it's a bad day to use a GPSR since the signal power is substantially lower.

---

 

Iron Chef, there maybe a way of looking at these type of things even in the past. It would require a location (Lat/Long & approx location), date and time (preferably UTC time but at least including local time zone).

 

I'm interested in having a look if you can supply the info but there's nothing guaranteed depending on the location relative to the data source (private e-mail if you wish). Generally in the U.S there is a fair amount of data available.

 

Cheers, Kerry.

[Guest Ron Streeter]

Yikes ! Where's my Cray computer?

 

Turn on the gps. Push GoTo. Find Cache. Log It. Go to next one.

 

30H/89F

[Guest TeamFamilyAffair]

y you ask? This is the first step of the FAAs (Federal Aviation Administration) plan to transferring ATC (Air Traffic Control) to a satellite based system). The Accuracy of a WAAS enabled GPS receiver is supposed to be 2 meters (6 feet) rather than the normal 15 Meters (45 feet) that everyone else enjoys. I have played with it quite a bit (I do have a pretty cool job) and it does make quite a bit of difference. The end reason for this - (for all you aviation type people) is that this allows a properly equipped airplane to make a precision approach to any landing strip or airport anywhere in the Continental United States in any weather.

 

Scott of TeamFamilyAffair

 

[This message has been edited by TeamFamilyAffair (edited 17 October 2001).]