[Xastir] GPS question, off topic.

[Gerry Creager]

As a rule of thumb, one gets an improvement of ~one order of magnitude 
with augmentation.  Not quite so good with WAAS as with truly local 
DGPS, and nowhere nearly as good as RTK.  We were able to get, in the 
old days, with SA on, <10m accuracy with DGPS, and 15cm accuracies with 
RTK, assuming a good correlator and stable oscillators.  This was when 
we could document >90m induced error with SA.

After SA was turned off, we've started seeing ~6m accuracy most of the 
time without augmentation for a fixed antenna.  I've seen somewhere near 
that (but not verified it rigorously) with a fully exposed patch antenna 
on a moving vehicle: I can keep it inside the perimeter of a 2 lane road 
and generally on the right side of the line, with no augmentation.  With 
WAAS, it's always inside the perimeter of the roadway and always on the 
same side.  Assuming I'm driving where I'm supposed to be, but that's 
another story (If you don't like my driving, say off the sidewalk...).

Let's assume, then, that the 6m number with cheap receivers (and they're 
getting better, too) is valid, oh, say, 90% of the time, with the 
outliers caused by local distortion of iono and tropo refraction.  If 
one were to then augment with WAAS, and see a 2x instead of 10x 
improvement, one's accuracy should be in the 3m range, and a 4x 
improvement would be ~1.5m, or pretty close to 1e-05 degrees.  So, we're 
not too far from being below a meter at this point.

Add in some info on tropo, which can be modeled and observed, and 
doesn't move very fast, and you've reduced your error budget toward the 
submeter range.  If one can then calculate iono, you're almost certain 
to get below the magic number, and the ionosphere, while constantly 
changing, is stochastic and predictable.  And relatively slow.  While 
most of the iono effect is caused by scinitllation, which is fast, the 
net effect of action on such a large target is effectively a slow system.

And, John, just driving around in a field shouldn't make the locals 
think too much... Try having the locals see cows in a pasture...

gerry

Richard Polivka wrote:
> A tidbit...
> 
> In decimal degrees, N43.00000 W88.00000 to N43.00001 W88.00000 is equal to about 1.1 meters.
> 
> If they are looking for sub-meter accuracy, they will need at least six decimals out, unless they are saying that their accuracy is to five decimals +/- 0.000005. That's specsmanship in my book.
> 
> Moving accuracy is not going to be too good as to get some honest accuracy, you can't be moving. 
> 
> The ionosphere is always moving and models will not really be able to figure it out accurately. I wonder if their system is some form of a two station RTK system using a stationary system to supply correction to the rover. 
> 
> If it sounds too good to be true....
> 
> 73 form 807,
> 
> Richard, N6NKO
> 
> 
> John Ronan <jronan at tssg.org> wrote: >
> Evening,
>> What would the GPS be used for? APRS Tracking? Farming?
>>
> Farming actually.
>> The Phoenix GPS claims sub-meter accuracy. Barring marketing lies,  
>> this is not possible with only WAAS. WAAS would likely get you to 3  
>> meter accuracy at best (Gerry N5JXS certainly knows more about this).
>>
> Ok thats what I was reckoning.
>> The eDif feature mentioned for th Phoenix appears to attempt to  
>> model ionosphere delays by analyzing multiple satellite signals  
>> over time. Normally ionospheric delay (signal bending) correction  
>> is done using a 2nd GPS frequency, on which the actual data is  
>> encrypted, but the clock information can be extracted. This 2nd  
>> frequency is what is normally referred to as "Military grade" GPS.  
>> The eDif claims to be able to get the same type of correction data  
>> out of a single frequency receiver. It also sounds like a costly  
>> add-on.
>>
> ok
>> The other differential positioning technologies mentioned are  
>> likely DGPS,  where you have a separate GPS receiver at a precisely  
>> known position which radios out the offsets (within some "local"  
>> radius) between the GPS-derived position and its precisely known  
>> position. DGPS receivers recieve and incorporate these offsets into  
>> their calculations.
>>
> ok
>> The question back to you,  I guess, would be: do your friend need  
>> sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs  
>> output positions only at 1Hz or less, depending on the NMEA  
>> sentences enabled.)
>>
> Ok.
> 
> Short answer is I'm not really sure
> 
> I'll try and address Curt and Gerry's questions as well.
> 
> Firstly.. WAAS signals aren't a problem, the only mountains are  
> North, and going south, next stop would be Spain. Most of the land is  
> pretty flat.
> 
> The system is being marketed to do several things (apologies, I'm not  
> at my desk, so the piece of paper I wrote the URL on is not available  
> to me.. scratch that.. found it. http://www.farmworks.co.uk/gps.php  
> click on GPS Swath Guidance ), one of which is to allow him to spread  
> fertiliser with as little overlap as possible.  This is because over  
> the course of a season, the cost of the overlap is quite high  
> apparently (in monetary terms).  It must be appreciable as he is  
> looking for a method to reduce that cost, but is sceptical of the  
> solutions presented.
> 
> Purely as an exercise, I was going to try the GPS-18 that I have  
> already, put GPSman running (or something) and drive one or two of  
> his fields.. just to see what he thinks...all the locals looking at  
> their APRS displays will think I've gone barmy...
> 
> Regards
> John
> 
> 
> 
> --
> John Ronan , +353-51-302938
> Telecommunications Software &  Systems Group,  http://www.tssg.org
> 
> 
> 
> 
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University
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