Re: [NTLK] newtontalk Digest V3 #422

From: Matthew Boulanger (matthewboulanger_at_mac.com)
Date: Sun Aug 10 2003 - 12:40:04 PDT


Eric-

Yes, selective availability (SA) was turned off, thereby giving everybody w=
ith a GPS unit much better quality and results with their GPS units. Prior=
 to President Clinton's decision to do such, accuracy for Civilian units wa=
s around 100 meters vertical and horizontal. With SA turned off, most hand=
held units will get on the order of 10-20 meters accuracy. This is what I =
suspect the Trimble GPS cards would get if used today. I suspect this beca=
use that is what most handheld units at the time were capable of. I say 's=
uspect' because I can't seem to find all the specs on these units. In real=
ity, my handheld Magellan unit (manufactured a few years after these Trimbl=
e cards) gets around 10 meter accuracy while in the field, on a clear/light=
ly cloudy day. The issue isn't with the signal 'quality' but with the hard=
ware and what is is able to provide. Most of the early GPS receivers simpl=
y were not built to take advantage of the signals with SA turned off (guess=
 they figured that SA would always be there).

The positional data from the GPS can be 'corrected' for errors as well, but=
 that generally requires extra hardware or software. Often times correctin=
g your data will result in sub-meter accuracy. This generally needs to be =
done in the office though, and after obtaining 'correctional data' (sometim=
es at cost). Some units allow you to correct you data in the field, but t=
hese are often survey quality pieces of hardware, and require a substantial=
 investment. =20

Today, there is a new correctional system in development called WAAS (EGNOS=
 in Europe, and a few other things in Japan, and Asia). The use of WAAS al=
lows you to get even further refined readings while operating, on the order=
 of 3-5 meters. As such, most handheld GPS units are taking advantage of t=
his technology. However, you need to buy one that is specially enabled for=
 this technology, it's not backwards compatible.

Now, theoretically, the US military maintains it's own encrypted signal usi=
ng the GPS satellites, which has been rumored to provide submeter accuracy =
on the fly. I've seen reports that this is simply not true, but I've also =
seen reports, and spoken with ex-marines that say it is true.

The important thing to remember is that virtually no GPS unit is going to g=
ive you 100% accuracy in the field, ever. The Trimble cards, while certai=
nly a collector's item, are quite dated compared to the newer handheld unit=
s available. A colleague of mine and I created cables to attach our Magell=
an handhelds to the Newton, and are very pleased with the results. His uni=
t is WAAS-enabled, while mine is not. When comparing our data on screen in=
 the office, my unit's points are routinely, randomly dotted within 5-10 me=
ters of his. After we post-process (correct) the data, we generally get a=
 3-5 meter circle of points surrounding our location. That is generally go=
od enough for our purposes. However, we also have a large backpack unit fr=
om Trimble that, when post-processed, will be within 0.5 meters...which is =
sometimes preferable (but we don't want to lug that backpack through the wo=
ods all the time!)

For more info on GPS, and it's capabilities, a good FAQ is :
http://www.cpc.unc.edu/services/spatial/gpsfaq.htm

-Matt

>Date: Sun, 10 Aug 2003 00:05:52 -0400
>Subject: Re: [NTLK] DARPA Newts
>From: "Eric L. Strobel" <fyzycyst_at_comcast.net>
>
>somewhere near the temporal coordinates of 8/7/03 12:20 PM, the entity kno=
wn
>as Matthew Boulanger transmitted the following from
>matthewboulanger_at_mac.com:
>
>> The Trimble card and antennae were pretty much off the shelf items avail=
abl=3D
>> e to the public. While they may have been, at the time, adjusted to dec=
ode=3D
>> the military GPS signals (allowing for meter-level accuracy in realtime)=
, =3D
>> they are extremely unlikely to be able to do so now (as the system is pa=
ssw=3D
>> ord enrypted, and has likely changed in the passing years).
>
>Actually, wasn't SA (selective availability) turned off a few years ago
>(meaning that you can now get the meter-type accuracy)?
>
>- Eric.
>--=20
>
>Eric Strobel (fyzycyst_at_NOSPAM^mailaps.org)
>
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>If aviation had grown as slowly as space travel, the first paying
>customer would have flown in 1943 -- in the 1,657th expendable Wright
>Flyer.
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

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