Well, I was going to keep this to myself and not make an issue of it, but it does have some merit I believe. I've been waiting for the time and opportunity to play with the addition of precise ephemeris to our bag of tricks in Ashtech Solutions 2.6, and decided what better way to play than with an O.R.G.I. session. I have a project currently in the works that I figured would be a prime candidate to work with. The project is some 14 miles North of Station POST (my office base point), and about 6 miles from an area HARN (pacs) point and 3 miles from one of our municipal controls. I first ran sessions as normal from the local controls to the project site, which was sufficient for the project needs, and then decided to run a multi-point O.R.G.I. session using POST and one of my project site controls. The O.R.G.I. session was run from 0600 - 1400 on day 02.329 (11/25/02) under fair (just not perfect) solar/ionospheric conditions. The data was processed using the same CORS sites (distances from 66 miles to 180+ miles away)as used in the original experiments of Nov. 2001, and same procedures....with the exception of using the UltraRapid orbit in the current test. The following are coordinates for your consideration.

Station POST

HARN Derived (Locus units Fall of 2000)
N 6835851.505
E 3100685.732
ELE 402.979

O.R.G.I. (Locus-Ave of 3 sessions 11/01)
N 6835851.421
E 3100685.746
ELE 402.852

OPUS (Trimble 4000ss- Ave of 2 sessions of 8 hours each 12/01)
N 6835851.422
E 3100685.774
ELE 402.799

NOV 2002 8 Hour O.R.G.I. Session (POST)
N 6835851.407
E 3100685.752
ELE 402.797

And for kicks here's the results to compare our project site point between a session using a single HARN and a local point (derived from area HARNS) and the O.R.G.I. session. Note the original difference between the HARN derived elevation on POST and the OPUS derived elevation on POST from above.

Point 0002 (Harn derived - no direct tie to POST)
N 6910020.935
E 3096453.517
ELE 366.441

Point 0002 (O.R.G.I. Session - simultaneous processed with POST)
N 6910020.859
E 3096453.604
ELE 366.168

Seems the 0.18' difference would put station 0002 still in the 3 cm range.

I am in the process of trying to figure out a way using 2 units in a simultaneous processing environment, with one on a well known CORS related point, to make good use of O.R.G.I.

J.D.




another small detail here is that I did use my own variation of the vertical phase center for the Locus antenna in this session. Had I used the 0.1424m factor, rather than 0.125m, in the November 2001 experiments I believe the vertical would have been near a perfect match.

Still not bad for L1 Only!

Modified By J.D. Billings on 11/26/2002 at 4:14 PM

JD, thanks for sharing. Quite interesting.

I have done my own tests using four Canadian Active Control points that are part of the Canadian Spatial Reference System (CSRS) with the rapid orbits and dual frequency receivers. Given the wide expanse of the Great White North, we do not have anything like the density of the CORS stations, so the baselines I was working with were in the 300km to 1500km range. With hour long sessions and the mask set to 10 degrees, I consistently get repeatable results in the decimetre range at 1500km and much better at 300km. I have more trials to run, but the results are much better than I might have expected.

Since this was a part of the experiment I figure I should include the coordinates for POST processed with the Broadcast ephemeris:

N 6835851.398
E 3100685.756
ELE 402.768

Errors compared to Opus Position:

N 0.018
E 0.024
Elev 0.031

All coordinates are reported in U.S. Feet for Texas North Central Zone SPC

So, using the O.R.G.I. "squish to the middle" style processing I really can't see a dramatic difference between the Broadcast and UltraRapid Orbit, although I will continue to use either the UR or Rapid orbit for future O.R.G.I. type positioning. It makes sense the actual computed ephemeris data would be much safer to use than "predicted" data.

I guess I should have made it clear what the whole O.R.G.I. experiment was begun for. This was for positioning with single frequency units from somewhat distant CORS. The data I have shown above is from L1 only. Not dual frequency units. The unit on station POST was a Locus unit. The unit on the remote site, 0002, was a static only (original beta unit) ProMark 2.

Just wanted to make it clear. Nothing but L1 allowed for O.R.G.I.

J.D.

J.D.

I knew ORGI is L1 only. I thought your results were excellent.

We do have a number of L1 receivers that I have tried on long baselines (yep, even to 1500km).

My original intent was to look at options for long baseline processing in the event that we lost a session from one of our receivers in the field. The cost of reoccupation can be quite high, so it made sense to examine what kind of results I could get with L1 and L1/L2 data to the Canadian CACS sites. I never seem to be closer than about 300km to a known reference station, often much farther.

My L1 results aren't as clean as yours, but admittedly, I limited the data to two hour sets since that is a typical L1 observation session for us.

We are putting up our own reference stations in selected areas through the province to support our field operations, so I should revisit L1 only data sets.


Modified By Scott Partridge on 11/27/2002 at 11:01 AM

Scott

I would really hate to have the conditions you have to work under in the "North Country". It has to be rough enough due to the weak sv constellation you would probably see, but the geomagnetic conditions have to be a real killer for you. I've finally decided that the ORGI type work can be of use as an additional check for HARN ties and remote positioning as well for most boundary related projects needing ties to SPC. I think there is a method that will prove successful in here somewhere.

Generally it isn't that horrible. We do get more than our fair share of solar activity problems but the constellation itself isn't an issue. The real kicker is that there is an awful lot of forested country up here with poplar and spruce trees as far as the eye can see. Site obstructions caused by big timber is the number one headache, followed closely by the difficulty in accessing some of these places. It can take quite a while simply to get from one GPS point to the next, so we do what we can to minimize revisits. Many of the well site projects we do are short duration, so the crew is only in the area for a day or two before moving on to the next one.

As Phil noted in an earlier post, I would rather that the crews spend more effort checking antenna heights than anything else since we often only get one shot at it.