GPS

Other than Time/frequency uses, I'm mainly interested in the Global Positioning System sub-topic of precision (near centimeter accuracy). This page is under constant construction, so come back later if you are interested in ongoing details. Caveats: I am NOT a surveyor, and my coordinates should not be used as any legal description!

Equipment: I have a dual-frequency Legacy receiver and RegAnt choke ring antenna from Javad Systems, along with other options (such as frequency input), accessories (power), and software - using Seco 2M height pole. I also have weight bags for the legs, and a traditional tripod (not normally used although I have a GPS Optical Plummet). I carry along a nice temperature-compensated pocket barometer (Thommen Classic, seems to read about 1mm low compared to my Princo Nova Fortin design mercurial barometer or published sources such as UW Seattle High Resolution Atmospheric Pressure), Psychro-Dyne psychrometer (and ASTM 63C & SAMA FT40 thermometers to supplement), Suunto's Tandem (compass declination set around 19 degrees E, for bearings to obstructions and to orient antenna north, clinometer to help note obstructions above 15-degree elevation), and 2M measuring tape. My base has a Paroscientific MET3A; maybe I should add a gravity meter!

Although I also try to find coordinates for my multiple GPS antenna masts (used for timing) and play around finding my property corners on a map, I mainly try to reproduce local "benchmark" coordinates suitable for GPS observations (typically National Geodetic Survey HARN stations; I don't care about orthometric heights although I might compare some to GEOID99). While doing this, I evaluate/research hardware, software, and my own ability to center over a mark. I avoid stations on a roadway and those which require special permission (such as airports). For convenience I've installed a PK nail within my property (not near a property line!) but I keep looking for the best spot suitable for observations and if found, I'd probably install proper monumentation.

For best results, I try to plan for at least 5.5 hour sessions, trying to avoid the couple times each 12 hours when VDOP exceeds 6 (if doing a shorter survey I try to aim for a "trough" where VDOP<3), and plan a second observation on a different day/time (especially different satellite geometry - for example a multiple of 12 hours plus 4 or 8 later). I also try to avoid surveying during passage of a weather front, using indicators such as http://www.rap.ucar.edu/weather/surface/us_ptnd.gif. I might look at gopher://solar.sec.noaa.gov/00/latest/daypre or http://www.sec.noaa.gov/nav/gps.html for A<20 and K<5 (and/or at http://www.nwra-az.com/ionoscint/sp_solmax.html to avoid nighttime high scintillation but maybe not since I'm at mid-latitude). If using single-frequency (such as on my property for short baseline), I might try between midnight and 7am local time to reduce errors due to the ionosphere (highest is around 2pm local, lowest is around 4am local).

In the field, I park my vehicle at least 10M away, I let my weather instruments get acclimated, collect data at minimum 15 second epochs down to 10-degree elevation, and re-check level/weather at the middle and end of observation. It is convenient to note the approximate UTC, day of year, and GPS week number.

After moving the data to my PC, I typically process using 16- or 17-degree elevation against my nearest Continuously Operating Reference Station plus at least two or three more using precise ephemeris. The precise ephemeris takes some number of days to become available at Index of -igscb-product- (igs*, or igr* if you're in a hurry) in ITRF and/or NIMA in WGS 84(G873) with velocities. Relative to my home, SEAW is the closest (~16Km SW, maybe 6mm East and lower than published) and has meteorological data but has cycle slips every 30 minutes. SEAT isn't much farther (~22Km SW), followed by RPT1 (moved due to earthquake?), WHD1 (NW - maybe 1.5cm higher than published), and SEDR (N). I like to add ALBH because it is an IGS global station with ITRF solution (and velocity!) in the WCDA (and is a different receiver type - actually they have a newer receiver and call it ALB2). LIND is around 144Km SE, and if I go ~229Km there is GWEN or GOBS. There is a 5-second epoch station in Yakima. At a distance of ~246Km to the NE there is DRAO which I like for the same reasons as ALBH (and it is considered to be on the "stable" part of the North American Plate). Within that larger radius there are others such as FTS1, NEAH, PABH, KELS (has a fair amount of L2 cycle slips), CHWK and NANO (and Tillamook which might be far enough away). If converting from Trimble raw, there are SATS and JRO (142Km SW and 188Km S, using Trimble Choke Ring antennas). New is SPN1 and soon we should have FAA's WAAS RS at ZSE, and maybe someday also Moscow and Brewster and Hanford. All those include a variety of antenna types/covers (and RINEX antenna reference point offsets?), receivers, and mounting. I might retrieve met data for near SEAT (but subtract 8mb to get station pressure), 5 miles from RPT (subtract 1.5mb), 10 miles from WHD (subtract 4.25mb), 10 miles from SEDR (subtract 6mb), 2 miles from LIND (subtract 56mb), Kelso, 21 miles from PABH, 8 miles from NEAH (subtract 55mb) and 25 miles from NEAH (for RH), 5 miles from FTS, and 14 miles from GWEN (& 23 miles from GOBS) (for summer defaults in Western Washington consider using 68%RH, 18 degrees C, and 1016mb which must be adjusted to station height).

Assuming you have lots of dual-frequency RINEX data, there are some offers of processing at AUSPOS, SCOUT, OPUS, and Auto GIPSY.

When looking at coordinates, I try to use ITRF at the same epoch for everything (or adjust for any different date). Everything is on the North American plate so hopefully there is not much difference in velocity/acceleration. WGS 84 (G873) is very close to ITRF (within a few cm), but of course differs up to a few meters when compared to the various NAD 83 adjustments. To go from NAD 83(Original) to NAD 83(1991) in my area you add about .005" latitude and subtract about .007" longitude (use Corpscon or NADCON21 but stated uncertainly is 5cm). To go from ITRF 96(Epoch 1997.0) to NAD 83(1997) or ITRF 94(Epoch 1996.0) to NAD 83(1996) in my area within about a centimeter you subtract about .021" latitude, subtract about .053" longitude, and add about .35M height.

If the coordinates were for one of my antenna masts, I then enter them into a timing receiver as my known static location (if a timing receiver requires MSL rather than HAE, I try to enter the value appropriate for their geoid model rather than the more true value). When trying to find my property corners on a map, I digitize a map that contains my property and nearby benchmarks. Using the benchmark coordinates I attempt to calculate the X and Y scale. Using that scale, I then verify that the resulting coordinates matched my expectations (of course such techniques are approximate).

Probably the favorite NGS HARN station that I liked to occupy is FBN order-A (with ITRF solution) Aviation 2 (aka Seattle) and there is also the nearby NOAA VERT
NGS FBN order-A (ortho order-2) NW VLBA
If it was closer, my second favorite NGS HARN station might be FBN order-A Sat Track Sta 003
NGS FBN candidate order-A: Mead (SX1472)
NGS CBN order-A: ortho order-2 Cap Sante (TR2695), Pt B (SY4354)
NGS CBN order-B: Paine Field (SY5643), Brown, Lion Park; Granite Falls
NGS FBN order-A (ortho order-1): Slug
NGS order-A (soon to be FBN?): BM 31530-39 (AE1858)
NGS order-B (ortho order-1): SEAT A (AF9780)
My current favorite non-HARN station is (City of Redmond) GPS90 1C1 (SY5751). I also like the 150 and 1058 marks of the Sand Point calibration baseline, and Magnuson 2. Vall (SY3544) is considered my nearest NGS GPS station.
Nearby ortho order-1 choices include X 67 1934 (SX0852), RD 10 (SX0870), and Z 67 (SX0857)
King County DOT (Survey Unit): They installed a rod with me at Cottage Lake Park and at Wilmot Park and at a Carnation park. 939 (aka 945?), 911 (47 46 11.5921408N, 122 04 48.0039581W, 105.262M), 912 (47 46 07.7798571N, 122 03 54.2369486W, 86.201M), 836
WA DOT (ortho 1cm): BM31522-94, GP31009-93, BM17203-2, GP31524-79

More photos...

The USCG NAVIGATION CENTER has various status/links. Rather than listing all the other GPS pages out there, I reference them indirectly through Sam Wormley's GPS Resources (numerous links including news:sci.geo.satellite-nav which seems to be mostly "noise" about Garmin, and http://www.op.dlr.de/~igex98op/monitor/monitor.htm which seems like nice place to watch recent status) and Navstar GPS Internet Connections (doesn't seem to be updated anymore - includes pointers to SOPAC, ION which hosts the industry meeting, UNAVCO, Global Positioning System Overview, Navtech GPS Store, GPS World, Leick GPS GLONASS GEODESY, etc). There is also Yahoo's GPS list (includes pointer to Oregon GPS Users Group). IGLOS makes GLONASS interesting. WAAS I forgot about Archives of CANSPACE until recently

Some links that I use which aren't easily referenced above include: Public Works Survey of Snohomish County, Washington State Geographic Information Council (WSGIC), NOAA OPSD Benchmarks, GeoLine Positioning Systems. Some other links that I'm looking at are Land Surveying and Geomatics.

I'm enjoying different WAAS GEOS such as Inmarsat 3 AOR/W move from 54W to 142W early 2006, Telesat 107W (Anik F1R PRN 138) late 2006, and PanAmSat 133W (Galaxy XV PRN 135) Fall 2006.

Old info below:

Those CORS use the Ashtech groundplane with radome (L2 is 3cm below L1), have published ITRF94(E96) and NAD 83 coordinates of their L1 phase center, but have 0 listed as their RINEX antenna reference point offset. Those IGS (and UW) sites use the Dorne Margolin-style choke ring antenna without radome (L2 18mm above L1), and have varying values listed as their RINEX antenna reference point offset. After studying the antenna issue, I try to derive the apparently unqualified coordinates of the IGS sites (so that I can use them as base stations).

Using post-processing software such as GPSurvey, I process the remote data using the closest base and then the other bases (hopefully there won't be much difference). Assuming that I have multiple solutions that are within spec, I typically try to "average" them together (giving less weight to the farther base stations) or perform network adjustment. Before trusting any data from my own property, I perform the procedure on a benchmark and check that the resulting coordinates are within spec (if I notice a small, systematic error in the vertical component, I consider revising my antenna height slightly). Since the base stations are RINEX, I try to check the software by converting the remote file to RINEX and computing the solution again to verify that it is the same, and then use a different program to compute the solution (with the RINEX file).