I went out to McClellan Park (and a bunch of other crazy surveyors) at 7am this morning to see NGS's airplane with the gravity meter in it. That is pretty cool. They are working on measuring gravity over the entire US. I mostly went to see the Pilatus single engine turbo-prop airplane 🙂
They need a pressurized twin for So Cal work. The Pilatus is single engine and is working up here but down there they need to go farther off shore which requires a twin. Too bad I don't have one of those lying around 🙁 Even if I did I can't imagine meeting the likely insurance requirements. The budget is 3.2 million but I'm not sure exactly what that includes, just the airplane or airplane and pilot (and I would think insurance).
A fascinating project for sure, and one that will takes YEARS to complete.
A good read (overview) can be found here:
http://www.ngs.noaa.gov/GRAV-D/pubs/GRAV-D_v2007_12_19.pdf
Loyal
> I went out to McClellan Park (and a bunch of other crazy surveyors)
I was one of them. Woke up at 2:30 for unknown reasons, couldn't get back to sleep, got up at 4:00 to take my son to the bus pickup for a ski trip, then hung around the house until 6:30 and off to the GRAV-D show-and-tell. Breakfasted afterward with a retired NGS Advisor (and occasional subcontractor) and some Caltrans folks. Had fun, learned stuff, now in dire need of a nap.
Though Dave K and I live within 20 miles or so of each other, today was our first face-to-face encounter.
P.S. Thinking about gravity makes my head hurt and disrupts my understanding of the word "up".
Howdy,
Sounds like fun. I hope that the airborne gravity observations go well and the data is made available to researchers soon. There is an interesting project planned for later this year here in Texas involving the validation of geoid slopes using a mixture of instruments. The zenith camera (to do astro-geodetic deflections) is of particular interest to me. For an interesting article on the technique (actually the abstract is linked) see: journal article
Information below was copied from the most recent NGS height mod meeting notes: site address
Relevant section of document:
Geoid slope validation project (Renee Shields) – Plans are ongoing at NGS to be completed in TX. Leveling, GPS, gravity, and a zenith camera will be used from Corpus Christi to Austin (300 km line). It should fill in gaps in wavelength data from GRACE/GOCE satellites. Geoid team and Dru Smith are developing the project to be completed later in the year.
Cheers,
DMM
I'm still puzzling over how you measure gravity in an airplane that is subject to continually varying accelerations. Can they compare acceleration to altitude changes with sufficient accuracy to cancel accelaration out of the equation?
I asked that very question.
The airplane has two IMUs in it (Inertial Measurement System) which apparently allow the computer to account for the roll, yaw and just general bumpiness of the airplane ride. They need good weather to fly; no more than light turbulance.
> They need good weather to fly; no more than light turbulance.
In fact, they had to scrub today's morning flight because of turbulence.
I believe the meter used by NGS is the one described in the following file: Micro g LaCoste Romberg TAGS meter
> I believe the meter used by NGS is the one described in the following file: Micro g LaCoste Romberg TAGS meter
That looks like the one we saw in the plane today.
There was also some talk about bringing a LaCoste A-10 absolute instrument to the project, though I didn't catch exactly how it would be used, as it's not an airborne unit. The interesting thing to me about the A-10 is that it works by actually measuring the trajectory, in terms of both time and distance, of an object in freefall in a vacuum. It has a stated accuracy of 10 microgals, and a precision of 10 microgals in 10 minutes at a quiet site. Pretty amazing.
Howdy,
Absolute meters are used to provide the tie absolute gravity at the aircraft parking spot. See the following link:
GRAV-D Alaska Survey 2008 Report Executive Summary
BTW, I had a chance to see the FG5 in operation in the 90's. An impressive piece of technology. It is more accurate though less portable than the A10. FG5 specifications are: Accuracy: 2 ?Gal (observed agreement between FG5 instruments) Precision: at a quiet site, 10s drop interval, 15?Gal/sqrt(Hz) [eg. About 1 ?Gal in 3.75 minutes or 0.1microGal in 6.25 hours] The meter builder's web site has some great documentation including manuals the directory is here.
That said, watching operation of the equipment is like watching paint dry (or a GPS receiver on a long session).
Cheers,
DMM
Howdy,
I happened to come across this image of the screen of the TAGS meter within a presentation. I copied it and post it
Resolution is not great but hopefully you can see the effects being monitored and quantified.
Enjoy,
DMM