Air and Water pressure measurements

Are you talking about setting up my own DGPS transmitter? If so, what is the rough cost estimate?

I just read somewhere that Garmin supports both DGPS and WAAS. I will look into enabling DGPS from USGC. The test site is a couple of miles from the ocean, but the hill itself might be blocking the signals. Unless I can extract the correction data elsewhere to reprocess the data.

Yes, that's the plan. I just need relative positions anyway. It might not matter if they are all off by the same amount.

No.

A receiver.

In the system that I'm describing, which is what I'm familiar with, there is no transmission of this data, other than sending it along a cable to a computer for storage, where it is compared with the stored data from other receivers.

You have one fixed reference receiver, and as many mobile data collection receivers as you like.

Are you logging the 10MHz P-code and bulk process the signals? Actually, that might be the way to do. We simply have to bury RF front-ends and 10MSPS A2D and wire them to a fixed station

Differential GPS will get you about centimeter resolution horizontally and 3 centimeter vertically for measurement made in one minute. There is a big difference in price between survey grade equipment and consumer equipment, between 10:1 to 100:1. There is also a difference in required operator skill as well.

Perhaps what you need actually is similar to what earthquake geologists use. Relatively inexpensive field equipment reporting continuously via ISM band to a small logging computer. Effective long term accuracy; mm/year velocity, millimeters displacement for any given period over one day, centimeters within one hour, etc. Cover the slope with sensors, let the data settle, watch the expensive homes slide after the warning.

Talk to a university geology professor.

What I meant are the residuals from the overdetermined equation solver in the receiver.

It works like this. After measuring the pseudo-ranges, you get a set of second order equations with 4 unknowns (x,y,z,time). That is, in order to solve for position and time, you need 4 pseudo-ranges. That gives you two solutions, of which one is outside the orbits and can be ignored.

If you now use more than 4 pseudo-ranges, you get an overdetermined equation system. Most receivers will use up to 12 pseudo-ranges here.

Solving for x,y,z,t will give you an 'average' result. It also gives you a set of residuals. In principle, this is the remaining error after the solver gives you the best possible solution.

The residuals can be used to estimate the worst case error of a fix. While sampling a position repetitively, you may throw away samples with too high residuals.

Some receivers can output residuals, look for RAIM capabilities.

Kind regards,

Iwo

Only in spirit. :-)

DGPS is very simple - you have a base station which knows what the correct ranges to all visible satellites should be. It calculates the differences and typically broadcasts them to the mobile units.

The mobile units look at their set of pseudo-ranges, throw out the ones invisible at the base station and add the range corrections to what's left. Computing the position from those eliminates the errors.

However, the relevance of the corrections decays with the distance from the base station. It doesn't take long before the corrections actually make the situation at the mobile unit worse. Depending on the system we are talking about

WAAS (EGNOS for Europe) attempts to create a continent-wide grid of 'virtual' base stations. The system estimates the shape of the ionosphere and calculates the corrections for each grid point. The receiver interpolates between points.

It specifically only creates a model of the ionosphere. Local effects, weather, etc. are not part of it.

It gives a continent wide improvement in accuracy, but can't beat DGPS for local measurements.

The same caveats (on a larger scale) apply. Outside the USA, WAAS may give worse results than not using it at all.

There is some sort of third category, carrier phase differential GPS. In a nutshell it's like DGPS, but you start at the base station and move away. These systems track the change of the RF carrier while you move and approach mm accuracy. The expensive surveying receivers mentioned by other people usually fall into this category.

Kind regards,

Iwo

Have you considered differential pressure measurement? Find a place which doesn't move and log the pressure. Also known as a barometer. ;-)

Iwo

We are inside the WAAS corrected zone.

This should take care of most errors except receiver carrier phase shift.

We can have a base station less than 1km away.

They are approximately $10K to $20K, which is slightly out of our budget range, at least for the moment. They requires dual frequencies (L1:1.5G and L2:1.2G) carrier phase recordings of C (Coast) and P (Precision) codes. The US National Geodetic Survey will decode them for us, but the problem is getting the equipments to record them.

With the right equipment, even down to mm.

Exactly, nobody wants to spend money before the disaster. Money is no problem, if and when nature reclaims the land under the houses. Unfortunately, we would not have the data for references after the fact. I have been watching this slope for years, and first alarms (some homeowners) sounded couple of months ago. I received the alarms couple of weeks ago.

Yes, air pressure is useless, but water pressure is still useful. We have turned our attentions to precision GPS measurements.

No, we are going to talk to an Engineering Professor. What we need is a digital phase lock loop based on the semiconductor atomic clock developed at

Perhaps the University Professor can get the chip and money to work on it.

Water can be isolated with a vertical pipe. Even if we can't, all we need is relative pressures at different points. During and after a rainstorm, it's not just wet soil, but underground water tanks. At least, that's the theory and we need data to confirm it.

...

I suppose just a big tarp is out of the question? ;-)

Thanks, Rich

A guy that I drank beer with on Fridays in Austin used COTS GPS equiptment in at least two projects that needed on the order of millimeter vertical resolution. One was to help re-position a subsiding bridge in (perhaps) Edinburgh or Glasgow, and one was to re-level a rocket sled track. If it is really important, email me and I'll see if I can get back in touch, if he is busy.

Thanks, I'll keep that in mind. We can just borrow equipments for the initial mappings, but we should record data continuously afterward. These equipments should be available locally, if we can get to the right person. Again, we are just working our way to the ears of the people in charge.

Just show them pictures of previous mudslides, and get funding to install drainage tiles, like an upside-down drainfield, that will let the water just flow down to the beach, rather than loosening the mud. And plant lots of deep-rooting plants, like elm trees. :-)

Or maybe even a mudwall, like they have in spots up and down the west coast: Big vertical iron plates, like corrugated steel magnified by 100.

And drainage.

Or a big tarp. ;-)

But to measure the movement of the ground, you'll have to go to those earthquake and/or volcano people, at least to get a baseline, and a better idea of what's already available off-the-shelf.

Good Luck! Rich

I found one of their articles on the web. It describes such uses as bridge deformation, and mm accuracy, in the 1997 epoch.

Got that.

Working on that.

Problem is that the natural flow of water goes into the houses. The city should not have approved the houses in the first place. They spent a million to redirect it sideway, which is a waste of money in my opinion. We need to pump and pipe the water out.

It's not really ground movement. More like mud floating down hill.

We know what's available. We just need fundings before the disaster.

Talk to the insurance companies. :-)

Good Luck! Rich

Who's insurance?

The builder's insurance already settled for a million ten years ago. The home owners' insurance companies wouldn't cover flood damages, or act of God, in general.

I guess we can go after the city's insurance (the tax payers) for approving such project in the first place.

For what is worth, here is a picture of the site.

As a jury, would you find the city liable?

can we see it on google earth?