Quote from: 3DGeek on September 02, 2017, 04:05:45 PM

The interesting thing about the FE'er's assertions that GPS is broken is that it was developed by the US military - primarily to allow cruise missiles and other long range/high-precision weapons to achieve ~2 foot precision on targets anywhere in the world.

The coordinates take you to real places if you attempt to follow them. The distance between the coordinates are based on a globe.

Quote from: 3DGeek on September 02, 2017, 04:12:50 PM

Quote from: Tom Bishop on September 01, 2017, 01:00:19 PM

Quote from: Rounder on September 01, 2017, 12:58:01 PM

This isn't like wiring a house, Tom.  You don't just bring a whole bunch of it and stop spooling it out when you make landfall, because undersea cable is very expensive.  And even if you did, undersea cables have repeaters emedded in them every so many miles to boost the signal.  The owners know how many repeaters are in the cable, which means they know how long the cable is.

Are you an owner? Do you have access to their records for us?

As usual, we have the passive-aggressive effort to sow confusion and doubt rather than draw back the curtains and open the window to allow enlightenment and debate.

The point here is that the people who lay undersea cables - and the people who pay for them to be laid - and the people who run the system by remotely querying those regularly space repeaters would all have to be bundled up into your increasing spiral of conspiracy.

So now, the big undersea cable companies are a part of the same conspiracy as NASA, SpaceX, GPS and cellphone providers, the Russian, Chinese, Indian, French, South African and (now) North Korean governments?

Is there anyone besides Tom Bishop who is NOT a part of this coverup?

Isn't this just the teensiest bit paranoid?

Where did I say anything about a conspiracy? You are making a lot of assumptions on how submarine cable layers operate and I am asking for further information to demonstrate what was claimed is true. How do we know that they didn't run out of cable at one point and learned that they needed to bring more cable for these things?

This is an interesting Website . Not sure how I wound up here, and not sure how i ended up reading this thread.

Anyway. Tom, cable laying ships are extremely precise. I did a brief stint (9 days) aboard one such vessel, MV Wave Venture (http://www.cablesm.fr/Wave%20Venture.pdf). at the time I was working as an engineering intern for a company which contracted this vessel to do some work. I spent a lot of time in the operations room as well as on the cable deck and learned about the cable laying process.

The supplies aboard the ship are precisely measured and inventoried. This is necessary as the ship is enormously expensive to operate and running out of cable or other supplies mid-tour would be disastrous.

If you go to that PDF I linked, near the bottom are photos of the two cable handling drums. Those large drums play cable in and out. Their circumference is known and their motion precisely measured. Up in the control room, there are readouts on rate of cable pay-out, tension on the cable, amount of cable played out (easily calculated from drum diameter and # rotations of drum).

If the ship moves ahead too quickly and tension rises, the cable will snap. This would be catastrophic. To this end, tension is monitored careful and ship movement must be precisely controlled. The ship uses a dynamic positioning system, based on GPS. The accuracy is around 2 meters (the 400-something ft long ship can maintain its exact position within 2 meters and a degree or two of heading using directional "azimuth thrusters" and high performance GPS receivers positioned around the ship).

The GPS positioning system agrees closely with the cable run length, measured physically using the cable drum over long distances. This experimental "proof" of GPS accuracy is performed on every cable laying run. If the ship has moved 1 mile on GPS, but an unexpected amount of cable has been played out, this would be obvious in the ship control room (would probably indicate an unexpected underwater feature and the ship would back up/pick up cable and figure out what went wrong before re-laying).  Note that the cable laying plan also takes into account underwater topology (based on oceanic surveying, done via sonar).

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On another note of interest. A lot of land surveying these days is done via LIDAR. Basically an aircraft (or for smaller areas, maybe a small UAV) flies over and a lidar sensor takes millions of point distance measurements (worked based on speed of light and reflections, does not rely on any notion of round or flat anything). The end result is a dense "point cloud" or high resolution 3D map of an area.

Here's a  little clip that shows a bit of the process and what the results look like:

In order for the measurements from the moving aircraft to be combined into a coherent 3D model, the precise location of the aircraft & LiDAR at each moment must be known. Otherwise, the map would be an enormous mess (jumble of points). There are two major methods of inferring the location of the lidar at each time step

- SLAM (simultaneous localization and mapping)
- Sensor-based (GPS + Inertial measurements)

The slam method is much more difficult and is the subject of current research. With the slam method, in each new time step, you try "sync up" your new laser scan with the last one, finding an overlapping match and thus inferring the new position of the laser.

The sensor-based method uses a combination of GPS and an IMU (accelerometers and gyros) to log the precise position and orientation of the sensor in 3D space. By combining the long-term accuracy of GPS and short-term precision of inertial methods, and using filtering techniques, centimeter-level accuracy can be achieved.

In either of these methods, if the position of the laser cannot be accurately determined in 3D space for every moment in time (lets say GPS or IMU failure or inaccuracy), the map will look like a mess with all kinds of overlapping, incoherent points. In this event, the instrumentation will have to be repaired and data will have to be re-collected.

Now, if GPS worked consistently but with a scale offset, ie measured wrong distances, there are two issues. Firstly, the sensor fusion would fail (the inertial measurement unit would disagree with the GPS measurements). Secondly, the created map would be accurate, but at the wrong scale (distances incorrect). While the former takes some knowledge of signal processing to understand, the second can easily be analyzed empirically.

You can personally download LiDAR data sets, tagged with GPS data as well as aircraft data (altitude, position, velocity, etc). To prove to yourself that these GPS data sets are of accurate scale (since you don't seem to want to trust anyone else), you could download a dataset for somewhere local to you and look at the 3D point cloud. Measure a distance in the pointcloud between two known locations (lets say measure out the distance between two buildings). Then, in real life, go out there and confirm this data empirically using whatever equipment you like (laser range finder, radar, measuring tape, whatever).

In this manner, you will have proven the following:

A) the physically measured 3D pointclouds agree with published aerial maps (ie what you would find on google earth)
B) Aircraft are able to accurately determine their location, speed, position, orientation and altitude
C) GPS is able to accurately measure distances within a tolerance of several feet of absolute non-compounding error.

The underlying assumptions here are:

- The speed of light is ~ 3*10^8 m/s (required for lidar measurements)
- Time can be accurately measured (again required for lidar measurements)
- You are capable of personally measuring distance in the order of a few hundred feet to empirically verify the data

This is real, undisputable physical data that you can empirically verify yourself.

My credentials:

Bsc. Electrical Engineering & Computer Science
BSc. Aeronautics & Astronautics
Massachusetts Institute of Technology

My graduate thesis involves navigation of autonomous vehicles and as such relies on accuracy of systems such as GPS from a small, directly measurable scale up to larger areas. We work in GPS-available and GPS-denied environments and must fuse data from inertial systems, Lidar, RADAR, GPS, cameras, etc. to have both aerial and ground vehicles navigate precisely.

Hopefully my suggested experiment gives you an avenue to prove to yourself the performance of GPS . I prove it to myself daily in my line of work. Once this business of distances is sorted, I believe the rest of the proof is clear cut using the geometry already presented by others. I should probably get back to work now.