Clarification about the term "GPS Shutdown"

  • Thread starter Thread starter Sam Wormley
  • Start date Start date
Which brings up the question of exactly how shutting off GPS will deter
terrorists. Precision is completely unnecessary. A missile with
inertial guidance is more than accurate enough for that. We aren't
talking about needing to put a warhead through a particular window. All
you need is to be able to target a city. Close counts in horseshoes,
nukes, and terrorist attacks. Chemical and biological weapons have even
less need for precision.

If shutting off GPS would stop an attack, then I have no objections. But
I fail to see how it would have any effect at all, other than to increase
the disruptions, making the attack even more effective.
Click to expand...

Osama bin Laden recently said that he'd bankrupted the Soviet
Union, and now he's going to bankrupt the USA.

Looks like it's working.

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To maintain RNP 10 tracks over the oceans, an INS needs an
update every six hours.
Currently, the only way to do that is with GPS.
Click to expand...

THAT IS NOT THE ONLY WAY. RPN 10 approval requires an airline to demonstrate
it's equipment meets certian accuracy requirements. Meeting those requirements
allows 6.2 hours of flight without position updates. Several means of updating
or increasing INS/IRS accuracy are mentioned, GPS is one but not the ONLY one.
See: http://www.faa.gov/ats/ato/rnp.htm
Quote from FAA order 8400.12A --- .
Effect of En route Updates. Operators may extend their RNP-10 navigation
capability time by updating. Approvals for various updating procedures are
based upon the baseline for which they have been approved minus the time
factors shown below:

(1) Automatic updating using DME/DME = Baseline minus 0.3 hours (e.g., an
aircraft that has been approved for 6.2 hours can gain 5.9 hours following an
automatic DME/DME update).

(2) Automatic updating using DME/VOR = Baseline minus 0.5 hours.

(3) Manual updating using a method similar to that contained in Appendix 7 or
approved by AFS-400 = Baseline minus one hour.[/QUOTE]

All of these require that the aircraft be within range of a
VOR/DME.
That means it cannot fly directly across an ocean - it has to
detour to get within range of such a ground installation.
GPS is listed as a SUPPLEMENTAL navigation system by the FAA - aircraft can
still navigate without it. Maybe someday GPS will be required on aircraft, but
not now.
Click to expand...

That's true.

And that day is rapidly approaching.
 
Terrorists don't have to depend on anything other than their will to terrorize.
One of the threat items is, OTOH, how they would exploit the advantages and
benefits of GPS. They are no different than anyone in their desire for
reliable, cheap solutions to problems that further their cause.

Stop evading the question, Mx, what is currently so GPS dependant that it would
fail entirely if GPS were out for a brief period (24 - 72
Click to expand...
hours).

You're going to predict a terrorist attack within 72 hours?

If you can do that, I'm sure that the US Justice Department
would like to talk to you.

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RE: > Regional/very localized shutdown! How are you going do that
techwize?

I don't know how they do it. But I know they have done it.

A large yacht under charter to the US Gov. crossed a line of longitude
on it's way into a hostile zone. As soon as it crossed the line of
longitude all three onboard civilian GPS units went blank. Several
days later, while exiting along a similar route, all three civilian GPS
units sprang back to life. I don't know how they do it. I have
pondered this. At first the math to shut down a small region with
clearly defined edges seemed a problem.

At first I thought it was possibly a jammer. Terrestrial or low
altitude would give sort of a fuzzy circle. I even thaught of a
targeted jammer based on a doppler or fazed array system. I don't like
this solution because terrain would be a problem. It's more likely
that it's an the orbiting systems.

In WWII the British built the first radio navigation system based on
the interference between two radio sources. This was the first
hyperbolic radio navigation. That morphed into to be Deca's DELRAC.
Later in the US a copy cat system called OMEGA was developed. They
worked by creating a doppler effect between two signals. The original
signal was not masked but the user figured out location by the
interference or masking of on against the other.

If such a system can be used to figure out location, then, used in
reverse, you could effectively interfere with the reception along a
hyperbolic line by skipping some of the transmition.

Since GPS uses the difference in the signals to fix location we know
that the GPS satellites transmit very accurately clocked of
transmissions. Therefor, with a lot of 3D spherical, and radio
propagation math, one should be able to figure out what bit of the
original signal to skip to selectively give a problem for some users
but not all. The location of the affected users will be defined by a
curve along the lost interference from the two satellites.

You interfere with a single location by transmitting most but not all
of each satellites signal.

Lets assume that GPS use three basic bits of information to work:
Satellite identifier, location and a clocked signal. The comparisons
of variations in the clocked signal identifies the difference in how
far a way two different birds are. The GPS receiver knows that it must
be on one of the hyperboloids of intersection. Projected on to a
mathematical sphere and you get a curved LOP. Now using several birds
you get several hyperboloids, and the intersection of each gives you a
3D fix in space, independent of the theoretical earth.

Your ability to build each hyperboloid depends on your ability to hear
both birds at once. If both birds had a hole in the transition, a
short missing bit, it would create a dark spot. Just the locations
where the two missing signals overlapped would be a problem. The shape
of the problem would be curved lines running along the hyperboloids.
Other GPS receivers located along different hyperboloids would still
get the signal, but users on the masked band of hyperboloids would be
in the dark.

It's a bit more complicated than that. You need multiple dark bands
caused by missing bits from all the birds in the sky. And they all
need to be synced to blackout the target.

Each bird would calculate it's own dark bands based on the location, or
locations, to be blacked out. They would need some sort of
synchronization algorithm.

The blackout hyperboloids's would be curved, and so there would be
other places along the LOP that also get blacked out. The simplicity
of the system is that other locations on the hyperboloids would be
covered by other satellites. Only at the targeted 'zone' would all the
blackout hyperboloids intersect, effectively providing targeted
blackout.

For operational use you need to blackout more than one point on earth.
Otherwise the receiver would just move off a little in one direction
and be back in business. Each blackout hyperboloids would need to be
expanded to a blackout band. The longer the missing bit of transition
the wider the hyperboloids, and therefor a wider band for a foot print.
Footprints would be shaped according to bird elevation and
orientation.

With enough computing power multiple narrow bands could be combined to
create a complex footprint with edges that approximate operational
needs. At first I thought the math would be horrible but then realized
it's the same math that the chip in your GPS uses to figure out its
HOPs.

Because the satellite keeps moving the math for the blackout
hyperboloids would have to be done in real time. Based on second hand
reports from the field I suspect the function has been operational for
at least ten years. Therefor I suspect the function is built in to
the satellites OS.
 
Pieter said:
The only unique function a
GPS can perform is telling you where you are now
Click to expand...

That's usually far more useful than knowing the approximate direction of
the magnetic poles.
 
Ed said:
In other words you don't know the facts and are just making up facts
as you go along.
Click to expand...

Other words are not necessary. If you put three balls on a pool table
and set them in motion, they aren't likely to hit each other very often;
if you do the same thing with 200 balls, they'll hit each other
regularly. Thus, precise navigation becomes more and more necessary as
the density of traffic rises. Simple math.
 
JM_Runs said:
Your ability to build each hyperboloid depends on your ability to hear
both birds at once. If both birds had a hole in the transition, a
short missing bit, it would create a dark spot. Just the locations
where the two missing signals overlapped would be a problem.
Click to expand...

I'm not sure what you mean by this.

The satellites transmit precise time of day and ephemeris information
saying exactly where they are at specific times. Receivers note the
positions of the satellites at the specific times when they broadcast a
message, then observe the delay in receiving the message and use this to
calculate the receiver's distance from each satellite. With a few
satellites, it's straightforward to locate the receiver in
three-dimensional space.

So how would you change these transmissions to leave certain specific
solutions "dark" in practice?

You could turn off the transmissions completely, or transmit false data.
So far, so good. The problem is that, at any given point on the Earth's
surface, there are typically half a dozen satellites visible, and often
a dozen. Each satellite has a footprint that nearly covers a
hemisphere. It would be very difficult to change transmissions in a
pattern that would make all satellites visible to a certain region look
different from the way they look to the rest of the world. And it would
incidentally be almost impossible to keep secret as well.

Now, if you had special transmitters on the satellites that could cover
specific areas on the planet, the situation would change. But I don't
know if that's the case now. I don't see how it could be done with
single transmitters for the entire footprints of the satellites.
It's a bit more complicated than that. You need multiple dark bands
caused by missing bits from all the birds in the sky. And they all
need to be synced to blackout the target.
Click to expand...

I'm not clear on how the transmissions would be changed to produce dark
bands only in certain locations. Remember, nearly half the
constellation is visible from many points on the planet at any given
time.
Because the satellite keeps moving the math for the blackout
hyperboloids would have to be done in real time. Based on second hand
reports from the field I suspect the function has been operational for
at least ten years. Therefor I suspect the function is built in to
the satellites OS.
Click to expand...

A sophisticated denial system could be built into the satellites. But
without highly directional transmitting antennas, I'm not sure I see how
it could be done. Of course, nothing prevents the installation of
exactly these types of antennas on the satellites to facilitate the
task.

Of course, Galileo would incorporate exactly the same thing. But it's
hard to say who would be controlling the off switch in the European
system.
 
Mxsmanic said:
...Of course, Galileo would incorporate exactly the same thing. But it's
hard to say who would be controlling the off switch in the European
system.
Click to expand...

The Germans would engineer the off switch, the Italians would build it, and the
French would claim to control the action.

---Bob Gross---
 
Mark McIntyre said:
So far here he's merely providing light amusement. It'd be more fun if
he knew anything at all about navigation. Like, for instance, the
points of the compass, the shape of the world etc....
Click to expand...

We could answer his point of "Which hemisphere are you on" with the
classic "In the southern hemisphere everything is upside down" ;-)

Juergen Nieveler
 
Mxsmanic said:
That's usually far more useful than knowing the approximate direction
of the magnetic poles.
Click to expand...

A gyroscopic compass doesn't show you the magnetic north, it shows
true north. Airplanes tend to carry at least one of those nowadays...

Juergen Nieveler
 
help.

Which is why they have more than one backup...


For the ATC, another radar. Or if everything breaks down, they might
call the Pentagon and ask them to launch an AWACS bird if they're
really desperate.
Click to expand...

There is no radar coverage over an ocean.
 
To maintain RNP 10 tracks over the oceans, an INS needs an
update every six hours.
Currently, the only way to do that is with GPS.
Click to expand...

THAT IS NOT THE ONLY WAY. RPN 10 approval requires an airline to demonstrate
it's equipment meets certian accuracy requirements. Meeting those requirements
allows 6.2 hours of flight without position updates. Several means of updating
or increasing INS/IRS accuracy are mentioned, GPS is one but not the ONLY one.
See: http://www.faa.gov/ats/ato/rnp.htm
Quote from FAA order 8400.12A --- .
Effect of En route Updates. Operators may extend their RNP-10 navigation
capability time by updating. Approvals for various updating procedures are
based upon the baseline for which they have been approved minus the time
factors shown below:

(1) Automatic updating using DME/DME = Baseline minus 0.3 hours (e.g., an
aircraft that has been approved for 6.2 hours can gain 5.9 hours following an
automatic DME/DME update).

(2) Automatic updating using DME/VOR = Baseline minus 0.5 hours.

(3) Manual updating using a method similar to that contained in Appendix 7 or
approved by AFS-400 = Baseline minus one hour.[/QUOTE]

All of these require that the aircraft be within range of a
VOR/DME.
That means it cannot fly directly across an ocean - it has to
detour to get within range of such a ground installation.
GPS is listed as a SUPPLEMENTAL navigation system by the FAA - aircraft can
still navigate without it. Maybe someday GPS will be required on aircraft, but
not now.
Click to expand...

That's true.

And that day is rapidly approaching.
 
There is no radar coverage over an ocean.
Click to expand...

Except when AWACS covers it, of course. However, there's no NEED for
radar coverage over oceans, flight plans are calculated to make sure
there's no spacing problem.

When was the last time you heard about mid-air collisions over the
Atlantic?

Juergen Nieveler
 
Robertwgross said:
The Germans would engineer the off switch, the Italians would
build it, and the French would claim to control the action.
Click to expand...

It doesn't matter who controls the off switch - if it's built
by the Italians, it will not work most of the time anyway. ;o)

Greets, Dirk
 

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