Mike

Instead of relying on the dubious claims of those marketing an extremely
inefficient jammer it would be better to actually do some simple
calculations.

Typical commercial receivers stop tracking with a Jam to signal ratio of
not more than 60dB or so:
http://www.gpsworld.com/gps/jamming-gps-778

The input signal level at the receiver input is around -160dBW.

A 1W ERP source with an isotropic hemispheric radiation pattern will
exceed the the required jamming signal strength for distances less than
several tens of kilometers.

This estimate is consistent with the fact that LO parasitic radiation
from TV systems on boats have been known to jam GPS for distances of
several kilometers.

Bruce

Mike Monett wrote:

Even 10 KM is pretty useful. If the thing were solar powered with a
supercap "battery" it could easly transmit for say 2 minutes per hour w/
significant power. It'd be hard to find if the on times were generated by
a multiple fedback CMOS shift register.

-John

================

The commercial jammers referred to in an earlier post advertise 10 to 45m or
so range, with significant power levels and battery life measured in a few
hours. Considering that these devices are illegal to begin with, I have to
assume that these figures are probably optimistic (optimistic advertisement
is probably the least of their concern.)

If I were a pilot, I would probably be more worried about the kid playing
with his Nintendo in 15A (or his father trying to retrieve his email with
his GSM smart phone) during approach than a jammer on the ground.

Didier

Didier

As jammers those devices are extremely inefficient.
They may well rely on the inefficient generation and radiation of a very
high order harmonic of the clock of an unshielded legal device.

A commercial GPS receiver may require a signal as small as 60dB (depends
on the operating mode, and receiver design) above the GPS signal at the
receiver input.
An ERP of a few microwatts should suffice to achieve the claimed range.

Bruce

Didier Juges wrote:

Bruce,

I agree with your calculation and conclusion, as far as commercial consumer
GPS receivers are concerned.

A data sheet that was linked in a previous post for an aviation-grade
commercial GPS receiver indicated resistance to signals -30dBm at the
receiver input. That is quite considerable, and much better than the
hand-held consumer units (by 30dB?)

I would expect planes and other potential "high value targets" to have
receivers of similar performance.

I don't disagree that it would be fairly easy to disrupt the consumer
devices, but other than a few missed appointments and frustrated gadget
freaks, and the occasional emergency vehicle not finding its way to the
scene of an accident, that would be more of a problem) I am not too worried
about the consequences of that.

The thread started with the loss of the LORAN system, and nobody (maybe I am
going out on a limb here) ever used a LORAN receiver in his car to find the
nearest restaurant :)

I think the people who should complain the most about the loss of LORAN are
the boaters, but they are the one who embrassed GPS the first and are it's
biggest advocates!!! I know, I live on the coast of Florida.

Didier

My first contact with LORAN was at the bankrupcy of a local company that
supplied tracking and location services for trucking companies. The trucks
had a receiver that sent data back to home base via two-way radio and home
pbase computed positions. I was only nteresteed in buying their mimis.

In the early 80s I was playing around with uP (8085) based LORAN receivers
(Made by Appelco/Raytheon). One version read out time differences and a
more advanced (w/ 2x 8085s) version read out Lat & Long directly. Either
would pinpoint my location to 100' class. It was accurate navigation in a
two-way radio sized box.

There were, of course, no map or chart displays. Imagine what 20+ years of
development would have brought.

-John

=============

On Sun, Nov 15, 2009 at 08:56:36PM -0600, Didier Juges wrote:

GPS has been widely used for time and frequency sync in radio

systems of various sorts - some depend on it for successful simulcast
from multiple transmitters (including many public safety radio systems)
and while there is SUPPOSED to be significant holdover, there is are
always those situations where nobody has EVER checked that it works...
And what will holdover do about LONG TERM outages of GPS ?  Perhaps
folks are willing to bet the whole infrastructure on the supposition that
we could NEVER see a complete long term loss of signal (using civilian
receivers with modest anti jam abilities) in a particular location or
region - I am not exactly sure what alternative might exist though I
guess there are some for time and frequency - terrestrial GPS beacons
from high points that could lock up a cell system in a region come to
mind...

And more and more and more stuff depends on more or less

ignorant and sometimes not very bright operators expecting that the GPS
positions are always there or they don't know what to do.  And if GPS
is almost always reliably there there may well be little or no practical
training about what to do if it fails.  Failure modes and paths in code
and procedures (and sometimes even actual hardware) which aren't often
tested or exercised usually fail when actually needed and often in
entirely unanticipated ways - maybe the backup ALSO depends on GPS in
some way the system designers never thought about (or knew could
happen)...

LORAN C represents a viable (albeit not often deployed) backup

to time and frequency control and could be implemented in modern
hardware as a backup location service at reasonably low cost for those
applications where that is important enough.

Actually readers of this list have, though not in recent times I guess.

An enhanced LORAN that has some of the accuracy and automation

of a GPS receiver exists... I suspect boaters care most about
convenience (and accuracy) and found GPS easier.  Certainly so compared
to earlier LORAN C gear.

And of course there is another issue with GPS, it is controlled

by the DOD and is supposed to have the ability to deny service in a
region if conditions are sufficiently apocalyptic to require it.  For
folks worried about this (more outside than inside the US obviously)
LORAN is a local resource they may control and certainly can if they
make a more or less modest investment in the ability to do so.  Some
(large) players are of course planning their own space based systems for
just this reason but LORAN can be implemented by most nation states...

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

On Sun, Nov 15, 2009 at 07:17:37PM -0800, J. Forster wrote:

E Loran could supply the same basic position information as input

to charting and mapping software as GPS does... most of the code doesn't
care where the position came from one wit....

But I am sure you know that (we are in violant agreement).

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

In fact, GPS has performed so well, it has become "part of the furniture"
and it is really hard now to assess the full impact its loss would have.

Another overlooked aspect would be the perceived impact of a number of
failures. Just look at the hoohaws over lead in toys, defective cribs,
tainted beef, tainted pet food, flamable kids sleepware, trace chemicals
in bottles and many other things.

LORAN is cheap insurance, IMO.

A modern dual receiver could compare LORAN and GPS positions to provide
very high confidence levels at low incremental cost.

-John

=================

I would be very surprised if "aviation-grade" receivers are better than
your new average handheld Garmin.

I would expect the opposite, since important receivers - say for timing of
power generation, timing of bankingsystems, etc typically runs forever
often beeing 5 to ten years old before uppgrading.

--

Björn

On Sun, Nov 15, 2009 at 10:33:07PM -0500, David I. Emery wrote:

I might add that there ARE some regional solutions to the timing

and frequency backup issue (time-nuts meat) that COULD be implemented
pretty easily.

One is locking ATSC TV signals to Cs standards backed by GPS. I

am pretty sure that it would not take a lot of effort to adopt existing
ATSC Tuner chip designs and maybe the actual current technology already
available  chips themselves to recover accurate time and frequency from
a ATSC signal locked to a good standard.

And TV transmitters are LOUD compared to GPS and therefor not so

easily jammed on a wide area basis.

I don't imagine the cost of Cs locking a few TV signals is all

that high either... most of the gear can accept external frequency
references and clocks... and already does to a considerable degree.

Obviously if one needs time of day to high precision one needs

to use a local GPS to determine the time offset of a TV signal as
received at a particular site, but this should not change much provided
the clock at the transmitter was really good.  And the ATSC transport
stream provides a rich channel for sending information about time offsets
and other sync status on a real time basis...

I suppose this could substitute for Loran as a regional backup

for telecoms networks as protection against GPS denial...

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."