Thank you guys for your comments and suggestions. As per your request I contacted the expert, here is his lengthy but informative reply:


EDWARD:

Background -
Salt River Project is the 3rd larger municipal level power provider in the
USA (behind the New York Power Authority and the Los Angeles Power and Water
Department.) We serve metropolitan Phoenix Arizona. (http://www.srpnet.com/)

As you can imagine - we have had experiences with GPS in an around
HIGH-voltage power transmission lines!!


In Response to your Inquiry:

The potential results are very PROBLEMATICAL, dependent on how close the GPS
receiver unit is in proximity of these power lines and their supporting
structures (the towers.)

Obviously farther away reduces the aggravating circumstances.

First, the degradation that you might (probably will) experience is very
dependent on the VOLTAGE and the number of lines in question (single, double
or higher number of circuits associated with the particular transmission
system - some have combinations of voltages, such as 230-kV coupled with a
115-kV and 69-kV under-built.)

Some larger systems are the multi-circuited 500kV Tx-lines (common in and
around the larger metroplexes in the UsofA), and now we are seeing 735 -
765kV and there is one 1-Mv system in upstate NY crossing into Canada.

The tremendous amount energy and field force associated with the EMF
(Electromagnetic Force) of the larger Voltages, coupled with multi-circuit
lines, obviously will seriously degrade and interfere with the (already very
weak) attenuated GPS signals.

Another concern would be to consider just how close you are going to be to
the actual lines themselves.
If you are contemplating working directly under a major high voltage line,
PLEASE RETHINK THAT STRATEGY.

The EMF will not only degrade and probably corrupt the GPS signal and data,
it may very well have serious deleterious effects on the on-board
electronics of both the receiver unit and associated data-collectors.

We would also suggest avoiding the use of a 'conventional' survey
total-station + data-collector directly underneath of the higher voltage
power systems.
Dependent on weather conditions, in a very dry atmospheric state, painful
static electric shocks between equipment with metal housings and the human
operators are not only possible but probable.

ANY type of work directly under the very high voltage systems is a potential
safety risk, and should not be considered lightly.


Second, - again dependent on the type of transmission system in question,
there may be multiple different types of support structures (i.e. towers).
Operating GPS receiver antennas around, near, (and especially under) the
lattice type of tower (classic 'Erector Set' type of construction) will net
you nasty and insidious GPS multi-path problems of a surprising amount, to
the point were the GPS receiver system will often not operate because of
massive amounts of conflicting data.


Having said all of this - there are situations (time constraints, costs,
etc.) where some GPS surveyors would attempt to work near, in, around or
even under a high voltage system.
If someone chooses to do so, then they need to set up some type of system of
redundant re-measurements to add a quality assurance verification that the
data captured is viable and without inherent problems that are hidden within
the data sets.
This probably means a second (or even third)set of measurements to change
satellite geometries to check for multi-path problems and to verify
repeatability.

We can not condone this approach for obviou

Edward,

Thanks for sharing that very important information. That's another one to print out and save.

J.D.

Edward,

Thanks for posting this. I figured Gregg would be a good info source on the topic.

On the safety issue, I always found it interesting that SRP's survey crews are IBEW members. They get quite a bit of safety training, and have MUCH more experience working around high voltage lines than the rest of us.
Modified By Brian D. Ewing, PLS on 8/8/2001 at 9:44 AM