Can power lines interfere with satellite positioning systems (GNSS)?
GNSS operates by sending radio-frequency signals (in the microwave range, 1.2 - 1.6 GHz) from a constellation of satellites to the receiver. If the power line interfered with these signals, it could potentially cause either a reduced accuracy in the calculation of the position, or even an erroneous position. Does this happen in practice? There are two possible mechanisms to consider:
Screening of the satellite signal by the physical presence of the power line
A power line is similar to any other physical structure. We know that buildings, trees etc can sometimes stop the GNSS receiver from picking up all the satellite signals. Potentially, the same effect could occur close to a pylon. But calculations show that the conductors of a power line are too thin to have any significant effect. This has been confirmed by measurements: driving under the conductors of a power line made no measureable difference to the signal strength detected by a GPS receiver.
GPS, GLONASS, and numbers of satellites
If a tree, building, or pylon does temporarily obstruct the signal from a satellite, it should only be one or perhaps two satellites that are "behind" the obstruction at any one time and are affected.
With GPS (the American system), there may be 8 satellites above the horizon and able to be used for fixes at any one time. If the GNSS receiver needs, say, 6 satellites to achieve its full fix, as some systems aiming for higher accuracy do, losing one or two satellites might become significant.
More modern receivers often detect GLONASS (the Russian system) satellites as well. Then there may be potentially twenty or more satellites available, and you would expect the temporary loss of one or two to be much less significant.
Radio-frequency interference produced by the power line itself
High-voltage power lines can produce corona, which does cause the line to emit some radio-frequency EMFs. All power lines can sometimes have arcing on their fittings, which likewise produces radio-frequency EMFs. But again, calculations show that it would have to be really extreme even to be big enough potentially to cause problems, and in practice, tests have not detected any interference with GNSS.
Some systems for obtaining greater accuracy from GNSS use a separate signal from another satellite or from a local transmitter set up either by the user themselves or more centrally for groups of users. Again, no interference is expected, except in rare circumstances where the pylon might block the line-of-sight in the same way as a tree or building.
More details on differential GNSS
These should not be subject to interference from the power line unless the line of sight between the GNSS receiver and the satellite broadcasting the correction signal is significantly blocked. In this respect, a pylon would be no different to a tree or building - except that because it's not so solid, it's even less likely to cause interference. Because the correction signal usually comes from a geostationary satellite, if interference happened at all, it would be in a fixed direction.
Most Real Time Kinematic (RTK) systems use a ground-based correction signal, either a local base station communicating with the receiver on a local radio link, an area-based correction signal using radio or mobile phone signals, or an internet-based service. These tend not to be so constrained by line-of-sight, so no interference is expected.
Right up close to a pylon, there might be some degradation in GPS performance, just as there can be some degradation close to buildings and trees. Other than that, there is no evidence of power lines interfering with GPS.
Wherever interference comes from - trees, buildings, or even pylons - more modern units are likely to be more immune, and RTK should reduce any problem even further.
Other effects of power lines on agriculture
One use of GPS is in agriculture, sometimes referred to as "precision agriculture". See also the evidence on whether power lines have any other effects, on animals, crops etc.
Abstracts of relevant publications
Drop down this box for the abstracts
Use of Global Positioning System (GPS) receivers under power-line conductors
Silva, J.M.; Olsen, R.G.
The use of Global Positioning System (GPS) technology continues to grow and accuracy augmentations will generate ever more innovative applications. The issue of GPS use under or near electric power lines has been raised since some GPS documents have vague warnings about such use. First, GPS and the satellite microwave signals used to determine position, velocity, and time are described. Then, the potential effects of electromagnetic interference and/or signal scattering from overhead conductors are evaluated analytically and with some practical measurements under transmission lines. This work demonstrates that it is unlikely that power line conductors will interfere with use of the GPS satellite signals.
Note: the following paper refers to a system of differential GPS used in America but now increasingly superseded by more accurate systems such as RTK.
Evaluation of the potential for power line noise to degrade real time differential GPS messages broadcast at 283.5-325 kHz
The new Nationwide Differential Global Positioning System network uses the 283.5-325 kHz band to broadcast differential GPS (DGPS) correction messages. Concern has been expressed that power line corona and gap discharge noise could degrade the performance of DGPS receivers using this band. Previous work on power lines and the AM broadcast band identified corona and gap discharges as broadband noise sources in the LF/MF bands. The potential to locally degrade performance of DGPS receivers relatively close to some power facilities appears possible for certain situations. The extent of any DGPS interference problem will depend on receiver/antenna design and placement, signal strength, power line design, weather conditions, and characteristics of the noise source. Also affecting DGPS receiver performance can be the presence of any nearby nonpower line RF noise sources;such as electronic devices or equipment internal to the user's vehicle.
People (in the UK at any rate) often use "GPS" for all satellite navigation systems. Strictly, however, "GPS" - "Global Positioning System" - is the name of the American system. The Russian system is called GLONASS, with the European system Galileo still in development. The generic term for all these systems is "Global Navigation Satellite System" (GNSS).