RF Interference


RF signals are able to travel without interference to signals at other frequencies.  The problems occur when signals at the same frequency interfere with each other.  There are three types of interference classified by the how close the interfering frequency is to the signal frequency.  Having interference on the same frequency as the signal causes in band interference, while near band interference would be located close to the signal frequency.  Out of band interference is just that, out of the signal bandwidth.  Approximate ranges for these values is given in Figure 1.GPS is touted as being jam resistant, which is one of the properties associated with spread spectrum transmissions. 


Figure 1 Approximate GPS L1 Interference Band Ranges



The easiest way to jam something is to transmit on the signal frequency.  We see this happen with GPS when a receiver is around a pseudolite.  A pseudolite is essentially a ground-based transmitter that acts like a satellite, which transmits a PRN on the L1 frequency.  Since the pseudolite is on the ground and not in space, the power budget for it is different than for the other satellites.  The problem with this is leads to the near-far problem that describes the operational bubble of a pseudolite.  At any given transmitting power, there will be a point where the pseudolite signal is too weak to receive.  Conversely, there will be a distance where the pseudolite’s signal is so strong that it jams out the other satellite’s signals.  If you increase the transmitting power to increase the effective range of the pseudolite, you also increase the range with which you overpower the GPS satellites.


The next type of interference deals with near-band interference.  In this case, while the jammer frequency is not within the signal bandwidth, it is very close to it.  This could corrupt the readings because it essentially gets through the ends of the band pass filter.  If you decrease the range of the filter to mitigate some of the near-band interference effects, you run the risk of attenuating part of the signal that you are trying to measure.


One of the least efficient ways to jam something is to transmit on a different frequency.  Although every signal can be jammed by another signal, given the right amount of power, this one is less common.  Out of band interference is usually removed by placing a pre-amp filter between the antenna and the pre amp.  While this can remove this type of interference, you also lose some signal characteristics.


Possible sources of interference with GPS


The most possible source of EM interference with a GPS receiver is through harmonics.   An analogy of the creation of harmonics can be extended to musical instruments.  If you were to pluck a guitar string, it would vibrate, and the length of the string would determine its fundamental frequency.  However, the string also vibrates such that it has additional nodes located along the string.  The number of the nodes determines the number in the harmonic series, which can be seen in Figure 2.  Harmonics are frequencies that are unintentionally generated by an intended, or fundamental, frequency.  These harmonic frequencies are integer multiples of the intended frequency and can fall within the GPS L1 code frequency.


Figure 2 Description of Higher Order Harmonics



The GPS L1 signal is susceptible to second harmonics from transmitters operating in the 781-794 MHz range, 3rd harmonics in the 521-530 MHz range, and so on.  In different countries these frequencies can be allocated for different uses, but in North America, the 781-794 MHz range refers to the upper portion of television channel 66, and the lower part of channel 67.  Additionally, the 3rd harmonics of channel 23; the 10th harmonic of the marine very-high-frequency (VHF) communications channels; and the 12th and 13th harmonics of aviation communications frequencies, all fall in the GPS L1 band.  That means that with enough power, GPS can be jammed by any of these. 

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