Telit Wireless Solutions SC872-A, Руководство пользователя

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SC872-A Product User Guide Electrical Interface
1VV0301202 Rev. 6 Page 37 of 53 2018-12-21
Multipath interference results when the signal from a particular satellite is reflected from a
surface (e.g. a building or the roof of a car) and is received by the GNSS antenna either in
addition to or in place of the line of sight signal. The reflected signal has a path length that is
longer than the line of sight path and can either attenuate the original signal, or, if received in
place of the original signal, can add error in determining a solution because the distance to the
particular satellite is actually shorter than measured. It is this phenomenon (as well as the partial
sky obscuration) that makes GNSS navigation in urban canyons (narrow roads surrounded by
high rise buildings) so challenging. In general, the reflection of a GNSS signal causes its
polarization to reverse. The implications of this are covered in the next section.
GNSS Antenna Polarization
GNSS satellites all broadcast a signal that is Right Hand Circularly Polarized (RHCP).
An RHCP antenna will have 3 dB gain compared to a linearly-polarized antenna (assuming the
same antenna gain specified in dBic and dBi respectively).
An RHCP antenna is better at rejecting multipath interference than a linearly polarized antenna
because the reflected signal changes polarization to LHCP. This signal would be rejected by the
RHCP antenna, typically by 20 dB or greater.
If the multipath signal is attenuating the line of sight signal, then the RHCP antenna would show
a higher signal level than a linearly polarized antenna because the interfering signal is rejected.
However, in the case where the multipath signal is replacing the line of sight signal, such as in
an urban canyon environment, then the number of satellites in view could drop below the
minimum needed to determine a 3D position. This is a case where a bad signal may be better
than no signal. The system designer needs to understand trade-offs in their application to
determine the better choice.
RF Interference
RF Interference into the GNSS receiver tends to be the biggest problem when determining why
the system performance is not meeting expectations. As mentioned earlier, the GNSS signals
are at -130 dBm and lower. If signals higher than this are presented to the receiver, the RF front
end can be overdriven. The receiver can reject up to 12 in-band CW jamming signals, but would
still be affected by non-CW signals.
The most common source of interference is digital noise, often created by the fast rise and fall
times and high clock speeds of modern digital circuitry. For example, a popular netbook
computer uses an Atom processor clocked at 1.6 GHz. This is only 25 MHz away from the
GNSS signal, and depending upon temperature of the SAW filter, can be within its passband.
Because of the nature of the address and data lines, this would be broadband digital noise at a
relatively high level.
Such devices are required to adhere to a regulatory standard for emissions such as FCC Part
15 Subpart J Class B or CISPR 22. However, these regulatory emission levels are far higher
than the GNSS signal strength.
Shielding
Shielding the RF circuitry generally is ineffective because the interference is received by the
GNSS antenna itself, the most sensitive portion of the RF path. The antenna cannot be shielded
because then it could not receive the GNSS signals.
There are two solutions, one is to move the antenna away from the source of interference, and
the other is to shield the digital interference source to prevent it from getting to the antenna.