Viavi 87339, Руководство по эксплуатации

Страница: 148 / 198

Principals of Operation
6 -31
SNC
Many GPS receivers indicate signal strengths in manufacturer specific units, which are
determined from measurements made on the signals by the signal processing hardware.
The values are the result of integrating the output of a signal cor relator, that is fed the
noisy input signal and a clean local replica of the expected PRN code. The integrated
result is a linear indication of the signal-to-noise-ratio, over the bandwidth of the
correlated signals.
In any particular receiver, this result can vary due to differences in receiver bandwidth
and integration time. Often the result is scaled to be consistent across a product range.
The resultant values are often referred to as SNC (Signal-to-Noise-Counts) and are
scaled to match a measurement made over a 1 kHz bandwidth. The 1 kHz comes from the
fact that many of the early receivers integrated for 1 millisecond, resulting in an effective
1 kHz bandwidth.
Converting SNC to SNR
Normally SNR (Signal to Noise Ratio) is expressed as a power ratio on a logarithmic
scale instead of an amplitude ratio on a linear scale.
To convert:
•
SNC in a 1 kHz bandwidth = (sA/nA).
•
Where sA = Signal Amplitude and nA is the Noise Amplitude.
•
SNR in a 1 kHz bandwidth [in dB] = 20*Log10(SNC) - 3db
Converting to C/N
0
A more technically precise and common measurement of GNSS signal strength is known
as C/N
0
(carrier-to-noise density) and is the ratio of received carrier (i.e., signal) power
to noise density.
Many receivers have the ability to display or output values in these units however, these
values are not measured directly, but are calculated from the directly measured SNC
count values, refer to table 6-4.
NOTE:
C/N
0
is not the same as SNR (signal-to-noise ratio), although the terms are sometimes
used interchangeably. Effectively, C/N
0
assumes that the noise has infinite bandwidth
(and thus power) and therefore characterizes it using a density, that is, as the amount of
noise power per unit of bandwidth.