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  SATELLITES IN VIEW--What the Data Means

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Author Topic:   SATELLITES IN VIEW--What the Data Means
jimh posted 04-15-2015 10:58 AM ET (US)   Profile for jimh   Send Email to jimh  
Many global navigation satellite system (GNSS) receivers can show information about the satellites they are receiving and using in their position solution calculation. This is nominally called Satellites In View. The data is usually presented in some graphical manner, with bar graphs rising from a baseline. The height of the bar graph is understood to be representative of the strength of the signal from the particular satellite. The satellites are usually identified by their pseudo-random noise codes (PRN). The PRN is an index to the sequence of pseudo-random noise modulating codes used in the carrier division multiple access (CDMA) scheme. A typical presentation looks like this:


Satellites in view display from Lowrance HDS-8

Most users of a GNSS receiver with some background in the concept recognize the polar graph of satellite azimuth and elevation. More sophisticated user probably are aware of the metric for Horizontal Dilution of Precision. But note that in the table on the upper right, data is given for "SNR" or signal-to-noise ratio. The SNR data is our topic in this discussion.

Signal-to-noise ratio is a figure of merit that describes the ratio, usually expressed in decibels, of the desired signal and the undesired noise accompanying it. In the above screen capture of a GNSS receiver status, the figure "32" is probably interpreted by most people as being indicative of a SNR of 32-dB. This is probably a reasonable assumption as the legend for the data suggests it is a signal-to-noise ratio, which would typically be reported in units of the decibel, however it is very likely to be a completely erroneous assumption. The presentation of the data in this way is misleading, and for that we may have to thank the consumer GPS receiver industry.

It is more likely that the data reported by the GNSS receiver is actually the figure of merit called Carrier-to-Noise density (usually written as C/No). There is an excellent and comprehensive, albeit rather technical, discussion of the difference between SNR and C/No. See

http://www.insidegnss.com/auto/novdec10-Solutions.pdf

An executive summary of that highly technical article is much simpler to understand: the actual SNR of the GNSS signals is not anything like 32-dB. It is actually a negative number, that is, the desired signals are actually buried in the noise, often more than 20-dB weaker than the noise (or less than one one-hundredth of the power of the noise).

In the cited article, an example is provided where the C/No is given and the SNR is derived. For a C/No in the range of 37, the SNR computes to be about -29 dB. Applying this same sort of transformation to the signals shown in the screen capture, we find that the actual SNR of the signals is nothing like 32-dB, but more like a negative number, perhaps smaller than -30 dB.

Normally any signal that was buried into noise 29 dB stronger would be unusable. This is another fabulous aspect of the engineering and signal theory used in GNSS. The complex digital modulation allows for a substantial gain in SNR during the demodulation, called correlator gain, which has the effect of hugely improving the SNR. It is only because of correlator gain that GNSS signals can even be used.

A brief mention of correlator gain is provided by other authors. The general concept of correlator gain can be understood through the recongition that most electrical or radio noise is random in nature, but the modulation patterns of the GNSS signals are not. By using complex digital filtering or correlation, a receiver can discover the non-random signals in the presence of the random noise, even when the desired signals are much weaker. This is another wonderful facet of the design of GNSS systems. Were it not for this property of correlation gain, the signal levels transmitted by the satellites would need to be much greater, which would require much more power in the satellite to generate.

jimh posted 05-09-2015 09:02 AM ET (US)     Profile for jimh  Send Email to jimh     
Perhaps the reason for presenting the signal strength metric in terms of Carrier-to-Noise density (C/No) instead of in a more common signal-to-noise ratio (SNR) is to make interpretation of the numbers simpler. Signal strength in C/No units is a positive number, and the higher the number the stronger the signal. This is simple to understand for a non-sophisticated user.

If the signal strength were presented as a SNR, the numbers would be negative numbers, and, ignoring the sign of the number, a smaller number would be a stronger signal. That may be counter-intuitive for the unsophisticated user.

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