What exactly does C/No (dBHz) mean in u-Blox GPS data?

Question

In this related question I show data collected from a 6.5 hour run from NMEA sentences from a u-BLOX NEO-6 GPS module (datasheet, protocol spec) with an active external antenna via Raspberry Pi.

I'm looking at the value called C/No. (dBHz) which is reported together with altitude and azimuth for each GPS satellite used in a solution.

If I estimate the bandwidth of a given GPS signal to be about 10 MHz and my data scattered around 25 dBHz, that puts "dB" at -45. That's way too large to be "dBm" of the raw signal at the antenna, though I'm using an external active antenna, so "dBm" is plausible for the input to the module.

So far I haven't found a definition for C/No but it is mentioned in Section 7.4 of u-BLOX's Application Note RF Design Considerations for u-blox GPS Receivers

7.4 Sensitivity test

Check the C/No values in the $GPGSV or the UBX-NAV-SVINFO messages. Under open sky a good design should reach up to 50 dBHz for the strongest signals. If it reaches 45dBHz it can still be acceptable but the source of the reduction should be investigated (e.g. small antenna, ...).

Designs with maximal signal strengths below 40dBHz usually provide degraded performance (long TTFF times, lower coverage, accuracy, dynamic).

Question: What exactly does C/No (dBHz) mean in u-Blox GPS data? How is it defined?

update: This document gives a partial description, but I'm not confident that I understand it exactly. NEP would be power per square root frequency, but is the ratio total carrier power divided by NEP x 1 sqrt(Hz)?

5. Signal Loss and C/No

In recent years, clever techniques have been developed to extract tiny GNSS signals from the background noise. But, the fundamental limitation to what can be achieved is limited by the ratio of the gain of the antenna element to the total receiver noise, referred to the input, or “G/T”. This is an absolute indicator primarily of antenna-plus-front-end performance, and determines the ultimate value of C/No for a given signal level. C/No is the ratio of carrier power to the noise power mixed with the signal, in a 1Hz bandwidth. This ultimately defines a limit for the GPS receiver sensitivity. So, simply put, antenna gain should be maximized (the “G”), and LNA noise figure minimized (“1/T”); a complicated way to state the obvious.

If the C/No ratio is diminished by any cause, be it bandwidth limitations or increased LNA noise figure, GNSS sensitivity will be reduced by the same amount. Once impaired, there is no way to recover C/No for a given receiver. Even additional gain does nothing because C and No are amplified equally, and so is to no avail.

Answer 1

Looks like a form of Noise spectral density. In fact there's another Wikipedia article for this: Carrier-to-noise density ratio

C/No is the ratio of carrier power to the noise power mixed with the signal, in a 1Hz bandwidth.

Basically, it's the ratio between the amount of signal power (practically measured as the carrier power with the obvious unit of watts) and the amount of noise power density (unit of watts per hertz) you receive into the receiver. Since it's in a 1Hz bandwidth, you can express the unit as dBHz (dB is the quasi unit for ratios of powers, per hertz of measured bandwidth)

This is just a figure to indicate how noisy the incoming signal is.

Answer 2

This is to confirm @hatsunearu's answer and to add a little more math.

According to Wikipedia's Carrier-to-noise-density ratio:

In satellite communications, carrier-to-noise-density ratio (C/N₀) is the ratio of the carrier power C to the noise power density N₀, expressed in dB-Hz. When considering only the receiver as a source of noise, it is called carrier-to-receiver-noise-density ratio.

It determines whether a receiver can lock on to the carrier and if the information encoded in the signal can be retrieved, given the amount of noise present in the received signal. The carrier-to-receiver noise density ratio is usually expressed in dBHz.

The noise power density, N₀=kT, is the receiver noise power per hertz, which can be written in terms of the Boltzmann constant k (in joules per kelvin) and the noise temperature T (in kelvins).

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So if the Noise temperature of your front end were 400K and the Boltzmann constant \$k_B\$ is about 1.381E-23 Joule/K then your noise per Hz N₀ would be about -201.6 dBW or -171.6 dBm. If the carrier power C were -140 dBm, then your C/N₀ would be 31.6 dB.

Carrier-to-noise-density ratio is different than Carrier-to-noise ratio .