|
From: | Kyeong Su Shin |
Subject: | Re: Galileo Frequency Shifting and Filtering |
Date: | Thu, 2 Mar 2023 04:58:06 +0000 |
Hello all:
Marcus is obviously right (thank you for pointing out the issues); I was actually thinking about the amplifier(s) in the active antenna, but I forgot to clearify that in my e-mail.
Adding additional amplification stages after that does not help much (assuming that the receivers already have necessary amplification stages).
Regards,
Kyeong Su Shin
보낸 사람: Marcus D. Leech <patchvonbraun@gmail.com> 대신 discuss-gnuradio-bounces+ksshin=postech.ac.kr@gnu.org <discuss-gnuradio-bounces+ksshin=postech.ac.kr@gnu.org>
보낸 날짜: 2023년 3월 2일 목요일 오전 4:08 받는 사람: discuss-gnuradio@gnu.org <discuss-gnuradio@gnu.org> 제목: Re: Galileo Frequency Shifting and Filtering On 01/03/2023 13:01, Kyeong Su Shin wrote:
I'll point out that: (A) Most "active" GPS/GNSS antennas have an LNA, and the antenna structure is a printed-on-high-Er-ceramic patch antenna. Such antennas are quite narrow-band, so, there's already filtering ahead of the pre-amp. Such antennas are also, unfortunately, lossy, even within the pass-band. If you care about noise figure, then your receiver noise-figure doesn't really matter as much as the LNA that physics and coaxial cable dictates you have right up at the antenna. Unless your receiver is literally dangling off the antenna outdoors, its noise figure (as long as it isn't totally horrific) doesn't actually matter that much because you'll have an LNA right up at the antenna. Active GPS/GNSS antennas satisfy this requirement already to a great extent. The problem is that in most environments (except perhaps rural and middle-of-nowhere), you *need* some filtering ahead of the LNA, which necessarily eats into the noise figure of the LNA. (See, as you say, Friis noise chain analysis). The only situation in which you'd see the signal (as an extended increase in the apparent noise floor) is when your front-end noise figure is really really low, and perhaps you're pointing at only a single GPS satellite. The "trick" in GPS and similar spread-spectrum systems is that if you know the spreading code and offset, you can "despread" the signal, vastly increasing the effective SNR on the underlying BPSK signal and proceed from there. But the signal architecture was *deliberately* designed so that you couldn't "see" it with ordinary equipment if you didn't know the spreading code.
|
[Prev in Thread] | Current Thread | [Next in Thread] |