Re: [gpsd-users] Acquiring a Navisys GR[67]01W (was: GR-301W)

[Gerry Creager - NOAA Affiliate]

On Wed, Nov 19, 2014 at 2:05 PM, Gary E. Miller <address@hidden> wrote:

Yo Thomas!

On Wed, 19 Nov 2014 20:56:19 +0100
"Thomas P." <address@hidden> wrote:

> GPS is a very weak signal and getting enough signal for any receiver
> through a wall is quite unlikely. So if one works, this is just on
> the edge and you have very thin wooden walls or roof.

My experience says otherwise.  I find the uBlox6 and SiRF III have no
trouble in most buildings I am in.  And not on the edge, quite strong.
I do agree that steel and/or tall building are very hard on a GPS
signal, but that is not something I often see.

GPS is expected to present a very weak signal (on the order of -124 to -144 dBm) at the surface, using a frequency hopping algorithm which imparts ~10 dB of processing gain. GPS receiver systems (receiver, any needed preamplification and antennas) are designed to receive these weak signals. Some do better than others. A good weak-signal system may well see enough coincident RF to get a signal and lock, but is likely to be subject to multipath error overall. 

 

> But I would not expect, 1dB more or less sensitivity such a chipset
> version has more or less would make a difference. (And newer chipsets
> typically have better sensitivity than olders)

My experience says otherwise.  Many of the newest GPS chips have
emphasized power saings over receiver sensitivity.  Way more than dB.
For example, I find the SiRF IV to perform very poorly compared to the
SiRF III.

A couple of dB does make a fair bit of difference, as does the system noise figure. I'm reminded of the old Trimble 4000-series geodetic receivers and their preamplified antennas. If you had a couple of those antennas powered up and in immediate proximity, you could set up an interference pattern that'd effectively block both receivers at GPS frequencies. They had preamps with gain figures >40 dB, a badly designed bandpass circuit and a tendency to reradiate.

Similarly, I'm aware of at least a couple of projects that are using incident GPS radiation for passive radar mapping and are designing their sensitivities down in the -160 dBm range. Or lower. I suspect those will hear just fine indoors. 

---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588


RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

--

Gerry Creager

NSSL/CIMMS

405.325.6371

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“Big whorls have little whorls,

That feed on their velocity; 

And little whorls have lesser whorls, 

And so on to viscosity.” 

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