[aprssig] How The FCC Plans To Destroy GPS - A Simple Explanation

Stephen H. Smith wa8lmf2 at aol.com
Mon Feb 7 15:00:05 EST 2011

Reposted from the Yahoo TinyTrack group:

"Seen this in the VX-8R and TH-D7 groups here on Yahoo. Since I haven't
seen it posted in here... I figured I would (as it would have a serious
effect on APRS tracking).


Mike - N1EVH"

Quick summary of the story:   The FCC has apparently fast-track authorized, through a questionable procedure, a "4G" high-speed wireless internet service to operate on frequencies directly adjacent the the 1575 MHz GPS channel.

This system, ostensibly a satellite-based service, would be augmented by thousands of terrestrial "fill-in" transmitters that could desensitize or block GPS receivers for a radius of several miles around each site.

I have already observed this phenomenon from 1.9 GHz PCS cellular base stations.  Some GPS systems with remotely-mounted active antennas containing 15-20 dB gain preamps (to offset the coax loss) will desensitize and go out of lock when they get within a half-block or so of a PCS cell site.  (All-in-one GPS devices, such as hockey-puck or mouse-type units or car navigators, where a non-amplified passive antenna is connected directly to the receiver front end, don't seem to be prone to this problem.)

In the case of PCS, the offending transmitters are 400 MHz or so away from the GPS channel.  This new threat is DIRECTLY ADJACENT to the the GPS channel, and most likely will be a far far worse problem.

Is this the BPL fiasco redux??

Or more directly comparable, the mess on 800 MHz where thousands of Nextel transmitters were licensed on frequencies interleaved with 800 MHz public safety channels.  The problem was that the public safety systems use relatively few base stations, often on mountains or tall towers many miles away from the users' handhelds; i.e. delivering a relatively low signal strength to users on the ground.  The Nextel systems were placing transmitters on low towers every few blocks in town, yielding signals on the ground hundreds or thousands of times greater. These were massively overloading the front ends of PS receivers on adjacent channels, desensitizing them without creating audible interference.

Hundreds of millions of dollars have been spent in a 800-MHz rebanding process (where Nextel is supposed to pay for new radio gear for the PS users on a different block of frequencies) that has dragged on for years now. So far, the rebanding effort has mainly benefited communications attorneys and radio design consultants hired by both sides.



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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