[aprssig] GPS repeaters

Jim Lux jimlux at earthlink.net
Mon May 23 12:38:20 EDT 2005

Someone wrote:
"No..... it's like a passive repeater.  You take an active (ie preamp'ed)
antenna on the roof and plug a passive antenna into it.  Bias the whole
thing with 5v to run the preamp on the external antenna.  The passive
antenna becomes a radiating antenna.  All the GPS signals become
"mirrored" inside the building.  The only problem I can see is getting
the coax thru the roof.  Think of it is cutting a hole in the roof so
that your portable GPS unit inside the building could see the sky."

Hmm... an antenna, an amplifier, and another antenna.  Make a fine and 
dandy GPS jammer if not perfectly implemented. (that is, if the receiver 
can see both your widget and the real signals, it won't be able to lock on 
and get a fix).  This was the basis of the notorious $10 GPS jammer 
allegedly used in Iraq (using a Cable Preamp from Radio Shack).

Not to mention that such devices are explicitly forbidden by the 
FCC.  Check out the recent enforcement actions against Sport Chalet, and 
others, for "GPS extenders", which are basically what you describe.. active 
antenna with a cable and a coupler to a "external antenna-less" GPS.


They (and lots of others, apparently) were selling the Vortech Re-Radiator 
GPS Antenna.

Pursuant to Section 15.201(b) of the Rules, 47 C.F.R. §
15.201(b), intentional radiators, such as the Vortech Re-
Radiator GPS Antenna, must be authorized in accordance with
the FCC's certification procedures prior to the initiation
of marketing in the United States.  Moreover, the Vortech
Re-Radiator GPS Antenna operates in frequency bands used for
GPS, which are within the restricted frequency bands listed
in Section 15.205(a) of the Rules, 47 C.F.R. § 15.205(a).
Section 15.205(a) allows intentional radiators to transmit
only spurious emissions1 in the restricted frequency bands.
Thus, the Vortech Re-Radiator GPS Antenna apparently cannot
comply with the FCC's technical standards and therefore
would not be capable of receiving a grant of equipment


There ARE legal ways to do this kind of thing:

1) Pseudolites
2) Cable connection from external antenna to GPS receiver
3) reradiating into a electromagnetically sealed box (we use this one for 

Not to mention, that if you didn't have the amplifier, you could probably 
just hook up passive antennas back to back to propagate the signal 
inside.  The quality of fix might be a bit weird, depending on the other 
paths available.  Also, everybody inside would should the same position, 
since effectively, they all have the same antenna location.

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