[time-nuts] Re-radiating a GPS signal...??

David McGaw n1hac at Alum.Dartmouth.ORG
Thu Apr 12 13:44:13 EDT 2012


Since I am used to figuring the cable (and additional propagation) 
delays, I did forget to mention that for timing.  :-)

On the other hand, I would not be using a repeater except for just 
functional testing of a payload system.

David

On 4/12/12 12:15 PM, bg at lysator.liu.se wrote:
> Not at all!
>
> The (first) receiving antenna defines the position you get out of a long
> antenna cable or a reradiating system. The delays in LNA, filters, cables,
> rerad antenna, free air between rerad antenna and final receiving antenna
> ALL goed into the receiver clock error. This is clear both from a
> theoretical point, from most standard GPS texts and from practical
> experience from multiple installations I have used over the years.
>
> If you disagree, please provide evidence.
>
> --
>
>    Björn
>
>> Not quite, the delay of the antenna cable is affecting less the
>> horizontal position (it depends also on the current received
>> constellation geometry), but mostly the height ASL of the fix point,
>> prolonging simultaneously all the paths from the satellites with a fixed
>> value.
>> Also, the propagation speed in a cable is significantly lower than in
>> free space - the perceived delay increase is ~1.5 times for usual cables
>> (~.67 velocity factor), and the computed fix point would have a lower
>> height ASL than the real one.
>>
>> Those relaying systems are merely good for an approximate location fix,
>> mostly for not loosing the GPS signal in covered areas so that the
>> reacquire of the real signal is faster, with almost no perceived
>> discontinuity.
>>
>>
>> On 4/12/2012 6:11 PM, David McGaw wrote:
>>> The time/position fix would be from the location of the receiving
>>> antenna of the repeater, degraded only by noise.
>>>
>>> This should work if both antennas have good back-side rejection
>>> (choke-rings are particularly good for this but perhaps any good timing
>>> antenna could meet this), the re-transmitting antenna is close to being
>>> directly under the receiving antenna, and the system gain is low enough.
>>> The problem I would see in a room that is not fully shielded is
>>> interference between the direct and retransmitted signals at the
>>> receiver under test.
>>>
>>> David N1HAC
>>>
>>> On 4/12/12 10:17 AM, MailLists wrote:
>>>> GPS being extremely time-dependent, any delay introduced will affect
>>>> positioning precision. Also, the signal is too weak for such an
>>>> amplification/echo cancelling signal chain.
>>>> Passive relaying, or using at most a simple amplifier with low enough
>>>> gain, and short signal delay, remain the only feasible concepts.
>>>>
>>>> On 4/12/2012 4:48 PM, Azelio Boriani wrote:
>>>>> Passive UHF TV repeaters were in use in Italy too. Nowadays, for the
>>>>> DVB-T
>>>>> TV, active gap-fillers are used instead. Active gap-fillers are
>>>>> same-channel repeaters with the necessary, sophisticated echo
>>>>> suppression
>>>>> technique. We have developed our echo suppression signal processor on
>>>>> a
>>>>> Xilinx Virtex5 FPGA: maybe something similar may be done for the GPS
>>>>> CDMA.
>>>>>
>>>>> On Thu, Apr 12, 2012 at 3:29 PM, Alan
>>>>> Melia<alan.melia at btinternet.com>wrote:
>>>>>
>>>>>> If the isolation is good and the "clear view" signal is reasonably
>>>>>> strong,
>>>>>> the passive system works well in hangers, metalclad warehouses,
>>>>>> ferry lorry
>>>>>> decks.
>>>>>> The passive system in the UK used to be refered to as the "Matlock
>>>>>> Repeater".
>>>>>>
>>>>>> Alan
>>>>>> G3NYK
>>>>>>
>>>>>> ----- Original Message -----
>>>>>> From: "Michael Baker"<mpb45 at clanbaker.org>
>>>>>> To:<time-nuts at febo.com>
>>>>>> Sent: Thursday, April 12, 2012 2:05 PM
>>>>>> Subject: [time-nuts] Re-radiating a GPS signal...??
>>>>>>
>>>>>>
>>>>>>> Time-nutters--
>>>>>>>
>>>>>>> So-- How do GPS signal re-radiators work?
>>>>>>>
>>>>>>> How do you place a GPS antenna on top of a building,
>>>>>>> pick up the signal with an LNA, amplify it to re-transmit
>>>>>>> on an inside antenna without the amplified re-transmitted
>>>>>>> signal getting back into the roof-top receiving antenna?
>>>>>>>
>>>>>>> I can see circumstances where a huge metal building
>>>>>>> (aircraft hangar?) might provide enough isolation to
>>>>>>> prevent problems, but in many cases I wonder about it...
>>>>>>> ----------------------------
>>>>>>>
>>>>>>> As an aside note-- I recall seeing, many years ago, a totally
>>>>>>> passive TV signal repeater on top of a tall hill in mountainous
>>>>>>> territory relaying a TV station signal to some homes in a valley
>>>>>>> just below. The passive repeater consisted of an array of
>>>>>>> high-gain UHF yagis pointing to the 40 mile distant TV station
>>>>>>> tower.
>>>>>>> The yagi array was coupled to another set of high-gain yagi
>>>>>>> antennas pointing down to the homesites in the valley. I was
>>>>>>> told that it worked pretty well.
>>>>>>>
>>>>>>> Mike Baker
>>>>>>> ----------------------
>>>>>>>
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