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

bg at lysator.liu.se bg at lysator.liu.se
Thu Apr 12 16:15:20 UTC 2012


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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>>>>>
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