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

Thu Apr 12 21:58:59 UTC 2012

It is well known that vertical accuracy is worse by about 1.8 times the
horizontal accuracy. It is true this is geometrically caused. In indoor
scenarios high sensitive receivers have a vertical bias due to excessive
multipath. More or less all received signals have bounced multiple times
to reach the receiver in the worst scenarios. This however has nothing to
do with a rerad-system, where your rerad signal will be much stronger than
the natural one. Otherwise there would be no need for a rerad-system in
the first place. Also a correctly installed rerad-system will not give
excessive multipath problems. Vertiacal biasing is not caused by
(correctly) installed rerad systems.

--

    Björn

> Purely geometrically, the fix solution is computed as the intersection
> point of spheres with the radii determined by the propagation time, and
> the centers by the positions of the satellites (practically not all
> spheres intersect in the same geometrical point, so an average is
> computed).
> If the GPS Rx would receive simultaneously all satellites, considered
> evenly distributed on a sphere, then the added path delays would mostly
> cancel out - but if only the visible satellites are accounted for, we
> will have an unbalanced system, approximated to an hemispehere, in which
> the horizontal error will be low, as the longer paths cancel mostly out,
> but for the vertical one it's not the case.
>
> Any GPS receiver will exhibit lower vertical precision than the
> horizontal one.
> The same phenomenon, of low precision, and biasing of position is
> evident if just a part of the constellation is used (an obstacle
> obscures a large part of the sky).
>
> The internal delays of the Rx are mostly fixed and known, so they can be
> accounted for, and compensated in the firmware fix solution, but the
> cable length is a variable (depending on the installation) factor, not
> accounted for.
>
>
> On 4/12/2012 7:15 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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