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

Thu Apr 12 17:48:22 UTC 2012

Timing GPS receivers have the cable delay parameter to account for the
cable delay.

>>added path delays would mostly cancel out
How can delays cancel out?

On Thu, Apr 12, 2012 at 7:33 PM, MailLists <lists at medesign.ro> wrote:

> 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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>>>>>>>
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