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Re: Satellite ranging



Anthony Monteiro wrote:

> At 06:50 AM 10/10/2005 +1000, Andrew Rich wrote:
>
>> So if I do this on our local voice repeater which is 40 kms away,
>>
>> So the round trip should be 266 us roughly. (delay due to gear ?)
>>
>> Sound right ?
>>
>> Can I do this with a 100 meter roll of hook up wire ?
>
>
> Hi Andy,
>
> You can certainly measure the delay to and from the repeater
> and your round trip delay is correct but as you noted,
> you would need to add in the delay introduced by the repeater
> itself. 

To clarify, most voice repeaters have an installed audio delay board
(analog or digital are both commonly seen) that delay the audio to the
transmitter slightly.  By wiring the COS/CTCSS trigger for the receiver
directly to the transmitter's PTT, but running the audio through a
slight delay, you have the effect of removing the annoying open squelch
blast of white noise at the end of a user's transmission. 

This is more common in Amateur practice than commercial, especially with
Amateur user gear where the radios typically don't react to CTCSS phase
changes.  Most commercial radios rapidly shut down their audio amp or
mute received audio upon detecting a phase shift on the CTCSS signal,
but for some reason Amateur gear (from the same manufacturers) rarely
does.  In commercial systems, the delay is used less frequently and a
CTCSS phase shift signals the user radios to mute quickly instead.

(There are two different phase shift standards in degress, one that was
popularized by Motorola, one by GE, but good modern commercial rigs can
detect both.)

And for even more fun, higher-end commercial repeater systems can be fed
accurate clock signals and create specific delay according to the needs
of the system designer for things like multi-casting (two transmitters
on the same frequency covering the same physical coverage area) where
the delay is adjusted and the carrier frequency of both transmitters is
locked to that external source, so that the receiver in the center of
the overlap area will receive the two transmitters at the same time,
causing no problems with FM capture in the overlap area.

Then of course, just when you think you have it all working, there's
reflections and multipath... (GRIN)... you can play "what-if" for hours
and hours with repeater (or satellite) systems. 

Accurate clocking and timing continues to be one of the areas humans are
constantly working on -- from accurate clocks to be able to tell
Longitude for sailors long ago, to trying to get all our radios of today
perfectly on some arbitrary frequency -- clock sources and standards are
fascinating. 

Nate WY0X
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