[LinuxPPS] time compensation

[Remco dB]

On Wednesday 02 June 2010 09:05:47 Paul wrote:
> Hmm, delay in the attenna cable? Your GPS unit will just think it is in
> a different point in space. What is the difference between a delay in
> the ether (I know it doesn't exist) and a delay in the cable? In fact
> the receiver will think it is 10% of the way up the antenna cable
> (velocity factor in coax is more like 90%). The unit corrects for time
> delays from the satellites to it's (assumed) point in space. Cabling
> from there to your computer does create a delay. Incidently the specs on
> some GPS modules are to 1mS (yes thats a milli not a micro second)
> 

The velocity factor for 'normal' coaxes (e.g. with PE dielectricum) is 0.66
(i.e. vf = 1 / sqrt(Er), with Er = dielectric constant) -> 1/sqrt(2.3) = 0.66 
for PE dielectricum. (For e.g. PTFE the vf will be 0.69, Er = 2.1).

This means that your receiver is 1/(0.66) = 1.5 more wavelengths away from the 
antenna than the length of the coax implies when PE coax is used. 

With the speed of light being approx. 3E8 m/s, this means that the signal 
arrives 1/3E8= 3.3 ns later. In the coax this will be 3.3 * 1.5 = 5 ns/m.

Therefore, for very precise timing, a correction has to be made for the cable 
length. E.g. with the Oncore driver (Reg :-) one can enter the the DELAY.
I interpret this delay as: ĺength of the coax in metres * 5 ns and that the 
Oncore itself (or the driver, #30) corrects this to 'additional delay', due to 
the fact that a part of the propagation path is within another dielectricum 
than air. 

Note that when there are other dielectrica in the atmosphere, e.g. clouds 
and/or heavy rain, the same theory applies and delay of GPS signals in general 
occurs (with this method it is possible to derive the total amount of moisture 
within a vertical column in the atmosphere e.g.). The dielectric constant of 
water = 80, so the velocity factor is 0.11 (!).

Milliseconds of delay therefore are not the result of coaxial cabling in my 
opinion. How the receivers process the GPS signals and 'convert' them to your 
DCD-pin, is of course another issue. 

Remco