[time-nuts] tube GPS receivers

[Jim Lux]

On 6/24/13 3:01 PM, Bob Camp wrote:
> Hi
>
> I'm not so sure that "slow" would work. With all the sat's moving various directions all the time, I suspect you need to do a solution fairly quickly. If you don't the stale data messes up the solution. Also you need the correlators to work fast enough to lock on to an essentially unknown code before the sat is out of view.
>
The sliding correlator is pretty easy, and would lock up quite quickly. 
Basically, you need to have a vacuum tube PN generator to generate the 
correct Gold/Kasami code for the satellite in question (e.g. you need 32 
of those generators). Each generator has a pair of 10 stage shift 
registers in it.  I haven't looked in my copy of Millman and Taub, but I 
think you could probably build the shift register with 2*N devices 
(maybe N tubes, if you use dual triodes/pentodes, what have you).  There 
might also be better choices for the tubes that have some form of 
latching behavior (thyratrons maybe..)

You slide the correlator until it locks, and then it automatically also 
tracks the doppler of that S/V.  I assume you'd use some sort of 
early/late tracker rather than a tau dither. I don't know what you'd use 
as the VCO, but there is probably some scheme (after all, FM 
transmitters existed before the invention of the Varactor solid state 
device)

You can track the raw observables (code phase and Doppler) without 
needing to do a nav solution at all. And those observables don't change 
all that quickly (after all, the Doppler only changes a few kHz during 
many hours as the satellite goes from horizon to horizon).

The trick is in how do you get the code phase into your nav algorithm. 
It's easy to get a pulse at 1 ms intervals when the code epoch comes by, 
but you really want to get a range estimate, and that means figuring out 
where you are in the bigger scheme of things. and, then getting that 
ingested into whatever computation scheme you're using.