[time-nuts] DSAC launches tonight

[jimlux]

On 6/24/19 11:33 PM, Poul-Henning Kamp wrote:
> --------
> In message <89d7213a-30a8-de38-a150-fc22ec733d14 at earthlink.net>, jimlux writes:
> 
>> and, as well, COSMIC-2 is being deployed - 6 satellites each carrying a
>> fancy GPS receiver that does two things: Precision Orbit Determination
>> (POD) using a choke ring antenna; Occultation measurements of the
>> atmosphere using a directive array of helical antennas facing the limb.
> 
> How do you orient a choke ring antenna in space ?  Is the distance
> for LEO to GNSS orbits long enough that you can still just "point
> it upwards" ?
> 

Yes,up,  or at the limb, depending on which satellites you want to see. 
Or a combination of two antennas which are canted from each other, but 
generally "up" (which is what COSMIC-2 does)

GPS receivers that are doing reflectometry/scatterometry have their 
antennas pointed down

GNSS is at 20,000 km, LEO is 500-1000 km, so the geometry isn't much 
different.

It's when you get to MEO or GEO that which way you point the antenna 
becomes important. You're typically adding some gain (because the link 
is 60,000km instead of 20,000).