Comments and Limitations of C-C Rider Approach (7-2004)

I have seen the proposal by Tom Clark W3IWI on C-C Rider, a C-band up / C-band down 
satellite approach, using an inverting transponder.  It seems like a good idea for 
HEOs but it has some heavy limitations for LEOs. 

The first is due to the Doppler shift not being cancelled for ALL users.  An example 
where it is would be an Illinois station talking to another Illinois station.  The 
satellite is a LEO over Texas.  Here Station A on the uplink is received by the 
satellite moving towards him Doppler shifted 100 kHz higher.  His signal is inverted 
and repeated from the satellite 100 kHz lower.  Station B sees the satellite moving 
toward him also and hears Station A near his original frequency ... negligible 
Doppler shift.  If he calls on the frequency he hears him, Station B will be 
received by Station A near his frequency.

But say the stations are in Minnesota and Mexico with the satellite over Oklahoma.  
Station A still sees the satellite approaching, his signal is received at the 
satellite 100 kHz high, inverted and repeated 100 kHz low.  The Mexican station B 
sees the satellite moving AWAY and receives its signals ANOTHER 100 kHz lower.  He 
hears the Minnesota station 200 kHz lower than he is.  If he calls on Station A's 
frequency, he will be received by the satellite 100 kHz lower still (300 kHz shifted), 
be inverted and repeated 300 kHz high, and then be received at Station A 400 kHz 
higher than Station A's original frequency. Now Station A hears himself with no 
Doppler shift and Station B hears himself with no Doppler shift, but A hears B 
400 kHz above A's frequency. That's very hard to handle for casual contacts through 
a LEO on a 10 minute pass.  

Of course, Station B could use software to calculate the satellite induced Doppler 
shift from the satellite position, Station A's position, and his position and 
transmit at such a frequency as to land near Station A's frequency.  But ...

If we use C-C Rider in a HEO satellite, we are in much better shape.  First, near 
apogee, the satellite moves slowly with much lower Doppler, and it's much farther 
away, making it impossible to have two stations in QSO located on opposite sides of 
the satellite.  When the satellite is 5 times further away, Minnesota and Mexico have 
very similar paths and Doppler effects are much smaller.  Station A hears himself 
with little Doppler ... and Station B too.  Sorry I haven't figured out the worst 
case shift.  

So, I think that C-C Rider is much more applicable to HEO satellites than LEOs.

John Matz KB9II

(BSEE, MSEE, Northwestern University, PE, ham for 40 years)