Satellite Coordination,Link Margins

Jim Cohen and Mike Davis

SATELLITE COORDINATION

Equivalent Power Flux Density (epfd) Algorithm

a i

r

N

iir

i

itP

dBi0G G·d

G·epfd

12

1010

410log10

Takes into account the gain and pointing directions of both theSatellite and the Radio Telescope. Must be run repeatedly to coverfull range.

Link Margins

• The satellite operators require a ‘Link Margin’, the maximum S/N in the intended receiver after all free-space link losses are accounted for.

• IRIDIUM used 16 dB as their link margin, for example. Transmitter power on a given connection can be increased as needed, up to this level.

• Note that if nth-order intermods are causing interference, increasing the transmitter power 1 dB generally increases the intermod level n dB!

Basic Requirements• A satellite system needs to place a certain power flux

density PFDsat on the ground in the satellite allocated band, to provide its legitimate service.

• The signal becomes detrimental if it exceeds PFDrec769 in a radio astronomy band

• PFDsat – PFDrec769 dB attenuation or more is needed between the satellite band and the RAS band. Basic to the work of Task Group 1-7.

• PFDrec769 is published, but• PFDsat generally is not publicly available. • Measure it?

RFI Survey at the RPA G. R. Harp and R. F. Ackermann

GEOs

LEOs

Sun

48 hours continuous scanning

Satellite SpectrumLeeheim, Germany Monitor Station

Detrimental Level,Rec. 769

Measured Signal Level

40 dB

RAS Allocation 10.68-10.7 GHz

FSS Allocation

Radio Astronomy Maps Before and After TV Satellite Operation

IUCAF Doc 7D85,

4 Feb 2002

After 1995

Before 1995

Reduction in RAS Channel Capacity caused by interference

Relative Channel Capacity (r/ri) = r

2/(r2 + i

2)

Recommended Level

99%

50%10 dB

7D86, 4 Feb 2002

CONCLUSION• Measure the satellite emission ON THE

GROUND, before launch.

• Do it carefully.

• Do it thoroughly.

• Once the satellites are up, it’s too late.