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Re: "spontaneous" LEILA activation

Rolf Niefind wrote:
>  May be the old french system for radioloacation  " S Y L E D I S " is back ??

Most likely it is the "Pave Paws" 70cm high power radar which gave
us a lot of trouble when trying to comand AO-13 via 70cm uplink...

I remember that the QRM was mostly there when the S/C was visibible
for US and UK..   It was clean when over asia etc..

If you put an unmodulated carrier on the transponder,m you can here this
kind of "clicking" which is probably due to overload of the input agc etc..

73s Peter

The following is from an article written by me in 1993 for the
AMSAT-DL Journal and translated by Don Moe, DJ0HC/KE6MN).
What we see on LEILA is probably related to this?


Since 1987 the US Air Force has been operating 
a total of four so-called "PAVE PAWS" radar 
systems located in the states of Texas, Georgia, 
California and Massachusetts. The first such in-
stallation went into operation in 1979. 
The radar system serves primarily to recognize 
launches of ship-based and intercontinental bal-
listic missiles (ICBMs) and to track their trajec-
tories. Additionally it also simultaneously ob-
serves and tracks more than 6000 satellites and 
other objects in Earth orbit. The data are sent to 
NORAD in Colorado for analysis. Orbital param-
eters and Kepler elements for nearly all satellites 
are administered and published there, including 
among others weather and naturally amateur 
radio satellites. This data is available at no charge 
over a variety of computer networks.
Each of these gigantic "PAVE PAWS" radar an-
tennas consists of so-called phased arrays [1]. 
These phased arrays are built around thousands 
of separate antennas in order to generate a direc-
tional radar beam to scan the skies. In contrast to 
customary radar installations in which a parabolic 
dish is mechanically turned and aimed, the direc-
tional control is determined by changing the 
phase angle between the individual dipole ele-
ments. Rather than taking minutes, a computer 
can electronically aim at any desired point in the 
sky within a few milliseconds! 
A "PAVE PAWS" radar is built around a three-
sided, 32 meter high building and can cover an 
arc of 240 degrees. Phased array antennas with a 
diameter of 31 meters are mounted on two sides 
of the building. Each side can hence scan 120 de-
grees of azimuth and 80 degrees in elevation.
The transmitter power is provided by transistor-
ized amplifier stages consisting of four 100 Watt 
transistors combined in each transmitter module 
to drive a single radiating element. This is re-
peated for a total of 1792 elements. 1792 times 
400 Watt requires over 700 Kilowatt of pure 
transmitter power, not considering the enormous 
antenna gain! Objects with a surface area of 10 
m2 can thus be observed at a distance of nearly 
7000 km!
Two older early warning systems for ballistic 
missiles (BMEWS) also existed in Thule, Green-
land and Fylingdale, Yorkshire, England. In the 
meantime both BMEWS systems have been con-
verted and modernized to PAVE PAWS radar 
The PAVE PAWS radar in Thule, Greenland 
went into operation in June, 1987. The English 
system followed a few years later. In contrast to 
the radar in Greenland, the English PAVE PAWS 
radar has even a third antenna array and can thus 
cover an entire 360 degree circle. Each of the 
three antenna surfaces consists of more than 2500 
individual antennas. 
Unfortunately the operating frequencies of PAVE 
PAWS radars lie within the 70cm amateur radio 
band, and in particular between 432 MHz and 
445 MHz. One of the three broad-band main car-
riers is almost exactly on 435 MHz and a second 
one is close to 439 MHz. The frequency spectra 
also overlap to some extent. The 435 MHz sat-
ellite segment is thus particularly affected. Ingen-
iously the range below 432 MHz is not occupied 
at all. Only around 422 MHz do we find another 
At the enormously high radiated power levels of 
these radar systems, consequences to satellite op-
eration are unavoidable. After the new PAVE 
PAWS radar installations went into service in 
Greenland and England, our situation became 
correspondingly earnest, particularly over the 
past few years since the launch of AMSAT 
OSCAR-13, but OSCAR-21 with RUDAK-II is 
also seriously affected.
To support command station operations, 
OSCAR-13 relies upon separate frequencies to 
allow the command stations direct access to the 
onboard computer. Depending on which 
transponder is active, the command station must 
call either the Mode-B command receiver on 
70cm or the Mode-L command receiver on 23cm. 
Since Mode-B operation predominates, particu-
larly during emergencies or during the launch 
phase, the 70cm command receiver plays a very 
important roll.
Shortly after the launch of AMSAT OSCAR-13 
in June, 1988, I discovered, as the primary com-
mand station in Europe, that during orbits when 
OSCAR-13 covered large portions of the USA 
only very poor command control was possible. In 
contrast, command control on easterly orbits was 
quite reliable even at very low power levels. 
Therefore we were able to accomplish our tasks. 
Initially we did not even consider radar interfer-
ence, but suspected faults in our own stations.
Over the course of time, the apogee of OSCAR-
13 has continued to drift further north and the 
satellite correspondingly covers more of the 
northern hemisphere for longer periods of time. 
Unfortunately this also includes the radar instal-
lations in the northern parts of the USA and of 
course the modernized radar sites in Greenland 
and England. There is practically not a single 
orbit where one of these PAVE PAWS systems is 
not in view of OSCAR-13. In the early stages, 
this was not too serious for the reasons just men-
tioned. Incidentally, in February, 1993 the apogee 
of OSCAR-13 has reached its northern-most ex-
cursion at 57.6  and is now gradually moving to-
wards the equator.
On the analog transponder of Mode-B, the radar 
interference can scarcely be noticed and nobody 
seems bothered by it. However the interference 
means the end of digital communications via 


During the first two years following the launch of 
AO-13, a reload of the software was indeed re-
quired on two occasions and the last time it took 
nearly 6 hours to reprogram the onboard com-
puter because of the radar interference. A portion 
of the upload had to be performed by Graham 
Ratcliff, VK5AGR, in Australia. Because Graham 
lives in the southern hemisphere, he does not suf-
fer from radar interference, but his access periods 
are much more limited than ours are in the north-
ern hemisphere.

The problems that we here in Europe have with 
the 70cm command uplink affect not only 
OSCAR-13, but also AO-21 with its RUDAK-II 
experiment, for example. In this case, the digital 
uplink frequencies are in the satellite band just 
above 435 MHz. The downlink is at the upper 
end of the 2m satellite segment. Although AO-21 
is in a significantly lower orbit compared to AO-
13, it is not spared by the radar in Fylingdale. 
Again we have noticed that command access is 
extremely poor when the PAVE PAWS radar is 
within view. Inspection of the disrupted uplink 
blocks have shown an agreement with the pattern 
of interference to AO-13. In FM mode, the pulse 
interference can be heard quite well and sounds 
like crackling noises. On AO-13 or AO-10 these 
same irregular pulses can be readily heard on a 
weak CW carrier as interruptions or crackling 


OSCAR-21, like AO-13, has a command receiver 
and uplink frequencies in the 70cm band also. 
The situation here is likewise quite difficult. Only 
low passes in the East can be used at all when the 
English radar is below the horizon. The success 
ratio for uploads of long data files is unfortu-
nately mostly below 30% so that much time is 
lost to numerous repetitions. On March 25 and 
26 the radar was coincidentally out of service for 
maintenance and the success ratio rose to over 
98%! Unfortunately this is only seldom the case 
and for obvious reasons at irregular intervals and 
without prior notice. 


[1]	"Phased-Array Radars", Eli Brookner, Scientific American, February 1985.
[2]	"Wind Profilers at 449 MHz", Rick Palm, K1CE, QST, March 1992
[3]	"Wind Profiler Frequencies", Richard Barth, W3HWN, QST, April 1992

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