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[Fwd: Link calculations for FM satellites]



Hi,

Does someone know what the Power output of the FM transmitter on
Sunsat is?  If it only supports 10-15 minutes of operations, it
must be a lot.  So a "real" FM satellite probably has 
-10dB or even less of output power.  Also remember that
MIR is using a *lot* of output power..

Anyway, it seems we have discussed this 5 years ago, see attached
message..

Oh by the way. More than 10 Years ago ICOM produced some excellent
SSB/CW portable radios, the ICOM IC202 (2m) and ICOM IC404 (70cm).
They did cost no more or even less than some of these fancy
HT's we buy today.  I started my satellite work on OSCAR-8 
(or was it OSCAR-7?) with the IC404 (TS700G for 3m) and I had 
a lot of fun!!

73s, Peter DB2OS



-------- Original Message --------
Subject: Link calculations for FM satellites
Date: Mon,  6 Mar 1995 19:50:38 -0500
From: Walter Daniel <wk02593@worldlink.com>
To: amsat-bb@amsat.org


The recent discussion of smaller satellites, using FM, and so on
caused me to turn to my engineering texts and calculator.  Here are
some preliminary calculations that may shed some light on the
design choices that have been made for Phase 3D.

MICROSAT CLASS FM TRANSPONDER IN LOW EARTH ORBIT

Let's say that a small, microsat-class satellite with a Mode B FM
transponder is launched into a polar low Earth orbit.  The satellite
is about 9 inches (23 cm) on a side, so the area of one face is
0.0529 m^2.  Assuming that all of that area is covered with solar
cells (which isn't the case) and that one side faces the Sun, the
power available when in sunlight is:

power = 1353 W/m^2 * 0.0529 m^2 * 0.10 = 7 W

Where 1353 W/m^2 is the solar flux and 0.10 represents the 10%
efficiency of silicon solar cells.

Assume that the orbit is 600 km circular (actually a bit low).  The
period is about 100 minutes and maximum eclipse time is about 36
minutes.  The power budget in sunlight would be:

3 W to spacecraft systems (computer, receiver, etc.)
3 W to charge battery (battery and charger not perfectly efficient)
1 W to transmitter

so the RF output would be 0.5 W (-3 dBW) with a transmitter efficiency
of 50%.  The satellite is passively stabilized, so no transmitter antenna
gain is present.  The system is designed for reception on an HT, so
no receiver antenna gain is present.

If the satellite is 20 degrees above the user's horizon, the
slant range is 1400 km.  With a wavelength of 2.05 meters (146 MHz),
the path loss is:

PL = 10 log( 4 * pi * 1.4E6 m / 2.05 m )^2 = 139 dB

An HT (especially with a rubber duck antenna) is not a very sensitive
receiver, so I'll assume a system noise temperature of 1000 K.
[Does anybody have a better estimate?]  The noise power density is:

N = 10 log( k * Ts ) = -199 dBW/Hz

and the FM receiver bandwidth is 15 kHz (42 dB-Hz).

The link equation for the downlink is:

xmtr power     -3 dBw
xmtr ant gain   0 dB
path loss    -139 dB
rcvr ant gain   0 dB
noise dens   +199 dBW/Hz
bandwidth     -42 dB-Hz
             -----
S/N            15 dB

This is barely acceptable for FM voice or 1200 bps packet.  Keep in
mind that I neglected polarization losses, nulls in the antenna
patterns, atmospheric losses, and feedline losses.

My conclusion is that this design would not be very useful.  The
link is marginal and the transponder could only be used when the
satellite is in sunlight.  The slow tumbling of the satellite would
cause the signal to fade in and out.  AO-21 had a better link than
this design, but then AO-21 was a small box attached to a larger
satellite that provided both active stabilization and more power.

MICROSAT CLASS FM TRANSPONDER IN ELLIPTICAL ORBIT

It has been suggested more than once that the large, expensive
Phase 3D spacecraft be replaced with several smaller satellites.  To
examine this possibility, consider the FM Microsat in an elliptical
orbit.

The power situation is better in that the spacecraft is in sunlight
most of the time.  The budget for the 7 W of power would be:

3 W to spacecraft systems
trickle to charge battery
4 W to transmitter

so the RF output would be 2 W (+3 dBW) with a transmitter efficiency
of 50%.  The satellite is only passively stabilized, so no transmitter
antenna gain is present.  The system is designed for reception with an HT
or 2m packet antenna, so no receiver antenna gain is present.

Doppler shift at perigee would make the downlink difficult to receive;
apogee would be about 40,000 km.  Let's compromise and make the
link calculation for 20,000 km slant range.  The path loss is:

PL = 10 log( 4 * pi * 2.0E7 m / 2.05 m )^2 = 162 dB

and I'll use the previous numbers for noise density and FM bandwidth.

The link equation for the downlink is:

xmtr power     +3 dBw
xmtr ant gain   0 dB
path loss    -162 dB
rcvr ant gain   0 dB
noise dens   +199 dBW/Hz
bandwidth     -42 dB-Hz
             -----
S/N            -2 dB

Once again, I've neglected polarization losses, nulls in the antenna
patterns, atmospheric losses, and feedline losses.  This signal is
buried in the noise; I believe that FM requires S/N of more than
10 dB for acceptable performance.  My conclusion is Phase 3D cannot
be replaced with small, unstabilized FM satellites.

What could be done to improve the link?  If the satellite were
stabilized, some transmitter antenna gain would be possible.  The user
on the ground would also need an antenna with gain for his or her
receiver.  The user would need a preamp and a more sensitive
receiver to get the noise power density down.  Finally, the modulation
would have to be CW (a few hundred Hz), SSB (3 kHz), and narrow-
bandwidth digital modes.  Wait a minute...I've just described a
typical AO-13 setup!

Phase 3D offers three advantages with its three-axis stabilization:
more power for the transmitters, higher transmitter antenna gain, and
lower pointing losses.  (Anybody who has tried to work AO-13 lately
knows all about pointing losses.)  With Phase 3D doing more in space,
life will be easier for users on the ground.  While an HT won't
let you work Phase 3D, you might be able to use a small Yagi pointed
at the same spot in the sky instead of large Yagis on rotors.

LAUNCH OPPORTUNITIES

One of the major factors that drove the Phase 3D design was
the launch opportunity that was generously offered by Arianespace.
The Ariane 5 is going to geostationary transfer orbit, so the only
choice possible is a high elliptical orbit.  High orbit dictates
stabilization so that more power, higher gain, and better pointing
are possible.  My conclusion is that the Phase 3D design is the
right choice (likely the ONLY choice) for this launch opportunity.

There don't seem to be many low Earth launch opportunities these days.
The success of the Microsats and the University of Surrey spacecraft
caused the launch companies to start selling their secondary payload space.
Even the days of riding piggyback on another satellite (e.g., RS-10,
AO-21) seem to be over.  RS-15 finally got into orbit, but as a small,
free-flying satellite with low power, high altitude, and no stabilization
--its downlink is weak and the fading (due to tumbling) makes the
transponder difficult to use.  There aren't too many high-power amateur
radio payloads going into orbit in the near future.  In the end, the AMSAT
societies have to use whatever launch opportunies become available.


Thanks for your time and sorry about the length of this...

73, Walt KE3HP


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