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		RADIOASTRON COMMUNICATION LINK PARAMETERS

			    L. R. D'Addario
			      27 May 1990


	At the Tenth Radioastron Review Meeting (Helsinki, 23-26 May 
1990), attempts were made to establish the requirements on both the 
space and ground portions of the three radio links:  phase reference 
uplink, phase reference downlink, and wideband data downlink.  
Discussions are still underway, but preliminary information was 
exchanged.  

	Representatives of NASA and NRAO wanted to have the Soviets
establish the spacecraft parameters and thus to specify minimum
requirements on the ground segment's EIRP (uplink) and sensitivity
(downlinks).   A draft document was prepared which, when completed
by the Soviet side, will give these requirements, among other
things.  But the Soviets preferred that the ground segment
parameters be given so that they could determine requirements on the
space segment (so as to minimize the transmitter powers, for
example).  It became clear in the discussions that neither side has
actually built the necessary hardware, not even in prototype form.  
Neither side can wait for the other to produce a prototype before 
proceding with its own design.  Therefore, regardless of which side
provides requirements to which, they will have to be based on
estimated rather than measured performance.  It seems to me that
this means that the performance of each side must be specified in
terms of guaranteed minimum values as well as nominal values, and
that -- to be conservative -- we must ensure that there is adequate
margin even when each side achieves only its minimums.  It will take
some iterative negotiation in order to get everyone to "sign off" on
acceptable values.

	In this memo, I attempt to provide a preliminary set of
minimum requirements for each side.  I propose that we adopt the 
criterion that, for each of the three links, there should be a 
minimum margin of 4 dB when the spacecraft elevation at the earth 
station is 5 degrees, the atmospheric loss is 3 dB, and each side 
achieves only its guaranteed minimum performance.

	Table I covers the wideband data link.  First some basic
characteristics of the channel are given; these are taken to be
fixed requirements (but note that the maximum range corresponds to
the 1-day orbit only, and that the maximum bit error rate is
somewhat arbitrary).  Next the space segment and finally the ground
segment parameters are given, each in two columns:  the first is the
current estimate of the achievable performance (as given at the
Helsinki meeting by V. Rogalski for the space segment and by myself
for the ground), and the second is my guess of what a prudent
engineer would be willing to guarantee.  The ground segment numbers
are based on the preliminary design of the Green Bank station.  The
resulting margins are then calculated. 
				-2-


	Tables II and III cover the phase links in the same way.  
Here the required SNR is based on allowing the thermal noise on each 
link to degrade coherence by 0.996 at 22 GHz.

	It will be seen that the suggested criterion is easily 
satisfied for the phase downlink, but not for the others.  For the 
phase uplink, I propose that this be improved by reducing the PLL 
noise bandwidth on the spacecraft from 1000 Hz to 400 Hz.  The 
latter corresponds to 16.6 m/sec velocity, which should be more than 
adequate.  For the data downlink, we will have to study which of the 
guaranteed values must be improved.  My guesses have degraded the 
estimated spacecraft performance by a total of 4 dB, whereas we are 
3 dB short of the proposed goal.  Further tests on the Green Bank
antenna may enable a better value of effective area to be
guaranteed.