The goal of this link is to provide a low-data rate radio link between satellites for the purpose
of sharing time synchronization and status information with one another. In addition, this
link will be responsible for communicating with the WAAS ground stations. In designing
this link, a major design decision had to be made regarding the type of antenna to implement.
At first glance, one would assume that dish antennas would be necessary to achieve the
necessary CINR. Since dish antennas are highly directional, however, some sort of mechanical
steering mechanism or electrically steered array (high complexity) would be required in the
implementation.

In order to cut down on complexity and cost, the omni directional conical spiral antenna
used in the GPS link will be utilized. With this antenna pointing directly at the planet, all
satellites visible via line-of-site will be within a -5db antenna gain of the omni. As a result, the
received SINR is extremely low. However, since there are no restrictions on data-rate, CDMA
can be used to accomplish reliable communication.

Worst-Case Calculation for Intra-Satellite Link

In designing the link around a worst-case scenario, it was necessary to analyze the orbits of
the satellites on the basis of determining the link distance and the corresponding antenna gain
at that particular angle in the beam-width. The two geometries illustrated below (maximum
distance vs. minimum distance) were used to determine the off-axis angle, which is necessary
for finding the worst case antenna gain.

From these plots, it is determined that our system will only encounter off-axis angles:
15.6° < |x| < 54°. From the antenna pattern show below, we would expect a worst-case gain of 0dB at -54°. Since there exists a slight inverse relationship between link distance and antenna gain, it is necessary to determine the worst-case when the effects of antenna gain and path loss are combined. The following details the analysis of relationship:

Maximum Distance

• Path loss at 24373Km: -147dB
• Antenna Gain at 15.6° off-axis: 6dB
• Combined Result: -141dB

Figure 2: Minimum distance between satellites

Minimum Distance

• Path loss at 6422Km: -136dB
• Antenna Gain at 15.6° off-axis: 0dB
• Combined Result: -136dB

Notice that the improvement in antenna gain wasn’t quite enough to offset the negative
effects of a larger path-loss. Because of this, the maximum distance between satellites was
used to access the link performance.

Note: This is not a perfect model for determining the worst case scenario, since only 2 data
points are observed. In fact, the main point of the analysis was to show that the path loss was
largely off-set by other link characteristics.

Note: The worst case analysis for the satellite to WAAS link was based around the assumptions
presented in the GPS link section.

The detail for both the transmitter and the receiver can be accessed via the menu on the left.