  Several space missions such as GRACE, GRAIL, ACES and others rely on
intersatellite communications (ISC) between two satellites at a large distance
one from another. The main goal of the theory is to formulate all the
navigation observables within the General Relativity Theory. The same approach
should be applied also to the intersatellite GPS-communications (in perspective
also between the GPS, GLONASS and Galileo satellite constellations). In this
paper a theoretical approach has been developed for ISC between two satellites
moving on (one-plane) elliptical orbits, based on the introduction of two
gravity null cones with origins at the emitting-signal and receiving-signal
satellites. The two null cones account for the variable distance between the
satellites during their uncorrelated motion.The intersection of the two null
cones defines a distance, which can be found from a differential equation in
full derivatives. This distance is the space-time interval in GRT. Applying
some theorems from higher algebra, it was proved that this space-time distance
can become zero, consequently it can be also negative and positive. But in
order to represent the geodesic distance travelled by the signal, the
space-time interval has to be "compatible" with the Euclidean distance. So this
"compatibility condition", conditionally called "condition for ISC", is the
most important consequence of the theory. The other important consequence is
that the geodesic distance turns out to be the space-time interval, but with
account also of the "condition for ISC". This interpretation enables the strict
mathematical proof that the geodesic distance is greater than the Euclidean
distance - a result, entirely based on the "two null cones approach" and
moreover, without any use of the Shapiro delay formulae. Consistency between
several other newly derived numerical parameters is noted.
