  Traditional Global Navigation Satellite System (GNSS) immunity to
interference may be approaching a practical performance ceiling. Greater gains
are possible outside traditional GNSS orbits and spectrum. GNSS from low Earth
orbit (LEO) has long been viewed as promising but expensive, requiring large
constellations for rapid navigation solutions. The recent emergence of
commercial broadband LEO mega-constellations invites study on dual-purposing
these for both communications -- their primary mission -- and a secondary
positioning, navigation, and timing (PNT) service. Operating at shorter
wavelengths than traditional GNSS, these constellations would permit highly
directive, relatively compact receiver antennas. PNT-specific on-orbit
resources would not be required: the transmitters, antennas, clocks, and
spectrum of the hosting broadband network would suffice for PNT.
Non-cooperative use of LEO signals for PNT is an option, but cooperation with
the constellation operator ("fusion" with its communications mission) eases the
burden of tracking a dense, low-altitude constellation from the ground and
enables a receiver to produce single-epoch stand-alone PNT solutions. This
paper proposes such a cooperative concept, termed fused LEO GNSS. Viability
hinges on opportunity cost, or the burden a secondary PNT mission imposes on
the communications constellation operator. This is assessed in terms of
time-space-bandwidth product and energy budget. It is shown that a
near-instantaneous-fix PNT service over $\pm60${\deg} latitude (covering 99.8%
of the world's population) with positioning performance superior to traditional
GNSS pseudoranging would cost less than 1.6% of downlink capacity for the
largest of the new constellations, SpaceX's Starlink. This allocation is
comparable to adding one user consuming 5.7 Mbps of broadband service to each
cell.
