Other Satellite Navigation Systems - Understanding GPS: Principles and Applications - page 627

using only three or four satellites. An initial design called for these to be placed in cir-
cular orbits at the full 35,800-km geosynchronous altitude with 45º inclination. This
design has carried over to the current QZSS concept for navigation services as well,
with the exception that the new design now calls for orbits with high eccentricity [71].
A number of possible constellation designs have been considered for placing
three satellites in HEO. The original plan for a 45º inclination and symmetrical fig-
ure 8 (circular orbit) was dropped due to the hazard of crossing the geostationary
belt (evidently not addressed in the original concept). The currently favored HEO
design will miss the belt by 400 km and remain near apogee for 12 hours a day,
above 70º elevation from central Japan. This design calls for three satellites, all trac-
ing the same elliptical orbit with timing to make the apparent intersection of the
resultant figure eight trace directly over Tokyo for hand-off of services from one
satellite to the next [72, 73].
At the time of this writing, the Japanese Aerospace Exploration Agency pre-
sented an updated concept for the QZSS constellation at an international confer-
ence. In an apparent reversal, the agency proposed a constellation of three distinct
and separate inclined HEO orbits, complemented by a return to as many as five
spacecraft at geostationary positions instead of the minimal three-satellite design.
This new design may conceivably rely on existing or new communications satellites
to host the additional navigation payloads [68].
11.3.5 Spacecraft Development
Specifications for the spacecraft and the letting of contracts were on hold at the time
of this writing, awaiting allocation of funding. The question of which governmental
element or industry consortium will assume the task of operating the QZSS system
once it is launched remains at an impasse. Monies expected from the private sector
in the amount of approximately $47 million needed in 2005 to start the spacecraft
design will likely be available when the question is settled as to who will fund and
administer operations. Success in this unstructured negotiation is doubly important
because it will establish guidelines for future public-private space projects. The gov-
ernment's share of the overall program cost has been estimated at $814 million for
the overall program, anticipating a first launch sometime after 2008, but none of
four ministries seen as stakeholders in QZSS has stepped forward to submit the first
year's increment in their budget proposal [69].
The spacecraft design itself has not been finalized but a 12-year lifetime is in the
current specification. This number and some additional technical requirements for
the spacecraft have emerged because they are implicit in the constellation design. As
with virtually all other space systems, the 12-year lifetime goal is challenged by the
demands of station keeping. Velocity increments required for satellite formation
phasing represent part of this challenge, which has already been considered [74, 75].
Not all QZSS concepts rely on having an atomic clock on board, but if one is
required, clock reliability and reliability in terms of mean time between failures
(MTBF) may be an issue. Parallel development of a hydrogen maser for the program
is proceeding under the High Accuracy Positioning Experiment project at the Japa-
nese National Institute of Information and Communications Technology. The abil-
ity to meet this lifetime specification will likely determine the choice of clock [76].