Global Positioning System Reference
In-Depth Information
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IIR specification
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Space vehicle number
II/IIA (Rubidium)
IIR (Rubidium)
II/IIA (Cesium)
Figure 3.9
One-day Hadamard stability ranking. ( Source: Lockheed Martin Corp. Reprinted with
permission.)
The Block IIR uses nickel hydrogen (NiH 2 ) batteries, which require no recondi-
tioning and accompanying operator burden.
When in Earth eclipse, automatic pointing of the solar array panels is accom-
plished via an onboard orbit propagation algorithm to enable quiescent reacquisi-
tion of the Sun following eclipse exit. This provides a more stable and predictive SV
bus platform and orientation for the L-band signal.
Block IIR has an expanded NSC capability to protect the user from spurious sig-
nals. It is enabled automatically in response to the detection of the most harmful
on-orbit RAFS and VCXO discontinuities.
Block IIR has a capability to perform autonomous navigation via intersatellite
crosslink ranging. This function is called AutoNav. It provides 180-day independ-
ent navigation performance without ground contact. Although the CS currently
cannot support full AutoNav operation, portions of this capability are undergoing
on-orbit testing. There is potential for increased accuracy when using AutoNav.
In addition to intersatellite ranging, other communications with on-orbit SVs
consist of crosslink commanding and data transfer to other SVs in the constellation.
The Block IIR SVs were also designed to operate through laser and nuclear threats.
Reprogrammability
There are several reprogrammable computers on board: the redundant SV bus
spacecraft processor unit (SPU) and the redundant navigation system mission data
unit (MDU). Reprogrammability allows the CS to change the flight software in
on-orbit SVs. This feature has already been employed on-orbit in several instances.
The MDU was provided with diagnostic buffers to give detailed insight into the
behavior of the TKS. It was also given a jumpstart capability allowing current TKS
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