Precise Positioning Service Performance Standard - GPS.gov

February 2007
GPS PPS PS
2.3 Overview of PPS SIS Performance Characteristics
The PPS SIS performance characteristics are described below.
2.3.1 PPS SIS Availability
The PPS SIS availability is the probability that the slots in the baseline constellation will be
occupied by healthy Navstar satellites transmitting a usable PPS SIS. For this PPS PS, there are
two components of availability as follows:
Note:
Per-Slot Availability. The fraction of time that a slot in the baseline constellation will be
occupied by a satellite that is healthy and transmitting a usable PPS SIS.
Baseline Constellation Availability. The fraction of time that a specified number of slots
in the baseline constellation are occupied by satellites that are healthy and transmitting a
usable PPS SIS.
1. For historical reasons, the term "operational satellite" applies to any Navstar satellite which appears
in the current almanac data in subframe 5 or subframe 4 of the NAV message (see Figure 2.2-1). A
satellite need not be healthy or transmitting a usable PPS SIS to be an "operational satellite",
appearing in subframe 5 or subframe 4 is sufficient. The almanac data in subframes 5 and 4
includes all "operational satellites" regardless of their health or ability to transmit a usable PPS SIS.
Not all Navstar satellites occupy a slot in the baseline constellation. Satellites that are not
occupying a slot in the baseline constellation (whether in a baseline configuration or an expanded
configuration) are considered "surplus" satellites. The PPS SIS from a surplus satellite is available
if that satellite is healthy and transmitting a usable PPS SIS. The PPS SISs from surplus satellites
do not count towards either the per-satellite availability or the baseline constellation availability.
Note:
1. The term "spare satellite" has certain connotations which do not apply to a "surplus satellite." In the
past, there were 3 spare satellites in the previous 18+3-satellite and 21+3-satellite constellation
baselines. Each of the 3 spare satellites had a pre-defined slot and the operating plan was to launch
a new satellite to fill those slots when they were unoccupied. In contrast, the current baseline is a
24-slot constellation, not a 24+3-slot constellation. Surplus satellites do not have pre-defined slots,
and there are no a priori plans to replace surplus satellites when they fail. Surplus satellites may be
"young" (recently launched) satellites waiting to move into an unoccupied slot, or they may be "old"
(nearly worn out) satellites providing their last few months or years of navigation service before they
finally expire.
2.3.2 PPS SIS Health
The PPS SIS health is the status given by the real-time health-related information broadcast by
each Navstar satellite as an integral part of the PPS SIS. The PPS SIS health is also sometimes
referred to as "satellite health" or “space vehicle health” or “SV health”. For this standard, there are
three possible PPS SIS health status indications as follows:
"Healthy". The PPS SIS health is healthy when all of the following four conditions are
present:
(1) There is no PPS SIS alarm indication present, where the PPS SIS alarm
indications are as defined in paragraph 2.3.4.
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