Tuesday, 13 August 2013

The GPS Standard Positioning Service

The GPS Standard Positioning Service

The Global Positioning System (GPS) is a space-based radionavigation system which
is managed for the Government of the United States by the U.S. Air Force (USAF),
the system operator.  GPS was originally developed as a military force enhancement
system and will continue to play this role.  However, GPS has also demonstrated
a significant potential to benefit the civil community in an increasingly large variety
 of applications.  In an effort to make this beneficial service available to the
 greatest number of users while ensuring that the national security interests
of the United States are observed, two GPS services are provided.
  The Precise Positioning Service (PPS) is available primarily to the military
of the United States and its allies for users properly equipped with PPS
 receivers.  The Standard Positioning Service (SPS) is designed to
provide a less accurate positioning capability than PPS for civil and all
other users throughout the world.


Purpose

The GPS SPS Signal Specification defines the service to be provided by GPS to the civil
community. This document is written to satisfy the following four objectives:
  •  Specify GPS SPS ranging signal characteristics.
  • Specify SPS performance, given a receiver designed in accordance with this Signal
Specification.
  • Stndardize SPS performance parameter definitions and measurement methodologies.
  • Define SPS performance characteristics.
The Signal Specification consists of this document and three Annexes. This document specifies
GPS SPS signal characteristics and the minimum requirements for receiving and using the SPS
ranging signal. The Annexes provide technical data that quantifies SPS performance. Provided
below is a definition of each Annex's purpose:

· Annex A:SPS Performance Specification. 

This Annex specifies GPS SPS performance
in terms of minimum performance standards, and conditions and constraints
associated with the provision of the service.

· Annex B: SPS Performance Characteristics.

 This Annex defines GPS SPS performance
parameters and their characteristics as a function of time, user location, system
design and changing operational conditions.

· Annex C: Means of Measuring GPS Performance.

 This Annex defines the specific
measurement processes which a user must apply to evaluate GPS performance, in order
to obtain results which are consistent with the parameter definitions and performance
standards established in this Signal Specification.

Scope

This Signal Specification defines SPS ranging signal characteristics and minimum usage
conditions. The Annexes establish the SPS performance which a minimally equipped SPS user
can expect to experience anywhere on or near the surface of the Earth, and the means to
evaluate that performance. SPS signal and performance specifications are independent of how
the user applies the basic positioning and timing services provided. Performance specifications
do not take into consideration the measurement noise or reliability attributes of the SPS receiver
or possible signal interference.
This Signal Specification and the Annexes establish new definitions and relationships between
traditional performance parameters such as coverage, service availability, service reliability and
accuracy. GPS performance specifications have previously been made to conform to definitions
which apply to fixed terrestrial positioning systems. The new definitions are tailored to better
represent the performance attributes of a space-based positioning system. Refer to Annex B for
a more comprehensive discussion of GPS performance parameter definitions and relationships.
Due to the nature of the system design and its operation, individual GPS satellite ranging measurements
will not necessarily exhibit unchanging SPS ranging error statistics. Furthermore, the
Department of Defense (DOD) does not guarantee that GPS ranging or positioning error statistics
will remain stationary, or that individual satellite ranging error statistics will be consistent throughout
the constellation.
The DOD will base its on-going measurement and assessment of all specified aspects of SPS
performance on data gathered from Control Segment (CS) monitor stations. If the minimum
performance standards are met at each of the monitor stations, the DOD will assume that
standards are being met on a global basis. Geographic variations in performance will be taken
into consideration in the assessment process.

Policy Definition of the Standard Positioning Service

SPS is a positioning and timing service, and is provided on the GPS L1 frequency. The GPS L1
frequency, transmitted by all GPS satellites, contains a coarse acquisition (C/A) code and a
navigation data message. The GPS L1 frequency also contains a precision (P) code that is
reserved for military use and is not a part of the SPS. The P code can be altered without notice
and will not normally be available to users that do not have valid cryptographic keys. GPS
satellites also transmit a second ranging signal known as L2. This signal is not a part of the SPS,
although many civil receivers have incorporated technologies into their design that enables them
to use L2 to support two-frequency corrections without recourse to code tracking logic. SPS
performance standards are not predicated upon use of L2.
Any planned disruption of the SPS in peacetime will be subject to a minimum of 48-hour advance
notice provided by the DOD to the Coast Guard Navigation Information Center and the FAA
Notice to Airmen (NOTAM) system. A disruption is defined as periods in which the GPS is not
capable of providing SPS as it is defined in this Specification. Unplanned service disruptions
resulting from system malfunctions or unscheduled maintenance will be announced by the Coast
Guard and the FAA as they become known.

Key Terms and Definitions

Terms and definitions which are key to understanding the scope of the GPS Standard Positioning
Service are provided below.

  • General Terms and Definitions

The terms and definitions discussed below are used throughout the Signal Specification. An
understanding of these terms and definitions is a necessary prerequisite to full understanding of
the Signal Specification.

  • Standard Positioning Service (SPS). 

Three-dimensional position and time determination
capability provided to a user equipped with a minimum capability GPS SPS receiver in
accordance with GPS national policy and the performance specifications established in this Signal
Specification.

  • Minimum SPS Receiver Capabilities. 

The minimum signal reception and processing
capabilities which must be designed into an SPS receiver in order to experience performance
consistent with the SPS performance standards. Minimum SPS receiver capabilities are identified
in Section 2.2.

  • Selective Availability.

 Protection technique employed by the DOD to deny full system accuracy
to unauthorized users.

  • Block I and Block II Satellites. 

The Block I is a GPS concept validation satellite; it does not
have all of the design features and capabilities of the production model GPS satellite, the Block II.
The FOC 24 satellite constellation is defined to consist entirely of Block II/IIA satellites. For the
purposes of this Signal Specification, the Block II satellite and a slightly modified version of the
Block II known as the Block IIA provide an identical service.

  • Operational Satellite. 

A GPS satellite which is capable of, but may or may not be, transmitting a
usable ranging signal. For the purposes of this Signal Specification, any satellite contained within
the transmitted navigation message almanac is considered to be an operational satellite.

  • SPS Signal, or SPS Ranging Signal. 

An electromagnetic signal originating from an operational
satellite. The SPS ranging signal consists of a Pseudo Random Noise (PRN) Coarse/Acquisition
(C/A) code, a timing reference and sufficient data to support the position solution generation
process. A full definition of the GPS SPS signal is provided in Section 2.

  • Usable SPS Ranging Signal.

 An SPS ranging signal which can be received, processed and
used in a position solution by a receiver with minimum SPS receiver capabilities.

  • SPS Ranging Signal Measurement. 

The difference between the ranging signal time of reception (as defined by the receiver's clock) and the time of transmission contained within the satellite's navigation data (as defined by the satellite's clock) multiplied by the speed of light. Also known as the pseudo range.

  • Geometric Range. 

The difference between the estimated locations of a GPS satellite and an
SPS receiver.

  • Navigation Data.

 Data provided to the SPS receiver via each satellite's ranging signal,
containing the ranging signal time of transmission, the transmitting satellite's orbital elements, an
almanac containing abbreviated orbital element information to support satellite selection, ranging
measurement correction information, and status flags.

  • Position Solution. 

The use of ranging signal measurements and navigation data from at least
four satellites to solve for three position coordinates and a time offset.

  • Dilution of Precision (DOP). 

The magnifying effect on GPS position error induced by mapping
GPS ranging errors into position through the position solution. The DOP may be represented in
any user local coordinate desired. Examples are HDOP for local horizontal, VDOP for local
vertical, PDOP for all three coordinates, and TDOP for time.

  • SPS Performance Standard. 

A quantifiable minimum level for a specified aspect of GPS SPS
performance. SPS performance standards are defined in Annex A to this Signal Specification.

  • SPS Performance Envelope.

 The range of variation in specified aspects of SPS performance.
Expected SPS performance characteristics are defined in Annex B to this Signal Specification.

  • Service Disruption

. A condition over a time interval during which one or more SPS performance
standards are not supported, but the civil community was warned in advance.

  • Major Service Failure.

 A condition over a time interval during which one or more SPS
performance standards are not met and the civil community was not warned in advance.


Peformance Parameter Definitions

The definitions provided below establish the basis for correct interpretation of the GPS SPS
performance standards. As was stated in Section 1.2, the GPS performance parameters
contained in this Signal Specification are defined differently than other radionavigation systems in
the Federal Radionavigation Plan. For a more comprehensive treatment of these definitions and
their implications on system use, refer to Annex B.

  • Coverage

. The percentage of time over a specified time interval that a sufficient number of
satellites are above a specified mask angle and provide an acceptable position solution geometry
at any point on or near the Earth. For the purposes of this Signal Specification, the term "near the
Earth" means on or within approximately 200 kilometers of the Earth's surface.

  • Service Availability. 

Given coverage, the percentage of time over a specified time interval that a
sufficient number of satellites are transmitting a usable ranging signal within view of any point on
or near the Earth.

  • Service Reliability.

 Given service availability, the percentage of time over a specified time
interval that the instantaneous predictable horizontal error is maintained within a specifiedreliability threshold at any point on or near the Earth. Note that service reliability does not take into
consideration the reliability characteristics of the SPS receiver or possible signal interference.
Service reliability may be used to measure the total number of major failure hours experienced by
the satellite constellation over a specified time interval.

Positioning Accuracy

Given reliable service, the percentage of time over a specified time
interval that the difference between the measured and expected user position or time is within a
specified tolerance at any point on or near the Earth. This general accuracy definition is further
refined through the more specific definitions of four different aspects of positioning accuracy:

· Predictable Accuracy. 

Given reliable service, the percentage of time over a specified
time interval that the difference between a position measurement and a surveyed
benchmark is within a specified tolerance at any point on or near the Earth.

· Repeatable Accuracy. 

Given reliable service, the percentage of time over a specified
time interval that the difference between a position measurement taken at one time and a
position measurement taken at another time at the same location is within a specified
tolerance at any point on or near the Earth.

· Relative Accuracy. 

Given reliable service, the percentage of time over a specified time
interval that the difference between two receivers' position estimates taken at the same
time is within a specified tolerance at any point on or near the Earth.

· Time Transfer Accuracy.

 Given reliable service, the percentage of time over a specified
time interval that the difference between a Universal Coordinated Time (commonly
referred to as UTC) time estimate from the position solution and UTC as it is managed by
the United States Naval Observatory (USNO) is within a specified tolerance.

Range Domain Accuracy

Range domain accuracy deals with the performance of each satellite’s SPS ranging signal. Range domain accuracy is defined in terms of three different
aspects:

·  Range Error. 

Given reliable service, the percentage of time over a specified time interval
that the difference between an SPS ranging signal measurement and the “true” range
between the satellite and an SPS user is within a specified tolerance at any point on or
near the Earth.

·  Range Rate Error.

 Given reliable service, the percentage of time over a specified time
interval that the instantaneous rate-of-change of range error is within a specified tolerance
at any point on or near the Earth.

·  Range Acceleration Error.

 Given reliable service, the percentage of time over a specified time interval that the instantaneous rate-of change of range rate error is within a specified tolerance at any point on or near the Earth.

Global Positioning System Overview

Sufficient information is provided below to promote a common understanding of the minimum
GPS baseline configuration. The GPS baseline system is comprised of two segments, whose
purpose is to provide a reliable and continuous positioning and timing service to the GPS user
community. These two segments are known as the Space Segment and the Control Segment.

The GPS Space Segment

The GPS Block II/IIA satellite constellation normally consists of 24 operational satellites. * The
Block II satellite and a slightly modified version, the Block IIA satellite, will be the mainstays of the
constellation over the next decade. From a civil user's perspective, the Block II and Block IIA
satellites provide an identical service.
Each satellite generates a navigation message based upon data periodically uploaded from the
Control Segment and adds the message to a 1.023 MHz Pseudo Random Noise (PRN)
Coarse/Acquisition (C/A) code sequence. The satellite modulates the resulting code sequence
onto a 1575.42 MHz L-band carrier to create a spread spectrum ranging signal, which it then
broadcasts to the user community. This broadcast is referred to in this Signal Specification as the
SPS ranging signal. Each C/A code is unique, and provides the mechanism to identify each satel -
lite in the constellation. A block diagram illustrating the satellite's SPS ranging signal generation
process is provided in Figure 1-1. The GPS satellite also transmits a second ranging signal known
as L2, that supports PPS user two-frequency corrections. L2, like L1, is a spread spectrum signal
and is transmitted at 1227.6 Mhz.
The Block II satellite is designed to provide reliable service over a 7.5 year design life through a
combination of space qualified components, multiple redundancies for critical subsystems, and internal
diagnostic logic. The Block II satellite design requires minimal interaction with the ground
and allows all but a few maintenance activities to be conducted without interruption to the ranging
signal broadcast. Periodic uploads of data to support navigation message generation are designed
to cause no disruption to the SPS ranging signal.

The GPS Control Segment

The GPS Control Segment (CS) is comprised of three major components: a Master Control
Station (MCS), ground antennas, and monitor stations.
The MCS is located at Falcon Air Force Base, Colorado, and is the central control node for the
GPS satellite constellation. Operations are maintained 24 hours a day, seven days a week
throughout each year. The MCS is responsible for all aspects of constellation command and
control, to include:
  •  Routine satellite bus and payload status monitoring.
  • · Satellite maintenance and anomaly resolution.
  •  Monitoring and management of SPS performance in support of all performance
standards.
  •  Navigation data upload operations as required to sustain performance in accordance with
accuracy performance standards.
  • Prompt detection and response to service failures.