GPS Performances

GPS
Title	GPS Performances
Edited by	GMV
Level	Basic
Year of Publication	2011

The US Global Positioning System (GPS) provides 24/7 positioning and timing services for worldwide users.

GPS supplies two different service levels, Standard Positioning Service and the Precise Positioning Service:

1. The Standard Positioning Service (SPS), is a positioning and timing service provided on GPS L1, L2 and L5 frequencies and available to all GPS users. The L1 frequency contains a coarse acquisition (C/A) code and a navigation data message. The L2 frequency contains a CM-code and CL-code signals whereas I5-code and Q5-code signals are transmitted in L5 frequency.
2. The Precise Positioning Service (PPS), is a highly accurate military positioning, velocity and timing service broadcasted at the GPS L1 and L2 frequencies. Both frequencies contain a precision (P/Y) code ranging signal with an encrypted navigation data message that is reserved for authorized users.

Introduction

The levels of performance that the user can expect from GPS are specified in the Standard Positioning Service Performance Standard, and the Precise Positioning Service Standard. However, the values provided by these documents are very conservative, being the actual performances usually better than these official values.

Moreover, the performance obtained with GPS depends strongly on the mode of operation. For instance, a stand-alone receiver that uses only the signals received from the satellites, the levels of performance are:

• C/A-code receivers ~ 5 -10 m.
• P/Y-code receivers ~ 2 -9 m

In case of using GPS in a differential mode, DGPS, the performances that can be expected are:

• C/A-code DGPS receivers ~0.7 -3 m.
• P/Y-code DGPS receivers ~0.5 -2.0 m.

More advanced techniques, such as Real Time Kinematics or Precise Point Positioning, might provide performances in the order of a few centimeters.

GPS Service Level Performances

The Performances of each Service are different and they are specified in the Standard Positioning Service Performance Standard, and the Precise Positioning Standard.

Standard Positioning Service (SPS) Performances

The Performance standards for SPS Service based on Single Frequency C/A-Code are:

Service Performances Standards for Standard Positioning Service(SPS)

GPS Performance Standard Metric		SPS User Performance	SPS Signal in Space Performance
Global Accuracy	All-in-View Horizontal 95%	<100 m	< 8 m
	All-in-View Vertical 95%	<156 m	< 13 m
Worst Site Accuracy	All-in-View Horizontal 95%	<100 m	< 15 m
	All-in-View Vertical 95%	<156 m	< 33 m
User Range Error (URE)		N/A	<7.0 m 95% of time
Velocity Accuracy		N/A	< 0.2 m/sec 95% velocity error
Time Transfer Accuracy		N/A	<30 ns 95% of time
Geometry (PDOP ≤ 6)		> 95.86% global	> 98% global
		> 83.9% worst site	> 88% worst site
Constellation Availability		N/A	>98% Probability of 21 Healthy Satellites

Precise Positioning Service (PPS) Performances

The Performance standards for PPS Service based on Dual Frequency P/Y-Code are:

Service Performances Standards for Precise Positioning Service (PPS)

GPS Performance Standard Metric		SPS User Performance	SPS Signal in Space Performance
Global Accuracy	All-in-View Horizontal 95%	< 36 m	< 13 m
	All-in-View Vertical 95%	< 77 m	< 22 m
User Range Error (URE)		N/A	<5.9 m 95% of time
Time Transfer Accuracy		N/A	<40 ns 95% of time
Integrity		N/A	< 1x10-5 Probability Over Any Hour
Geometry (PDOP ≤ 6)		>95.7% global	>98% global
Constellation Availability		N/A	>98% Probability of 21 Healthy Satellites

In general, PPS performance standards are in line with SPS ones. Some advantages of PPS Service opposite to the SPS Service are that PPS access to WAGE (Wide Area GPS Enhancements) rapid ephemeris updates and corrections, and the use of dual frequency to correct the delay suffered by the signal in its transmission through the ionosphere in real time, which implies a significant performance improvement. The P/Y code is encrypted to avoid spoofing and the access to the service to unauthorised users.

Modernized Signals Performances

Since the beginning of the transmission of the CNAV navigation message, by April 2014, the L2C signal is suffering several improvements. On March 2015 the U.S. Air Force reported that signal performance of CNAV matches or slightly outperforms Legacy performance: average user range error (RMS URE) from 25 February – 3 March 2015 was 0.50 m for Legacy and 0.57 m for Modernized; best week for Modernized signals since the broadcast initiated April 2014 was 0.42 m for 6 – 13 January 2015.

Combined Services Performances

GPS can be interoperable with other GNSS systems. When combining GPS with other GNSS constellations this enhances positioning performance. This is due to an improvement in availability, i.e. the number of satellites in view is larger, and geometry.

There are other ways to enhance the GPS positioning solution, such as GNSS Augmentation systems, or Differential GNSS techniques, that are explained in more detail in the corresponding articles. With DGNSS the accuracy is improved to the order of 1 m, and the GNSS Augmentation systems assure integrity.