GPS Performances

GPS
Title	GPS Performances
Author(s)	GMV
Level	Basic
Year of Publication	2011

The GPS is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to civilian and military users on a continuous worldwide basis.

GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service:

1. The Standard Positioning Service (SPS), is a positioning and timing service provided on GPS L1 frequency and available to all GPS users. The L1 frequency contains a coarse acquisition (C/A) code and a navigation data message.
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 a navigation data message that is reserved for authorized use by the use of cryptography.

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.

GPS Service Level Perfomances

TO DO!!!!

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	< 9 m
	All-in-View Vertical 95%	<156 m	< 15 m
Worst Site Accuracy	All-in-View Horizontal 95%	<100 m	< 17 m
	All-in-View Vertical 95%	<156 m	< 37 m
User Range Error (URE)		N/A	<7.8 m 95% of time
Time Transfer Accuracy		N/A	<40 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 are:

Service Performances Standards for Standard Positioning Service(SPS)

		Standard Positioning Service
		Based on Single Frequency
Coverage		Global
Availability		>99 %
Accuracy (95%)		Horizontal: 17 m
		Vertical: 37 m
Time Transfer Error		< 40 nsec (95%)

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.

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 assures integrity.