GPS Services

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

GPS provides two different positioning services: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS).

Precise Positioning Service

As defined by the United States Government the GPS Precise Positioning Service (PPS) is a positioning and timing service provided by way of authorized access to ranging signals broadcast at the GPS L1 and L2 frequencies. The L1 frequency, transmitted by all Navstar satellites, contains a coarse/acquisition (C/A) code ranging signal, with a navigation data message, that is available for peaceful civil, commercial, and scientific use; and a precision (P) code ranging signal with a navigation data message, that is reserved for authorized use. The P-code will normally be cryptographically altered to become the Y-code. The Y-code will not be available to users that do not have valid cryptographic keys. Navstar satellites also transmit a second P- or Y-(P(Y)-) code ranging signal with a navigation data message at the L2 frequency.

In order to restrict civilian user access to full system accuracy, the following protections were introduced:

• S/A or Selective Availability: intentional satellite clock degradation (process-δ) and ephemeris manipulation (process-ε). The effect on horizontal positioning implies going from about 10m (S/A=off) to 100m (S/A=on). The process-δ acts directly over satellite clock fundamental frequency, which has a direct impact on pseudoranges to be calculated by user's receivers. The process-ε consists in truncating information related to the orbits.

GPS accuracy before and after SA removal

USA President Bill Clinton ordered the termination of GPS Selective Availability on May 1st, 2000. In September 2007, this decision was confirmed by the fact that the U.S. Government decided to procure the next generation of GPS satellites, GPS III, without the SA feature. This decision eliminates an important source of uncertainty that generated considerable concern among civil users.

• A/S or Anti-Spooffing: it consists in P code encryption by combining it with a secret W code, resulting in the Y code, which is modulated over the two carriers L1 and L2. This serves the twofold purpose of protecting the code so that it can only be used by authorised receivers and avoiding adversaries to forge a misleading signal that could confuse military receivers. The use of A/S does not prevent civil users to take benefit from the C/A code.

Authorised users receive the corresponding encryption keys to access the PPS free from SA and A-S, and hence obtain the maximum accuracy from GPS. Should a PPS receiver have not been fed with valid keys, it could still behave as a SPS receiver.

The P(Y)-code, which is reserved for military use and authorised civilian users, is modulated over both carriers L1 and L2, and defines the Precise Positioning Service (PPS): it is reserved for military use and authorized civilian users. With a chipping-rate of 10Mbps and a wavelength of 29.31 m, the P(Y)-code is repeated every 38 weeks; a weekly portion, known as PRN, is assigned to each satellite.

Please refer to GPS Performances for more information about the performances provided by the PPS service.

Standard Positioning Service

As defined by the United States Government is the GPS Standard Positioning Service (PPS) is a positioning and timing service that it is available for peaceful civil, commercial and scientific use. It includes the C/A-code signal, the CM/CL-code signals, and the I5-code/Q5- code signals. The C/A-code signal is transmitted by all satellites and comprises an L1 carrier modulated by a coarse/acquisition (C/A) code ranging signal with a legacy navigation (LNAV) data message. The CM-code and CL-code signals are transmitted by some satellites and comprise an L2 carrier modulated by both a civil moderate length (CM) code ranging signal with a civil navigation (CNAV) data message and a civil long length (CL) code ranging signal without a data message. The I5-code and Q5-code signals are transmitted by some satellites and comprise an L5 carrier modulated by both a civil in-phase (I5) code ranging signal with a CNAV data message and a civil quadrature-phase (Q5) code ranging signal without a data message.

The Standard Positioning Service is therefore based on:

• The Coarse/Acquisition code (C/A(t)), which is modulated only on L1. It has a chipping-rate of 1.023 MHz, and contains 1 023 chips, so that the code is repeated every millisecond and each chip lasts about 1 µs, meaning a chip-width or wavelength of 293.1 metre.
• CM/CL-code signals are both modulated in L2:
  • Civil Medium (CM) code is 20 milliseconds in length at a chipping rate of 511,5 kpbs.
  • Civil Long (CL) code is 1.5 seconds in length at a chipping rate of 511,5 kpbs.
• In-phase (I5) and Quadraphase (Q5) signals are modulated in L5. They are independent but time synchronized, 1 millisecond in length with a chipping rate of 10.23 Mbps.

Please refer to GPS Performances for more information about the performances provided by the SPS service.