GPS signals include ranging signals, used to measure the distance to the satellite, and navigation messages. The navigation messages include ephemeris data, used to calculate the position of the satellite in orbit, and information about the time and status of the satellite constellation.
The original GPS design contains two ranging codes: the Coarse / Acquisition code or C/A, which is freely available to the public, and the restricted Precision code, or P-code, usually reserved for military applications.
Whereas the C/A PRNs are unique for each satellite, the P-code PRN is actually a small segment of a master P-code approximately 2.35 × 1014 bits in length (235,000,000,000,000 bits) and each satellite repeatedly transmits its assigned segment of the master code.
To prevent unauthorized users from using or potentially interfering with the military signal through a process called spoofing, it was decided to encrypt the P-code. To that end the P-code was modulated with the W-code, a special encryption sequence, to generate the Y-code. The Y-code is what the satellites have been transmitting since the anti-spoofing module was set to the "on" state. The encrypted signal is referred to as the P(Y)-code.
The details of the W-code are kept secret, but it is known that it is applied to the P-code at approximately 500 kHz, which is a slower rate than that of the P-code itself by a factor of approximately 20. This has allowed companies to develop semi-codeless approaches for tracking the P(Y) signal, without knowledge of the W-code itself.
In addition to the PRN ranging codes, a receiver needs to know detailed information about each satellite's position and the network. The GPS design has this information modulated on top of both the C/A and P(Y) ranging codes at 50 bit/s and calls it the Navigation Message.
The navigation message is made up of three major components. The first part contains the GPS date and time, plus the satellite's status and an indication of its health. The second part contains orbital information called ephemeris data and allows the receiver to calculate the position of the satellite. The third part, called the almanac, contains information and status concerning all the satellites; their locations and PRN numbers.
Whereas ephemeris information is highly detailed and considered valid for no more than four hours, almanac information is more general and is considered valid for up to 180 days. The almanac assists the receiver in determining which satellites to search for, and once the receiver picks up each satellite's signal in turn, it then downloads the ephemeris data directly from that satellite. A position fix using any satellite can not be calculated until the receiver has an accurate and complete copy of that satellite's ephemeris data.
The navigation message itself is constructed from a 1,500 bit frame, which is divided into five subframes of 300 bits each and transmitted at 50 bit/s (therefore each subframe requires 6 seconds to transmit).
For the ranging codes and navigation message to travel from the satellite to the receiver, they must be modulated onto a carrier frequency. In the case of the original GPS design, two frequencies are utilized; one at 1575.42 MHz (10.23 MHz × 154) called L1; and a second at 1227.60 MHz (10.23 MHz × 120), called L2.
The C/A code is transmitted on the L1 frequency as a 1.023 MHz signal using a Bi-Phase Shift Key (BPSK) modulation technique. The P(Y)-code is transmitted on both the L1 and L2 frequencies as a 10.23 MHz signal using the same BPSK modulation, however the P(Y)-code carrier is in quadrature with the C/A carrier; meaning it is 90° out of phase.
Besides the redundancy and increased resistance to jamming, a critical benefit of having two frequencies transmitted from one satellite is the ability to directly measure, and therefore remove, the ionospheric delay error for that satellite. Without such a measurement, a GPS receiver must use a generic model or receive ionospheric corrections from another source (such as the Wide Area Augmentation System or EGNOS). Advances in the technology used on both the GPS satellites and the GPS receivers has made ionospheric delay the largest source of error in the signal. A receiver capable of performing this measurement can be significantly more accurate and is typically referred to as a dual frequency receiver.
The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users. A goal of 2013 has been established with incentives offered to the contractors if they can complete it by 2011.
Modernized GPS civilian signals have two general improvements over their legacy counterparts; a dataless acquisition aid and Forward Error Correction (FEC) coding of the NAV message.
A dataless acquisition aid is an additional signal—called a pilot carrier in some cases—broadcast alongside the data signal. This dataless signal is designed to be easier to acquire than the data encoded and, upon successful acquisition, can be used to acquire the data signal. This technique improves acquisition of the GPS signal and boosts power levels at the correlator.
The second advancement is to use Forward Error Correction (FEC) coding on the NAV message itself. Due to the relatively slow transmission rate of NAV data (usually 50 bits per second) small interruptions can have potentially large impacts. Therefore, FEC on the NAV message is a significant improvement in overall signal robustness.
Unlike the C/A code, L2C contains two distinct PRN code sequences to provide ranging information; the Civilian Moderate length code (called CM), and the Civilian Long length code (called CL). The CM code is 10,230 bits long, repeating every 20 ms. The CL code is 767,250 bits long, repeating every 1500 ms. Each signal is transmitted at 511,500 bits per second (bit/s), however they are multiplexed together to form a 1,023,000 bit/s signal.
CM is modulated with the CNAV Navigation Message (see below), where-as CL does not contain any modulated data and is called a dataless sequence. The long, dataless sequence provides for approximately 24 dB greater correlation (~250 times stronger) than L1 C/A-code.
When compared to the C/A signal, L2C has 2.7 dB greater data recovery and 0.7 dB greater carrier-tracking, although its transmission power is 2.3 dB weaker.
In CNAV, two out of every four packets are ephemeris data and at least one of every four packets will include clock data, but the design allows for a wide variety of packets to be transmitted. With a 32-satellite constellation, and the current requirements of what needs to be sent, less than 75% of the bandwidth is used. And only a small fraction of the available packet types have been defined. This enables the system to grow and incorporate advances.
There are many important changes in the new CNAV message:
Defined in IS-GPS-200D
Very little has been published about this new, restricted code. It contains a PRN code of unknown length transmitted at 5.115 Mbit/s. Unlike the P(Y)-code, the M-code is designed to be autonomous; meaning that a user can calculate their position using only the M-code signal. From the P(Y)-code's original design, users had to first lock onto the C/A code and then transfer the lock to the P(Y)-code. Later, direct-acquisition techniques were developed that allowed some users to operate autonomously with the P(Y)-code.
In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a full-Earth antenna. This directional antenna's signal, called a spot beam, is intended to be aimed at a specific region (several hundred kilometers in diameter) and increase the local signal strength by 20 dB, or approximately 100 times stronger. A side effect of having two antennas is that the GPS satellite will appear to be two GPS satellites occupying the same position to those inside the spot beam. While the whole Earth M-code signal is available on the Block IIR-M satellites, the spot beam antennas will not be deployed until the Block III satellites are deployed, tentatively in 2013.
An interesting side effect of having each satellite transmit four separate signals is that the MNAV can potentially transmit four different data channels, offering increased data bandwidth.
The modulation method is binary offset carrier, using a 10.23 MHz subcarrier against the 5.115 MHz code. This signal will have an overall bandwidth of approximately 24 MHz, with significantly separated sideband lobes. The sidebands can be used to improve signal reception.
Two PRN ranging codes are transmitted on L5: the in-phase code (denoted as the I5-code); and the quadra-phase code (denoted as the Q5-code). Both codes are 10,230 bits long and transmitted at 10.23 Mbit/s (1ms repetition). In addition, the I5 stream is modulated with a 10-bit Neuman-Hofman code that is clocked at 1 kHz and the Q5-code is modulated with a 20-bit Neuman-Hofman code that is also clocked at 1 kHz.
The PRN codes are 10,230 bits long and transmitted at 1.023 Mbit/s. It uses both Pilot and Data carriers like L2C.
As of July 2007, the modulation technique has been finalized. The chosen method is to use BOC(1,1) for data with TMBOC(6,1,4/33) for the pilot. The Time Multiplexed Binary Offset Carrier (TMBOC) is BOC(1,1) for all except 4 of 33 cycles, when it switches to BOC(6,1).
Defined in IS-GPS-800
The L1C navigation message, called CNAV-2, is 1800 bits (including FEC) and is transmitted at 100 bit/s. It contains 9-bit time information, 600-bit ephemeris, and 274-bit packetized data payload .
|Band (Frequency)||Phase||Original Usage||Modernized Usage|
|L1 (1575.42 MHz)||In-Phase (I)||Encrypted Precision P(Y) code||Encrypted Precision P(Y) code|
|Quadra-Phase (Q)||Coarse-acquisition (C/A) code||Coarse-acquisition (C/A) code and|
L1 Civilian code and
Military (M) code
|L2 (1227.60 MHz)||In-Phase (I)||Encrypted Precision P(Y) code||Encrypted Precision P(Y) code|
|Quadra-Phase (Q)||L2 Civilian (L2C) code and|
Military (M) code
|L5 (1176.45 MHz)||In-Phase (I)||Safety-of-Life (SoL) Data signal|
|Quadra-Phase (Q)||Safety-of-Life (SoL) Pilot signal|
WIPO ASSIGNS PATENT TO SABANCI UNIVERSITESI FOR "INDOOR POSITIONING SYSTEM BASED ON GPS SIGNALS AND PSEUDOLITES WITH OUTDOOR DIRECTIONAL ANTENNAS" (TURKISH INVENTORS)
Jul 12, 2011; GENEVA, July 12 -- Publication No. WO/2011/080541 was published on July 07. Title of the invention: "INDOOR POSITIONING SYSTEM...
US Patent Issued to Intel Corporation on Aug. 10 for "Method for Acquisition of Gps Signals and Gps Receiver with Sample Time Error and Frequency Offset Compensation" (Israeli Inventor)
Aug 14, 2010; ALEXANDRIA, Va., Aug. 16 -- United States Patent no. 7,773,034, issued on Aug. 10, was assigned to Intel Corporation (Santa...