An article on how GPS Works

SiRF Technology, Inc. is bringing breakthrough GPS technology, along with other wireless innovations, to a wide range of consumer applications. The SiRFstarI/LX™ GPS chipset and software is distinguished by permitting designers to build small, low-cost, low power GPS-enabled consumer products. What’s more, the technology supports navigation in cities with narrow streets and tall buildings, and in areas of dense foliage (e.g. forests). These are the two prime areas for a large volume, consumer GPS product market, and precisely the areas in which current GPS technologies fail to work well. The LX designation indicates new low power extensions that have been added to reduce the power consumption of the GPS solution for use in handhelds and other power sensitive applications.

SiRF’s GPS technology features are unique to the commercial GPS market. These include: Rapid satellite signal re-acquisition (SnapLock™ acquisition), single satellite navigation (SingleSat™ positioning), Dual Multipath Rejection, high sensitivity and dynamic range (FoliageLock™ reception), and TricklePowerä advanced power management mode.

SnapLock, SingleSat and Dual Multipath Rejection are the keys to effective and accurate navigation in cities. These three features, plus FoliageLock reception, permit users of SiRF-enabled GPS products to use them in areas where other systems fail to work, and to re-acquire satellite signals more quickly after exiting a blocked area, such as a tunnel. Because of the company’s expertise in GPS system architecture, spread-spectrum technology and RF system design, SiRF has been able to implement these features in cost-effective silicon-and-software solutions.

The Global Positioning System

The US Department of Defense (DoD) has built an elaborate system for pinpointing locations virtually anywhere on the planet. A constellation of 24 NAVSTAR satellites circles the Earth every 12 hours emitting radio signals that contain information about their positions. Specialized receivers, on or near the planet surface, receive these signals and calculate their positions relative to these satellites.

With simultaneous data received from four satellites, one’s position (e.g. latitude, longitude, altitude and time) can be calculated. Under ideal conditions, the location is precisely and accurately determined. However, under real conditions, there is always some degree of error. Errors can be caused by the degradation applied by the US DoD to the satellite signal (called Selective Availability or SA), and by signal delay in the ionosphere. Despite the opportunity for error, positioning can be calculated to within a few hundred feet or less in most cases.

Four equations; four unknowns

Essentially, the way the GPS works is a receiver picks up signals from four satellites and measures the time it took for those signals to arrive. From this timing information, one can calculate the distance between the receiver and each satellite. The four satellites’ ephemeris data provide the satellite’s X, Y, and Z positions. The range, R, is the receiver measurement made by calculating the time it took for the signal to reach the receiver. The user’s position, (Ux, Uy, Uz), and the clock bias, Cb, is then calculated (see figure 1).

Spectrum sharing

Each of the 24 satellites transmits a set of signals using spread spectrum technology. Spread spectrum technology enables low-powered satellites to produce signals that can be detected at very low received-signal levels. Essentially, the carrier signal is modulated by a unique coding sequence which has the effect of spreading the signal’s frequency spectrum. Using a replicated code sequence, a GPS receiver searches that spectrum looking for a match. The signal can then be "unspread" and decoded. By transmitting several signals over the same spectrum, but using distinctly different coding sequences, each signal can share the spectrum without interfering with any of the others.

Error processing

The GPS assumes that signals will be traveling between satellite and receiver in a straight line. The signal will actually be delayed upon going through the ionosphere, and the receiver timing references will have some small error. Both of these errors are predictable and correctable. The aforementioned SA process also induces an error. However, using data from more than four satellites can mitigate that error. Nevertheless, the SA-induced error is presently a fact of life in each position calculation. Fortunately, SA will hamper very precise positioning accuracy, but not to a point where it undermines the requirements for personal navigation. Furthermore, the DoD is scheduled to eliminate SA by the year 2007. In the meantime, systems with higher accuracy requirements can receive locally generated differential corrections to enhance performance.

Multipath error

Multipath error, on the other hand, can produce very large deviations. Multipath is caused by satellite signals that arrive at the receiver after having bounced off some nearby structure (e.g. a tall building), or the ground. Because the path is not straight, the time delay will be longer, and the distance from the satellite will also seem to be longer (see figure 2). This can produce location errors that are unacceptable, particularly in urban automobile navigation applications.

Signal attenuation

Non-restricted GPS signals are transmitted at 1.575 GHz, a microwave frequency. Such signals are blocked by steel and concrete structures (e.g. buildings and tunnels), and attenuated by passing through trees and leaves. The GPS specification for minimum detectable signals renders reception marginal when the signal is attenuated by foliage. The denser the foliage, the more marginal the signal. As such, receivers that just meet this specification are not reliable for use in forests or even tree-lined streets. To ensure being able to detect signals in a forest, the receiver must provide sensitivity that exceeds the current standard. For example, the receiver must be able to detect signals whose power has been attenuated to a level of about 5 percent of the initial level.

Applying the GPS

To build a GPS-based navigation product, one must design a radio that can receive the spread-spectrum signals. The detected signals are then converted from RF signals into appropriate digital input formats. These digital inputs are processed and converted into position information, and the information is then processed to produce the required application output (e.g. a blinking cursor on a map overlay, a readout of latitude and longitude, etc.)

SiRFstar Technology

The SiRFstarI/LX architecture is implemented in a chipset and modular software system. This high performance, highly integrated architecture is optimized for mainstream automobile navigation and a wide range of consumer products requiring low power consumption.

GRF1/LX

The GRF1/LX chip is a RF Front End component that converts GPS signals from their 1.575 GHz frequency into baseband signals. To accomplish this, the IC integrates an LNA, mixers, amplifiers, a synthesizer and an analog-to-digital converter. The GRF1/LX also incorporates an on-chip voltage-controlled oscillator (VCO) so that it requires only an external crystal rather than an external oscillator. The single local oscillator (LO) design requires a minimum of discrete components to implement a suitable GPS front end (see figure 3).

Figure 3. The SiRFstarI GRF1/LX provides all the functions required to provide front end RF GPS signal reception and conversion.

The GRF1/LX is designed to interface to standard active antennas (or passive antennas with an on-board LNA), and provides two-bit interface to its digital signal processing companion chip, the GSP1/LX. With its high-speed, parallel processing, signal-processor architecture, the GSP1/LX is optimized for GPS designs. Its SnapLock feature offers 100 millisecond satellite signal re-acquisition, a speed that is 20 to 30 times faster than alternative ICs. Since positioning accuracy improves with each additional satellite data stream and potentially a maximum of 12 satellites may be visible at one time, SiRF designed a parallel 12-channel architecture that permits simultaneous processing of up to 12 satellite signals.

The GSP1/LX is designed to interface with any standard 8, 16 or 32 bit microprocessor, and its 8- or 16-bit memory interface supports either dynamic (DRAM) or static (SRAM) memory chips. It features a 2-bit interface to the GRF1/LX and can produce 10 positions-per-second output. The GSP1/LX has 2 full duplex serial ports that offer programmable data rates up to 38.4Kbaud. (see figure 4).

Figure 4. The GSP1/LX is a 12-channel, parallel processing, GPS signal processor. It is designed to interface with standard microprocessors and memory chips, and together with the GSW1/LX modular software, provides SnapLock, SingleSat , Dual Multipath Rejection and TricklePower functionality.

In addition to the chipset, the SiRFstar solution includes optimized and modular GPS software. The GSW1/LX’s modules include a customer-controllable receiver manager, tracking loops, data demodulation, navigation filtering, I/O and GSP interface, a standard API interface, plus drivers for PC oriented applications.

The SiRFstarI/LX chipset and software solution can be used to build navigation modules, integrated navigation solutions, and GPS-enabled PC products (see figure 5).

Differentiating Features

There are several differentiating features that make SiRFstarI/LX technology especially well suited to a wide variety of high-performance, low-cost, low power consumer GPS products and applications.

SnapLock acquisition

SiRF’s SnapLock acquisition feature provides re-acquisition of satellite signals in only 100 milliseconds, as well as fast initial search. SnapLock acquisition results from a parallel spectrum search to find code correlation, involving 20 code samples. Alternative devices take typically two to three seconds to re-acquire a lost signal, and may take minutes to do the initial search.

SnapLock acquisition is a critically important feature for automobile navigation. Cars lose satellite visibility in cities because they are blocked by tall buildings and tunnels, but they get a clear view in intersections, or when exiting a tunnel. The average time in an intersection is one to three seconds, but a re-acquisition time of two or three seconds leaves no time for collecting signal data. SnapLock acquisition re-acquires the signal and collects a measurement for a position update in one-tenth of a second. Thus, an intersection offers enough time for both re-acquisition and positioning when a system is based on SiRFstar technology. This high-speed re-acquisition is also a key part of the power management scheme. Since the signal can be re-acquired in 100ms the chipset can be power cycled at a rate faster than the standard 1Hz update rate, causing no apparent loss of data but at greatly reduced power consumption.

SingleSat positioning

When driving in an urban area, a car’s satellite visibility is often blocked by intervening buildings. For other GPS systems, when less than three satellites are visible, no positioning calculations can be made. However, SiRF’s SingleSat positioning mode allows positioning calculations, for short periods, when only a single satellite is visible. SingleSat positioning works by using a single satellite’s data to determine how far along a current path the car has traveled. Any errors in position can be corrected as soon as SnapLock reacquires three or more satellite signals (e.g. when the car passes through an intersection). Car navigation systems employing SiRFstar technology will thus provide more position fixes than other systems when navigating in an urban setting.

Dual multipath rejection

Multipath errors occur when signals reach a receiver along an indirect path. Low level reflected signals bouncing off of far-away objects are simply eliminated. Errors caused by nearby reflected signals are filtered. Without such a rejection scheme, multipath-induced errors often cause random, large-scale errors in positioning for car navigation systems being used in urban areas. SiRFstar’s Dual Multipath Rejection capabilities significantly reduce multipath errors eliminating these large-scale deviations.

Foliage lock sensitivity

The GPS standard signal threshold is -160 dBW. It allows for receiving a signal that is much reduced in power. However, car navigation and personal navigation products used in tree lined or wooded areas will often receive satellite signals that are below this threshold. FoliageLock sensitivity is 20 dB lower than the threshold standard. Thus, signals that are indistinguishable to other GPS receivers are detectable with those based on SiRFstar technology.

Reducing Power Consumption

The LX extensions to the original SiRFstar architecture reduce power through new hardware and software. New foundry technology and peripheral integration in both chips reduce the overall system power consumption in hardware. The GSP1/LX also contains a high-precision real time clock that allows the software to keep very accurate time (to a few microseconds) during power down to enable very fast restarts. In addition, new software in TricklePower mode puts the power to the GPS chipset under software control. By using the SnapLock reacquisition capabilities, the chipset can be turned off for up to 800ms of every second and still reacquire, track and produce a new solution in the remaining 200ms. This allows the receiver to provide a continuous 1Hz update and only use approximately 1/5 of the power. In addition, the software has a push-to-fix mode which allows the receiver to autonomously turn on and collect the necessary data to provide a SnapStartä position fix in under 2 seconds. The background consumption of the push-to-fix mode has the chipset operating only 2% of the time.

In sum…

By offering a GPS solution with these and other features, and by offering that solution in both board and chip-level form factors, SiRF Technology is ensuring that designers have the flexibility to build a variety of products, with success-factor features, in sizes and prices that will stimulate the demand for GPS-enabled consumer products.