Manufacturers of GPS receivers and related equipment seem to be adopting a somewhat laid-back attitude regarding the impending introduction to the GPS of the new civilian signals, L2C and L5.
The first satellite that will send out L2C signals is scheduled for launch next year. According to Peter Ramm of the Victorian Department of Natural Resources and the Environment, manufacturers of GPS equipment generally believe that they will be able to move quickly to place new, suitable equipment in the market.
Ramm, who is manager of VicMap, met a number of manufacturers at a recent conference. They told him they are confident that they can develop a chip that is capable of receiving and processing the new signals. What's more, they can do so in the time between the satellite's launch and the availability of the new signals.
In fact, Trimble Navigation has already reported carrying out extensive tests with existing receivers, which they claim will be capable of handling the new signals.
Late last year, Trimble said its R-Track GPS receivers had been used to verify the interoperability of the new Block IIR-M satellite payload with current survey equipment. The company claims that its R7 RTK GPS system is the only L2C-ready survey equipment currently available to test the new satellite signal. It said the system has shown that the Block IIR-M satellite data can be acquired, tracked and logged.
The tests were conducted by ITT Industries in New Jersey, where the IIR-M satellites are being tested before launch.
Simulators were used in the tests. These devices can generate spread spectrum signals at the frequencies of the L1, L2C and L5 signals. They can simulate anywhere between four and 30 GPS satellites.
The new civilian signals, together with the signals from the European Galileo system, are good news for users. More power and increased accuracy will benefit most applications, and there will be up to 70dB of anti-jamming capability.
When discussing the modernisation of the GPS, it is useful to distinguish between the GPS frequencies, and the signals that are carried on those frequencies. Currently, Block II/IIA and Block IIR satellites emit the civilian C/A code on the L1 frequency, and the military P(Y) code on both the L1 and L2 frequency.
The new Block IIR-M satellites will emit the same signals as the previous two blocks, but will have a new signal added - L2C on the L2 frequency.
The fourth generation of satellites, Block IIF, is scheduled for launch beginning in 2006. They will have all the capabilities of the previous blocks, plus the new military M signal on both L1 and L2, and a new civil code at a third frequency, L5.
All of the GPS frequencies consist of strings of chips, or bits, which together make up a unique code.
L1 has a carrier frequency of 1575.42 MHz, a code length of 1023 chips, and a bit rate of 50 bits per second. L2 has a carrier frequency of 1227.60 MHz. L5 has a carrier frequency of 1176.45 MHz, a code length of 10,230 chips and a bit rate of 50 bits per second.
L2C has two codes, the CM and the CL. The CM code is 10,230 chips long and repeats every 20 milliseconds. It is bi-phase modulated with message data. The CL code is 767,250 chips long and repeats every 1.5 seconds.
The accuracy of stand-alone positioning can be expected to improve from its existing 10 to 20 metres (using the C/A code), to 5 to 10 metres (using both C/A and L2C). Most of the improvement comes from dual frequency ionospheric correction. Using C/A, plus the additional civilian code at L5, drives accuracy to between one and five metres.
The implication is that while GPS accuracy will improve dramatically, differential correction will still be necessary in many cases. Such systems will deliver horizontal accuracy between a half and five metres. This will typically be used in applications such as professional mapping and GIS data collection - to map public utilities, to map logging roads, wildlife habitats or bush-walking tracks.
Surveying GPS equipment requires a horizontal accuracy of between 1 and 2 cm. Such systems are used, for example, for cadastral and property surveys, and for construction stakeout. This accuracy can be obtained in real time, using a portable base station.
Several factors were taken into account in the development of the new L2C signal. The signal had to serve the increasing number of dual-frequency civil users, who employ high-value receivers for their professional or commercial applications. They represent a purchase value of more than 1 billion dollars.
However, the most important objective was to eliminate the need for the semi-codeless tracking technique. A replica of the C/A code would meet this requirement, but L2C will improve performance by having no data on one of its two codes. This improves threshold tracking performance by 3dB, and provides full-wavelength carrier phase measurements without the 180 degree phase ambiguity that is inherent in GPS signals which carry data.
Since L2 is shared between civil and military signals, L2C is limited to a single bi-phase component in quadrature with the P(Y) code. Even while L2C is also limited to a 1023MHz clock rate to maintain spectral separation from the military M code, there is an important advantage in having two codes.
The advantage stems from the fact that L2C time multiplexes two codes of different length. The composite signal is clocked at 1023MHz, and alternates between chips of each code.
Another objective was to make L2 suitable for many single-frequency GPS applications that have, until now, only been served by the L1 signal.
L5 was designed with two equal power signal components - one with data, and one without. Although each component has only half the total power, the 6dB threshold advantage of tracking a dataless signal gives an overall 3dB tracking improvement. Since L5 is not shared with military signals, it achieves the power split by using two equal-length codes in phase quadrature, each clocked at 10.23MHz.
One of the great practical advantages of this extra complexity is better signal acquisition indoors. The downside is that it will require much more work from the receiver during acquisition.
Work already done for indoor GPS will help to some extent - with highly parallel time domain search with multiple correlators, and highly parallel frequency domain search with Fast Fourier Transforms.
For general outdoor use, the signal designers recommend using the 10,230 chip data signal for acquisition of the first satellite. Most receivers using L2C will be dual-frequency receivers. Such receivers will be able to acquire the L1 code first. Acquisition of the L2C signal will then only entail an ambiguity of less than a code-chip.
The GPS L5 and the Galileo E5A signals share a common frequency and a common basic code length of 10,230 chips. Both have a common chipping rate of 10.23MHz, and use tiered codes. Tiered codes generate a very long code by multiplying a medium length fast code by a short slow code. Thus, a pre-acquisition can be achieved within one chip of the slow code, and the results stored and post-processed.
The acquisition can be done in two stages, at first integrating for only the period of the primary code. This is extended to the full composite code only when high sensitivity is required. For tracking purposes, after the acquisition, the long code would be used all the time.
Advances in computing power and digital signal processing during recent decades have made it possible to use the new signals economically.
This is a result of the judicious selection of equipment, and the use of multiple frequencies or wideband signals only where the application justifies the cost.
It is believed that many, if not most, receivers will be dual standard - Galileo plus GPS. The critical issue will be the economic implementation of both techniques.
Receivers capable of accessing more than 50 satellites - Galileo and GPS - and capable of using signals attenuated through walls by 25dB, will make it possible to use GNSS inside buildings.
We can only begin to guess how this capability will change the world of positioning. For both surveyors and navigators, new levels of accuracy and reliability are in the offing. For manufacturers, the possibilities seem limitless.
