4.1.4 GPS Receiver Design:

TRENDS IN GPS INSTRUMENTATION

It is impossible to be precise about general trends in GPS instrumentation. The task is no easier if attention is focussed only on a specific market, such as that for GPS surveying receivers. Nevertheless, speculation based on R&D activities being undertaken at present gives some clues:

• The third generation of GPS receivers presently available already exhibit significant gains in miniaturisation, reduction in power consumption, and portability, over the earlier models. This trend can be expected to continue. Surveying receivers are likely to decrease in size quickly to that of a car radio and then to a mobile phone. However, there is also a trend to pack more electronics into the receiver box. Some navigation receivers are "handheld", and "sub credit card" sized units have made their appearance. The miniaturisation of much of the electronics (particularly the tracking channels) onto VLSI circuits means that less power is needed for the receiver to function. But the challenge, as with powering notebook computers, is to develop small, lightweight, longlife batteries.
  
• There have been many predictions of low-cost GPS receivers. Many handheld navigation receivers are available for just a few hundred dollars. It is doubtful whether the cost of GPS surveying receivers will ever fall below a level that is typically ten to a hundred times the cheapest receiver, for a number of reasons:
  • The market (present and projected) volume is relatively small.
  • The tendency to pack more electronics and sophisticated features into the receiver "package".
  • There is a considerably greater software investment in a GPS surveying product than is the case for navigation receivers.
    
• There is a tendency towards product differentiation, with many different configurations of tracking channels, data recording options, and, in particular, software applications. Although some of these trends may be due to manufacturers wanting to give their product "an edge" in the marketplace, it is equally valid to suggest that this is in response to the different demands (some very specialist) of the market. The dilemma confronting GPS manufacturers is whether to maintain a broad-based development program, and market a product that is versatile enough to satisfy many applications (including the provision of interchangeable components such as antennas, RF units, memory, etc., to accomplish this), or to focus on specialist applications (military, navigation, precise navigation, vehicle market, geodesy, surveying, timing, etc.). To date most manufacturers are split between those with products for the surveying/geodesy market, and those focussing on the low-cost, high-volume navigation market. Only a few attempt to address all markets.
  
• There is a tendency, particularly for surveying receivers, to incorporate more channels and strive for "all-in-view" continuous tracking. Multiplexed or fast switching tracking channels are unlikely to be used in future surveying receivers. In the operational GPS constellation sometimes10 satellites or more are visible above the horizon at any one time. Third (and subsequent) generation instrumentation will be able to track all of these, and optionally on both frequencies (requiring at least 20 channels). There is only a trend to develop instruments that can also track the signals from the Russian GLONASS system . However, it remains to be seen whether GLONASS does have a future at all!
  
• There will be no strictly codeless surveying receivers. Proprietary technology is, however, being used extensively to make L2 phase and L2 pseudo-range measurements possible (section 4.2.3), as the Y code generating algorithm implemented under Anti-Spoofing will remain secret into the next decade (at least until the "GPS modernization" program is completed around the middle of the first decade of the new millennium).
  
• There are many trends evident in GPS navigation receiver instrumentation and applications. The core unit of a navigation instrument will be mass produced on one or more chips by a small number of chip manufacturers. These low-cost components will then have "value-added" features grafted to them. An exciting marriage is between satellite navigation and satellite communication, and between satellite navigation and cellular communications, combining real-time positioning with instantaneous transmission of position. GPS receivers will be fitted to many different platforms (some unmanned, for example, railway rolling stock and ship cargo containers), and their locations remotely monitored at a central site via a coms link. In addition, for land navigation these could include electronic map displays to aid the driver. What is clear is that for many applications the navigation data provided by a GPS receiver will merely be the first link in a complex spatial information display and control system.
  
• There is likely to be more self-checking (or "intelligence") capabilities built into GPS instruments. In addition, much of the pre-processing of the data will be carried out within the receiver, including operations such as cycle slip editing, data compression, automatic and continuous operation of the receiver, and real-time carrier phase-based positioning.
  
• The trend towards kinematic positioning with centimetre-level accuracy is very strong (section 5.5.5). This mainly requires advances in the development of carrier phase data processing software, but also in antenna and tracking technology. Although an exciting prospect for airborne and marine applications, its potential to replace static GPS surveying on land will revolutionise further the practices of surveying and mapping. Instead of requiring half an hour or more of phase observations at two (or more) sites, "rapid static" and "stop & go" GPS surveying techniques represent a tremendous boost in productivity. Carrier phase-based positioning with only a few epochs of data will be the norm. In addition, real-time relative position to this accuracy is already possible with the appropriate communication link and processing software.
  
• Perhaps the most significant trends will be in software development. Many more applications may be addressed with the appropriate (and often specialised) software. Such software will take advantage of the advances in instrumentation and make, for example, real-time high precision positioning a simple and routine operation. There is already the possibility of receiver output compatible across different instrument types. Such a common output format has already been devised, the so-called RINEX format (GURTNER, 1994), and has been adopted (albeit sometimes reluctantly) by most surveying receiver manufacturers. The significance of this is that it is (almost) as easy to use a mixture of instrument types for a GPS survey, as a single brand of receiver hardware and software. Similar advances in real-time carrier phase-based positioning will require the definition and adoption of a standard format for data transmissions (similar to the RTCM standard for pseudo-range-based differential GPS).
  
• The next generation of GPS surveying equipment will begin to incorporate advances in antenna technology in order to more fully exploit the accuracy of measurement of the GPS signals. These trends include:
  • Accurate calibration of antennas: Some of the present antenna designs are unsuited to the GPS surveying application, while others have proven well suited. It is important that these antennas be accurately calibrated in order to assure suitable performance.
  • Increased antenna flexibility: Some antenna designs, while well suited to some applications, are poorly suited to others. Although a variety of interchangeable antennas could satisfy specific application requirements, it would be preferable if a single "generic" antenna could be used for all applications.
  • Receiver software to include phase centre offset calibration data: Data processing software is stating to incorporate antenna phase centre calibration data.
  • New antenna design: Antennas are required which are designed with GPS phase stability specifications in mind. Such antenna design should also incorporate amplitude pattern control for minimisation of multipath interference.
  • Improved manufacturing: Mechanical tolerances required for millimetre level phase centre stability are more stringent than those in effect for communication antenna fabrication.
    
• The choice of GPS instrumentation is unlikely to ever be a straightforward decision. Many factors will need to be taken into account, and not all of them of a technical nature. The hardware/software development cycles by different GPS manufacturers are generally out of synchrony, hence just as an instrument from one manufacturer is "maturing", new improved (but untried) systems will be released.

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© Chris Rizos, SNAP-UNSW, 1999