Q. What are the main features that separate one GPS receiver from another?

A. Mike Etherington, Magellan Systems Corp.: What separates one GPS receiver from another are the applications for the products. User-need determines what features are designed into a GPS receiver, and those features determine the price of the unit. Arthur Lange, Trimble Navigation: There are two kinds of capabilities within a GPS receiver: the capabilities of the GPS engine and the capabilities of the user application. The differences between the capabilities of GPS engine are described by the following parameters: time-to-first-fix, number of fixes per second, maximum velocity, code phase noise, multi-path resistance, differential accuracy, power consumption, and the like. Differences in these GPS engine parameters result from trade-offs in the engineering design phase regarding cost and performance, and have measurable effects on the GPS engine performance.
      Price and performance are related: the lower cost GPS receivers usually have lower performance than more expensive GPS receivers. The differences between the user application software of GPS receivers results in one GPS receiver being well-suited for a particular application and another GPS receiver not usable in the same application.

John Stenmark, Leica Inc.: There are too many interesting features to mention in this space. Describing the many different types and capabilities of GPS equipment could fill an entire issue. Instead, consider the applications. Before looking at receiver feature lists, you should take time to examine your own needs. This will quickly narrow your search.
     What type of work will you use GPS to accomplish? Navigation, GIS, surveying, machine control, monitoring are all common uses and may employ similar or radically different GPS equipment. What are your requirements for accuracy? Do you need positions accurate to 1 meter, 10 meters, 100 meters, or 0.01 meter? How much experience with GPS do you or your clients have? Who will process and analyze the GPS results? Where will the equipment be used? In remote locations, or close to a central facility? Carried by hand, in a backpack, or mounted in a vehicle or on a rooftop? Do you need positions immediately, or can you wait a few hours to see the results. Finally, how much money is in your budget?
      Now that you have a clear idea of what you will use your GPS equipment for, your can focus on various receivers and procedures. While high-end GPS receivers can perform virtually all types of work and provide all levels of accuracy, you may be better off with simpler, lower cost equipment that is best suited for your job. That said, there are two key features to look for in a GPS product: upgradability and flexibility. The various products differ greatly in these areas. We must expect that GPS technology will continue to improve. Be sure that your system can expand with time. Don't work within a focus that is too narrow. As you examine your needs, anticipate expansion of needed functionality. Choose your equipment with an eye towards where you want it to be in two years. While this may require a larger investment up front, the payoff over time will be substantial.

Dr. Frank van Diggelen, Ashtech Inc.: The features that matter depend on the application.
• Geodetic, Scientific, Reference Stations and Survey: measurement precision; position accuracy, real-time (centimeters) and post-processed (millimeters); reliability of accuracy; speed with which reliable high accuracy is achieved; satellite tracking during ionospheric variation related to sunspot activity1; jam immunity; and number of channels (12 for all-in-view).
¥ Precision positioning and machine control: position accuracy, real-time (decimeters to centimeters); reliability of accuracy; speed with which reliable high accuracy is achieved; update rate; satellite tracking during ionospheric variation related to sunspot activity1; and jam immunity.
• GIS: ease of use; position accuracy, real-time and post-processed (decimeters or better); compatibility with GIS software; and reliability under adverse conditions - e.g. accuracy under trees. • Navigation: position accuracy (meters or better); attitude determination capability; and reliability.
• Consumer: cost and ease of use.
      Note1: Increased sunspot activity is coming! The 11 year sunspot cycle has just passed the point of least activity. Ionospheric interference with GPS will increase through the next five years, this is of particular concern for real-time centimeter applications requiring dual-frequency measurements.

Q. If multipath is still regarded as one of the major limitations in real-time performance, then what can be done about it, and what research efforts are being focused on the problem?

A. Wendy Corcoran, NovAtel Communications Ltd.: There are three ways to reduce the effects of multipath: 1. Select a site free of multipath effects; 2. select a GPS antenna that has a flat profile (i.e.: microstrip) or uses a choke ring which both reject low angle reflections; 3. chose a GPS receiver with Narrow Correlator capability. Multipath can never be totally eliminated but there have been efforts at minimizing its effect. NovAtel GPS is one of the few companies that has focused its research and development around the tracking loops of the GPS receivers to minimize multipath for commercial applications. The Narrow Correlator technique that was patented by NovAtel GPS is an example of this. In all GPS receivers, the incoming signal is compared with an internally generated signal. When these two signals are aligned, the time offset is taken and multiplied by the speed of light to compute the range to the satellite. The matching of these two signals is called correlation. In standard correlator the amount of C/A code signal captured is less than in narrow correlators and the spacing of the samples during correlation is wider verses the narrow spacing. In standard correlator GPS receivers the pseudorange measurements are not as accurate, there is more noise in the measurements and it is highly susceptible to multipath distortion. In narrow correlator GPS receivers, multipath effects are reduced, noise is reduced and the pseudorange measurements are more accurate.
      Other methods of reducing multipath have also been patented at NovAtel GPS, including MET and MEDLL. MET (Multipath Elimination Technique) is described as a software choke ring. NovAtel GPS has worked on identifying and eliminating multipath effects in the tracking loops of the GPS receiver. This option reduces multipath by an additional 25-50 percent over narrow correlation. MET is available on all NovAtel GPSCards. The second multipath patent is MEDLL (Multipath Estimation Delay Lock Loop). MEDLL is a multicard rack which reduces the effects of carrier and pseudorange multipath by up to 90 percent. If you would like more information, NovAtel has a technical paper called RF Signal Progation and Multipath which explains all the bases of multipath and the techniques available to estimate and eliminate its effects.

Etherington: Magellan does not regard multipath as a major problem for a real-time system. Multipath resistant antenna design and software approaches have mitigated the effects of multipath.

Lange: Given a particular GPS receiver, multipath interference can be reduced by using an antenna ground plane. The antenna ground plane attenuates the GPS signal reflections from the ground and other objects below the level of the GPS antenna. The effects of multi-path are often reduced when the GPS receiver is moving, since the multi-path reflections become random and are rapidly changing with motion. Research efforts are being focused on designing special digital filters which can detect the presence of multi-path interference and cancel its effects.

Stenmark: Multipath is well known as a problem in both real-time and post-processed GPS. Multipath introduces systematic error into the GPS measurements and affects both differential and single point GPS observations. Zero baseline tests show that multipath has its largest effect on differential code. Its effect on differential phase baselines is much smaller.
      It is very difficult to predetermine multipath conditions at any given location. It is normal to have different systematic error at each GPS site. This is made even more difficult in a kinematic application where the changing antenna location may result in dynamic changes in the effects of multipath.
      From the user's standpoint, prevention is the best medicine. Multipath problems can be avoided by careful site selection. Take care to stay clear of buildings and other structures or landforms that may reflect the GPS signals. Use of ground planes and choke-ring antennas is also helpful. Unfortunately, neither of these options is very attractive or practical for real-time GPS.
      It is certainly reasonable to assume that GPS developers have dedicated resources to this issue. At this time, it is premature to discuss specific approaches to further resolving the multipath difficulty.

van Diggelen: For high accuracy, multipath is indeed one of the major limitations in performance. Ashtech undertakes continuous R&D in all areas that affect Global Positioning performance, both in the receiver and antenna designs. However, it is Ashtech policy not to release specific details of research in advance of product releases.

Q. What is the effect of powerlines on the GPS signal? Can GPS be used under or in near proximity to strong magnetic fields?

A. Corcoran: Very few tests have been done to investigate the effects of powerlines on GPS signal reception. Powerlines primarily effect the RF section of each GPS receiver and since each manufacturer has different methods of shielding to try and minimize external interference, each receiver will react differently in that environment.
      The AC voltage carried by powerlines has a frequency of 60 Hz regardless of the type of powerlines. Powerlines located in neighborhoods have a lower voltage than the high voltage towers in remote areas. The housing powerlines should not effect the GPS signal but the high voltage line may. The rule of thumb is to be 100 to 200 feet from the lines to avoid interference but given most of these lines are located on towers that clearance may already be sufficient.

Etherington: Magnetic fields have no effect on the GPS signal, nor do power lines; even electric transmission towers and their stronger EMF emissions have minimal effect.

Lange: In North America the energy from powerlines is 60 Hz and the harmonics of 60 Hz, (50 Hz in Europe) with the harmonic energy falling off rapidly as the frequency increases. Magnetic field power line harmonics are not a factor at GPS frequencies.
      However, certain computers radiate electromagnetic energy at the GPS frequency and can interfere with a GPS receiver. This can be verified by moving the GPS antenna closer to the suspect electronics and observing the GPS receiver SNR values. If the SNR decreases with the GPS antenna near the suspect electronics, then the electronic device is radiating measurable GPS interference. The remedy is to move the GPS antenna away from the offending electronics.

Stenmark: Overhead power transmission lines create electromagnetic fields. We have seen little or no effect of powerlines on the GPS signal. Overhead telephone lines pose no problems. Operation of GPS receivers near strong magnetic fields (e.g. electromagnetic cranes) also shows little difficulty.
      One possible concern is cable television transmission systems, which often occupy the same infrastructure as electric power and telephone networks. CATV signals can generate harmonics with the GPS frequencies. If the CATV signal is strong and poorly insulated, then problems may arise. The operator should monitor the performance of the GPS equipment when this situation is encountered.
      This brings us to a catch-22: These types of transmission facilities are often exactly the items that the GIS/GPS operator is assigned to locate. If the towers or poles are occupied by a 'leaky' CATV distribution system, then life can become difficult for the GPS operator.

van Diggelen: Powerlines and strong magnetic fields can cause problems in some receivers. However, internal and independent tests have shown Ashtech receivers to have very little, if any, degradation in performance under these adverse conditions.

About the Participants:
Wendy Corcoran serves as manager, survey and mapping products at NovAtel Communications Ltd. in Calgary, Alberta, Canada. She may be reached at 403-295-4900. Mike Etherington is a marketing and sales manager at Magellan Systems Corp. in San Dimas, Calif. He may be reached at 909-394-5000. Arthur Lange is GIS product manager at Trimble Navigation in Sunnyvale, Calif. He may be reached at 408-481-2994, or via e-mail: [email protected] John Stenmark serves as manager, product support, Geodesy North America at Leica Inc. in Englewood, Colo. He may be reached at 303-799-9453 or 800-475-3422 in the U.S. Dr. Frank van Diggelen is a marketing manager at Ashtech Inc. in Sunnyvale, Calif. He may be reached at 408-524-1508, or via e-mail: [email protected]

BACK