The GPS Q & A Column is intend-ed to directly address and answer commonly asked user questions. This monthly column is designed to help users realize the full potential of GPS. Its goal is to broaden the understanding of GPS in general, while educating current and future users about its capabilities.

Q. What features should I consider when choosing a GPS receiver?

A. Wendy Corcoran, NOVATEL Communications: The most important thing consumers of GPS can do for themselves is to have a very clear description of what their application is before purchasing a GPS receiver. Many people believe the accuracy is the most important feature but even if a GPS receiver could obtain the accuracy required, it may not be able to given a different application. For example, some GPS receivers can obtain accuracies within a few centimeters, but in an F-18 or on rough seas with the boat pitching and heaving, these same accuracies are not possible. Make sure you explain the application thoroughly to the GPS vendor and have them commit to the standards you require.
      Other features would be:
a) Number of Channels - Some applications do not require that all satellites in view be tracked but in a majority of cases, the more satellites tracked, the more assured you will be that there is enough data to process, there is redundancy and in some cases, the occupation time at a station can be reduced. Having all satellite tracking capability is especially important for differential reference stations. Purchase GPS receivers with dedicated or "parallel" channels also. This means that only one satellite is tracked per channel and that the time is not shared on that channel with other satellites. Parallel channels have better tracking characteristics and any other technique is old technology.
b) What does the GPS receiver measure? - There are four types of measurements that can be made with a GPS receiver - C/A code, P Code and the carrier frequencies L1 and L2. More measurements usually translates to higher accuracies and better performance but higher prices. Again, make sure you know what accuracy you require because you do not want to pay for something you do not need.
c) Data storage - Ask if the data is stored and if so how many hours of operation can you achieve given the accuracies you need. Many times sales people will tell you the GPS receiver can store up to 10 hours of data but what they do not tell you is the data rate used and the number of satellites this is based on. The 10 hour figure could be computed based on tracking five satellites, logging data every 30 seconds. For your application you may require all satellites, logging at 1 second. Be clear about your application and what data rate would be required for the accuracy you need, then find out how many hours of storage there is. If you do not need memory storage, don't pay for it.
d) What is the update rate? - Depending on the application, you may only need to log data every 15 seconds (static applications) or 10 times a second (aircraft positioning). Make sure the GPS receiver your interested in can handle the data rates you require.
e) Hours of battery operation - Know how long you will be able to operate in the field based on the batteries used. This is the biggest problem in GPS use because people do not plan enough battery power for the job. If you require more operational time than the battery supplies, you have to calculate the additional batteries in the price you will pay for a GPS system.
f) Real time differential applications - Some purchasers of GPS equipment require an accurate GPS position in the field and cannot wait until the field data is processed with another receiver after the job is done. Make sure the equipment you are purchasing can work in real-time. In most cases, this is an additional cost so be aware of this and ask up front.
g) Ask what is included in the price - Some manufacturers include a complete field setup in the price they are quoting but make sure this is the case. If this is just the price of the GPS receiver, you need to ask what additional equipment is required for a complete field setup and get quotes for all of these components. This goes back to the initial comment of knowing your application and describing it accurately to a GPS vendor. If they know about GPS they will know what you need for a complete system. If they cannot answer your question, talk to someone else - it's important that you know this information otherwise you will be paying again later just to get the equipment operational.
      The ultimate feature is ease of use, otherwise known as "user friendly." A GPS receiver can have all the features you require but if you need a Ph.D. to understand the screens or process the data, it may not be the one for you.

Craig Hudson, II Morrow: First, you need to understand the difference between single channel sequential GPS receivers and multi-channel (minimum of at least four channels) parallel designs. The first issue is TTFF. All low end single channel designs require 2 or 3 minutes for satellite acquisition and position computation. Multi-channel designs will have acquisition times under 1 minute. After all, time is money!
      Then, compare all multi-channel receivers for ease of use. Complex 56 keypad designs are cumbersome when most of your data is entered by using the scroll keys navigating through pull down menus. A clean, intuitive and programmable interface will save thousands of keystrokes in the field. Evaluate how much data is actually "entered" in the field and ensure the "arrow" keys are not the smallest on the keypad. Explore programmable PC interfaces, which allow the building of pull down menus for handheld attribution in the field.
      "A picture is worth a 1,000 words," so evaluate the data presentation on the handheld display. Some receivers have a two or three line alpha-numeric display, while others offer large "bit-mapped" graphic displays. This can be important for visually validating field collected data, while still in the field.

John Bohlke, SOKKIA Corp.: The following features should be taken into consideration: horizontal accuracy; user interface; data collection capabilities; software compatibility (RINEX); RTCM capability; ruggedness; storage capacity; battery life; and weight.

Frank van Diggelen, Ashtech Inc.: Look for these features: accuracy; durability; reliability; and application specific software. For high accuracy: dual frequency receivers that track all signals (L1 and L2, full wavelength carrier wave and pseudo-range).

Q. What is differential GPS and how does it work? I'm aware that two GPS receivers running at the same time can be more accurate than only one receiver. Where does the error go? How is this possible?

A. Corcoran: Differential GPS is when two GPS receivers are tracking the same satellites at the same moment in time and the data collected at these two (or more) stations are processed together. When processing the data, errors that are contained in one station's data is assumed to be contained in the other station's data as well. When the two are compared, the common error can be eliminated or reduced making the overall measurements more accurate. For example, if Station A is tracking satellite 3, there are errors in the range measurements because of the signal's travel through space. Maybe the position satellite 3 is transmitting has an error and satellite 3's clock has a drift or bias. If Station B is tracking the same satellite, its range measurement will have the same error. When these two range measurements to satellite 3 are compared (in fact subtracted or differenced), these errors can be removed to improve the range measurements taken. This happens for every satellite that is common between the two stations, then position is computed.

Hudson: All GPS receivers are affected by several error sources, both imposed and inherent in the design of satellite positioning. These errors can place your computed position anywhere within a "football field" of your actual position. You can locate one receiver at the 50 yard line (a known position) and then: either record the satellite information for post processing, back on the bench after the game is over; or transmit the satellite information to all receivers in the field for real time correction and real time accuracy.
      DGPS can compensate for many of the satellite and system errors and bring higher accuracy to your data. Sometimes to the single "hash mark!"

Bohlke: Differential GPS involves collecting data with a fixed GPS receiver on a known position while using a second roving receiver to collect various unknown positions. The two sets of data are processed together in order to increase the positional accuracy. The fixed receiver at the known position will log signal errors so they can be "de-applied" to the roving receiver data during processing to improve positional accuracy.

van Diggelen: The Global Positioning System is operated by the U.S. Air Force and it provides a worldwide, 24 hour, 3d positioning service to anyone with a GPS receiver. However, the available accuracy to civilian users is deliberately degraded to 100m. This deliberate degradation is known as Selective Availability (SA). To achieve higher accuracies it is necessary to have differential corrections from a GPS reference station.
      Differential corrections are generated by a GPS receiver which is fixed at a known position. The receiver (known as a base, master or reference receiver) uses the knowledge of its own position and the satellite positions to calculate the errors and corrections. The corrections can be applied to any other receiver (the remote, slave or rover receiver) in the same general area. The effect of the corrections is to cancel the errors caused by SA, as well as any other common errors (such as satellite clock errors, orbit errors and errors caused by the effects of the atmosphere on the signal).
      To use DGPS you require either your own reference receiver, or access to a differential service. If you use a GPS/GIS receiver as a rover, then a second GPS/GIS receiver can be used as a reference receiver. It is common practice to use the same model receivers for reference and rover. Differential services provide differential corrections on a radio broadcast, or on bulletin boards. Commercial services charge. The Coast Guard DGPS service and some bulletin boards are free.
      Differential corrections are used in two ways:
      1. A radio receiver monitors corrections transmitted by a DGPS service. These corrections can be used for real-time differential operation.
      2. Corrections are collected and stored at the reference receiver for post-processing with the data stored in the field.
      Mode 1 applies to the Coast Guard DGPS network. Mode 2 applies when you set up your own reference station, and you do not have a real-time radio link in the field, or when you get corrections from a bulletin board.
      Differential corrections are most effective when the rover receiver is close to the reference (within 100 miles). Differential corrections may be applied at a rover receiver which is far from the reference. The SA errors will still be cancelled but the atmospheric and orbit errors will not. This will lead to an extra error of about a meter. The use of differential corrections over long distances is known as Wide Area DGPS.
      The errors that are removed are common to both the base station and the remote station. The base station works out its error and the remote station subtracts this error from the GPS measurement, thus removing the common error.

Q. What kind of accuracies can I expect from GPS data?

A. Corcoran: The accuracies attainable from GPS data vary according to the measurements taken by the GPS receiver. There are four types of GPS measurements that can be made: C/A code, P code, and the two carrier frequencies, L1 and L2. Accuracies are usually reflected by what is measured, but other factors can be influenced also such as distance from a reference station and the time required to obtain those accuracies.
      Under the C/A code measurements there is also an exception. Some of the C/A code receivers being manufactured today use a special technique called Narrow Correlation which will give them similar accuracies as a P code receiver (differentially). The following page contains a brief overview showing the types of accuracies that can be attained with the typical types of receivers on the market. (See Figure 1.)

Hudson: Autonomous positioning, using "raw" satellite data, can position the user within a "football field" of their actual position. However, the use of differential techniques can restore the positional accuracy to within a few meters. The user needs to understand that DGPS accuracy will vary across applications and environments. GPS accuracy will degrade as satellite visibility (affected by buildings, foliage and mask angle), GPS signal reflections (multipath), distance to base station and actual satellites move across the sky (DOP). For many GIS/LIS applications, meter level positioning can be achieved without becoming a GPS expert. However, sub-meter accuracy will require a broader understanding of GPS. The user should evaluate the GPS equipment in the actual environment and not accept controlled sales demonstrations.

Bohlke: mm-10m, depending on the system used, how the data are processed, satellite configuration, the environment, etc.

van Diggelen: Typical accuracies (95 percent accuracy) are:
1. The raw GPS pseudo-range measurement is accurate to 100m.
2. The differentially corrected pseudo-range is accurate to 1m.
3. The differentially corrected carrier-phase is precise to 1cm.
      All GPS receivers produce positions from pseudo-ranges. Some GPS systems (with appropriate software) produce positions (with centimeter accuracy) from carrier-phases.

Q. What is RINEX? What is it used for? By whom? Why?

A. Corcoran: RINEX stands for Receiver INdependent EXchange format. It is a common format for GPS data agreed upon by government and industry. The purpose of creating RINEX was so that all GPS manufacturer's data could be combined together for processing. Each GPS manufacturer has their own data formats and therefore one manufacturer's data could not be combined with another until RINEX. With the data in one format, RINEX, surveys can be done with various manufacturers equipment. This also allows the consumer the flexibility of choosing another GPS manufacturer, if they desire, instead of having to remain with the same company they purchased from before.

Hudson: If post processed DGPS solutions are required for your application, then RINEX should be in your vocabulary. The Receiver Independent Exchange format was established to allow any manufacturer's GPS data to "communicate" in a common and open format. Originating from a European application back in 1989, RINEX has gone through revisions to accommodate the expanding use of GPS in solving real world problems. A GPS consumer should always ensure that their GPS data is compatible with the RINEX format. Some manufacturers have "proprietary" binary storage formats that will marry you to their products for life. Make sure that RINEX post processing software is included with the cost of your receiver, and not a hidden cost.

Bohlke: RINEX is the standard file format for GPS data. A GPS operator can use RINEX files to process data from unlike receivers. This standard file format offers a user more flexibility when purchasing GPS equipment.

van Diggelen: Receiver INdependent EXchange format. Used to transfer data from receivers to software applications. Used by everybody in the GPS world, especially geodisists and surveyors. Why? It is an ASCII format, is easy to read, and it has become a standard adhered to by all manufacturers.

About the participants:
Wendy Corcoran is a product manager, Survey and GIS, at NOVATEL Communications, Ltd. in Calgary, Alberta, Canada. She can be reached at 403-295-4789.
Craig Hudson joined II Morrow of Salem, Ore., in 1988 and currently serves as GIS product manager. He can be reached at 503-391-3411 or 800-742-0077 in the U.S., or 800-654-3415 in Canada.
John Bohlke is a systems engineer with SOKKIA Corp. in Overland Park, Kan. He can be reached at 913-492-4900 or 800-4-SOKKIA in the U.S.
Frank van Diggelen is a marketing manager for Ashtech Inc. in Sunnyvale, Calif. He can be reached at 408-524-1508.

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