Orbital Precession, Optimal Dual-Frequency Techniques, and Galileo Receivers

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“GNSS Solutions” is a regular column featuring questions and answers about technical aspects of GNSS. Readers are invited to send their questions to the columnists, Professor Gérard Lachapelle (lachapel@geomatics.ucalgary.ca) and Dr. Mark Petovello (mpetovello@geomatics.ucalgary.ca), Department of Geomatics Engineering, University of Calgary, who will find experts to answer them.

Gérard Lachapelle and Mark Petovello with Penina Axelrad, Kristine Larson, Dr. Gary McGraw and Tony Murfin

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Q: Is it true that the GPS satellite geometry repeats every day shifted by 4 minutes?

A: It is true that the GPS satellite orbits were selected to have a period of approximately one half a sidereal day to give them repeatable visibility. (One sidereal day is 23 hours, 56 minutes, and 4 seconds long or 236 seconds shorter than a solar day.) However, because of forces that perturb the orbits, the repeat period actually turns out to be 244 to 245 seconds (not 236 seconds) shorter than 24 hours, on average, and changes for each satellite.

The selection of a half sidereal day orbit causes the satellite ground track and the satellite visibility from any point on earth to be essentially the same from day to day, with the satellites appearing in their positions approximately 4 minutes (236 seconds) earlier each day due to the difference between sidereal and solar days. This was a particularly useful property in the early days of GPS when session planning was important to ensure adequate satellite coverage. With this easily predictable coverage, GPS users could schedule repeatable campaign sessions well in advance just by shifting their experiments forward each day by 4 minutes.

(For the rest of Penina Axelrad and Kristine M. Larson’s answer to this question, please download the complete article using the PDF link above.)

Q: How can dual frequency code and carrier measurements be optimally combined to enhance position solution accuracy?

A: The smoothing of GPS code pseudorange measurements with carrier phase measurements to attenuate code noise and multipath is a well-established GPS signal processing technique. Unlike carrier phase real time kinematic (RTK) techniques, carrier-smoothed code (CSC) positioning solutions do not attempt to resolve carrier phase ambiguities. As a result, they offer a number of design and operational advantages for those applications that do not require RTK accuracies.

Ionospheric effects are a limiting factor in how much smoothing of pseudorange errors can be accomplished with single-frequency measurements. The use of dual-frequency code and carrier measurement combinations in CSC processing to attenuate pseudorange errors and as a precursor for carrier phase ambiguity resolution has gained increasing importance, particularly with the availability of all-in-view dual-frequency GPS receivers in the survey and military markets. Interest in these techniques will increase with the advent of additional GNSS signals as the result of GPS modernization and implementation of Galileo, along with the proliferation of differential services.

(For the rest of Dr. Gary McGraw’s answer to this question, please download the complete article using the PDF link above.)

Q: What is the availability of Galileo receivers?

A: With the launch of the GIOVE-A (Galileo In-Orbit Validation Element - A) Galileo test satellite in December last year, the European Galileo satellite navigation system is making progress. How will we be able to recognize the benefits of Galileo? We will require enough Galileo satellites to make a difference when used with GPS alone, and we will require dual-mode Galileo/GPS receivers.

First, let us recap what Galileo will provide to users. And second, let us summarize what benefits we can expect to see, not only from Galileo alone but from a combined GPS/Galileo constellation of approximately 60 satellites.

Galileo will offer several worldwide service levels, including open access and restricted access for various segments of users.

(For the rest of Tony Murfin’s answer to this question, please download the complete article using the PDF link above.)

Author Profiles

The columnists with:

Penina Axelrad is professor and associate chair of aerospace engineering sciences at the University of Colorado at Boulder, and a member of the Colorado Center for Astrodynamics Research. Her research focuses on aerospace applications of GPS.

Kristine M. Larson is professor of aerospace engineering sciences at the University of Colorado at Boulder. Her research focuses on the development of high precision methods in GPS and their application to geophysics.

Dr. Gary McGraw is a principal systems engineer with the Advanced Technology Center of Rockwell Collins in Cedar Rapids, Iowa, where he works in navigation system technology development with an emphasis on precision approach and landing systems. He received the B.S. degree in electrical engineering and mathematics from Iowa State University, and the M.S. and Ph.D. degrees in electrical engineering from UCLA.

Tony Murfin, vice president for business development at NovAtel, Inc., based in Calgary, Alberta, Canada, is responsible for the aviation business group and other strategic business initiatives for the company. He has been responsible for the Wide Area Augmentation System receiver programs and is now involved in the European Galileo receiver program. Murfin has a B.Sc. from the University of Manchester (UK) Institute of Science and Technology and is a UK Chartered Engineer (CEng MIEE).