A Little Q&A Follow-up and Feedback on My Last Column

November 19, 2009  - By

I received some feedback on my last column entitled “What’s the Difference Between a Used Car Salesman and a GPS Salesman?” Most of the comments were positive in that the technical content was reasonably deep and thorough. However, I did receive a couple of e-mails from folks who were offended by the comparison.

The joke has been around for a long time. As I mentioned, I recall hearing it in the early ’90s. I believe it was while I was at a conference somewhere in British Columbia, Canada. Anyway, I used to be a GPS salesman of sorts and I never took offense to it. I figured if I was doing my job correctly, there was nothing to be offended by. But, the fact is the joke has maintained staying power because a number of people do exist who fit that description. Fortunately, they don’t seem to hang around very long in the industry. On the flip side, over the years I’ve met many competent GPS sales professionals that have earned my trust. Many of whom I consider my friends.

Leftover Webinar Q&A

There are some lingering questions left over from the last webinar (September). There are still a few questions left after this that I’ll post in future newsletters.

Question #1: If GLONASS has a full constellation by 2020, would it be fair to say a L1, L2, and GLONASS receiver would be fine because you would really only have a L1 and
GLONASS receiver thus therefore enough sats?

Gakstatter: The question is referring to the semicodeless sunset I’ve written about in the past. You can read about it here.

Back to the question. Generally speaking, I would say yes but it’s going to depend on the receiver firmware design. If the receiver is optimized to depend on L2, then the firmware would have to be altered. Then, the question is whether the manufacturer is willing to update the firmware.

Question #2: If SBAS is made for aviation safety purposes, why it does not cover all of the world? Will the coverage expand in the future?

Gakstatter: Building an SBAS is very expensive and time-consuming project. If I recall correctly, the US WAAS total expenditure to date is well into several billion dollars. The FY2010 annual budget for WAAS is just under US$100M.

The nature of SBAS is that they are regional systems. Currently, there are three. WAAS covers most of North America. EGNOS covers most of Western Europe and North Africa. MSAS covers the region around Japan. GAGAN is still in the planning stage and will cover India and surrounding areas.

This still leaves South America, Australia, and Africa as major land masses not covered by SBAS.

MSAS has the potential to cover Australia and EGNOS has the potential to cover Africa. Whether that happens or not is more political and financial than technical issues.

Also, once GPS L5 is broadcast by a full constellation, the requirement for SBAS for aviation will diminish because GPS in aviation will transition to dual frequency (L1/L5) thus mitigating the effects of the ionosphere.  However, there would still be a question about position integrity, which is a central function of SBAS. The debate within some countries that do not currently have SBAS coverage is whether to invest in a SBAS or hold out for GPS L5. However, a full satellite constellation broadcasting GPS L5 will not be operational until ~15 years from now.

Question #3: Can we obtain SBAS in other regions in the future?

Gakstatter: India is far along with their GAGAN SBAS. At one point, a test signal was being broadcast. Within the next few months, India reportedly will be launching a geostationary satellite for GAGAN.

Russia is reportedly in the early stages of developing their SBAS called SDCM (System of Differential Correction and Monitoring).

The interoperability among SBAS is virtually seamless. A GPS receiver utilizing WAAS in North America is able to use EGNOS in Europe and MSAS in the Japanese region. Most receivers will automatically tune to the local SBAS given the SBAS satellites in view. However, some must be manually set to “look” for the SBAS satellite(s) of that region.

Question #4: GNSS improves the productivity. What about the precision? Are there any reports testing/comparing GPS vs GNSS?

Gakstatter: At this point, the ideal situation would be to only utilize GPS satellites. The quality, integrity and monitoring is world-class. Unfortunately, for RTK users there just aren’t enough of them in orbit to be able to work consistently throughout the day.

The quality and reliability of GLONASS measurements aren’t as good as GPS yet. The ephemeris data and clock corrections are worse and there isn’t world-wide monitoring of the satellites. Russia’s program managers have stated they are striving to reach the same measurement quality as GPS.

So, the short answer is no, I wouldn’t expect GPS/GLONASS to improve positioning precision. However, it also depends on the scenario. If the comparison is between a GPS constellation of 5 satellites with a PDOP of 5 against a GPS/GLONASS constellation of 5 + 5 with a PDOP of 2, then I’d vote for the latter.

There is an extensive report available from The Survey Assocation in the UK. Although it focuses on Network RTK, there’s a valuable discussion in it regarding GLONASS. You can download the report here.

Question #5: By 2020, is the DOD going to completely get rid of L2 signal?

Gakstatter: No, not at all. L2 was never intended for use by the civilian community. However, in the 1980’s, some really smart scientists in the commercial sector figured out a way to gain access to the encrypted L2 signal via a technique is referred to as semicodeless.

Since that time, the Department of Defense (DOD) has respected that technique because it’s been vitally important in the development of the GPS commercial markets. By respected, I mean they have intentionally not made changes to the GPS that would disrupt the semicodeless technique.

The DOD has now has come to a point that dancing around the semicodeless issue has become a bigger liability than they want to assume. That’s understandable since L2C will allow dual frequency GPS receivers to utilize L2 without using semicodeless techniques, the DOD wants to be released of their liability with respect to L2 semicodeless.

So, after December 31, 2020, the DOD will no longer guarantee that semicodeless receivers will operate as they do today. It is not a “brick wall” in that it will stop working at midnight on December 31, 2020. In fact, a semicodeless receiver may work perfectly fine most of the time after December 31, 2020, but if it stops working properly, the user assumes the liability.


One of the reasons my column is late this week is that I caught a bug earlier this week. It’s nothing serious; it’s just slowing me down a bit because I don’t have the energy I normally have. The fact is that I just don’t get sick beyond my tendency to attract strep throat once every few years.

This year, I’ve opted out of the flu shot despite a specific call from my mother putting the heat on me to get one. I’ve never gotten a flu shot and probably never will. But, I do admit this is the first year I’ve actually considered it. At one point a few weeks ago, 25% of the primary and middle school-aged kids in our school district
were absent due to illness. There were significantly fewer Trick-or-Treaters in our neighborhood this year; I believe due to illness.

I’ve always made an effort to wash my hands, face, nose regularly (no, I’m not obsessive-compulsive) and pay attention to those around me when I’m in public places like malls, movie theaters and airplanes. I absolutely hate getting stuck sitting next to a person on an airplane who is coughing and sneezing. I remember specifically sitting next to a person on a flight back from Phoenix, Arizona, a half-dozen years ago. She looked like death warmed-over…coughing and sneezing horribly. Sure enough, the next day I could feel my body losing the battle. I ended up contracting strep. Ugh.

Anyway, I want to remind you given that many of the US readers are traveling next week for the Thanksgiving holiday to take care of yourselves and your loved ones.  You’ll be traveling amongst those who haven’t escaped “the bug,” but have still decided to travel. Here are some tips from the Center for Disease Control and Prevention to keep you and your loved ones healthy.

This article is tagged with and posted in Opinions, Survey

About the Author:

Eric Gakstatter has been involved in the GPS/GNSS industry for more than 20 years. For 10 years, he held several product management positions in the GPS/GNSS industry, managing the development of several medium- and high-precision GNSS products along with associated data-collection and post-processing software. Since 2000, he's been a power user of GPS/GNSS technology as well as consulted with capital management companies; federal, state and local government agencies; and private companies on the application and/or development of GPS technology. Since 2006, he's been a contributor to GPS World magazine, serving as editor of the monthly Survey Scene newsletter until 2015, and as editor of Geospatial Solutions monthly newsletter for GPS World's sister site Geospatial Solutions, which focuses on GIS and geospatial technologies.

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