The New GPS 24+3 Constellation: What Does it Mean to the Surveying and GIS User?

January 19, 2010  - By
Image: GPS World

Last week, the U.S. Air Force announced it is reconfiguring the GPS constellation. The Air Force is changing the constellation from a 21+3 configuration to a 24+3 configuration. The result will be more satellites in view, on average.

This is great news for the GPS surveying and GIS mapping user. In my  opinion, it was the only achievable short/medium-term solution to the GPS “brownout” problem that has plagued GPS surveying and mapping users for years, and has worsened in recent months.

In short, a GPS “brownout” is a time of the day when a GPS user is unable to utilize his or her GPS receiver because there aren’t enough satellites in view to achieve the desired accuracy. GPS “brownouts” primarily affect high-precision RTK users because that technology requires that the GPS receiver is tracking at least six satellites for a reliable position. With the current GPS constellation, there are times during the day when this is not possible given the satellite configuration and local conditions (obstructions such as trees, buildings, and terrain). This problem puts a serious damper on GPS productivity.

Even though there are currently 30 operational GPS satellites, they are configured in a 24-satellite constellation. Essentially, several satellites are “paired up” so they add no value to users on the ground. They are designated as back-up satellites in case of a failure. I wrote a detailed article on this subject in October 2009 titled GPS Constellation Management: Playing Not to Lose that summarizes the problem.

The New 24+3 Configuration

Announcements from various publications and online newsgroups have different interpretations of the Air Force announcement. Some are emphasizing increased accuracy and others are citing increased coverage in Afghanistan. While both are correct, the major benefit to the surveying/mapping user community is increased worldwide satellite visibility. In other words, more GPS satellites will be in view at a given time during the day.

More satellites in view = greater RTK and mapping productivity.

The reason that increased accuracy is mentioned in the announcement is because PDOP values will be lower in general due to the increase of satellites in view…and there’s a direct correlation between accuracy and PDOP. Just how many more satellites will be in view is not clear yet. I’m working on producing some mission planning charts that will illustrate the benefits of 24+3 compared to 21+3.

The three satellites being repositioned are SVN24, SVN26, and SVN49. SVN24 and SVN26 are two of the oldest satellites (Block-IIA) in the GPS constellation. SVN24 was declared operational in August 1991. SVN26 was declared operational in July 1992. SVN49 is a newer Block II-RM that was launched last March and has never been declared operational due to an anomaly discussed here before. More on SVN49 further down.

The time to reposition each satellite is significant. SVN24, with the furthest distance to travel, began its journey last week and will take 12 months to reach its destination slot according to the Air Force. SVN49 will begin its journey on January 21, 2010, and will take four months (May 2010). SVN26 will begin its transition on February 8, 2010, and will reach its destination slot in approximately three months (May 2010), according the Air Force.

Which Users Will Benefit the Most?

After (and maybe during) the transition, RTK users will see an increase in the number of visible GPS satellites throughout the day. As I mentioned above, I’m still working on producing satellite visibility charts to better and more accurately illustrate this, so stay tuned. Of course, the benefit is going to vary depending on where you are located.

Another group who will benefit is GIS mapping users, especially those working in difficult GPS conditions such as in forestry, urban/municipal areas, and areas where there is rugged terrain. An increased number of GPS satellites in view will allow GIS mapping users to operate in areas where it may not have been possible before and perform better in areas that were difficult.

Consumer GPS users will benefit the least. Even during times of GPS “brownouts,” the pushback from consumer GPS has been minimal. Automobile navigation systems perform without a hiccup for the most part and handheld receivers behave reasonably well. The primary reason is that both of those types of receivers aren’t selective about the satellite signals they accept. Accuracy is way down the list of important design features in those receivers. That’s not the case with RTK and professional GIS mapping receivers. RTK and GIS mapping receivers require high-quality measurement data from GPS satellites.

Will RTK Users Still Need GLONASS?

Certainly, GPS-only (non-GLONASS) RTK users will see an increased benefit with the 24+3 configuration, particularly those who are operating in relatively clear-sky environments like precision agriculture. However, as we’ve seen with this technology, users will keep pushing the GNSS envelope to use it in marginal conditions where GPS 24+3 won’t be enough. GLONASS will still contribute more satellite measurements, on average, than GPS 24+3. Therefore, GLONASS will still be a desirable feature.

But, I think we may see mainstream GNSS receiver manufacturers selling the GLONASS option at a lower price (or offering it for free) as it will be required in fewer instances. Russia is continuing to launch GLONASS satellites three at a time with the most recent launch being in December 2009 and the next scheduled one being later this spring.

SVN49 – What No One Is Talking About

One of the three GPS satellites being repositioned for 24+3 is SVN49. If you’ve kept up with the GPS constellation over the past six months, no doubt you’ve read about the SVN49 problem. It was launched last March and still hasn’t been declared healthy due to an irreparable problem. There has been much debate about what to do with SVN49. You can read about it here and here and here.

Most likely, the problem that SVN49 has will not affect RTK users. In other words, if SVN49 is set healthy, RTK users will be able to utilize it like the other normally operating GPS satellites.

However, GIS mapping users won’t benefit from SVN49. The Federal Aviation Administration (FAA) has said it won’t incorporate corrections for SVN49 in WAAS. Although the U.S. Coast Guard (USCG) and Department of Transportation (DOT) haven’t commented, I doubt they will broadcast DGPS corrections for SVN49 either. For code phase post-processing, I doubt manufacturers will modify their post-processing software to accommodate the SVN49 anomaly.

So, for GIS mapping users, it’s likely going to be a 24+2 configuration instead of a 24+3 configuration.

Either way, this is good news across the board for the GPS surveying and mapping user community.

I agree with what Don Jewell wrote in his column recently, that this subject is going to be written about and discussed a lot over the next few
months as SVN26 and SVN49 are repositioned, and over the next year as SVN24 reaches its destination.

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