I didn’t plan it this way, but my coverage of GPS 24+3 turned out to be a three-part series, with this column being part three. One reason it turned into a three-part series is because I’m learning more about it along the way, but its mostly because details weren’t released all at once.
The good news is that I (along with help from others….thank you) was able to generate an almanac that simulates 24+3 reasonably well. The idea behind doing this is that I could compare the satellite visibility plots in satellite visibility software using both the original almanac (I chose January 1, 2010) and a GPS 24+3 modified version of the same almanac. For those plots, I could present to you what you can realistically expect the improvement to be with the 24+3 satellite configuration.
A quick note before diving into the 24+3 configuration. At the end of this column is a brief discussion about solar activity and GNSS/GPS. Last week, there was a solar event and some users have voiced concerns about that. I’ve addressed those in a section at the end of this article.
You can view my first two columns relating to the 24+3 configuration by following these links:
Plus a news article: New Details of 24+3 GPS Configuration Released
I’d like to update you on some bits of information that I’ve learned about 24+3 since my last column. I asked the HQ Air Force Space Command some questions about 24+3 and they kindly responded.
EG: Will the satellites (SVN24, SVN26) remain healthy during their repositioning journey?
HQ AFSC: Yes. The satellites will be set unhealthy for the initial Delta-V, but will return to healthy status approximately 24 hours after initiation of the Delta-V. Initial Delta-V for SVN24 was accomplished on 13 Jan 10 and returned healthy on 14 Jan 10. SVN 24 will take up to a year to reach its final destination. Initial Delta-V for SVN 49 was accomplished on 21 Jan 10 and will arrive at its expanded position in Jun 10. Initial Delta-V for SVN26 will begin early Feb 10.
EG: Why the two-year timeframe to realize the benefits when all repositioning will be complete in 12 months?
HQ AFSC: The two-year timeframe is a conservative estimate which takes into account potential operational necessities which could extend the time required for completion. We must take a disciplined approach to cover possible failures and ensure continuity of coverage during the transition.
We will be adding GPS IIF vehicles to the constellation and older vehicles may fail during the transition timeframe. As vehicles are added and removed, the current plan is subject to change in order to provide the best service to all civil and military users. Some of these decisions could require additional time to complete the expanded constellation. However, benefits will likely be realized well in advance of 24 months.
EG: What is the reasoning behind using SVN49 as a key component of the 24+3 configuration since it won’t benefit a significant portion of the civilian user community, namely aviation and marine navigation as well as other SBAS (WAAS) and DGPS users? In my understanding, the FAA’s and the Coast Guard’s user bases are primarily single-frequency pseudo-range, users who won’t be able to use SVN49.
HQ AFSC: SVN49 was selected because it is a brand-new satellite with four good clocks. Although issues with SVN49’s navigation signals may make it unusable for all civil use, it could still put out a valid set of signals for military use. The Air Force team is continuing to work “open book” with civil and industry GPS experts to determine the possible outcome of SVN49. Although SVN49 is not currently healthy, GPSW and 50th SW are actively working a mitigation that may allow setting the vehicle healthy in the future. As a mitigation in case we are unable to set SVN49 healthy, SVN30 will be rephased to the same slot following a successful launch and on-orbit checkout of IIF-1. We expect to have either SVN30 or SVN49 healthy and broadcasting from the expanded slot within a 24-month timeframe. At this time, no decisions have been made and no options have been ruled out regarding SVN49.
Satellite Visibility Plots
As promised, I’ve (with help) been working on creating an almanac that simulates the 24+3 constellation. My goal was to be able to show you what the benefit to you will be with the new GPS 24+3 satellite configuration.
The method I used was to modify an almanac from January 1, 2010. The reason I chose that day is because it was before the satellite repositioning began. The first satellite began its repositioning journey on January 13, 2010.
Within the almanac, I adjusted the position of three of the satellites in the almanac to reflect the new orbit locations they are going to assume.
- SVN 24 is moving from slot D5 to slot D2F
- SVN 26 is moving from slot F5 to slot F2F
- SVN 49 is moving from slot B5 to slot B1F
Following is a graphic I’ve published before that illustrates the satellite repositioning:
Using the original January 1, 2010, almanac to plot a satellite visibility chart and then using the 24+3 modified almanac to plot another chart for the same location, I was able to generate the following comparisons between the current GPS satellite configuration and the 24+3 satellite configuration. Please note the following:
- A 15-degree elevation cutoff was used to account for obstructions (terrain, buildings, trees).
- The modified almanac does not take into account the other three satellites that are being slightly repositioned (SVN46, SVN55, SVN56) so the modified almanac represents a worst-case scenario.
- The original almanac is the first plot. The modified 24+3 plot is directly below it.
Portland, OR USA (N45 41, W122 11) Original Almanac:
Portland, OR USA (N45 41, W122 11) 24+3 Almanac:
Miami, FL USA (N25 46, W80 11) Original Almanac:
Miami, FL USA (N25 46, W80 11) 24+3 Almanac:
Tokyo, Japan (N35 42, E138 30) Original Almanac:
Tokyo, Japan (N35 42, E138 30) 24+3 Almanac:
London, England (N51 30, W000 07) Original Almanac:
London, England (N51 30, W000 07) 24+3 Almanac:
Moscow, Russia (N55 45, E37 37) Original Almanac:
Moscow, Russia (N55 45, E37 37) 24+3 Almanac:
New Dehli, India (N28 54, E77 13) Original Almanac:
New Dehli, India (N28 54, E77 13) 24+3 Almanac:
Rio De Janeiro, Brazil (S22 27, W42 43) Original Almanac:
Rio De Janeiro, Brazil (S22 27, W42 43) 24+3 Almanac:
Bangkok, Thailand (N13 49, E100 28) Original Almanac:
Bangkok, Thailand (N13 49, E100 28) 24+3 Almanac:
Perth, Australia (S31 49, E116 10) Original Almanac:
Perth, Australia (S31 49, E116 10) 24+3 Almanac:
A Quick Note about Solar Activity and GNSS/GPS
I’ve read media reports and I’ve been asked about a solar event that occurred last week (Thursday, February 12) and what possible effect it had on GPS operations.
I consulted with Joe Kunches of the NOAA Space Weather Prediction Center to understand how significant of an event it was.
“There was some activity but I would not think it would have an impact on GPS,” stated Kunches.
I asked him at what point would GPS operations be affected.
“As for flares (Radio Blackouts on the NOAA Scales), I’d say 10 to 20 times stronger than last week (R3 to R4 and above) would be sufficient to affect GPS on the dayside, but not for long,” said Kunches.
So, although there were media reports about the solar event last Thursday, if you had trouble with your GPS it wasn’t due to solar activity.
However, solar activity is a serious issue for GPS users, especially those using high-performance L1 receivers (sub-meter). You can be sure that I’ll will be covering this subject in-depth as we move further into the current solar cycle.
Thanks, and see you next time.
Follow me on Twitter at http://twitter.com/GPSGIS_Eric.
If you haven’t seen the announcement regarding my Webinar this Thursday (February18, 10 a.m. Pacific Time, 1800 hrs GMT), you might be interested. The title is “GPS for GIS – 101.” It’s a beginner’s (and refresher’s) guide to using GPS for GIS data collection. I’ve invited Craig Greenwald as Guest Commentator.