GPS 24+3 Configuration: A Closer Look

February 3, 2010  - By 0 Comments

In the few years I’ve been writing this column, very few subjects have warranted back-to-back newsletter coverage. The new GPS 24+3 onfiguration is one of them. The reason I’ve continued with this discussion is because it will significantly affect your GPS operations, especially if you’re using RTK or DGPS.

What is the new 24+3 GPS configuration?

If you didn’t read my last column, you might want to read it so you have a common frame of reference. Essentially, the effect of the 24+3 configuration will be to increase the visibility of more GPS satellites throughout the day at a given location. In addition to have more satellites in view, you will generally see lower PDOP values which can result in an increase in accuracy; but certainly the increased satellite visibility is the major upside with 24+3.

Remember that the GPS satellites are configured in 6 orbital planes (A, B, C, D, E, F) with X number of satellites in each plane that are referred to as “slots.” For example, slot A1 is the first satellite in the A plane, slot B4 is the fourth satellite in the B plane. Note that the slots aren’t necessarily in numerical order. Following is a graphic presented by the U.S. Air Force in September 2009 to provide an illustration of the planes, and slots within each plane. GLAN is the Geographic Longitude of the Ascending Node.

On the graphic above, note that many of the satellites are paired together. When GPS satellites are paired together, there is little benefit to the user on the ground because the satellites aren’t “spread out”. Ideally, the user on the ground needs the satellites to be “spread out” in the sky which will result in a lower PDOP value (better constellation geometry) and ultimately better accuracy. The satellites are in this configuration today because GPS policy defines a 21+3 configuration. Since there are 30 operational GPS satellites in orbit (six more than required), the six spares are placed near other operational satellites. This isn’t optimal for the user on the ground.

The concept behind the 24+3 configuration is to spread out the satellites more than the current configuration to benefit users on the ground. This involves significantly repositioning three GPS satellites (SVN24, SVN26, SVN49) and slightly repositioning three other GPS satellites (SVN56, SVN46, SVN55).

Following is a tabular listing of each slot in the 21+3 configuration. Please note that the graphic above is a rough graphic for illustration purposes (referencing GLAN) while the tabular data below are the actual values.

 

 

Notes:

Epoch: 00:00:00 UTC, 1 July 1993
Greenwich Hour Angle: 18h 36m 14.4s
Orbital Slot IDs are Arbitrarily Numbered
* Orbital Slots Marked by an Asterisk are Expandable

 

In the 24+3 configuration, slots B1, D2, F2 are split to B1F/B1A, D2F/D2A, and F2F/F2A. The F designation is Fore and the A designation is Aft.

Following is the tabular data for the expanded slots:

On the B plane, SVN49 is repositioning to slot B1F while SVN56 is moving slightly to slot B1A.

On the D plane, SVN24 is repositioning to slot D2F while SVN46 is moving slightly to slot D2A.

On the F plane, SVN26 is repostioning to slot F2F while SVN55 is moving slightly to slot F2A.

You can refer to the graphic at the beginning of this article to reference the current location (approximate) of each SVN as well as the slot id. The SVN number is to the left of the symbol while the slot id is to the right.

SVN24 has the furthest distance to travel. It began its journey late last month and will arrive in January 2011. SVN49 and SVN26 will both arrive at their destination slots in May 2010.

If they were in a hurry, the satellite travel time could be reduced, but according to folks I’ve spoken to they have to conserve fuel. After the satellite reaches its destination slot, it must have enough fuel to occasionally maneuver as well as retain enough fuel for an end of life boost which could happen many years in the future.

The description of 24+3 configuration is detailed in the September 2008 release of the Global Positioning System Standard Positioning Service Performance Standard document. You’ll see this is the source of the two tables I’ve shown above.

 

Exactly how many more GPS satellite will my receiver “see”?

I was hoping to publish satellite visibility charts in this column for different regions of the world to illustrate the upside of 24+3. This is where the “rubber meets the road.” I’ve been experimenting with a modified GPS almanac in satellite visibility software to generate these, but I want to confirm the accuracy of the plots before I publish them. I’m close, but not quite there yet.

Also, I want to publish a separate satellite visibility chart for DGPS users. Remember from my last column that SVN49 is a tricky one. It’s still unhealthy since it was launched into orbit last March. Most likely, it will never be usable by SBAS (WAAS, EGNOS, MSAS) and DGPS receivers and will effectively reduce the 24+3 configuration to a 24+2 configuration for those users. Mind you, even if SVN49 is not usable by SBAS and DGPS, the new configuration will still be an improvement over the current configuration.

Look for continuing coverage on the 24+3 configuration. It will be the most relevant GPS topic for day-to-day GPS users in 2010.

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