Friday, May 21, 2010

SVN49 and Old Possum's Book of Practical Cats


I've been trying to piece together the story of SVN49. What's SVN49? Is it one of those unpronounceable high-altitude fixes that Center uses to steer you around uncharted military airspace (those of you who fly in the flight levels in the west know what I mean)? Or an oceanic fix? A proposed regulation? SVN49 won't affect me in my Cherokee Six or glider, right?

Wrong. SVN49 is a GPS satellite. Everyone in the world uses GPS, if not for navigation then for timing, so everyone should be worried.

Why haven't you heard of SVN49? A GPS satellite is like a cat. T. S. Eliot wrote



The Naming of Cats is a difficult matter,
It isn't just one of your holiday games;
You may think at first I'm as mad as a hatter
When I tell you, a cat must have THREE DIFFERENT NAMES.


We can counter with



The Naming of Sats is a difficult matter,
It just isn't one of those rocket nerd games;
You may think that these geeks are as mad as a hatter
When they claim a sat must have THREE DIFFERENT NAMES.


Obviously one of the names is SVN49; SVN stands for space vehicle number. The second name is NAVSTAR 63; NAVSTAR was the original name of the GPS program. And the final name, the one that belongs to "no other sat," is PRN01.

PRN01 should ring a bell; you've probably seen NOTAMs like

!GPS 12/064 GPS NAV PRN 25 OTS WEF 0912181500

Aviation GPS receivers usually refer to satellites by their PRN. PRN stands for Pseudo-Random Noise, and is one of the engineering marvels of the GPS system. The original GPS system had all of the satellites communicating simultaneously on two frequencies, called L1 (1575.42 MHz) and L2 (1227.60 MHz). You've heard the mess that results when two airplanes transmit on the same frequency: lots of noise followed by someone saying "Blocked." But that's the standard GPS communications environment.

The receiver gets the navigation data from one satellite at a time using CDMA (Code Division Multiple Access), which is also called spread spectrum encoding. Many cellphones use the same technique to allow simultaneous conversations on the same frequency. The technique was invented by, and patented by, the actress Hedy Lamarr during World War II. Her patent expired before the advent of GPS, so while her estate gets no royalties her name is revered in GPS circles. PRN01 refers to the specific binary code assigned to good old NAVSTAR 63.

SVN25, or PRN25, is out of service for a very good reason: it was launched in 1992, and has given its all for the cause. Much like the LGU VOR, which has been off the air for years and years, the satellite lives on in the NOTAM telling us not to use it. RIP. SVN49, however, is still begging at the door to be let in. It has been assigned PRN01, so we can tell its signals from all other signals, but its status is ominously unhealthy, so your receiver ignores it.

We all know that the FAA NOTAM system is a mess, but that's not the only NOTAM system involved. The US Coast Guard Navigation Center publishes NANU's. NANU sounds like something that Robin Williams said on Mork and Mindy, but in fact it is quite serious. NANU stands for Notice Advisory to Navstar (you remember Navstar, right?) Users, and this is where DUATS and Flight Service get their GPS NOTAMs. Here's the deal on SVN49:

!NOTICE ADVISORY TO NAVSTAR USERS (NANU) 2010068 NANU TYPE: GENERAL
*** GENERAL MESSAGE TO ALL GPS USERS ***
THE 50TH SPACE WING IN COOPERATION WITH THE GLOBAL POSITIONING SYSTEMS WING, WILL BE TESTING SVN49 (PRN01) SIGNAL CHARACTERIZATION OVER THE NEXT 6 MONTHS. SVN49 (PRN01) WILL REMAIN UNHEALTHY UNTIL FURTHER NOTICE, AND THE BROADCAST NAVIGATION MESSAGE WILL REMAIN IS-GPS-200 COMPLIANT. USERS CHOOSING TO TRACK AN UNHEALTHY SVN49 (PRN01) SHOULD CONTACT GPSOC (MILITARY) OR NAVCEN (CIVIL) IMMEDIATELY FOR ANY ISSUES THAT MAY BE ATTRIBUTED TO THESE ACTIVITIES.


How SVN49 got to be unhealthy for 15 months is a cautionary tale about engineering. The first thing to keep in mind about engineering is that at some point you have to freeze the design and start building: that's version 1. But even while you build version 1, you think of improvements, which eventually make their way into version 2. Etc.

One of the improvements to GPS is the addition of a third frequency, L5 (1176.45 MHz). This will give users improved correction for transmission errors, but the choice of this frequency is controversial.

In any event you have to test, so Lockheed-Martin engineers took a spare L5 transmitter and bolted it to a spare power bus on SVN49. Nobody has explained why a satellite has a spare bus, which is a lot more complex than having a spare USB port on your computer, but that is the case.

Now this was not a willy-nilly fly-by-night operation: I have no doubt that LMCO engineers did all of the requisite analysis, and that the proper drawings and other documentation were made. (It's very important to document every little detail of a satellite; imagine your embarrassment when the thing gets to orbit and doesn't work and you don't really know what you did when you built it.)

But despite this due diligence, the L5 signal interferes with the L1 and L2 signals, and the navigational data from the satellite is, so far, unusable.

Now there's more than one way to skin a cat, and lots of people are putting lots of work into figuring out how to make SVN49 useful. For the moment, though, it's still begging at the door, waiting to be let in. Let's hope that soon it will be PRNg.

Labels: , , ,

4 Comments:

At May 22, 2010 at 12:53 AM , Blogger Dave Starr said...

Now there's a topic not many other pilots have on their mind.

But given that much ballyhooed LMCO engineering and the fact that they only discovered the interference _after_ launch,it's a good one.

Most of the known world is happily plugging along following the signals of a system designed to replace a an arcane nautical timing system with virtually no original thought regarding aviation interests.

Not totally confidence inspiring. Buy the way, NAVSTAR 63 has at least two more names, its IRON number:

The Satellite IRON or Inter-Range Operating Number is assigned to a satellite upon launch for the purpose of tracking the object via space radars such as TRADEX and ALTAIR on Kwajelin Atoll. Can't find my IRON lst at the moment, but it's out there.

Iy also has a serial number ... 340, and a companion in crime, SVN50 which was originally going to be a another L5 tester, but at the last minute had the L5 system disconnected, literally by pulling the Cannon plug out, to avoid the uselessness of launching yet another test vehicle for a system known not to work.

Let's hope the block IIF bird currently on hold as I write this gets into orbit and works well for pilots, mariners and Boy Scouts everywhere.

 
At May 22, 2010 at 6:00 AM , Blogger John Ewing said...

Nice post about something more pilots should be concerned with, but aren't: The health of the GPS constellation.

My understanding is that the US (and LMCO) had a "time-to-market" issue with SVN49: Namely, they had to launch SOMETHING that used the L5 bandwidth in order to meet the International Telecommunications Union deadline so the US could claim primary rights to that part of the RF band for GPS use. If that was indeed the case, then I'm sure someone at LMCO is claiming the project was a success.

BTW, I thought SVN meant SPACE vehicle number ...

 
At May 22, 2010 at 6:12 AM , Blogger Frank Van Haste said...

Doc:

Thanks for that fascinating stuff. I can't believe that there isn't a capability in software to power down the L5 unit and leave SVN49 with just the L1/L2 capability of the earlier birds. Since there are enough birds on orbit for the system to function without it, I guess LockMart is continuing to troubleshoot in the expectation of curing the interferences (or learning to do it better next time).

By the way, what happened to L3 and L4?

Hope you're well,

Frank

 
At May 22, 2010 at 6:29 PM , Blogger Dr.ATP said...

Thanks for your great comments...they bring out some of the issues that I had in the first draft, but didn't think would be interesting. That shows how good my judgment is...

Dave: pulling the Cannon connector? And then, what? zip-tying the wire to keep it from banging around? I thought that this was supposed to be rocket science!

John: the motive for the quick launch was in fact to establish that the GPS system was using L5, so in that sense it was a "success." They also wanted to give the user segment a chance to test its equipment and algorithms on the L5 signal, another "success." I changed "Satellite" to "Space," too; thanks.

Frank: all I know about L3 and L4 I learned at Wikipedia. I vaguely remember some discussion of them at Institute of Navigation meetings, but that was a few years ago.

The Department of Transportion is accepting comments on this issue until May 28. None of the proposed solutions are appealing, including "Modify receiver software to use lookup table corrections." See the Federal Regiser, Vol. 75, N0. 58, Friday, March 26, 2010, p. 14658.

 

Post a Comment

Subscribe to Post Comments [Atom]

Links to this post:

Create a Link

<< Home