The new GPS Block IIR-M satellite (PRN 7, SVN 48) was set healthy in record time on Monday March 24, 2008. According to GPS World:

That is apparently a record timeframe for a satellite being set to usable after launch. The previous satellite launched, SVN57, which went into orbit December 20, 2007, and was set to usable January 2 — a matter of 13 days — had also set a record at that time. After the successful launch of SVN48, the Air Force originally said that it anticipated it would set the satellite to usable sometime next month.

The standard timeframe for setting a satellite healthy after a launch is a month, but if all goes well, it will usually be set healthy when it's ready.

So, why the delay once it's on orbit? 

Once the GPS satellite gets into the correct orbital slot, turn on procedures are followed to activate the payload, upload programs and navigation data.  The rubidium atomic clock is also turned on.  The atomic clock plays an important role for navigation users, you wouldn't know where you are without it.  Once the clock is turned on, it has to be set to the correct time.  This isn't as easy as turning the dial on your watch - these clocks are accurate to nanoseconds.  The GPS satellite operators go through processes with arcane names such as baseband reset and PRN sync to get the timing set correctly.  Then they have to watch the clock.  The control segment Kalman filters process the clock states continuously, watching and waiting for the initial start-up clock errors to settle down.  Once all the payloads are on and functioning correctly, all the data is uploaded and current and all of the clock and ephemeris Kalman filter states have settled down, the satellite can be set healthy for all to use.

The fact that this process takes so little time now, is a testament to the manufacturers of the satellite and the clocks and to the operators in the GPS Control segment. Well done!

Now if only I could get a dentist appointment that fast...

Ok, you've had some time to digest the first two parts of this series on assessed navigation accuracy.  First I discussed what ephemeris and clock errors were, then I went over how those errors are created.  Now, in this final installment, I'll show how these errors are are turned into accuracy measurements for your GPS receiver.  For now, let's assume that our only errors are those we've discussed - ephemeris and clock errors.  There are other errors that affect your receiver and those are covered in the GPS Error Budget nog.

Now that we know what our typical ranging errors are, we need to understand how those are translated into a receiver error. The other piece of the error puzzle (and there are only two pieces) is Dilution of Precision (DOP).  DOP is the effect that arises from measuring signals from spatially separated sources - be they lighthouses, GPS satellites, semaphore technicians or pulsars.  DOP typically has the affect of diluting the accuracy of your ranging measurements.  I'll cover DOP much more thoroughly in a future nog as well - because we want to get to the good stuff now!

Notionally, your receiver's accuracy can be modeled as:

receiver error ~ DOP x URE

This is not a mathematically robust description, but it does show the general behavior of your errors.   The higher your DOP or URE, the higher your receiver's error.  Lower either, and you're likely to find that geocache quicker.  The mathematically correct construct for DOP is a matrix, and the UREs are a vector.  The elements of the URE vector are the total root sum square (RSS) errors along the line-of-sight from your receiver's antenna to each GPS satellite.  So far we've only discussed the ephemeris and clock errors, but any error can be RSS'd into the URE and be used in the receiver error calculation.  We'll see this when we look at the GPS error budget.

So, there it is, on the table - now you know how accurate your receiver's position estimates are.  As long as you know all of the satellites ephemeris and clock errors at a given time (and all the other errors we glossed over) and you can do the matrix multiplications on the fly.  Piece a cake!  Right - we don't think so either.  That's why we automated all the data collection and math for you in our tools.

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Smile! It's comic day...

Here's one that may have actually happened to you (click to enlarge):

Finally,

What do you call a GPS receiver with three antennas?

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SpaceFlightNow is reporting a successful launch and deploy of the latest Block IIR-M GPS satellite.

Spacecraft separation confirmed! The U.S. Air Force's Global Positioning System Block 2R-19 spacecraft has been released from the Delta 2 rocket's third stage to complete this late-night launch from Cape Canaveral.

The deploy occurred at 2:18:03 a.m. EDT this morning.

Launch timeline and more details available from their web site.

biz.yahoo.com A U.S. Air Force modernized Global Positioning System Block IIR (GPS IIR-M) satellite built by Lockheed Martin is ready for launch aboard a Delta II rocket on March 15 from Cape Canaveral Air Force Station, Fla. The Block IIR-M spacecraft series is designed to provide enhanced navigation capabilities for military and civilian GPS users around the globe.The satellite, designated GPS IIR-19M, is the sixth in a line of eight GPS IIR satellites that Lockheed Martin Navigation Systems, Valley Forge, Pa. has modernized for its customer, the Global Positioning Systems Wing, Space and Missile Systems Center, Los Angeles Air Force Base, Calif.

When the latest launched GPS satellite is set healthy, another GPS record will be broken. For the first time since its inception, GPS will have 32 active satellites. A similar record was broken when 31 satellites became active back on February 26th of this year. That was a significant milestone in two ways. First, it was the first time more than 30 GPS satellites have been active at the same time. Second, it's the first time that more than 30 GPS satellite could have been active at the same time. In August of last year, the legacy Master Control Station (MCS) ground hardware and software was replaced by the new Architecture Evolution Plan (AEP) system. The new system is a client server-based computing system that has the capability to handle more than 30 satellites. The legacy system could only handle a maximum of 30.

32 active satellites is significant because only 32 of the original 37 1-week long segments of the original gold code were selected to be used by the GPS satellites. PRN (Pseudo Random Noise) codes are further explained by another author:

The mentioned PRN-codes are only pseudo random. If the codes were actually random, 2¹⁰²³ possibilities would exist. Of these many codes only few are suitable for the auto correlation or cross correlation which is necessary for the measurement of the signal propagation time. The 37 suitable codes are referred to as GOLD-codes (names after a mathematician). For these GOLD-codes the correlation among each other is particularly weak, making an unequivocal identification possible.

The new GPS satellite will use PRN 7 according to the GPS Operations Center, the last remaining PRN currently unused. When the new satellite is active, I'll put up some statistical DOP accuracy information up.

Last but not least I want to wish everyone a happy PI day! Grab your ruler and head towards your nearest circle to celebrate.

That reminds me of a funny story involving masking tape, my office floor and trig several years ago. Ask me about it!

UPDATE (3/15/08):

InsideGNSS is reporting that when PRN 7 becomes active, 2 additional GPS satellites will be deactivated:

As a result, the GPS constellation will have more functional spacecraft in it than ever — 32 if this week’s launch goes as planned. Two of those satellites, however, will be switched off and kept as spares, including SVN27 that IIR-M 19 will replace. The “baseline” GPS constellation or fully operational capability (FOC) calls for only 24 on-orbit satellites — 4 in each of 6 planes.

additionally, DOP will not be optimal, considering many of the satellites are in spare slots:

Because many of the satellites are clustered together in the orbital planes (see figure at beginning of article) to ensure back-up capability rather than spread out in an optimal configuration, DOP values look more like those of a constellation with fewer satellites.

Technorati Tags: navigation accuracy,gps,navigation,dgps,waas,ephemeris,atomic clock,ephemeris error,survey

So, if you read part 1 of this topic, you know that we need to determine the navigation satellite's ephemeris and clock errors - because those somehow affect the accuracy of your receiver. As mentioned in the previous post, these errors are the same (mostly) as those broadcast by dgps systems - so how do dgps systems determine the errors? Its actually pretty cool. We know how a receiver works - it moves and it's position updates as it moves around - the position being calculated by the governing navigation equations. Suppose the receiver was fixed at a known (surveyed) location? It's location is no longer an unknown in the navigation equations - now we can take the satellite position as an unknown and solve for it instead. Since the satellite is broadcasting it's precise (predicted) position, we can difference the two and get the ephemeris error. Easy.

What about the clock error? Well, we'd need a 'surveyed' clock state to compare the satellite's clock state against. This is achieved at some monitoring stations (as these fixed stations are known) by using a local atomic clock to compare states against. Other techniques exist as well, such as syncing to the USNO time standard.

Two problems occur when calculating the difference in this manner:

1. They difference data becomes spatially decorrelated
2. The data is noisy

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