tag:blogger.com,1999:blog-34282885239436341782024-03-05T04:03:07.439-07:00The NogThe nav blog, writing about navigation issues and ideas from the users to the satellites.Unknownnoreply@blogger.comBlogger14125tag:blogger.com,1999:blog-3428288523943634178.post-23184565074374248982012-05-03T10:38:00.004-06:002012-05-03T11:03:21.235-06:00Improving Galileo's Accuracy with only 18 SatellitesI'm just back from the European Navigation Conference in Gdansk Poland, where I <a href="http://teddriver.net/home/Papers/Operational%20Considerations%20for%20Improved%20Accuracy%20with%20an%20IOC%20Galileo%20Constellation.pdf" target="_blank">presented a paper</a> on improving the accuracy of the Initial Galileo navigation constellation. Europe now has all 18 satellites funded to reach their Initial Operating Capability (IOC), currently slated for 2015 sometime. With only 18 satellites, there is 100% coverage (at least 4 satellites in view at any time, globally), but the orientation of those satellites will not always be ideal. In fact, there will be times during the day when Galileo will suffer from Dilution of Precision (DOP) spikes for times longer than 30 minutes. This will be seen as Galileo outages by receivers. These times are dispersed throughout the day; other times during the day navigation accuracy is not bad. <br />
In my paper, I outline how the Galileo Control Center (GCC) can improve average user accuracy by improving the upload times for each satellite. Essentially, the more frequent the uploads of predicted satellite orbital and clock states, the more accurate the Galileo navigation satellites will be.<br />
I use the notion of a Signal-In-Space User Range Error (SISURE) that defines how accurate each Galileo satellite is. The lower the SISURE, the more accurate a user's position. When the GCC uploads its predictions more frequently, the SISURE will be smaller. This is because the SISURE is a difference between the actual satellite position and time and the predicted satellite position and time, uploaded to the satellite and broadcast to the users for use in calculating their positions. The older the predictions are, the higher the SISURE is, the larger the user's error is.<br />
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Below is a picture of 24 hour average navigation accuracy with each Galileo satellite having a SISURE of 2 meters. Note that because of the geometry of the 18 satellites, there are large mid-latitude bands that suffer more from DOP spikes. These will go away when more satellites are launched. The average accuracy across the globe for this configuration is 5.7 meters.<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMQTIcoYgIZQocwWv-VhVOSqs-sdtgizZ4B9mAmZ2oWpyM5_JFt3_-LmogwhRGDbsWx9Jeu8XE91NuDYgwY6dOsg-AykwRghs4QgNYDfM9MwObMQNQqLQR1Y7JY20qw6n5VDfixEYdkwxh/s1600/BaseAccuracy.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="283" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjMQTIcoYgIZQocwWv-VhVOSqs-sdtgizZ4B9mAmZ2oWpyM5_JFt3_-LmogwhRGDbsWx9Jeu8XE91NuDYgwY6dOsg-AykwRghs4QgNYDfM9MwObMQNQqLQR1Y7JY20qw6n5VDfixEYdkwxh/s400/BaseAccuracy.jpg" width="400" /></a></div>
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Now, if the GCC can maintain an upload schedule that limits the SISURE to 0.5 meters, a more accurate result can be seen:</div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAsr8lIFMoZlKCKs1loiGD9VwOOVjR3RpRrszk11DRC3CNlOcWixSmruW1cWg1cZnxuPySOl1Iuu5tB3_qqb_aciMnkLrNhMuLwM1qzciEDXHVV_2WUDxvXWKmsk4kQJRrmPwvSi5GGOOH/s1600/ImprovedAccuracy.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="283" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAsr8lIFMoZlKCKs1loiGD9VwOOVjR3RpRrszk11DRC3CNlOcWixSmruW1cWg1cZnxuPySOl1Iuu5tB3_qqb_aciMnkLrNhMuLwM1qzciEDXHVV_2WUDxvXWKmsk4kQJRrmPwvSi5GGOOH/s400/ImprovedAccuracy.jpg" width="400" /></a></div>
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In this scenario the average global accuracy drops to 1.9 meters - a big improvement. Notice also how the DOP spikes are minimized in the mid-latitude bands.</div>
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This is only one of many other methods that can be done to improve accuracy with an initial Galileo constellation. Working to keep the user's navigation error small will gain a lot of trust for Galileo in the user base, making it a better system from the start.</div>
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The paper is located here: <a href="http://teddriver.net/home/Papers/Operational%20Considerations%20for%20Improved%20Accuracy%20with%20an%20IOC%20Galileo%20Constellation.pdf" target="_blank">Operational Considerations for Improved Accuracy with an IOC Galileo Constellation</a></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-36817617941786147932012-02-16T10:06:00.000-07:002012-02-16T10:06:12.722-07:00LightSquared's Approval Revoked!LightSquared's conditional approval to operate ground-based 4G LTE transmitters (cell towers) was revoked by the FCC this week. This is great news for GPS users. GPS has an incredibly weak signal and can be easily interfered with, or jammed. 4G services by their nature are high powered relative to GPS signals, and even though the frequency bands used by both were not the same, they were next to each other - causing <a href="http://www.gps.gov/news/2012/02/lightsquared/" target="_blank">unacceptable interference</a> to GPS receivers. <br />
LightSquared secured their spectrum quickly through the FCC, and interference issues were not taken into account by either party. Because GPS is the equivalent of a national utility used by both civilians and the military, the FCC and LightSquared both <a href="http://news.cnet.com/8301-1035_3-57378764-94/lightsquared-blew-it-and-heres-why/" target="_blank">screwed up</a> by ignoring the interference possibilities. Additionally, GPS receiver manufacturers could have put filters in place (though not completely effective), to alleviate some of the interference from any outside transmissions. However, there have been no realized threats to the spectrum - why would the receiver manufacturers build something into a receiver, at their expense for a problem that didn't exist?<br />
The good news is, your GPS receiver, my GPS receiver, the FAA's <a href="http://www.faa.gov/nextgen/" target="_blank">NextGen</a> air traffic control system, banks, power companies, farmers, cellular service providers and all other users of GPS can now stop worrying about whether their GPS receivers will work when LightSquared turned on their system.<br />
GPS receiver manufacturers should take note - you got lucky this time, but now IS the time to start building in safeguards and fortifying your defenses against jamming, unintentional, or intentional. There will be other spectrum contenders to come.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-58993577475765633552012-02-12T06:40:00.000-07:002012-02-12T06:40:26.158-07:00PRN 26 off the air - what happened?On February 9, GPS PRN 26 was taken out of service. Notifications were sent to GPS users via <a href="http://celestrak.com/GPS/NANU/description.asp">UNUSUFN </a>NANU <a href="http://celestrak.com/GPS/NANU/2012/nanu.2012006.txt">2012006 </a>(Unusable until Further Notice notifications). When these types of notifications are sent out, it means the Air Force wants this satellite of the air immediately. <br />
Here's a look at PRN26's atomic clock behavior for three days prior to it going out. Erratic clock behavior is one of the likely causes of an outage and is typical of an aging clock. According to Richard Langley's <a href="http://gge.unb.ca/Resources/GPSConstellationStatus.txt">GPS status page</a>, this is a Rubidium atomic clock and was turned on in July of 1992 - which makes it almost 20 years old - 10 years beyond its design life! This is the second oldest operating clock in the entire GPS constellation. (The oldest clock belongs to PRN 32, turned on in December of 1990)<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjr_fJWm32rnd305hb5j66b0m4t49Prfll0M8fiKCOLHSnuUKkiJEwQHFJ-JA_PPuxy0qvbvvLEjoysfPubU-76W8WMZk-WXusH91vQ6T4Og-WrWypdHlOgbyvTT8hK9jpfAlY1kkVxtyGK/s1600/PRN26Feb7892012.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="345" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjr_fJWm32rnd305hb5j66b0m4t49Prfll0M8fiKCOLHSnuUKkiJEwQHFJ-JA_PPuxy0qvbvvLEjoysfPubU-76W8WMZk-WXusH91vQ6T4Og-WrWypdHlOgbyvTT8hK9jpfAlY1kkVxtyGK/s400/PRN26Feb7892012.png" width="400" /></a></div>
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Ideally, this pink line will be kept as close to zero as possible. The amount of error in shown in this plot is incorporated into your GPS receiver and can be seen as errors in your position determination. The US Air Force adjusts the satellite's clock values to keep this line as close to zero as possible. A typical adjustment can be seen about 12 hours into Feb 07. Normally the clock values move around a bit, but about 6 hours into Feb 08 the clock jumped dramatically. Each dot on the line represents 15 minutes, and clock errors of several meters (1 meter = 3.34 nanoseconds) in that short of a time are unusual. You can see that the clock is settling down between jumps but keeps having erratic episodes until, around 1800 hours on Feb 09, it starts changing by 1 meter every 15 minutes. Finally, the Air Force took it off the air at <a href="http://celestrak.com/GPS/NANU/2012/nanu.2012006.txt">1917 UTC</a>. </div>
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There's always a balance between keeping a satellite on the air (more satellites in view means a better position determination for you) and turning off a satellite that is harming your position. So, as the Air Force watches each 15 minute update, they have to decide to keep it on, or turn it off. Good call on this satellite and let's hope PRN 26 is healthy again soon.</div>
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<br />Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-66310583615429500852012-01-06T07:15:00.000-07:002012-01-06T07:15:42.903-07:00LightSquared facing the death knell?<div>Has the death knell for LightSquared arrived? The latest legislation signed by President Obama has a portion dedicated to GPS interference by ground-based data services. For ground-based data systems to exist, they must prove that they do not interfere with GPS. With Sprint putting its investment in LightSquared on hold, LightSquared is facing a major challenge. Here's the link to the eWeek article on the new legislation:</div><a href="http://www.eweek.com/c/a/Enterprise-Networking/LightSquared-GPS-Interference-Issue-Faces-Congressional-Oversight-860245/">LightSquared GPS Interference Issue Faces Congressional Oversight - Enterprise Networking - News & Reviews - eWeek.com</a>, and here's a link to the Sprint status with LightSquared: <a href="http://www.nypost.com/p/news/business/sprint_waits_on_lightsquared_HqchSN4yLQ6KgtXrSarfhP">Sprint waits on LightSquared</a>.<br /><br /><a style="font-size:13px" href="https://chrome.google.com/webstore/detail/pengoopmcjnbflcjbmoeodbmoflcgjlk">'via Blog this'</a>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-55859703100413137652011-11-08T19:01:00.001-07:002011-11-08T21:48:33.291-07:00How long can you use an almanac?<em>Note: this is a reissue of a Nog originally published in 2008.</em> <br />
When you turn on your GPS receiver - what happens? Well, lots of stuff. But primarily, GPS receivers work in a two stage process: <br />
<ol>
<li>Look for available satellites to track </li>
<li>Do everything else</li>
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In this article, I want to focus on step 1, we'll get to step 2 later. This is probably common knowledge, but for the record I want to state it. The GPS receiver uses an almanac downloaded from a single GPS satellite to help it determine what satellites are above the horizon as it searches for signals. Makes sense to not look for satellite signals that aren't even visible - it decreases your <em><a href="http://en.wikipedia.org/wiki/Time_to_first_fix">time to first fix</a></em> (TTFF). Since the receiver is simply determining whether a satellite is above the horizon, the almanacs don't have to be very accurate. They are typically a coarser version of the precise ephemeris broadcast by each individual satellite. <br />
What I do want to discuss is how long you can use an almanac for analysis. Your receiver will usually download a new almanac when it sees that a new one is available, so it always has the freshest data. When you do analysis though, sometimes you may not have the latest almanac (or the one correct for the time of analysis - a related problem). So, is it ok to use any old almanac for analysis? Since they are not the most accurate ephemeris representations, should I be using them for analysis anyway? I'll answer these questions and show some interesting graphics that bring the point home.<br />
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For analysis sake, let's say we're interested in Position Dilution of Precision (PDOP) prediction. PDOP is calculated from standard formulas and requires a source of ephemeris for the satellites. You can calculate PDOP from precise, actual ephemeris, or almanac generated ephemeris, or <a href="http://www.iht.com/articles/2008/02/05/america/satellite.php">whatever quality ephemeris</a> you have. I'll compare the PDOP calculated using the almanac to 'true' PDOP - that calculated from the precise ephemeris. <br />
For our first analysis, let's look at 24 hours of PDOP values - using a precise ephemeris for the actual PDOP and a three week old almanac to generate the ephemeris for the predicted PDOP. <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEihfQBsNhQ9mgfId4OReRXsuuW2vvCfmgPf5RFOR2YXJOXoV_92YUUtNvbbG8lDwd6k1ro49W21UiXp19Vph_ckh9BybRXuBKN9j15FLC2EZJSMbwkx5DimHZrCfIRINb-HaCZto-CbtDJi/s1600-h/3weekpdop%25255B5%25255D.png"><img alt="Three Week PDOP" border="0" height="368" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZ1Z_3g5TKWkENy9od1XuCmy4sVHOFmXKY5v_VNt1pNVsz7PxAy4ZbX_8jr94FYhOvZ04jZKyuWiYqqkgFqwgRNnTk5qn5-hhBe6mGVxGwBxVYfZD-NLFa9-mwOLFP94NQZ1x9lE9zaLke/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="Three Week PDOP" width="475" /></a> <br />
The actual PDOP is in blue, with the almanac predicted PDOP in green. <br />
We can see some differences here, though in general the predicted PDOP follows the same curves as the actual PDOP. The areas of trouble will be where the yellow lines are above the pink lines. In that case, the actual PDOP is greater than the predicted PDOP, and relying on your prediction can cause trouble. <br />
Let's take a look at the differences in PDOP values for a 6 week time span. I used an almanac to predict PDOP starting on the day the almanac was produced (0 days of almanac age) up to 6 weeks in the future (almanac age of 6 weeks), and differenced those predicted PDOP values against the actual PDOP values. <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZSz6EUQfb8pjGjPulavIMfZONgxC6tpNZ3e8hA2oe5BQ5-d5Tj1peq6sEqc16W1eu7SUYYc6BkzsafraOEnjvM4SzqnRqtJE6BIuwgbGgPDg7qGck0RV4YegVjhXgb51P5QNGZdPY-Kt7/s1600-h/6weekalmanacprediction%25255B11%25255D.png"><img alt="6weekalmanacprediction" border="0" height="367" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEimMV1DMLMU1RkRviNdyy0tfNqtVe20J_Dig7n-bKCUxiSJMPPDycmlqHVmeJIWFUsL50rRhAHF1jPlnjOm5cT0uiBuKbnNVIkppTpo8XsfofpY7qNl80v3PYBpVndVAWICHTnPzM_ASmi_/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="6weekalmanacprediction" width="480" /></a> <br />
This graph shows something interesting. Despite an occasional spike, it looks like <em>almanacs can be faithfully used for about two weeks to predict PDOP</em>. The differences are essentially zero for that time span. After about two weeks, things start to go down hill. We saw hints of this in the three week old graph above. Differences in PDOP value start to increase, with the major spikes occurring more often and secondary spikes beginning to occur. Note that the negative values here are the most dangerous - when actual PDOP values are greater than predicted values. It's nice to see that the spikes appear with a greater magnitude in the positive direction than the negative. <br />
What happens if we go longer with an almanac? Here's a PDOP plot for a day, using an almanac that is 21 weeks old. <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhk_5PzZcjYkjfItU6oxDd1bm9FocAXSspjdkv9s3iIfeljz-wbDG0iVBtHIzFuAEQ9Vl0Wu6EVwPMeYGsqqIB7bkWrNi4Qcol_l6-5uo7owbCWxTODo8jYRvHQrWeXMMze_g-yuDbdFE2O/s1600-h/21weekpredictedpdop%25255B5%25255D.png"><img alt="21weekpredictedpdop" border="0" height="373" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEih6F_syQhMRMPf1mpNL7zvLdtqei0qxO5snRexx_A1MmcgeafHXS3dtITjhQHh6duhVaSWdqpq0thVW4FmQWj-rlMip4TRDhMh_NMruQ7kwcQXy5Ptp1CkHN7EZz7GwuQ8e698KdLVni8q/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="21weekpredictedpdop" width="481" /></a> <br />
We start to see some serious problems here. The biggest problem happens around hour 8, where the actual PDOP is roughly 2.08 and the predicted is roughly 1.5. Another more stealthy error occurs at hour 20. Here, the predicted PDOP shows a drop in value, from roughly 1.93 to 1.6. These PDOP drops are associated with additional satellites coming into view and denote good times to get GPS position measurements. The actual PDOP here is rising though - indicating a failing prediction and completely incorrect information. <br />
How does the difference graph look at this time span? <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjKCUy0c6kHSIZvO-bgBCzY8UFIrxMjTRyvyCK646JxRWq7a2fszkfiGjUhrO0ElAvlsu0yJyZH1DTIi5wZIsBY6sU-lHfZZjffNIT8AAlfWeKKvSAXCDvJM77Il7qOJQ-4fMun45wngBxT/s1600-h/22weekalmanacprediction%25255B5%25255D.png"><img alt="22weekalmanacprediction" border="0" height="386" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg1NGZFKJUd7l5paKw3CQ0YYSOffEZE5uMVPnlp6sYipTQ7FwhXaJJoJLLXMH8VoOPVUPDXwGN-0D5wMEWJK7yIqMDrrB79lyEjScs9VQVI5cV1TRbS10I75Z4euZvTgST7KLj_gFQ3SB6V/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="22weekalmanacprediction" width="488" /></a> <br />
Yep - lots of problems in this regime. As the almanac ages, the negative values increase in magnitude, meaning hazardous prediction conditions. <em>Do not use an almanac this old!</em> <br />
I want to show two more graphs that will drive this point home. The first is a PDOP difference variance graph - basically how bad the PDOP values vary from each other as a function of time. <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5pK_MTKj6o8ta3WFmnD4SbtW57xOeRxmn2UCRgy9X5i3XnsI53RI0F9STkBBWX-L0WmPXKTSN_Gzfu-abcAZQvAkZIWiFdDAgtA45y-VJPnjfMBDwgbmMcuM6xLCKl86ev-foQFGP1P4-/s1600-h/pdoppredictedvariance%25255B4%25255D.png"><img alt="pdoppredictedvariance" border="0" height="381" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggEpnZwAjN9PwR_CkygjMPrSIImHaHs-Ns_cCcKw9d4NXJgjEvYaPVag4okXgIW9nK-YW1bz3Y9npTsj9GrHfjwxPagNZiDILNcx8nnBA8jyPvRbH3SiqTtvpXzkJC5vcBCM2K4zKKE8Vj/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="pdoppredictedvariance" width="492" /></a> <br />
As we saw above, the first two weeks, show essentially zero variance. After that, the variance increases linearly. This shows us again that almanacs should not be used after the two week mark. Maybe almanacs should come with an expiration date. If only you could tell they were bad by smelling them like you can with bad Nog. Oh well. <br />
The last graph shows how well the predicted PDOP values correlate to actual PDOP values. This is a cross-correlation analysis and essentially compares the PDOP values at each point in time (in this case, every 15 minutes), and determines how well they compare to one another. A high value means they are well correlated, a low value means there is little correlation between them. There are over 14700 correlations done here; one each 15 minutes for 22 weeks. We expect the correlations to be high at the same time every day. <br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVZtcnWjS3lawqOznSP8aDy0XEPkSomUw958B-Z15ytSFHASzlTikoY3sH-xTlSI-fH2Bi9hl2IfbMSALAtmXEniJv9ksm5R3dIkFtf230iFo3K-ZjJ7x3DAOhU30vioJP5PiNwuAxeuj5/s1600-h/almanaccrosscorrelation%25255B6%25255D.png"><img alt="almanaccrosscorrelation" border="0" height="386" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvSavo0VLQCCPjpdZX6Gg_oth2nnvCIaocRfKZ90kpjVvrTn6Y31ZiMyRbHEC3ibzMJ8TfZ4Bn1ANIewVTH4qQyh1PB4kBrQmWGkipOn4aHg_JhahfChDA6EOacQhaF2yhkQJuzx9IwCMe/?imgmax=800" style="background-image: none; border-bottom-width: 0px; border-left-width: 0px; border-right-width: 0px; border-top-width: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="almanaccrosscorrelation" width="492" /></a> <br />
We see correlation spikes at the same time each day, indicating that the PDOPs do correlate then. Over time however, we see that the correlations decrease, until at 22 weeks, there is no correlation between predicted PDOPS and actual PDOPs. The first two week period is denoted by lags up to 1344. Looking at this graph, you can see that even in the first two weeks, the PDOP correlation begins to degrade. Using the most current almanac will keep you on the left side of this graph, and allow your PDOP predictions to correlate best. <br />
To summarize, you can use almanacs to predict PDOP well, for up to two weeks even. Almanacs should not be used after that time period, as dangerous prediction conditions can arise. Also, to get the best performance from an almanac, use the latest one available. <br />
So we've seen how you can use almanacs to predict PDOP, and if you've surfed around the Nog, you've probably noticed this analysis defines how well you can predict GPS accuracy as well. PDOP is a good indicator of GPS position accuracy but it's not the only player in the game. For a more in-depth analysis of predicting GPS accuracy, look at my paper on the subject <a href="http://www.agi.com/downloads/support/productSupport/literature/pdfs/whitePapers/Long_term_prediction_GPS_accuracy_ION07.pdf">here</a>. <br />
Cheers!Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-7753067841807591062011-08-27T08:23:00.001-06:002011-08-27T08:24:14.056-06:00Recalled SVN-35 Not Doing WellOn August 16th, 2011, according to the 2SOPs Satellite Outage File (<a href="http://adn.agi.com/SatelliteOutageCalendar/SOFCalendar.aspx" target="_blank">Outage Calendar link</a>) and NANU <a href="http://celestrak.com/GPS/NANU/2011/nanu.2011062.txt" target="_blank">2011-062</a>, PRN-30 (SVN-35) was set healthy, bringing back a satellite previous removed from the constellation. SVN-35 had been labeled a spare, making it reusable in case it was needed. That’s just what happened over the last few months. According to <a href="http://insidegnss.com/node/2731" target="_blank">InsideGNSS</a>:<br />
<blockquote>SVN-35, also a Block IIA satellite, had been decommissioned from active service back in 2009 to make room in the constellation for the launch and eventual deployment of the latest new GPS Block IIR vehicle. <br />
Even while it was removed from the almanac of the active constellation, SVN-35 maintained accurate timing and navigation signals; so, when the need arose for a spare, 2 SOPS analysts knew just where to go.</blockquote>However, over the last few days, SVN-35 is showing it’s age.<br />
<a name='more'></a><h2>The Clock</h2>The atomic clock onboard SVN-35 that’s currently active is showing signs of instability. Atomic clocks are quantum mechanical devices and by nature they have random outputs. Fortunately we have developed the technology to the point where we can use the devices for positioning, navigation and timing (PNT) applications that require sub-nanosecond accuracy. When the clocks start to age however, the random outputs become less controllable, causing larger PNT errors.<br />
<h3></h3><h3>Predictable Behavior</h3>The Second Space Operations Squadron (2SOPs), using Master Control Station algorithms, predicts the behavior of all the clocks on orbit in the GPS constellation. They upload those predictions to the satellites which then broadcast them to your GPS receiver. Your receiver then calculates clock parameters necessary for your position determination. When a clock becomes unstable their predictions no longer match the behavior very well, leading to potentially large PNT errors.<br />
<h2>The Last 10 Days</h2>Over the last 10 days, since SVN-35 was set healthy again, the output of the clock is becoming less predictable. The figure below shows the 10 day history of SVN-35’s clock error, the larger the error, the less predictable it’s behavior:<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiELg3z6v3o-veBuj1TmW1yyya2UU5E9H8XdWRGyEsB3T9sNVnjobjkVpRpcVSRiHOOaZHG-5bG3-iRGTX1uUnA974xUOj5kdbe8nLjiChV_KU07xr-t0FgEALuW4e03dbz2hDFuvu-CH0n/s1600-h/PRN30Full%25255B11%25255D.png" target="_blank"><img alt="PRN 30 Last 10 Days" border="0" height="434" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0nG4AvKhC-cXxXlB8ljgcJlSCkuary14wyOq-AH6cxSDwYwOcOeaz3Z-c1IvT6_UBi2TwqpqYkRLg9FzZ1H-68ZWLjW1asQC-a_BEiPM5vBaOth7bYQ3-YqFXUChXP9CfwJALW-M5bL03/?imgmax=800" style="background-image: none; border-bottom: 0px; border-left: 0px; border-right: 0px; border-top: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="PRN 30 Last 10 Days" width="473" /></a><br />
Each time the error goes back to zero (or close to zero), 2SOPS has intervened and uploaded new clock predictions to the satellite. There are times when it behaves well (just after Aug 22) then there are times it behaves badly (Aug 23). This instability is a key indicator of a dying clock.<br />
Here’s a picture of the last three days:<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcNifGiDjZdZ41XvkjYieI_4mZOOd8n2e5pOq_Ii4wghHhL6L94cNlX3S0zQUsJB4vPyVdONYK2O0V94T4N1iC9CLF98jyMh1NoSzppifYrQlj5IE4Wati4uG3jKlL3-OZr2_OuN7MaPtJ/s1600-h/PRN30Last3Days%25255B4%25255D.png" target="_blank"><img alt="PRN 30 Last 3 Days" border="0" height="421" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhm4iOod8uVL9O5BG99YxnZNSS0QE9bqVBb2PFjgzBYc2OHcK1EVOZQ4aB1sW3yRpoiW5b9yZ6Ux_99n-l8GPUBiMjBORqXWAwwcshG81QGW8i8QJ13MUGjJJfLN_J7k0FA99Y0JI3LSXt/?imgmax=800" style="background-image: none; border-bottom: 0px; border-left: 0px; border-right: 0px; border-top: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="PRN 30 Last 3 Days" width="467" /></a><br />
On August 23, 2SOPs uploaded new predictions to SVN-35 six times, that’s 6 times the normal tempo. A normal tempo is once per day. Here’s a picture showing a comparison of SVN-35 (PRN-30) to arguably the best clock in the constellation, SVN-41 (PRN-14):<br />
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgBymC5ZJw0p8tAKoBZ91e1EMqK2N_7QPDro6sVxArrWUqK2QUR2VrWFvjQ4wDC4VeZDfdAiTeHYwMjnS-2lSw-Ie1wvBjqvgNVlDs3nXQVyqoD4jXF9WjPp1xFo8T_AP7vG8mLcIc2qZg-/s1600-h/PRN30PRN14Compare%25255B3%25255D.png"><img alt="PRN30PRN14Compare" border="0" height="420" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEih_kvlvHw6iuqy8qHZcyN78D7oU9LRp9J1fWRDPQ1ULMtXDtvDK4DJZBNBT-m3mDEqoaR1EuEfeFxmt2UZC8QQj7ktsUTHHFAipG1Fj4GDfSDwg4s9zL8Q92pIiMvev9W3t5357g9Knp4h/?imgmax=800" style="background-image: none; border-bottom: 0px; border-left: 0px; border-right: 0px; border-top: 0px; display: inline; padding-left: 0px; padding-right: 0px; padding-top: 0px;" title="PRN30PRN14Compare" width="466" /></a><br />
You can see that SVN-41 has a much more stable, predictable clock that SVN-35 has.<br />
<h2>Unusable</h2>On Friday, August 26th, 2SOPs again set SVN-35 unusable until further notice (NANU <a href="http://celestrak.com/GPS/NANU/2011/nanu.2011070.txt" target="_blank">2011-070</a>), and has <a href="http://celestrak.com/GPS/NANU/2011/nanu.2011069.txt" target="_blank">delayed maintenance</a> on SVN-43 (PRN-13) presumably because of this. Will SVN-35 come back? Can its clock be saved once again? Are there additional clocks on SVN-35 that might be better? We may not know for awhile. Unlike with a jury however, the longer it takes for SVN-35 to return, the less likely it is for a favorable outcome.<br />
<br />
PS: All of the graphs were produced from my GPS Satellite Performance website here: <a href="http://adn.agi.com/GNSSWeb/PAFPSFViewer.aspx">http://adn.agi.com/GNSSWeb/PAFPSFViewer.aspx</a>. A GPS Satellite Outage Calendar is also available, where you can track each satellite’s outage history and future outages: <a href="http://adn.agi.com/SatelliteOutageCalendar/SOFCalendar.aspx">http://adn.agi.com/SatelliteOutageCalendar/SOFCalendar.aspx</a>.Unknownnoreply@blogger.com2tag:blogger.com,1999:blog-3428288523943634178.post-60757492062911892852011-08-12T21:23:00.000-06:002011-08-12T21:23:48.634-06:00GPS OCX PDR Still Pending - 106 issues noted<div>Raytheon has 106 Preliminary Design Review (PDR) Issue Notices or PINs to work through - issues that were raised at the June OCX PDR. A team within Raytheon has been working them, to a completion date at the end of August.<br />
An independent OCX PDR review team from the SMC, chartered by Lt. Gen. Pawlikowski, examined plans for completion of the PINs and stated that Raytheon is on track to complete them.<br />
From and article in Inside GNSS: <a href="http://www.insidegnss.com/node/2724">GPS OCX PDR Still Pending against Backdrop of Defense Cuts | Inside GNSS</a></div><div><br />
</div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-14419864116619002022011-07-30T13:58:00.002-06:002011-07-30T14:02:19.334-06:00FAA Releases Impact Report from LightSquared InterferenceThe FAA, as requested by the Executive Office of the President's Space-Based Positioning, Navigation and Timing (PNT) Executive Committee's National Coordination Office (NCO) Director, (what a mouthful) has <a href="http://www.insidegnss.com/pdf/07122011_FAA_-_LightSquared_Aviation_Impacts.pdf">delivered a report</a> on the expected impacts from interference due to LightSquared's terrestrial broadcasts.<br />
These impacts are big, including the estimated loss of 800 lives - or rather, lives that would have been saved by moving to GPS technologies - something that is impacted by LightSquared interference.<br />
Here's a list of the expected impacts (from their Executive Summary):<br />
<br />
<a name='more'></a><ul><li> LightSquared's initial operations at the lower 10 MHz channel even at "reduced" power levels would impact the aviation use of high-precision GPS receivers.</li>
<li>[The FAA] estimate[s] it would take 10 years to design, develop, certify and install modified equipment in the civil aviation fleet.</li>
<li>LightSquared's planned operations would result in the loss of GPS-enabled operational, economic, and public safety benefits across the National Airspace System.</li>
<li>The FAA would be compelled to return to dependency on ground-based navigation aids.</li>
<li>Billions of dollars in existing FAA and GPS user investments would be lost.</li>
<li>The FAA would also need to replan Next Generation Air Transportation System (NextGen) investments, which would result in additional development costs and delays.</li>
</ul><div>The last bullet references NextGen. NextGen is the new system the FAA is planning to fly Air Traffic in more direct routes, and allow for closer spacing of Aircraft. NextGen relies on Automatic Dependent Surveillance-Broadcast (<a href="http://www.faa.gov/nextgen/portfolio/trans_support_progs/adsb/">ADS-B</a>) technology - a replacement for radar. ADS-B use satellite radio navigation technology (GPS) to create and transmit positioning and velocity information from plane to plane and to the ground. Relying on this technology means they are relying on GPS, and the spectrum on which GPS is broadcast. LishtSquared's plans to broadcast near or in in part of that spectrum, interferes with ADS-B message creation, meaning the FAA will need to relay on older technologies, like radar and other Direct Measuring Equipment (DME).</div><div>Additionally, The FAA states that this interference would cost, over the next 10 years, an additional $70,000,000,000 ($70 Billion) and 30,000,000 (30 million) tons of CO<sub>2</sub> for the following reasons:</div><ul><li>Loss of benefits from delayed NextGen technologies and procedures</li>
<li>Loss of existing GPS efficiency benefits</li>
<li>Loss of existing GPS safety benefits</li>
<li>Aircraft retrofit costs</li>
</ul><div><a href="http://www.insidegnss.com/pdf/07122011_FAA_-_LightSquared_Aviation_Impacts.pdf">Read the report</a> for more information.</div><div>It's imperative that LightSquared's transmission and spectrum issues be worked out, so that it does not interfere with our global utility. The fact that it has come this far is already beyond belief - take a stand, make sure everyone you know understands this issue and <a href="http://www.congressmerge.com/onlinedb/">reach out</a> <a href="https://writerep.house.gov/writerep/welcome.shtml">to congress</a> - let them know that you depend on GPS, and you don't want it to disappear.</div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-32449904988194540422011-07-28T12:34:00.000-06:002011-07-28T12:34:20.243-06:00OCX in Trouble? Already?The recent PDR (Preliminary Design Review) by the US Air Force of the next generation GPS Operational Control Segment, (OCX) under development by <a href="http://www.raytheon.com/capabilities/products/gps_ocx/">Raytheon</a>, was<a href="http://www.gpsworld.com/gnss-system/news/ocx-hits-a-bump-does-not-pass-design-review-11928"> not approved</a>. The US Air Force chose to "Withold Approval" until actions from the PDR could be reviewed.<br />
This is not good news for Raytheon and may point to trouble down the line for OCX. In the current economic environment, adding costs to an already costly program is not in the cards. If the current OCX design is not sufficient, what next? Will more funding, if required, be appropriated?<br />
Keep an eye on this one - it could be a storm.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-19601317390020068452011-07-28T12:24:00.000-06:002011-07-28T12:24:19.960-06:00Sprint moves LightSquared a step closer to entering 4G market<div>Sprint is on<span class="Apple-style-span" style="font-family: inherit;">board with Li</span>ghtSquared, meaning LightSquared's business is not going away. The FCC still ha snot approved their spectrum though - risky move by Sprint?</div><div>Sprint President of Network Operations Steve Elfman says:</div><div><blockquote>"We believe LightSquared, in cooperation with the FCC and adjacent spectrum users, is taking proactive steps to address and resolve these issues in a timely manner,"</blockquote></div><div>See the full story here:</div><div><a href="http://arstechnica.com/telecom/news/2011/07/sprint-deal-moves-lightsquared-a-step-closer-to-entering-4g-market.ars">Sprint deal moves LightSquared a step closer to entering 4G market</a></div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-8501907610593123982011-07-16T07:33:00.002-06:002011-07-16T07:35:49.096-06:00GPS IIF-2 LaunchThe newest next generation GPS satellite lifted-off today at 2:41 AM EDT. Aboard the Delta 4, the satellite was in it’s proper orbit within 3 1/2 hours.<br />
The satellite will undergo a checkout period prior to being set healthy. Typically this takes a few weeks.<br />
Here’s a shot from Universe Today: <br />
<a href="http://www.universetoday.com/wp-content/uploads/2011/07/DSC_5808-Copy.jpg" target="_blank"><img alt="GPS IIF-2 - Launch" height="283" src="http://www.universetoday.com/wp-content/uploads/2011/07/DSC_5808-Copy.jpg" title="" width="480" /></a><br />
<h2>Technical Data</h2><br />
<a name='more'></a>SVN: 63<br />
PRN: 1<br />
Orbit/Slot: D/2A<br />
<a href="http://www.celestrak.com/GPS/NANU/2011/" target="_blank">2SOPs NANU</a>: <a href="http://www.celestrak.com/GPS/NANU/2011/nanu.2011047.txt" target="_blank">2011047</a><br />
The orbit slot is one of three bifurcated slots. Per Richard Langley and <a href="http://gauss.gge.unb.ca/CANSPACE.html" target="_blank">CANSPACE</a>:<br />
<blockquote>The D2 slot is one of three orbital slots that are expandable into fore (F) and aft (A) slots. SVN46/PRN11 is currently in slot D2F. The other two bifurcated slots are B1 and F2.</blockquote><h2>Launch Info</h2>In previous GPS satellite launches aboard less powerful boosters, it took a day or more to get to the proper orbit, using an additional Apogee Kick Motor (AKM) attached to the satellite itself to propel it to it’s destination in Medium Earth Orbit (MEO). Now, with more powerful boosters, satellites are directly placed into their correct slots. From <a href="http://spaceflightnow.com/delta/d355/status.html" target="_blank">Spaceflight Now</a>:<br />
<blockquote>T+plus 100 minutes. Unlike the previous generations that rode on smaller rocket Delta 2 rockets, the dozen next-generation GPS 2F craft will be delivered into orbit atop the Delta 4 and Atlas 5 fleets of boosters in the Air Force's Evolved Expendable Launch Vehicle program. <br />
The Delta 2's, although highly reliable, weren't powerful enough to inject the GPS satellites directly into the orbiting constellation. The three-stage vehicles released the craft into a highly elliptical orbit stretching from 100 miles at its closest point with Earth to some 11,000 miles at its highest point, which is the altitude where the GPS network resides. The newly-launched satellites were themselves fitted with a solid-fuel kick motor that ignited a few days into flight and finished the job of propelling the craft into a circular orbit. <br />
United Launch Alliance's powerful Delta 4 and Atlas 5 rockets will haul the GPS 2F satellites directly to their desired destinations, bypassing the circuitous route of the past. So instead of taking days to reach the GPS orbit, the new 2F-2 satellite will get there in three-and-a-half hours today.</blockquote>More articles: <br />
<a href="http://spaceflightnow.com/delta/d355/status.html" target="_blank">Spaceflight Now</a> <br />
<a href="http://www.universetoday.com/87531/ula-delta-iv-thunders-off-pad-with-gps-iif-2-satellite/" target="_blank">Universe Today</a>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-18259204153951948322011-07-11T15:33:00.000-06:002011-07-11T15:33:03.145-06:00Iridium Offers New Air-Traffic Service<a href="http://online.wsj.com/article/SB10001424052702303678704576438104055934190.html">Iridium Offers New Air-Traffic Service - WSJ.com</a><div>Look like they'll provide a navigation service as part of a digital data stream for traffic control for long-haul carriers. Stay tuned for more on this.</div>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-58077604035387040192011-07-09T09:41:00.002-06:002011-07-09T09:46:58.377-06:00Do you know LightSquared?LightSquared (LSQ) is a company that is building a next generation broadband network, those of the "4G LTE" type. This is a good thing because it allows us all to have faster, more widespread communications, staying online longer. These networks work by transmitting signals from cell towers. The issue with LightSquared is that they are planning on transmitting on a frequency that is right next to GPS frequencies.<br />
<a name='more'></a> There's more to the story than that though. First, you need to know that GPS signals, for better or worse, are extremely low power. In fact, they are below the level of pure noise (~ -150 dB for you engineers). Meaning, if you were to listen for a GPS signal, on it's frequency (1575.42 MHz for L1), the static you'd hear would over power the GPS signal. There are ways to track GPS of course, but that's another topic.<br />
Back to LightSquared. The original frequencies planned for use by LSQ were in the range 1525 - 1559 MHz, right next to GPS. The broadcasting power has to be significant as well, to provide coverage to the cellular users. Here's the crux: with their high power and proximity to the GPS band, <i><b>most GPS receivers would no longer work.</b></i> The LightSquared transmissions would overpower the small GPS signals. Here's <a href="http://arstechnica.com/tech-policy/news/2011/07/report-lightsquared-deployment-would-have-severe-operational-impact-over-us.ars">a quote</a> describing the problem:<br />
<blockquote><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif; font-size: 13px; line-height: 17px;"></span><br />
<div style="margin-bottom: 1.308em; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif; font-size: 13px; line-height: 17px;">One engineer involved in the debate (he takes the GPS side) provided us with an analogy:</span></div><blockquote><div style="margin-bottom: 1.308em; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif; font-size: 13px; line-height: 17px;">What the FCC did was say, "While this was supposed to be a suburban neighborhood, we are changing the rules after 40 years. Company X has bought all the houses to the left of your house, and we are changing the zoning from your lot line left." As they bulldoze the houses (LSQ pays off Inmarsat to scoot over), you find out that they are going to allow them to build a concert stadium next to your two-post fence to the end of the block (put in GROUND stations broadcasting 3 billion times more powerfully than originally intended).</span></div><div style="margin-bottom: 1.308em; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif; font-size: 13px; line-height: 17px;">What happens? There's now going to be a U2 concert an inch past your lot line 24/7. There will be people fighting in the parking lot and the sirens of cops and ambulances going on all day and night forever.</span></div><div style="margin-bottom: 1.308em; margin-left: 0px; margin-right: 0px; margin-top: 0px;"><span class="Apple-style-span" style="font-family: Arial, Helvetica, sans-serif; font-size: 13px; line-height: 17px;">Now—are they in your yard? Not at all. They are on their side of the lot line.</span></div></blockquote></blockquote>GPS is used by the military, civilians and in countless technologies worldwide. GPS provides timing for bank and stock transactions (because GPS keeps time to the nanosecond level - a billionth of a second). GPS will also be crucial to the next generation air traffic control system: <a href="http://www.faa.gov/nextgen/">NextGen</a>. If GPS signals are interfered with, these things don't work.<br />
Since then, LSQ has proposed a<a href="http://static.arstechnica.com/lightsquarednewplan.pdf"> new frequency plan</a> that will alleviate some of the interference, but not all, especially for high-accuracy GPS users (surveyors, etc.).<br />
Stay-tuned for more and keep a close eye on your spectrum, it's valuable!Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3428288523943634178.post-70335024722271415842011-07-08T21:44:00.001-06:002011-07-09T07:11:25.042-06:00Welcome back!Welcome back to The Nog! I'm re-opening the navigation blog I started <a href="http://blogs.agi.com/navigationaccuracy">here</a>. My company discontinued blogs by individual authors, but I wasn't quite done writing about navigation topics. I'll continue writing in the same style I did on the original blog, but I'll be a bit more open about the topics I'll discuss and present a few more opinions.<br />
Soon, I'll start bringing over the most popular blog entries from the original location, reposting here.<br />
Let me know what you want to hear about, the nav topics are wide open.<br />
Good to be back!Unknownnoreply@blogger.com0