Expert Advice & Leadership Talks

Expert Advice: Moore’s Law and GNSS

July 1, 2010By

I started my relationship with GNSS and Moore’s Law in 1985, writing software for GPS tracking loops on the Advanced Range Instrumentation Aircraft program at the Applied Physics Laboratory of Johns Hopkins University for the U.S. Air Force. The project’s purpose was to navigate a large jet to accurately fly a pattern to drop buoys into the ocean. That receiver had seven circuit boards (six trackers and one navigator) mounted on a VME backplane in a 19-inch rack mount in the back of a C-130, and was about the size and weight of suitcase. read more

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Expert Advice: Quasi-Coherent Delay Lock Loop Tracking and Generalized Binary Coded Symbols in Multipath

May 1, 2010By

The original GPS signals, and indeed most GPS signals including L5, utilize conventional pseudonoise (PN) signal code division multiple access (CDMA), some with both in-phase and quadrature-phase modulation. In the late 1990s, I generalized Manchester PN symbol-spreading by defining split-spectrum binary square wave symbol-spreading, in a series of limited-distribution papers for the Air Force GPS Independent Review Team (IRT). These split-spectrum signals have been developed and analyzed much more fully by many others, and they are now termed binary offset carrier (BOC) modulation. The BOC codes can provide a noise-error advantage by placing more of their spectral energy at an offset frequency, thereby increasing the Gabor bandwidth. They can also provide spectral separation from other GNSS signals in the same frequency band, for example, L1. read more

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Expert Advice: Jamming: A Clear and Present Danger

April 1, 2010By

A packed audience attended the National Physical Laboratory in the United Kingdom for a February 23 meeting titled, “GPS Jamming and Interference: A Clear and Present Danger,” organized by the Digital Systems Knowledge Transfer Network. In his keynote address, David Last described a dark, silent and dangerous world without GPS. His final insight was this: “Navigation is no longer about how to measure where you are accurately. That’s easy. Now it’s how to do so reliably, safely, robustly.” read more

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Expert Advice: Are We There Yet?

March 1, 2010By

At the start of a new decade, let’s examine the state of the GNSS consumer market and technology. In the December 2009 issue of GPS World, I described the developments that put GPS in cell phones over the last decade. That technology revolution has brought GPS a very long way. Having come this far, we can ask that most famous of all navigation questions: Are we there yet? read more

Expert Advice: Integrity: Lessons from the 2008 Financial Collapse

February 1, 2010By

Deterministic risk modeling, the basis of the Efficient Market Hypothesis (EMH) at the core of modern quantitative finance, is known to be fundamentally flawed, but its elegance and convenience has blinded researchers to growing evidence of its weaknesses. The near-complete acceptance of the EMH led to models that dramatically accentuate its flaws, which in turn led to absurd but eagerly accepted conclusions for loan-default risk. These models proved dramatically vulnerable to changes in the housing market in 2007–2008 and led directly to the ensuing crash. read more

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Expert Advice: Availability Gaps: Solutions for Aviation

December 1, 2009By

Recent attention given to aging GPS satellites and availability gaps from lagging constellation replenishment have provoked deep concern, particularly within the aviation community. Available remedies include exploitation of well known but unused methods plus new techniques; those discussed here have future relevance, with or without availability gaps. Even with far greater coverage from multiple GNSS, crises could emerge from severely stronger interference levels or other unforeseen events. Advance preparation for any such occurrence would avoid the waste, confusion, and blind alleys that generally arise with the sudden appearance of an emergency. read more

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Expert Advice: GPS Constellation Maxed Out at 30

November 1, 2009By
Figure 2. The number satellites set healthy since 1998 (courtesy of Analytical Graphics, Inc.)

It appears that the GPS satellite constellation has a glass ceiling, so to speak. GPS was designed as a 24-satellite constellation, with four satellites in six orbital planes arranged to provide maximum observability around the globe. According to the government’s Space-Based Positioning, Navigation, and Timing website, “The U.S. government is committed to provide a minimum of 24 operational GPS satellites on orbit, 95 percent of the time. The U.S. Air Force launches additional satellites that function as active spares to accommodate periodic satellite maintenance downtime and assure the availability of at least 24 operating satellites. As of August 28, 2009, there were 35 satellites in the GPS constellation, with 30 set ‘healthy’ to users.” read more

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