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Navigating the Skies

By Jonathan Ray | December 1, 2012
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The GPS network is everywhere in our cars, on our phones, and, yes, in aircraft. Recent events have called the safety and security of the GPS network into question. But panelists on the Avionics Magazine Webcast, which represented aircraft operators, airborne GPS equipment manufacturers and the architect of the next-generationa satellite constellation, say the GPS network needs to be protected, but overall the future looks bright. Below is an excerpt of the Webcast.

Tom Hendricks, president, National Air Transportation Association: I’ve had the opportunity to testify up on the Hill on this four times, and it has been a seminal event for the aviation community to rally around protecting this valuable resource that we have. And our concerns are not directed at a specific company, rather it’s the technical aspects that we see that will present challenges, potentially, to aviation users.

GPS has become a ubiquitous aspect in our daily lives, so much so that I’d equate GPS in aviation in a similar fashion as I would to cell phones for the general public. We would simply have a hard time understanding how to move forward without these ground-breaking technologies.

As a new arrival to the general aviation community with my role at the National Air Transportation Association, I can say that GPS has made a huge difference to general aviation through the use of LPV approaches, giving us the ability to shoot approaches in all weather conditions to runway ends we never dreamed of in the past. For airlines, the GPS-based terrain alerting and warning system has all but eliminated controlled flight into terrain for U.S. carriers.

As we move forward with evolving our national airspace system into NextGen, GPS is providing the backbone for this much-needed change. Future communications through data com, navigation, and surveillance through ADS-B all rely upon this foundational capability to move forward.

Continued unimpeded use of GPS is indispensable to the future of aviation.

The GPS constellation is located about 12,600 miles in space, transmitting at levels lower than the power of a 50-watt light bulb. And as much as pundits may wish to change the facts, the laws of physics do remain constant here. So, claims of mitigation strategies must meet this reality of a very low-power signal that we’re trying to derive very accurate positions from.

Additionally, aviation certification standards are correctly very high. So, any impacts to our aircraft must pass a very high level of scrutiny. Any transmissions from any source that impede GPS signals are of great concern to the aviation industry. As a community, we must continue our public discourse on this subject, because of the critical importance of GPS. It was very interesting to watch the community realize that there is a threat to this important asset, and to see them rally around the possible solutions forward.

Technology is going to continue to evolve. So, deploying new and refreshing existing GNSS systems will help improve our already very safe air transportation system, and, as a community, we need to ensure that we come together whenever these threats evolve to this very ubiquitous system in the future.

Christopher Hegarty, director of CNS Engineering & Spectrum, MITRE Corp.: The theme of this webinar is on the future of the GPS, and although there was some tantalizing description of some of the threats from spectrum facing GPS, I thought I would focus on some of the more mainstream activities that the GPS avionics community is involved in right now.

First of all, to directly address the question of what is the future of GPS, let me state in my view it is very bright indeed. There are billions of users of GPS today worldwide and the government is investing tremendous amounts of money in GPS, with over $1.3 billion planned for next fiscal year.

The future of GPS avionics is actually tied with the future of the overall global navigation satellite system, or GNSS. Russia’s system –– GLONASS –– is now fully populated with 24 satellites, a similar constellation size to GPS. They have a second civil signal in use today, and they’re planning to add new code division multiple access signals in the future.

Europe is planning on a constellation of 30 satellites. They have two satellites in orbit today, and they’re planning to have this whole system up and running by 2020.

China is planning a constellation of overall 35 satellites. They now have 15 in orbit. They’re actually launching this quite rapidly. This will involve a mix of satellites, some in medium earth orbit, similar orbits to GPS and GLONASS, but some also in other types of orbit, geo-stationary, and inclined geo-synchronous.

There are two regional constellations planned, with seven satellites each one in India, one in Japan. There are also a number of satellite-based augmentation systems being deployed around the world. There are three operational today the WAAS within the United States, EGNOS in Europe, MSAS in Japan, and three more that are underway, at least in early stages of deployment. And also there are also augmentation systems, referred to as ground-based and aircraft-based systems.

The vast majority of the avionics that’s out there and flying today –– over 100,000 certified pieces of equipment –– is only processing one type of signal on one frequency, just the signals at 1575 megahertz, and only from GPS, and, in some cases, SBAS systems like WAAS.

ICAO first published standards and recommended practices or SARPs for GNSS in 2001, and these have since been amended 11 times. The main thing to understand about these SARPs is that they currently only address GPS and GLONASS signals in the L1 or 1575 megahertz band, as well as the augmentations I mentioned before, but only that one frequency.

What is new in the work that is currently underway is actually to develop multi-frequency, multi-constellation standards for the next generation of avionics. This development is going slow, and that’s uncertainty in the deployment of the different satellite constellations that are underway. Everyone who has watched the GPS schedules or any other satellite navigation system has seen these schedules only move one direction and they move quickly in one direction, and that’s to the right.

One other challenge that we run into quite a bit is cost/benefit. People in the aviation community are not flooded with money at the present time and they need to be sure, if you want to sell a piece of avionics to an airline, you really do have to have a very compelling cost/benefit story for every incremental improvement that translates into cost.

Rex Hygate, business development manager, Airline Solutions, Esterline CMC Electronics: What I’m speaking about is slightly more near-term, and only affecting the L1 GPS, and the upcoming requirements for, specifically, the ADS-B [automatic dependent surveillance-broadcast] mandate in the U.S. Most airliners today are using TSO-C129 GPS receivers, a standard that was made in the late 1990s. TSO-C129 has recently been canceled by FAA. For these aircraft to operate in the U.S. airspace past 2020, they’re going to need to look at some form of an update.

The option that they have is to get what’s called a TSO-C196 SA-aware GPS, a multi-constellation receiver, or what’s called a TSO-C145 SBAS or WAAS GPS receiver.

TSO-C129, as it stands right now, won’t meet the ADS-B accuracy requirements, and it was developed for lower accuracy than what they’re looking for. What they’re looking for in the U.S. is effectively using the GPS signal transmitted by ADS-B to the air traffic control systems in order to effectively replace secondary radar in an awful lot of places.

The other thing that’s come up is that the noise in the L1 area has increased over time and this has nothing to do with LightSquared. The WAAS system transmits on L1. Galileo also transmits on L1, and the result is that there’s extra noise that was not anticipated when they wrote TSO-C129 back in the ‘90s. This noise means that an older receiver may miss acquisition of a low-level satellite that otherwise, back in 1999, they would have seen. Simply put, their performance has degraded from what it was when the receivers were built.

So, the next step is what they call TSO-C196, SA-aware GPS receivers. These are being delivered on many new aircraft now. They’re significantly better than the 129, but they don’t take into account the WAAS accuracy improvement. They do take into account the latest noise criteria, so they have filters to take care of the additional L1 noise, but they don’t have the integrity that a WAAS receiver would give, and, therefore, in order to be used in 2020, the airline would have to check to make sure that the GPS constellation is adequate for their flight.

Another option is to wait for the multi-constellation GPS receivers, which will generally be built on L1/L5 or Galileo. The trouble is, for the 2020 mandate, when all aircraft operating in the U.S. have to able to meet it, the satellite constellations could barely be complete by 2020. The ground infrastructure is still not entirely complete and the cert standards are not complete yet. Some of them haven’t even started.

Given this, the chances of having airline standard receivers being available by 2020 is quite unlikely. In addition, the civil certification path for aviation safety of life operation for Compass and GLONASS is still not 100 percent clear.

The standards for airline operation are quite strict, and this is what would be driving an awful lot of the schedule.

The best solution is to get a TSO-C145 WAAS receiver. The WAAS satellite relays the information from all of the ground stations that checks. So, the receiver doesn’t just receive the position from the satellite but it also receives the ground stations that say the position is correct. This allows you to have an extremely high integrity on a WAAS SBAS signal, which allows it to be used in the 2020 mandate unlimited. It fully and simply meets the mandate requirements. And for me, near term, this is the biggest GPS issue for airline operators.

J ohn Frye, GPS III Capability Insertion Program Managerm Lockheed Martin: The GPS system today is really comprised of a constellation of satellites that originally was envisioned to be 24 satellites, and, in fact, it’s evolved to become a system where there’s 34 satellites in orbit. 31 of those are healthy and contributing to system performance. Satellites are contained in six different planes, nominally four satellites per plane, and they are in a orbit altitude where it takes about 12 hours to get around the planet once.

So, why GPS-III? What does the third block of satellites bring? What we’re looking to provide to the users is a combination of things. First is improved position navigation and timing accuracy, and the other is to provide higher power signals for both civil and war fighter applications. We have an enhanced M-code earth coverage power level, which, again, is a benefit to the war fighter in the future when the M-code user equipment is made available to the war fighters.

And then, an important element of the program that the Air Force had called out for was a graceful growth path. They wanted a satellite platform that would allow them to add capability over time and not have them locked out from adding new features within a timeframe of a decade or so. This has been a problem with past platforms.

But overall, the objectives of the program are to deliver this high-confidence acquisition, and avoid previous space program acquisition problems. In working with the Air Force, we’ve laid out a program that addresses this is in a number of ways.

The program decided from the outset to build what we call SVO-0, or our GPS non-flight satellite test bed. So, what we’re doing is building a satellite up from engineering development models and qualification models, from the box level all the way up through satellite design and test. Through this process, we are driving out any uncertainties, unknowns or problems in our manufacturing process, any issues in our designs, and verifying that all of our test equipment is working correctly and we’re not troubleshooting the test equipment on our first flight vehicles, but actually troubleshooting it on engineering model boxes, so that we don’t have a problem with the potential of damaging precious flight units that we’ll be using eventually, down the road. The contract was awarded back in May 2008. We have moved through a preliminary design phase and critical design phase for that SVO-1 space vehicle. The same basic design is envisioned to be used for the first eight satellites.

We’re sort of in the production and test mode right now. Where we’re at specifically in terms of the GPS non-flight satellite test bed, all that equipment’s been delivered, assembled, and is in test, and we’re working through single-line flow, thermal vacuum testing, and, again, cross-checking all of our test equipment.

The first space vehicle is being built up today, and will move into its test phases some time next year, with our objective being to have that first space vehicle available for launch second quarter of 2014, and we are on track and on plan to deliver in that timeframe.

GPS modernization is well underway, and the constellation will be maintained through the launch of all the IIF and the GPS III satellites. Between the IIF satellites and the GPS III satellites, Boeing and Lockheed are doing our fair share of ensuring that satellites are available for launch.

To listen to the complete Webcast, visit

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