Alternate Position, Navigation and Timing (APNT) – What is this?

In any conversation about satellite navigation and the use of enablers like GPS, talk inevitably shifts to the risks and the ease with which GPS for instance can be jammed. It is easy to sketch doomsday scenarios with a full-scale GPS outage once NextGen and SESAR are operational, making the industry essentially dependent on signals from space. The response is alternating between brushing away the risk or suggestions that satellite navigation is perhaps not the best path for the future of air traffic management.
The fact of the matter is, there have been cases where the GPS signal was effectively unusable in certain parts of the US, with the duration of the incidents varying between 1 hour and 72 hours. The incidents were all traceable to temporary adverse conditions but it is only a matter of time before malicious intent will join the list of causes. There is certainly no shortage of cheap but effective jammer devices, some of which fit inside a cigarette box.
Adverse conditions may arise for example as a result of meteorological or space-based phenomena or trucks passing near the location of an antenna situated in a crammed environment. Portable jammers may be activated anywhere…
One of the main attractions of the move to a space based ATM paradigm is the potential cost saving offered by the chance to eliminate the ground navigation infrastructure. The vulnerabilities of the space based system at the same time require that measures be introduced that cost-effectively mitigate the risks posed by those vulnerabilities.
Air navigation service providers the world over are obliged to set up a system that enables them to continue providing the services required even in the case of various contingencies. No-break power supplies, robust, redundant communications lines, contingency control rooms and the ability to transfer control to neighboring centers in case of a full scale failure or natural catastrophe are just a few examples of routine measures in place to soften the impact of contingencies.
In the past, the failure of a VOR/DME serving a busy intersection, failure of an ILS serving a busy runway or total equipment failure on board a single aircraft were serious events and made both controllers and pilots sweat but it was hardly the end of the world.

Once the transition has been made to space based systems, a major failure or interference event could very well shut down the total ATM network over a whole continent, leaving thousands of passengers and crew in acute danger.
The obligation of the service providers is to continue providing a safe service while also minimizing the economic impact in adverse circumstances.
Realizing that the future ATM system, if it is to meet the expected demand, must rely increasingly on space based position, navigation and timing (PNT) services, it is clear that effective measures are needed to address the vulnerabilities inherent in those services.
Enter APNT or Alternate Positioning, Navigation and Timing.
What is APNT supposed to do?
Obviously it is not realistic to expect that the APNT environment would provide the same capacity and the same flexibility of operations as will be normal in the unimpeded NextGen and SESAR environment. APNT must ensure that operations can transition safely to a reduced performance regime and continue to operate there safely for the duration of the incident. APNT must also ensure that this contingency facility is cost effective in implementation and operation and that it requires no or minimal changes to on-board equipment. The original business case for NextGen and SESAR did not consider the cost associated with the establishment of an effective APNT system and hence it is important to ensure that APNT does not kill the (fragile enough) business case of those new concepts.
In many ways, the US FAA has taken the lead in figuring out what the best APNT solutions could be. Although some of their conclusions reflect the situation in the US, for instance in respect of the capabilities of General Aviation, the arguments listed for the various options are generally applicable world-wide.
So what are the available options for APNT?
Use DME-DME.
This is the least expensive option for the airlines and is of course based on existing systems and technology. The impact for general aviation can be serious as no suitable avionics are available. Furthermore, DME-DME equipped aircraft without inertial capability have certain restrictions on using RNP en-route and even with inertial capability there are restrictions on RNP approaches. An important consideration is that DMEs can get overloaded if the number of simultaneous users exceeds a certain limit.
Wide area multi-lateration (WAM).
This option fits nicely with the plans to rationalize the surveillance infrastructure and has practically no impact on surveillance related avionics. At the same time the capacity limits discussed in the past in connection with systems based on Mode S Extended Squitter (1090ES) apply here also and this may result in a shortfall of achievable availability in certain circumstances. A common time reference, independent of GPS, would also be needed. Experience with MLAT systems has shown that the number of antennas needed on the ground for a reliable system is substantially higher than originally thought. If WAM is selected as the APNT option, the ground infrastructure, including the communications and signal processing elements, is likely to represent substantial costs. We must also keep in mind that MLAT was invented as a surveillance solution. Using it for navigation requires as yet undefined avionics changes.
DME Pseudolites (DMPL).
This is the least mature concept with the greatest impact on avionics. In this solution the aircraft calculates its position using a minimum of three ground stations (DME) with integrity monitoring also built into the on-board system. Position is then broadcast similar to ADS-B. Here again a GPS independent timing reference is needed.
The issue of time synchronization.
The more the ATM system, including its ground and airborne elements, becomes integrated, the more critical time synchronization becomes. Although we tend to think of GPS as a tool to establish a very precise position anywhere on the planet a perhaps even more important novelty it brought was a ubiquitous time reference available to all. The whole GSM network would collapse if this time reference were taken away for any length of time. However, in the work on APNT we are dealing with this exact case: what alternative time references could we find and how would this work in the aviation context? A lot of work still remains.
Once we have both GPS and Galileo, do we still need APNT?
With two independent satellite systems (and possibly more if we consider also the Russian and Japanese constellations) and the planned improved jamming resistance of GPS one may be excused for thinking that APNT is not that important after all. In any case, less important than it would be if GPS were the only game in town. Obviously, the risks and their impact on safety and economics of operations must be evaluated taking Galileo and the GPS enhancements into account and any APNT agreed in the end must be scaled to answer those risks without costly overkills. It is extremely unlikely that any analysis will result in APNT being shown as unnecessary.
The importance of world-wide interoperability.
In the nice old (and much simpler) days global interoperability of avionics systems was almost a given. I say almost because people did manage to deploy incompatible things, just think of the Soviet version of the ILS… But on the main, NDBs, VORs and DMEs all worked to the same standards and the boxes on board worked everywhere. With the introduction of 8.33 kHz channel spacing for the first time ever a situation arose where a basic tool like your trusty VHF radio might work well everywhere in the world except in Europe. A widespread and expensive retrofit campaign was the result. Controller/Pilot Digital Link Communications almost ended up with similar regional differences but luckily for the industry the US and Europe got together and agreed the details of CPDLC applications and services. Not everything is available everywhere, but if it is available, it will work according the agreed standard.
Regional differences for APNT harbor the potential for truly expensive and troublesome situations. Both ground and airborne elements need to be standardized lest aircraft are required to carry two or three different kits to match the differing tastes of local APNT preferences. Or… to give the ground folks a reason to maintain two or three parallel infrastructures to save airborne costs, a solution that is even more expensive in the end.
The future?
The jury is still out on this and it will take a while and interesting discussions to be sure, for the industry to agree a world-wide solution (or at least a set of interoperable solutions) to the APNT problem. Stay tuned…