Satellite Navigation Evolution (May 2016)

In my first article, I contrasted the rapid progress of LPV approaches in the US with the much slower adoption of these approaches in Europe. This article looks to the future of satellite navigation.  

Reliable and consistent satellite navigation using GPS has become a popular and mainstream method in our cars, boats and aircraft. The good news is that ongoing investment and technical enhancements continue to improve the precision, resilience and capabilities of these systems.

There are four distinct aspects involved:

  1. New satellites are making use of extra frequencies
  2. Satellite augmentation is increasing its footprint and precision
  3. Additional satellite constellations are coming online
  4. Receiver technology has become more powerful and lower cost

Newer frequencies and formats

Today, civilian GPS shares the same single frequency of 1575.42 MHz using the L1C/A signal used by most navigation equipment worldwide. The signal is affected by environmental factors such as atmospheric conditions in the ionosphere which can delay it by a few nanoseconds, introducing inaccuracy and uncertainty to the calculated position. Satellite augmentation systems such as WAAS and EGNOS publish correction data that compensates and allows more accurate and precise results.

The third generation of GPS satellites is being launched and brought into service. Satellites launched since 2005 support the L2 frequency of 1227.60 MHz for nonmilitary use and provide two separate civilian codes known as L2CM and L2CL. As of October 2015, there were 17 operational satellites transmitting L2 signals. Full service would require at least 24, expected sometime around 2018.

Additionally, the L5 frequency of 1176.45 MHz broadcasts at higher power and is specifically aimed for aeronautic use. The satellites are equipped with a completely independent datapath, antenna, chipsets and power source for L5, so that if L1 and/or L2 fails then L5 should remain operational. The L5 signal is pre-operational today, broadcast by 10 satellites, and expected to be fully supported sometime around 2021.

A new L1C signal is being introduced using the original 1575MHz frequency with the original L1C/A remaining available for backward compatibility. The new format improves reception in more challenging environments. This is a longer term upgrade and will require all new satellites. Launches of those commence in 2017 with an estimated fully operational timeframe of the late 2020s.

Through a technique called trilaning, the combined use of all three GPS frequencies could enable sub-metre accuracy without augmentation.

Satellite Based Augmentation Systems expand scope and precision

Satellite Based Augmentation Systems (SBAS) are regional systems that broadcast additional information from geo-stationary satellites. They also share the same 1575MHz frequency and coding formats, simplifying receiver design. The data is used to compensate and correct for known signal transmission errors and significantly improve position accuracy.

Regional systems include North America (WAAS), Europe (EGNOS), Japan (MSAS) and India (GAGAN). Today, these augment the L1C/A 1575MHz signal only.

LPV 200 Target Service AreaFor those of us based in Europe, in September 2015 the EGNOS system was declared safe for use with LPV approaches down to 200 feet AGL in much of Western Europe, excluding north of latitude 60 degrees or south of the Mediterranean. Existing approved RNAV equipment will be able to take advantage of this reduced minimum as soon as approaches are published.

In the longer term, dual frequency augmentation supporting both L1 and L2 should allow the LPV200 service to be extended into the Middle East and Africa. There is also an intention to combine GPS and GALILEO dual frequency signals to achieve an enhanced service capable of lower than 200 feet throughout the European Union.

Additional Satellite Constellations are coming online

Other nations have invested in their own satellite systems. The Russian GLONASS system benefitted from substantial investment in recent years which brought it back into full service. A Russian Government mandate that all smartphones had to support it (instead of or in addition to GPS) pulled through mass market/low cost receivers. There are many GPS/GLONASS receivers available, and both are now incorporated by default into the most popular smartphone models, such as Android and iPhone.

The Chinese BeiDou-2 system, also known as COMPASS, is that country’s second generation system designed for 35 satellites to provide full global coverage. In November 2014, it was approved by the IMO (International Maritime Organisation) for international navigational use with 2D fixes. At least 19 of the planned 35 satellites are now in orbit and it should be finished by 2020. The system has two levels of precision for military and civilian purposes of 10cm and 10m respectively.

The European GALILEO constellation was originally underfunded and incurred long delays. More recently, the pace of satellite launches has increased with 10 of the 30 required for full service now in orbit. A timeframe of full service within the next five years is not unrealistic.

Receiver design

All four systems are converging towards the same L1C signal format in the longer term, which will further simplify receiver design and cost. Certified equipment is generally GPS L1C/A only and will need to evolve, incorporating chipsets and software designed and matured in mass market devices.

Portable receivers are already available which can decode multiple systems in parallel to obtain high precision. Most of the latest smartphones and tablets already incorporate both GPS and GLONASS, typically achieving sub 5m accuracy. Portable devices such as the Garmin Glo and Bad Elf improve performance to 1-2m.

Canadian EOS positioning systems recently launched their Arrow 200, a portable multi-GNSS Bluetooth 372 channel device which claims to achieve 30cm using all four with satellite augmentation, and is compatible with most mainstream navigation App/tablets. Today’s $6,000 price tag will restrict mass take-up but illustrates the potential.

Consequences

Over the next few years, more active satellites will increase the number available at any one time or place, especially in areas with difficult terrain. Multiple signal frequencies will improve resilience and robustness while also increasing accuracy. Technical improvements will continuously increase performance and widen the scope of potential use cases.

Within the aviation community we can expect to see growing reliance on GNSS and increased use of Performance Based Navigation. While existing equipment will remain compatible with current technology for many years, we can only hope that certification procedures will not hamper rapid adoption of these forthcoming developments.

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