4. Background to Available and Emerging Radio Navigation Services

4.1 GNSS

The space-based positioning and navigation system provides worldwide, all weather, passive, three-dimensional position, velocity, and time data. The GNSS user community has grown exponentially in recent years and that growth is expected to continue. Rapid growth has occurred in all modes of transportation. The GPS and the Russian GLONASS system, have been the principal elements of GNSS so far. Formerly, the only widely available GNSS service was GPS served by the NAVSTAR constellation. Though the Russian Global Navigation Satellite System (GLONASS) share the same principles with GPS, because GLONASS constellation consists of 11 active satellites, it is virtually impossible to use this system stand alone, nevertheless receivers able to decode both GPS and GLONASS signals can take advantage on basic GPS receivers positioning particularly in the urban environment, exploiting the increased number of satellites available. Though dual receivers are currently available, they haven't a wide diffusion and their cost is very high (some thousand dollars), so this system is not particularly attracting for telecom manufacturers.

Additionally, systems of pseudolites that may share the GNSS frequency or operate on an offset frequency have been proposed as an availability enhancement for high accuracy LAAS (Local Area Augmentation Service).

Additional signals are planned to enhance the ability of GPS to support civil users. These signals will assist in the mitigation of ionospheric-delay estimation errors and serve as backups for the GPS Service.

4.2 FUTURE GNSS

Over the next few years Europe will be commissioning its own GALILEO service which will operate along with GPS 2 available from 2007 (and in 2015, GPS 3) GLONASS and possible a Chinese owned service still yet to be specified. The first element of GNSS Modernization will enable Civil GPS 2 users to correct for ionospheric errors using a second frequency in addition to the current signal. These corrections, when combined with switching off Selective Availability (SA)(Now done), will enable user equipment that meets benchmark standards to achieve horizontal accuracies in the 4 meter range. In addition, there will be a third civil signal for safety-of-life applications. It is proposed that GALILEO should also be operational by 2008 and therefore must now be recognised as a recognised service for provision of position information seemingly providing levels of accuracy similar or better than GPS 2. GALILEO will provide for an independent, global, European-controlled satellite based navigation system. The system will provide a number of services to users equipped with GALILEO receivers. It has been proposed that GALILEO will offer differing levels of service to suite differing needs, this infers, controversially, that there will be costs for higher integrity and Safety of life applications levied on the user. The levying of costs might preclude those who can benefit most from using it.

The first level is an Open Services (OS), free for all users, providing positioning navigation and timing performances comparable or better than those of existing and planned global navigation systems.

The second is for Commercial Services (CS), based on the open services signals, but providing value added positioning navigation and timing data to users (e.g. integrity), with a liability regime.

The third and probably the most interesting for the mobility sector will provide public interests Services - certified services - for Safety Of Life (SOL) for transport and other safety critical applications, Public Regulated Services (PRS) for the enforcement of Member States and EU policies implementation; and Search and Rescue (SAR) complementing COSPAS-SARSAT fort the detection of distress alarms from user beacons.

The final make up of the GALILEO services has not yet been finally defined and are subject to further refinement until 2003.

The global GNSS modernisation effort focuses on improving position and timing accuracy, availability, integrity monitoring support capability and enhancement to the control system to ensure a robust, highly dependable navigation and timing source for all users. Although current GPS users will be able to operate at the same, or better, levels of performance that they enjoy today, users will need to modify or procure new user equipment in order to take full advantage of any GALILEO and GPS 2 / 3.

Since the September 11th atrocities there are serious security issues that are presently being discussed both in the context of GPS and GALILEO.

4.3 AUGMENTATION AND HYBRID GNSS SOLUTIONS

The definitions of these parameters are provided in the report of the European GNSS Maritime Advisory forum describing maritime applications and associated requirements.

Augmentation System Performance Parameters (EMRF)
Parameter	DGNSS/ AAS	WAAS & EGNOS†	LORAN-C/ Eurofix
Predictable accuracy (m)	<10**	3.5 (2.5)*	3 - 5
Integrity Alert limit (m) Time to alert (s)	Yes 10 10	Yes 20 6	Yes 7.5 LAAS mode 4-6 RAAS mode <4
Availability (%)	99.8**	98.7	99.8***
Reliability (%)	Not yet known	Not yet known	Not yet known
Fix interval (s)	2.5 - 5	1	Not known

Notes:
† EGNOS accuracy is based on simulation results being performed during system design and exceeds the specified requirements
* Figures in brackets refer to use of GPS and GLONASS at the user receiver. Figures outside of brackets refer to augmentation of GPS alone.
** Accuracies of the order of 1m can be achieved and an availability of 99.9% is specified for stringent operations
*** Availability increases above 99.8% depending upon the number of stations in range

Current Un-augmented satellite based radio-navigation services such as GPS will not meet all users performance requirements for the harbour entrance and approach phase of marine navigation, or for many land transportation applications. A user must have at least five satellites in view above a mask angle of 7.5 degrees in order to provide Receiver Autonomous Integrity Monitoring (RAIM). This condition is not always satisfied with the existing GPS constellation, resulting in so-called "RAIM holes" and limiting GPS to use as a supplemental navigation system. To meet the requirements for Fault Detection and Exclusion (FDE), at least six satellites with good geometry are necessary.

Adverse effects of these variances may be substantially reduced, if not practically eliminated, by differential techniques. In such differential operation, a reference station is located at a fixed point (or points) within an area of interest. GPS signals are observed in real time and compared with signals expected to be observed at the fixed point. Differences between observed signals and predicted signals are transmitted to users as differential corrections to upgrade the precision and performance of the user's receiver.

4.4 WIDE AREA AUGMENTATION

The US WAAS, Japanese MSAS and European EGNOS are safety-critical systems consisting of the equipment and software that augments GPS. They are satellite and ground based system that augments the existing satellite services provided by the American Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS) for those users who are equipped with an appropriate receiver.

The United States WAAS service will provide GPS augmentation of much of North America and the Hawaiian Islands. Japan is developing the Multi-Function Transport (MTSAT) Satellite Based Augmentation System (MSAS) to provide GPS augmentation of some of the Far Eastern area. The European EGNOS service will originally provide augmentation over the European area. All of these systems will be interoperable, with the result that the user will only require a single receiver to use all three systems. However these systems cannot be used with GLASNOS. Within these areas the navigation performance is expected to be similar to that within the EGNOS Core Coverage Area, i.e. around 5-7m accuracy. However, it is important to note that, whereas EGNOS will provide a multi-modal service by design, WAAS and MSAS have been developed as and will be operated as aviation systems. The WAAS is provided by a signal-in-space and via the US LORAN service to airborne and terrestrial WAAS users to support precision navigation.

4.5 EGNOS EXTENSIONS

The EGNOS Programme is exploring, through a set of inter-regional partners, the feasibility of extending Core Coverage to other regions. The following illustration shows the extensions currently being investigated in the Caribbean and South America, Africa and the Middle East. However, the degree of augmentation and, hence, accuracy in the regions of extension is not yet defined and will depend on the number and density of reference stations that are implemented.

Approximate SBAS Core Coverage Areas including possible EGNOS extensions

4.6 WAAS LORAN & EUROFIX

The EUROFIX system in use within European today and with the WAAS LORAN service and shortly to be implemented in the US, both requires the LORAN infrastructure. Despite discussions on providing EUROFIX signals by LORAN-C station in Europe and in the United States and by Chayka stations in the Commonwealth of Independent States, the US have decided on a system developed by Stanford University that is not compatible with the EUROFIX. EUROFIX within Europe requires a political decision as to the whether funding can be provided to keep the NELS LORAN service commissioned beyond 2004.

To ensure maximum coverage the co-operation of Japan, Korea, China, Saudi Arabia and India will be necessary if the maximum practicable coverage is to be provided.

4.7 LOCAL AREA AUGMENTATION SERVICES

DGNSS, accuracy performance degrades as the distance between the user and the ground station increases. Close to the station the accuracy performance is likely to be much better than the 10m quoted above, whereas at extreme ranges the accuracy performance could be worse than 10m. This effect is dominated by the temporal decorrelation of the differential corrections calculated at the DGNSS reference due to signal fading and interference, especially under night-time conditions

The USCG declared Full Operational Capability (FOC) of the Maritime DGPS Service on March 15, 1999. Necessary steps to include DGPS as a system that meets the carriage requirements of the Navigation Safety Regulations (33 CFR 164), for vessels operating on the navigable waters of the U.S are being undertaken.

A Nationwide DGPS (NDGPS) Service is being established to provide coverage for all areas of the U.S. not currently covered by the USCG Maritime DGPS Service. This should be in operation from 2007.

Differential GPS (DGPS) marine stations in Europe are located mainly in the Baltic, North, Barents and Irish Seas as well as Icelandic waters and along the Atlantic coastlines. When the network is complete, stations will provide overlapping coverage in most areas, and will cover the Canary Islands and the Western and Central parts of the Mediterranean Sea. To provide coverage at the Eastern end of the Mediterranean Sea would probably require an additional 10 DGNSS stations.

The operational ranges of the stations vary, depending upon the particular requirement of the station and, currently, the need to avoid interference to other stations. However, the use of the frequency band has recently been revised to eliminate limitations due to possible interference from other stations. Nominal operational ranges are generally given as being in the order of 185 km but the usable ranges are in the order of 370 km.

In addition to marine coverage, work is ongoing to investigate the quality of radio-beacon DGNSS service over European land-masses, particularly in the UK and France. The early indications from the UK work are that high quality services are already available, even in city areas, at least in the day-time. Complete coverage in the UK could be provided by the addition of a small number of extra stations. However, it should be noted that a significant increase in the number of inland beacons will be needed for large countries with small coastlines (e.g. Germany). This concept is similar to the National Differential GPS (NDGPS) Service being implemented in the US.

Operational and planned European Maritime DGNSS/LAAS Coverage

4.8 TERRESTRIAL RADIO-NAVIGATION SERVICES

4.8.1 Loran-C CHAICKA & NELS

Formerly developed to provide military users with a radio-navigation capability having much greater coverage and accuracy than its predecessor (Loran-A). It was subsequently selected as radio-navigation system for civil marine use in the U.S. Within North west Europe it is run as the NELS (North West European Loran-C System ) and partially available within the Mediterranean under the SELS (Southern European Loran-C System (missing stations in Spain and Turkey). Loran-C can also be used for precise time interval and highly accurate frequency applications.

Loran uses medium wave (90-110kHz.) transmissions from a chain with three or more terrestrial transmitting stations to compute a position. Loran-C is installed world-wide and supports a large user community. It provides 0.25 nautical miles (674m) absolute accuracy (for its position lines) and 18-90m repeatable accuracy (95% confidence) and 99.7% availability (one chain with 3 stations. Further the Loran-C signal can be modulated to broadcast differential GPS correction data and GPS integrity information. Within the US this makes up part of the LORAN WAAS integrity service, in Europe, EUROFIX. However, the US and European services are not compatible.

Loran-C offers the advantages that the signal can be received even inside buildings and the TOE mode allows combined use with GPS in the way that position solutions can be calculated even if one system can not provide a position solution.

Current Eurofix coverage

NELS coverage

In other parts of the world Loran-C stations or similar systems (like the Russian Chayka) are under operation. The coverage area for the NELS, Eurofix, Chayka and South European Loran-C chains are shown.

Current Chayka coverage

Incomplete SELS Coverage

While the US, the Administration continues to evaluate the long-term need for continuation of the Loran-C radio-navigation system, the Government will operate the Loran-C system in the short term. The U.S. Government will give users reasonable notice if it concludes that Loran-C is not needed or is not cost effective, so that users will have the opportunity to transition to alternative navigation aids.

Within Europe the future of LORAN is unknown. Though from a safety point of view there should always be terrestrial radio-navigation coverage, presently state of the art NELS service is paid for by a few member states, and though it is far superior to the first generation Loran C, few receivers are available due to political uncertainty as to sponsorship of the system and at present the service, unless there is a clear direction for commitment for payment, the service may be stopped in 2004. Within the Mediterranean, only Italy has upgraded its 2 stations, the Spanish station and Turkish Stations needed to give Mediterranean coverage (SELS). They have not been re-commissioned since the departure of the USCG in 1995, and therefore again unless political direction is given, SELS service will also cease. Under the Agreement each member nation of owns the facilities on its own territory and appoints a National Operational Agency (NOA) to manage them and look after its national interests. Observers attending Steering Committee meetings include representatives from Italy, the Russian Federation, the United States and the Far East Radionavigation Service (FERNS), all of which provide LORAN-C or Chayka services.

Approximate coverage of LORAN-C, Chayka and LORAN-C/Chayka chains worldwide

4.8.2 Radio-navigation using Mobile Telephone Network

Mobile phone positioning systems have been developed or are currently under development, taking advantage of the positioning potential of cellular networks. Several commercial products are already being offered mainly in the US market, especially in the Emergency Call Services field. The location methods used to locate a mobile telephone can be split into two categories: network-based solutions and handset-based solutions. The combination of both types of solutions (hybrid solution) is also used as it enables to combine the advantages of the two techniques while limiting their drawbacks.

The network-based solutions rely on positioning capabilities that are intrinsic to the network (identification of the originating cell, signal attenuation measures, angle of arrival measures, timing advance measures…) and enable to locate unmodified cellular phones. They require, however, the installation of specific software and/or hardware tools in the network.

The handset-based solutions are based on an active participation of the mobile telephone to the calculation of its position. These techniques generally require the installation of specific software and/or hardware tools in both the network and the handset.

Handset based solutions currently envisaged for the short term include the integration of independent positioning systems receivers on the wireless phone (e.g. satellite navigation, …) and several enhancing positioning techniques (DGPS, A-GPS,…). In addition, independent positioning systems (e.g. GPS) can also be used to synchronise the network (e.g. network synchronisation by GPS in some EOTD solutions)

Network location based services based solutions do not require a change on the handset, they should work with already existing handsets without major modifications. The methods include Cell-ID methods, Timing Advance (TA), Up-link Time of Arrival (TOA) and Angle of Arrival (AOA).

Handset based technologies can use several signals available for location purposes in most areas. These include OTDOA (Observed Time Difference Of Arrival), E-OTD (Downlink Enhanced Observed Time Difference), Bluetooth, satellite based technologies, Loran-C signals inertial systems and dead reckoning.

A hybrid solution using Cell-ID, TA (MR) and LCS technique could be the used to give measurement reports and Cell-ID data that network would provide to support more sophisticated methods like GPS, TOA, E-OTD or OTDOA. They could be used to remove errors of the other methods (multi-path or interference). The MR plus Cell-ID method can be used as a fall back when the others fail.

A hybrid solution that uses GPS plus E-OTD/OTDOA in some cases, can reach better level of accuracy and wider coverage than either method separately. E-OTD/OTDOA method has better accuracy and availability in urban areas and in-door whereas GPS has better accuracy and availability in rural and sub-urban areas. GPS has no coverage indoors whereas E-OTD/OTDOA have.

The AOA location method makes use of the angle of arrival of the radio signals to estimate the location. The AOA technique is not covered by any standards, thus, it has to be implemented as a proprietary solution, and it is very expensive to implement.

Bluetooth systems are suitable for indoor applications within buildings, ships, trains for location of equipment, freight and people.

4.8.3 Inertial systems and dead reckoning

There are many applications requiring a position or velocity update that should be independent of radio-navigation services. There are occasions where the user is not within coverage of satellite or terrestrial services due to geographic location, in a shadow caused by tall buildings, mountains, canyons or tunnels or during an interruption to a service. In some applications it is necessary to report velocity to an accuracy that can not be provided by single antenna GPS, and where a twin antenna GPS or combined GPS fluxgate compass is cumbersome (due to the fixed baseline and horizontal separation required for the antenna) or too expensive for the application (€800 to €2000).

There are a variety of velocity sensors available on the market including vibrating gyros, Silicon single and three-axis accelerometers; advanced accurate solid-state fibre optic true-north seeking gyro-compass, "solid-state" micro-machined quartz angular rate sensors, linear servo accelerometers and MEMS low-cost, high accuracy Silicon Micro-Ring Gyro inertial navigation system (about 2 cubic centimetres in size). The prices range from a few tens of Euro to hundreds of thousands of Euro.

The accelerometer is fine for some applications where the requirement is to detect a velocity change rather than to accurately measure it. To accurately measure a velocity change then the Inertial Navigation solution is far the best. However the high cost of the inertial unit is a main obstacle to include it in the precise navigation complex for versatile areas of application ($150,000). In time and with high demand the cost of the INS should become affordable, but this would require RTD to focus on mass volume applications rather than defence and aeronautical applications as present.

The quest today for a number of companies is to use the simple inertial measurement unit (IMU) with rough sensors for precise navigation. Companies, world-wide are developing low cost inertial devices which use the cheap compact sensors. This group of instruments is called motion sensors. However, they will have the weak stand-alone accuracy and poor run-to-run stability and as such they are not suitable as a sole system and will require periodic update.

4.8.4 Psuedolites and Synchrolites

There are an increasing number of applications requiring precise relative position and clock offset information. In situations with limited or no visibility of the GPS satellites, ground transmitters that emulate the signal structure of the GPS satellites (pseudolites) can be used as additional or replacement signal sources. Transceivers (which transmit and receive GPS signals) can be used to improve standard pseudolite positioning systems. If their locations are known, transceivers can be used to remove the need for the reference antenna typically necessary in standard differential systems. In addition, transceivers mounted on vehicles can allow continuous inter-vehicle positioning without the presence of signals from GPS satellites.

Centimeter level position information is becoming increasingly important for autonomous vehicle control. Carrier-phase Differential GPS (CDGPS) readily provides such information. The feasibility of pseudolites has created interest in using existing GPS technology and equipment in situations not normally feasible for GPS satellite only systems, either by augmenting the existing satellite constellation or by replacing it altogether. However, the cost is very high due to the need for using an atomic clock.

Synchrolites provide an economical alternative by using a receiver-grade clock. Synchrolites transmit a signal that has the same frequency as a received signal except modulated with a different code. Transceivers are an effective way to expand the capability of a GNSS positioning systems. This can be done by eliminating differential reference stations, allowing pseudolites to self-survey their own locations, or enabling relative positioning between multiple vehicles.

Though presently examples of applications employing such advanced capabilities include open-pit mining, rover navigation for Mars exploration, and formation flying for space-based interferometers, they also are being used in commercial aviation applications and providing RTD is focussed on high volume sales applications can provide cm accuracy for a diverse number of local transport and positioning applications such as berthing, docking and ensuring bottom clearance of marginal ships to guidance of road and rail vehicles.