Certain emerging navigation performance requirements for maritime applications are challenging to satisfy using systems available today. Very stringent performance requirements have, in particular, been identified for port areas, and cover aspects including service robustness, accuracy, integrity and availability.
One promising answer may be to augment GPS coverage with a groundbased Harbour Navigation System.
In the Spring of 2008 EADS Astrium demonstrated a prototype for such a System, using transmitters based on navigation Pseudolite technology.
This paper focuses on the potential for a Harbour Navigation System to augment GPS for harbour operations. The paper was derived from several recent publications [1-4] by the same author and colleagues from Astrium’s Portsmouth offices.

Emerging maritime user requirements
Certain emerging navigation performance requirements are considered beyond the capabilities of GPS to support.

Figure 1: Maritime HNS Configuration

Identified applications in port areas (for example automated docking) have very stringent performance requirements as shown in Table 1. The figures are based on an analysis conducted in Project MARUSE [5] , and are compatible with figures from the International Maritime Organisation (IMO).
GPS performance today is suitable for the Oceanic Navigation Phase; for the other Phases, however, GPS cannot reliably meet requirements for accuracy or integrity. In addition, the demanded availability and continuity are impossible to meet using GPS alone in Port areas unless user vessels have a continuously clear sky view. In many Ports, various objects including high buildings, gantry and other cranes, and bridges, may obstruct the sky view for user vessels, potentially blocking GPS satellite signal reception and causing a navigation coverage outage.

Harbour Navigation System
HNS concept The Harbour Navigation System (HNS)

Figure 2: Transmit Station Physical Realisation

concept created by Astrium is for a Navigation System compatible with, but independent from, the GPS System. Users are able to navigation using HNS alone, GPS alone, or a combination of the two systems together.
EADS Astrium recently developed and demonstrated the use of an HNS. This included:
• A number of Transmit Stations mounted at accurately known locations overlooking the operation / demonstration zone. These transmit GNSS-like signals, typically pulsed to minimise any interference.
• A single Monitoring & Control Station with line of sight visibility of each Transmit Station.
In Figure 1 the configuration of the HNS is illustrated. This includes several synchronised Transmit Stations, a Reference Station, a Monitor and Control Station, and the wireless network interconnecting them (although wired interconnect may be equally valid for fixed installations).
A navigation solution can be obtained at a user receiver from a mixture of GPS and HNS signals. In addition to any onvessel equipment, the HNS has its own GNSS receiver to provide references to the M&C facility. These receivers used conventional GPS L1 transmissions as well as the HNS transmissions to maximise system robustness.

Table 1: Example Maritime Application Requirements (extract from [5])

Figure 3: Demonstration Scenario (figure courtesy of Dr Alan Grant General Lighthouse Authorities of the United Kingdom and Ireland)

HNS benefits

The HNS can bring a number of benefits to Harbour Operations.
These include the following:
• The HNS is a locally controlled and fully independent Navigation System.
• The HNS is scalable to larger or smaller coverage area. Transmitter powers can be increased or decreased to change the coverage provided. The HNS is also scalable to add more Transmitters if wanted, for example to provide a shaped coverage.
• The HNS is fully interoperable with GPS, but HNS operation permits local navigation if GPS were unavailable or suffered some problem.
• The primary usage model is based on a combined GPS / HNS Receiver on Ships or Pilot Portable Units. RF parts of the receiver are identical, as are signal processing parts, and items such as the display. New software is, however, needed to process the HNS measurements. Hence the cost impact on User Equipment is minimal. Operationally it is foreseen that users will use GPS anywhere, and will then switch to combined GPS & HNS in harbour areas.
• The HNS facilitates robustness to interference and to GPS “Black Holes”. Power Control can increase signal levels; in addition, the HNS can operate on different frequencies if user equipment is configured to support this. In terms of GPS “Black Holes” (regions where local obstructions cause problems with GPS reception), the HNS mitigates the problem by placing Transmitters close to where there are GPS problems
• There is also potential to extend HNS capabilities by relaying GPS differential corrections and/or RTK and/or Integrity data over the HNS transmissions using spare capacity in the transmission channel.

Transmit stations

The Transmit Stations are a variation of navigation pseudolite technology. They are compact and self-contained, and produce GNSS satellite-like transmissions at a source level that is preset and generally maintained constant. Transmitters are generally synchronised in groups to work together as a single, coherent system.
The design places few restrictions on the location of their deployment, ensuring that planners have the freedom to create optimal signal environments for maritime or other sector applications.

Monitor and control station

The Monitoring & Control (M&C) Station coordinates the operation of Transmit Stations so that they form a coherent navigation system. At the heart of the M&C Station is a processing element with the ability to accept and process data from a GNSS Reference Receiver. The data received includes measurements of Pseudorange, Carrier Phase, Doppler, C/No and lock time; data such as raw Navigation symbols and tracking & receiver status.
The M&C also generates commands for dissemination to each Transmit Station, for passing of navigation parameters and miscellaneous data. Linked with this is the mechanism for accepting and processing acknowledgements and other health & status data from each of the transmit stations received via the communications link.

Reference station

Core to the Reference station was a Septentrio GeneRx Receiver, capable of tracking GPS satellites as well as transmissions from the pseudolites. Its main function are to measure Pseudoranges from the transmit stations, to collect the transmitted Navigation Data, to time-stamp this data and to relay it to the M&C Station.A Communications Link interconnects the M&C Station with each of the Transmit Stations at their remote locations. For the prototype equipment, X8200 Radio Modems were used.
Serial data can be transmitted with baud rates from 1.2K to 115.2K over distances of 10km to 20km line of sight, and can operate in both licenseexempt and licensed bands. Antennas used are 10dB Yagi at the Transmit Stations and 0dB omnidirectional antenna at the M&C Station. For an operational system this link type may be appropriate, or may be replaced by a domain-specific or fixed-line link.

HNS signals
The Signals transmitted for the Oban demonstrations were based on Galileo transmissions. In the HNS, there will of course be no ionosphere between the Transmit Stations and the Receivers; it is therefore of limited value to use multiple transmission frequencies.
The transmissions used were identical in terms of frequency and modulation to Galileo transmissions [6]. They comprise two pairs of I&Q components referred to as E5A and E5B, which are AltBOC modulated onto an RF carrier centred at 1191.795 MHz.. The navigation message content was adapted to cope with the Transmitters being stationary.