Hydrographic Society Benelux

On 19 April, the Hydrographic Society Benelux, together with The Netherlands Institute of Navigation and the working group Applied Space Geodesy of The Netherlands Association of Geodesy, organised a workshop entitled ‘GPS - when things go wrong’. The workshop was held at Delft University of Technology and attracted over 200 people. The goal of the workshop was to make GPS users aware of the vulnerabilities and limitations of GPS, in view of the recently published study by the US Department of Transportation on these issues, see www.navcen. uscg.gov
The presentation by Prof. Van Willigen (Reelektronika) dealt with GPS vulnerabilities, such as jamming, spoofing and meaconing. Simple jammers, with a power of only one watt, already prevent receivers within a range of 85km acquiring GPS signals. Such jammers are for sale on the Internet. Prof. Van Willigen also demonstrated a very low (1 mW) power prototype jammer, as developed by TNO/FEL in The Netherlands. A GPS receiver located outside the workshop building immediately stopped tracking all satellites. The problem with jammers is that they are inexpensive, difficult to track and a challenge for large groups of people, ranging from technical enthusiasts to terrorists. His conclusion was that GPS is an offer that is ‘too good to refuse’, but at the same time too vulnerable to unconditionally rely on.
Dr Van Nee (Woodside Networks) discussed the backgrounds of multipath from a signal processing point of view. Dr Van Nee is the inventor of the Multipath Estimating Delay Lock Loop (MEDLL), implemented in some of the currently available commercial GPS receivers. The MEDLL greatly reduces multipath by a factor of about ten. Despite these developments, multipath is still considered one of the limiting factors for precise and reliable GPS positioning.
The ionosphere is another limiting factor, as was pointed out by Mr Van Toor (Thales Geosolutions) and Mr Visser (Fugro-Omnistar) in their presentations. Mr Van Toor's presentation in particular dealt with ionospheric scintillations, caused by irregularities in the ionosphere. The current maximum in the solar cycle causes receivers to lose lock on the GPS signals and instabilities in the positioning accuracy, especially in areas around the equator and in polar regions. In addition, differential data links are interrupted, resulting in corrections that do not arrive in time at mobile users. Similar problems were reported by Mr Visser. The approach to overcome at least some of these problems is to no longer rely on the GPS ionospheric correction model, but to use dual-frequency receivers instead. Other solutions include the use of larger antennas and better amplifiers.
Dr De Jong (Delft University of Technology) discussed the integrity monitoring of GPS/ Glonass observations and navigation messages using statistical testing procedures. These procedures work well in real-time, but are also very useful for offline analysis of receiver performance. He showed some examples of the performance of a receiver (precision of observations, number of cycle slips, number of available and missing observations) as a function of the installed receiver firmware.
Mr Jonkman (Ballast Ham Dredging) gave an overview of the error sources that are important for Real-Time Kinematic (RTK) GPS positioning. Apart from multipath, these are incorrect initialisations of the carrier phase ambiguities and ionospheric effects. Incorrect initialisations may result in large position biases for periods of up to several hours. Although Mr Jonkman's examples included only short baselines, these baselines already suffered from residual ionospheric effects at the few cm-level in the height component. Closer investigations revealed that these errors were highly correlated with variations in the number of electrons in the ionosphere. His conclusions were that for RTK GPS one should not only carefully choose reference stations, but also consider the integration of GPS with other sensors to increase the reliability of the estimated positions.


Workshops
New GPS equipment
14 June 2002
Thales Navigation, Papendrecht, The Netherlands

Harbour approach
13 September 2002
Belgium, exact location to be announced later

Special projects offshore
15 November 2002
The Netherlands, exact location to be announced later

Contact
T.H. Heringa
RWS-Directie Noord-Holland
Postbus 175
1970 AD IJmuiden
The Netherlands
Tel. +31 255 545 622
Fax. +31 255 545 645