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Potential interference To Galileo From 23cm band operations - continued
8. Potential interference from Galileo to 23cm amateur operations The Galileo signal at the earth’s surface is very weak and spread
over a wide bandwidth, and will only be a source of interference to EME
stations with large antennas. As a typical 23cm EME system uses a large,
typically >3m, antenna, the satellite will only be present in the beam
for a short time. However, fortunately the EME standard is for left hand circular polarisation (LHCP) on receive and so there is an additional attenuation of the cross polarisation performance of the dish and feed, typically 20dB. Thus the operator will not experience a noise increase. With a 10m dish the increase will just be noticeable. There is a further factor to be considered and that is the spectrum shape of the Galileo signal: this tapers towards the band edges and so there is a further (estimated) 6dB reduction in the noise received. Systems using noise measuring receivers to measure moon noise (for dish pointing or system calibration) or to observe radio stars in this band will be more adversely affected. For example a 500kHz wide receiver with a 10m dish and receive system would see a noise increase of about 30dB as a satellite went through the beam which would make it virtually useless. 9. The operation of GNSS receivers and their typical response to interference. In order to assess how amateur transmissions might interfere with Galileo receivers it is essential to understand a little about how these receivers might operate and about their capability to reject interference. The signal structure of GPS and Galileo is similar and so the receiver
characteristics of both will also be similar. A separate signal (in Galileo) also carries data giving the satellite orbit and other essential information which the receiver then decodes. When the receiver has gone through this process with four satellites, it is able to calculate its 3D position and velocity, and its clock is synchronised to the system standard time. Measurements to additional satellites will improve the accuracy of the measurements and provide resilience against intermittent loss of signal. Modern GPS receivers perform some of this process in digital form: in 5 years time virtually all of it will be digital. When a receiver is tracking a signal from a satellite the bandwidth of the code and carrier tracking loops can be very narrow. The code loop might be as low as 0.1Hz, the carrier loop 1kHz or less. The satellite’s motion is highly predictable and so a stationary receiver, once the carrier is locked on, can easily follow the Doppler change. However, if the receiver is moving, for example in a vehicle, then a sudden change of direction could cause the carrier loop to lose lock. To prevent this, either the receiver must allow the loop to operate at a wider bandwidth or the tracking loop must be “aided” by inputs form another sensor. In a fighter aircraft, for example, this aiding comes from the inertial reference system, in a vehicle it could come from a much simpler low cost gyro or dead reckoning system. These forms of coupled sensors are expensive. It is obvious from the foregoing that while a receiver is in tracking mode with the carrier and code operating as narrow bandwidth loops, it has a high ability to reject interference due to the narrow bandwidths. The loop characteristics are similar to a flywheel and a short interruption of one or more signals can be accommodated by the receiver. There are many techniques that can be used to extend the ability of the receiver to keep tracking the satellite(s) in the presence of interference. Some examples are: · Tracking the code alone if the carrier lock is lost. Where receivers are most vulnerable is in the acquisition phase. If a receiver starts absolutely from scratch, i.e. unknown position, velocity and time (PVT), then it will have to search with wide loop bandwidths in order to find the signal and lock to the code. The more information it has about its PVT the faster it can acquire and the narrower the loops can be. Once a receiver is giving good PVT data then it is more difficult to interfere with, or jam. Where problems can arise is when the receiver is forced into a re-acquisition mode and where interference prevents it from then re-acquiring. There is, obviously, a vast difference in receiver performance between those designed for leisure walking and those designed for civil aviation or for the most demanding military environments. A simple small receiver does not have the room for quality front end filtering or high dynamic range for example. Finally spoofing must be mentioned, this is a technique for interfering with GNSS operation by transmitting either a simulated signal or a delayed version of a real signal with the aim of making the receiver display an incorrect position. In the proposed PRS it is intended to include cryptographic techniques to prevent this abuse. 10. Practical Interference Scenarios This section will examine some interference scenarios. A pertinent question, (perhaps even an FAQ) is …“so why hasn’t
this problem occurred with GPS which has been in use for a decade or more?”
The answer is that the simple GPS L1 (1575.42MHz) receivers are, indeed,
vulnerable to interference but that their (approximately) 2MHz wide frequency
channel is clear because it is protected to aeronautical standards. These
receivers can be disrupted by relatively simple jammers designs for which
are available over the internet but there are few reported instances of
problems. A study of interference to Civil GNSS applications by out of band interference has been undertaken for the Australian Global Navigation Satellite System Coordination Committee, reference [5]. Testing of the performance of typical GPS receivers in the presence of potential interference sources was carried out using commercial receivers. The study concentrated on interference affecting the GPS L1 signal and it looked, in particular, at the possibility of interference from the harmonics of UHF TV transmitters to GNSS. By a mixture of measurement and simulation the study determined that the typical third harmonic radiated from a 480kW TV Transmitter would disrupt GPS operation over a 3.5 km radius. There are plenty of high power TV transmitters in the UK who’s second and third harmonics fall on the L1 frequency, but the writer has not heard of problems being reported. At the other end of the scale there is a report of a 2mW jammer disrupting
GPS operation over a 1nautical mile radius in a sea trial. This would
represent about -100dBm at the receiver, exactly the level predicted by
theory. In the lower part of the GNSS band both GPS and GNSS have to cope
with the pulsed signals from the aeronautical distance measuring equipment
(DME) and from TACAN and JTIDS / MIDS which are pulsed navigation systems
and data links respectively. There is no doubt that GNSS will play an increasingly important, if not essential, role in the transport infrastructure operation in both Europe and the USA. The EU seems determined to possess its own system, independent of the USA and GPS. However it still remains to be seen whether it can get the private sector to finance and run it for profit or whether it will have to heavily subsidise its operation. All NATO countries have access to GPS and so, in the light of other priorities for military equipment spending, it seems very unlikely indeed that there would be pressure from the European military for an independent system, especially when the US have said they would jam it, (or worse!) if it were perceived as a threat. The Galileo funding issue is not yet settled for development or for operation. If Galileo goes forward as planned, with a year or two’s delay, then as its usage becomes a more integrated and critical part of the infrastructure, the demand to have greater security, availability and reliability from the service will grow. This is happening already in the USA as the planned use of GPS for aircraft precision approach and landing comes closer to realisation. Air transport is more important in their infrastructure and so there is a need to see a way through to a highly robust civil GPS system. The report by John A. Volpe, Transportation Systems Center, reference 4, reviewed this area and made many recommendations for improving robustness and availability, including research into interference mitigation and interference location. Everyone is beginning to recognise that the current GNSS does have vulnerabilities and that interference mitigation has to be an important and necessary component of the receiver system design where these are to be used in crucial systems. Even if Galileo does not proceed we have to recognise that the 1260-1300MHz band will be used at some time for GNSS and that these systems will always have a rather weak signal at the earth’s surface. Sharing the allocation with radar is/was relatively painless for the Amateur Services. Radars, even civil ones, are designed to cope with interference by employing a whole library of techniques developed over many years. Furthermore, because the number of radars is small and they are large installations and easily physically protected, the techniques can be kept secret where necessary. Although there are some who are calling for these bands to be effectively swept clear of interference sources this is (in my own view) impractical, especially where they are not protected by the stringent aeronautical regulations. Therefore if it is required to have a robust PRS service in the E6 channel then those receivers will have to incorporate very extensive interference rejection measures. The limit will be set by what can be released from military anti-jam technology into this para-military area, bearing in mind the virtual impossibility of keeping large numbers of the PRS equipments secure. 12. What can the Amateur Services do about it? While Galileo might be delayed, it is unlikely to be stopped, although it is a possibility. Even if it is, then at some time (probably post-2007) another GNSS will take the allocation. This might lead to some limitations on continuous transmissions such as beacons, TV repeaters and FM repeaters below 1300 MHz. Non-continuous signals such as ssb/cw ought to be much less of a problem to a robust PRS receiver and one can argue that 23cm amateur transceivers will be available for many years to come and probably constitute the largest quantity of potential jammers available to any person or organisation wishing to cause disruption. Therefore the PRS receivers should protect against them and therefore we should be allowed to continue. We must argue that, to a moving vehicle, the signal from a typical amateur ssb/cw transmission will be very intermittent and therefore the receiver should be little affected by it. It would be useful to take some measurements of these sorts of signal levels. Obviously the terrain masking effect would be less for the police helicopter scenario but it would still be present to a degree. EME operations are typified by a higher erp than normal "tropo"
stations. However, the beam widths are small and so the duration of interference
is short and a well designed receiver in a police helicopter for example
would “flywheel” through it. The side lobe levels are about
the same erp as a tropo station and the antennas, being large, are at
low height, which considerably increases the intermittency of the signal
at a distance. Similar considerations apply to satellite operations in
the 23cm band which take place in the 1260-1270 MHz sector. At the IARU region1 meeting at Davos in September 2005 a similar paper to this one, C5.13, was presented by the RSGB but no actions were placed. In my opinion it would be useful in formulating our case to remain in the band if we had more information about the following issues. Which nations have definite plans to use the E6 services and for what
applications they anticipate it being used. It seems to me that Galileo receivers which are robust enough to coexist with ATC radars should also be able to cope with intermittent amateur signals.
[1] J-L Issler, G Hein, J Godet, et al. "Galileo
Frequency and signal Design", [2] House of Commons Transport Committee report on Galileo, HC 1210, Nov 25th 2004. See http://www.parliament.uk/transcom [3] "Results of interference susceptibility tests
of a 1250-1300MHz band aeronautical primary radar system with RNSS signals"
ITU document 8B/60, August 20th 2004 [5] " Study of Interference to Civil GNSS Applications by Out of Band Interference", Consultancy Commission No P2001/0283, carried out for the Australian GNSS Coordination Committee, November 2001 [6] M. De Angelis, "Analysis of Air Traffic Control Systems Interference - Impact on Galileo Aeronautics Receivers", Institute of Navigation National Technical Meeting, January 2005 [7] The web site of the Royal Institute of Navigation. www.rin.org.uk/SITE/UPLOAD/DOCCUMENT/Vuln-Owen.pdf
P K Blair. OBE, FReng, FIEE, G3LTF
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