Radio interference monitoring, spectrum protection and signal security

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Radio interference monitoring, spectrum
protection and signal security

• AFREF technical workshop
• 10-13 July 2006
• UCT
• Ludwig Combrinck Ruth Neilan

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Ready to install a GPS station?

• You have sorted out all requirements such as
• Power (UPS etc)
• Internet
• Security
• Multipathing etc.

But what about radio frequency interference?
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All sites have the potential for RFI
Many sources of RFI exist and each site needs an
independent assessment of potential RFI
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Of course, GNSS is a RFI source itself!
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Not only GLONASS..

• Notification of GPS L3 RFI world-wide SUBJECT
  GPS Testing Operations. FREQUENCIE(S) AFFECTED
  1381.05 2.5 MHz DATE (UTC) TIME (UTC) Comments
  (YYYYMMDD) (HHMM) (TEXT) -------------------------
  ----------------------------------------------
  20050621 0000 - 2400 Previously Unscheduled, late
  notice. 20050622 0000 - 2400 Previously
  Unscheduled, late notice.
  
• We have been alerted to the following,
  late-notice military GPS-L3 Transmissions which
  may cause RFI to observations scheduled for the
  use of L-band (20cm) on the date(s) and time(s)
  listed below. The 1381 MHz frequency is within
  the VLA L-band 21/20cm "LL"/"21", "L1", and "L2",
  continuum default frequencies, and any L-band
  spectral line set-up that includes 1381.05 2.5
  MHz.

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And many people are worried

• Development of an L-band Spectrum Management Tool
  Suite.
• Monticone, F. Box, T. Kim, R. Snow, and M. Tran
  (USA) Keywords  distance measuring equipment
  (DME), tactical air navigation (TACAN), FAA, NAS,
  GPS L5, spectrum management
• Abstract. Managing the use of spectrum by
  multiple diverse systems is a formidable task
  requiring the use of sophisticated tools. A prime
  example of a spectrum band shared by multiple
  Federal Aviation Administration (FAA) and
  Department of Defense (DoD) systems, that the
  research described in this paper addresses, is
  the 960 1215 MHz band, contained within the
  L-band. The L Band Steering Committee formed by
  the FAA and DoD and an ad hoc working group of
  the Interagency Global Positioning System (GPS)
  Executive Board (IGEB) have been independently
  formed to address concerns regarding the
  interference-free co-existence of L-band systems.
  This paper describes the effort that the MITRE
  Corporations Center for Advanced Aviation System
  Development (CAASD) has undertaken to develop a
  spectrum management tool suite to help address
  these concerns.

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The official picture

• GPS uses a set of rather weak radio signals from
  satellites in 20,200 kilometer high orbits and,
  as such, is susceptible to being overpowered by
  strong terrestrial interference. It operates in
  one of the restricted frequency bands of Title
  47 C.F.R. Part 15 and requires protection from
  harmful interference by international treaty.
• (RTCA Paper No. 086-01/PMC-139)
• Prepared by
• RTCA Special Committee 159
• March 27, 2001

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Accuracy degrades with rf noise and pulsed rf
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Other techniques have far more serious problems
though..
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What to look out for

• Proximity of other radio devices
• Antennas, VHF, UHF, even HF could cause
• intermodulation products
• Other space geodesy equipment (DORIS)!

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Airport areas (normally convenient) could be
problematic

• Primary surveillance radar systems for air
  traffic operating in the frequency bands 1240 to
  1350 MHz and 2700 to 2900 MHz with transmitter
  powers up to 2 MW and antenna gains up to 37 dB.
• Fortunately these services are being phased out
  as they are outdated and expensive to operate.
• Secondary surveillance radar systems for air
  traffic operating on an international standard
  frequency on 1030 MHz at lower power (1 to 2 kW
  transmitters) with 37 dB gain antennas.
• Distance measuring equipment (DME) transmissions
  for air traffic operating in the frequency band
  960 to 1215 MHz with 1 to 2 kW transmitters and
  low gain antennas.
• Secondary Surveillance Radar (SSR) and Airborne
  Collision
• Avoidance System (ACAS) operating on an
  international standard
• Frequency on 1090 MHz.

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Other users

• Point-to-point links operating in the frequency
  bands 1350 to 1525 MHz
• The point-to-point links are usually highly
  directional and may only be troublesome if a
  transmitting beam happens to be directed at the
  GPS antenna.
• RFI in this case has to be combated by distance,
  topographical shielding and regulatory
  enforcement and protection.

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Simple technique to locate RFI

• Spectrum analyzer (2 GHz)
• Log-periodic antenna/s

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The hunt is on..
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Quick scan may not reveal all

• May need a couple of days of observation to
  detect intermittent transmitters

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For AFREF

• We could set up a special mobile/portable
• RFI detection unit to validate/characterise
  sites.

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The end