Satellite Interference Geolocation

Page index:

• Introduction
• High-level description
• A typical Scenario
• Conclusion
  

Introduction

In  a  continuously  growing  satellite  telecom  environment,  transmissions  and networks  are  increasingly  subject  to  interference  from  a  variety  of  ground-based transmitters. This has become the major cause of service impairment and degradation. A clear need exists to quickly identify and remove unwanted signals on scarce, premium capacity satellite resources. Protecting on-orbit assets and reducing the  illegal use of space capacity requires a tool capable of effectively identifying and removing any possible service degradation sources. By deploying a combination of Integral Systems, Inc.’s (ISI) Monics and satID products, users can easily manage both  Interference Detection and Geolocation, the two primary actions required to protect on-orbit assets.
 

High-level description

Geolocation is a technique for determining the origin of signals on communications satellites. It mitigates against accidental  interference on military satellites  (usually caused by either human error or equipment  failure), and  is increasingly being incorporated into commercial satellite operations to locate sources of deliberate jamming. ISI’s unique  “satID”  tool helps  satellite providers and users protect  their on-orbit assets against  communication  link interruptions.
 

A typical scenario

Interference  is  often  accidental,  resulting  from  faulty  equipment  or  incorrect  ground  station  operation,  but interference can also result from deliberate jamming or the illegal use of available bandwidth. In all cases, identifying and resolving unwanted or pirate signals is a clear priority in today’s telecom satellite industry.

Using the lobes property of any given parabolic antenna, satID can intercept the identical signal pattern received by a primary satellite and by a secondary satellite that  shares  the  same  transmit  frequency, polarization,  and  coverage  zone  through  a reference site(s)  located within the downlink coverage  zone(s)  of  the  two  satellites.  The signal, which has travelled over two different satellite  links  (using  signal  overspill  for  the secondary  satellite)  is  downconverted  and digitized at two monitoring stations.

To  calculate  the  position  of  the  target,  the two sampled signals must be correlated.

• From the correlation, the Differential Time Offset  (DTO) between the two signal paths can be determined. Since the positions of the ground stations and satellites are known, there exists a finite number of places on the earth’s surface from where the signal could have originated to produce this time difference. These possible locations can be visualized as a line of position referred to as the “timeline.”
• In a similar way, the Differential Frequency Offset (DFO) can be measured after calculating Doppler shifts affecting the signal as a result of relative satellite motions. Again, using our knowledge of how satellites are moving at any given  time,  a  frequency  line of position  can be  calculated. The  target’s estimated  location  and  associated 95% confidence ellipse is then calculated directly from the measurements.

Geolocation’s biggest challenge is often detecting a suitable signal from the adjacent spacecraft; sometimes the signal  is more  than  40  dB weaker  than  the  interference  signal  itself.  Precise measurement  of  time  delay  and frequency shift is also a challenge, as is reducing biases from raw time delay and frequency shifts to the uplink-only values of Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA), and converting these two values into accurate geographical coordinates.

To obtain uplink-only values, a GPS  reference signal of known origin  is used. This also  improves measurement accuracy. The reference signal can be transmitted on either of the two satellites, provided it follows the same path as the interference signal, it could be a purposely generated carrier or also an existing service for which GPS accurate coordinates are known.

Finally, accurate satellite ephemeris are also needed to improve system accuracy. satID  has  the  unique  ability  to  correct  available  orbital data  to  enhance  overall system accuracy – this is called “Ephemeris Error Correction.”

SatID’s  software  offers  a  user-friendly  interface  to  define  scenarios,  take measurements,  and  report  results  that  can  be  displayed  through  a  variety  of cartographic tools, including Google Earth.

When an interference source cannot be clearly identified through station “switch-offs” in the given geolocated area, a Helicopter Direction Finding (HDF) activity can be performed. By employing a specially equipped airborne platform and using the geolocation results of the satID system, a visual identification of the ground station generating the interference can

be performed. The efficacy of this method is clearly proven by its 100% success rate.

Conclusion

satID has proven its effectiveness through many different scenarios: determining coverage configurations, identifying interference modulation types, and sweeping for signals. In particular, satID offers a “de-sweeping tool” as part of its baseline package. This enables the geolocation of sweeping carriers without the need for permanent monitoring by a system operator. satID is available as either a standalone system managed by the satellite operator or in the very near future, as a service provided by ISI on a worldwide scale.