===== How does GPS jamming work? =====

First, a reminder that in JAS there are two types of jamming: The ability of an aircraft to jam the radar associated with a SAM site (to reduce the radar’s range), and the ability to jam the GPS guidance package on a munition (reducing the accuracy of the precision munition). Here, we’re discussing the latter.

Second, a clarification: In JAS, GPS jamming affects only the effectiveness (CEP, and thus SSPD) of GPS-guided munitions. It has no impact on navigational accuracy of any platforms.

A munition is designated as GPS-guided by:

    *
      Data View: airData » AirExecutionFactorsData » AirPackagesData » [SCL ID] » Scl » [Munition Index #] » GpsGuided
    *
      SCL_WEAPON table, GPS_GUIDED_IND field

A jammer is created by:

    *
      Campaign Planning View: Characteristics and Performance » Assets List View » [desired Jammer Asset ID] » Overview Tab » select “Is GPS Jammer”

This creates default CEP error distribution Uniform, Min 0, Max 0 (stored in table UNFRM_ERROR_ DSTRBTN_JAMMER), and radius of 1 in (in table GPS_JAMMER). Alternately, users can select Normal with Mean and Standard Deviation, and radius (stored in table NORMAL_ERROR_DSTRBTN_JAMMER and GPS_JAMMER).

For a target to be supported by GPS jamming means the target is located within the max range of the jammer as provided in the GPS_JAMMER table at:

    *
      Data View: assetData » GpsJammerData » [Jammer Asset ID] » Radius
    *
      Campaign Planning View: Characteristics and Performance » [Jammer Asset ID] » GPS Jamming Tab » Radius

When that target is adjudicated by a GPS munition, a random draw is done for the selected error distribution/parameters to determine the munition’s CEP produced by the jammer. If the target is within the radius of multiple jammers, the largest of their individually produced CEPs is chosen.

Once this reference CEP is chosen, it then is used to determine the munition's SSPD. Most data will show that the SSPD decreases as the CEP increases (see figure below for example).

{{:jas:gpsjamming.jpg|}}

GPS SSPD data is stored for each Standard Target ID, delivery Air Platform, Munition Asset ID, Stick Size and CEP. If no entry exists for the combination of factors, an SSPD of 0.0 is used. Notice that, unlike SSPD data, weather band is not a factor for GPS munitions. If the reference CEP from above falls between CEP values in the GPS SSPD table, it is rounded down to the smaller CEP (generally the higher SSPD). If the reference CEP is smaller than any in the table, the table's smallest will be used; if it's larger, the largest.

Notional GPS_SSPD Data Table A2G_DELIVERY_ASSET_ID CIRCULAR_ERROR_PROB_RAD MUNITION_TYPE_CD WPN_PER_PASS_NUM STD_TARGET_ID SINGLE_SORTIE_DMG_PROB B1B 1.1 GBU_xx 1 123 0.98 B1B 2.2 GBU_xx 1 123 0.97 B1B 4.4 GBU_xx 1 123 0.90 B1B 8.8 GBU_xx 1 123 0.70 B1B 16.16 GBU_xx 1 123 0.25

CEP Examples Reference CEP SSPD Used 0.5 0.98 3.1 0.97 20 0.25 