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SSR inherently measures Slant Range of aircraft. In order to determine the True range of the aircraft, an adjusted is made based upon the height of the aircraft. In cases where the aircraft altitude is unknown (no Mode C), a default altitude or other approximation must be used. Significant error in Mode C data (pressure encoder error, data-link error or Mode A reply to Mode C interrogation) leads to an incorrect adjustment. These can lead to significant position errors where the aircraft is close to the radar.
Radar may be used to provide emergency navigation to an aircraft when the pilot is unable to navigate by normal means. This service may be required in the event of failure of various items of equipment (navigation sensors or display) or pilot performance limitations (ie VFR pilot in IMC).
 whether there are 2 aircraft or one because each aircraft is uniquely identified by a 24 bit code

ADS-B does not have an equivalent mechanism and hence is not subject to such errors.
 
Assuming that ADS-B uses positional information from the aircraft’s Navigation Sensors, a Controller using ADS-B could provide emergency navigation (vectors) to an aircraft provided at least one navigation sensor was functioning.
In the event of total loss of navigation, Separation can be assured using vertical separation, temporarily using half the usual vertical standard if normal vertical standard is not immediately available.
Thus the dependence of ADS-B on the aircraft’s navigation system should not preclude the deployment of ADS-B. Suitable procedural means exist to provide emergency response to this most infrequent event.

 
 

 

Radar may be able to provide emergency
Assuming the pressure encoder and static pressure
Emergency Altimeter Service
altitude/Flight Level information to an aircraft
port associated with the Transponder is operating when the pilot is unable to determine altitude by
normally, a Controller using ADS-B, could also the normal means. This service may be required in
 
provide emergency altitude information to an the event of failure of all altimeters available to the
aircraft.  pilot. This service is only available if the Pressure

Encoder and Static Pressure Port associated with the Transponder is operating normally
  
Discussion on additional issues identified by a comparative assessment 
a. Probability and impact of a common mode failure of nav and surveillance. This should be assessed for single aircraft or multiple aircraft
scenarios.
     See hazard analysis issues/mitigators.
 

b. Station overlap (mosaic v different ADS-B stations)
c. 
Station overlap is not a concern for ADS-B systems. The most recently received data can be used. If two stations receive data from an
aircraft at the same time, the data will be identical because it is the aircraft that constructed the data. As a result the ground station
display can disregard any second message. Management of such data within ADS-B systems is significantly easier than for radar, as in
the latter system the time of detection, the range from the radar (to derive the positional error) etc must be taken into account. Because
radars measure SLANT range and azimuth, if two radars measure the position of an aircraft at the same time, they need to rely on
altitude data to map them to the same display position.  
 

d. Navigation performance requirement for ADS-B. 
 
The use of a navigational uncertainty category value (NuC) is defined in ICAO XXXXXX. The comparative assessment methodology
requires the proposed system to meet or exceed the capability of the reference system. Therefore the navigation performance of aircraft
being provided with ADS-B services should comply with this requirement. The table above details the different positional accuracy
　
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