Originally Posted by
DaveReidUK
...basic Mode S only gives you surveillance ID (squawk) and altitude (DF5 and DF4, respectively). So, yes, you can calculate RoD, but the download data contains no velocity information so I don't see how it's possible to derive GS.
Internal signal processing in a radar receiver is very sophisticated. As well as track/position information sent from the aircraft (if any) the receiver can determine (calculate) similar information from analysis of the RF signal itself.
By way of illustration we can look at a couple of the fields in the ASTERIX radar data exchange format. This is the format used for radar data throughout Europe and beyond. Verbatim extracts from the standards document:
Data Item I001/120, Measured Radial Doppler Speed
Radial component of the ground speed as measured by means of Doppler filter banks in radar signal processors.
Data Item I001/200, Calculated Track Velocity in Polar Coordinates
Calculated track velocity expressed in polar coordinates. Two components: CALCULATED GROUNDSPEED and CALCULATED HEADING.
So even with plain vanilla Mode C calculated track information
may be available depending on the capabilities of the individual radar equipments.