General analytical formula of GDOP for TOA target localisation

For original 2D time-of-arrival (TOA) target localisation, since the unknown parameters containing a specific time of the signal that the target transmits, the unknown vector to be estimated is inherently with 3D frame, which makes the analytical characterisation of the geometric dilution of precision (GDOP) more challenging and the problem remains unresolved. A general concise analytical formula of GDOP with closed form for this case is proposed, which can be used to analytically evaluate the accuracy of TOA target localisation for most general application scenarios.