The instantaneous impact point (IIP) describes the touch-down point of a sounding rocket under the assumption of an immediate end of the propelled flight. It is representative of a situation in which the rocket motor is instantaneously switched off by the mission control centre following e.g. a guidance error during the boost phase. As part of the range safety operations during a sounding rocket launch, a real-time prediction of the IIP is performed to monitor the expected touch down point in case of a boost termination. The computation and display of the IIP allows the range safety officer to discern whether the rocket would eventually land outside the permissible range area and thus necessitate an abort of the boosted flight or even a destruction of the malfunctioning vehicle.
Supplementary to traditional radar tracking, the IIP prediction is nowadays based on GPS navigation data, which offer an inherently higher accuracy and reduced data noise. Following successful application in a variety of sounding rocket flights, GPS has therefore recently been recommended as the baseline for range safety tracking systems both for cost and performance reasons and is expected to provide the primary sensor for flight terminations systems in the near future.
IIP Model Development
To comply with the increased tracking performance, the accuracy of existing IIP prediction algorithms has been assessed and a simple, yet accurate, analytical IIP prediction method for real-time applications has been developed. The model is based on a plane-Earth parabolic trajectory model with first order corrections for surface curvature, gravity variation and Earth rotation. Despite the implied simplifications the resulting model is more complete and of higher accuracy than conventional IIP algorithms based on a flat Earth approximation with Coriolis correction. Overall the agreement with the full modelling of conservative forces is high enough to introduce IIP prediction errors of less than 1.5% of the ground range for sounding rockets reaching altitudes of up to 700 km and flight times of about 15 min. On the other hand the model is less complex than a perturbed Keplerian trajectory model or numerical integration and thus well suitable for real-time computations. Following its ground based validation, the model has successfully been incorporated into the Orion GPS receiver for high dynamics applications, which can thus directly generate range safety related information onboard a sounding rocket. Despite these encouraging developments, the IIP prediction still suffers from an insufficient modeling of atmospheric flight phases which poses the most sever constraints to the achievable accuracy. While a limited progress has been made to describe the IIP shift caused by drag during the ascent trajectory, the ballistic coefficient of a rocket after a destruction or malfunction remains essentially unknown.
Montenbruck O., Markgraf M., Jung W., Bull B., Engler W.;GPS Based Prediction of the Instantaneous Impact Point for Sounding Rockets;Aerospace Science and Technology 6, 283-294 (2002).
Montenbruck O., Markgraf M.;A GPS Tracking System with Onboard IIP Prediction for Sounding Rockets;AIAA Guidance, Navigation and Control Conference, Aug 11-14, 2003, Austion, Texas (2003).
Montenbruck O., Markgraf M.;Test Procedures for Instantaneous Impact Point Prediction;GTN-TST-0010; Issue 1.0, DLR/GSOC (2002).