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Document Title: 13
User’s Manual for the GPS Orion-S/-HD Receiver
Document No. Issue 1.0
GTN-MAN-0110 June 22, 2003
DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.
nals and ensures near-continuous tracking throughout the boost and free-flight phase of the
ballistic trajectory.
The command interface of the Orion-HD receiver supports a total of six different instructions
to support the handling of ballistic trajectory information:
• The LT (Load Trajectory) command initiates the upload of a set of trajectory polyno-
mials.
• Each trajectory polynomial is then loaded in the form a single F51 command mes-
sage.
• The sequence is terminated by the ET (End Trajectory) command.
• The reference epoch for the trajectory polynomials can be configured using the LE
(Load Epoch) command, unless it is automatically detected through a hardware lift-off
signal (see below).
• Using the TT (Transmit Trajectory) command, the currently loaded trajectory informa-
tion can be dumped. When issued, the receiver outputs an F50 message providing
the reference epoch and sequence of F51 messages containing the individual trajec-
tory polynomials.
• Finally, the aiding can be activated (or deactivated) through the AM (Aiding Mode)
command.
Both the reference epoch and the trajectory polynomials are stored in non-volatile memory
and made available upon a reboot of the receiver.
The aiding is designed to support a rapid acquisition and re-acquisition after temporary signal
losses. It controls the initial configuration of a previously void tracking channel but has no
impact on those channels that have already achieved a continuous code and carrier lock and
follow the signal dynamics with their respective tracking loops. When aiding is activated, the
Doppler and visibility prediction depends only on the a priori trajectory polynomials, and the
time since the reference epoch. As such, a faulty or outdated navigation solution has no im-
pact on the initialization of new channels and safe acquisition can even be achieved if during
boosted flights that do not allow a linear prediction of the latest state vector. On the other
hand, erroneous values may be predicted in case of a major deviation from the nominal flight
profile. The choice of aided versus unaided operation must therefore be based on a careful
risk assessment. Aiding is clearly advisable, if continued tracking cannot be assured due to
e.g. a changing field-of-view or switching between antennas. Unaided operation, on the other
hand, may be preferable, if a stable initial acquisition and continued GPS visibility can be
assured but the actual flight profile is not know with good confidence before the mission.
3.2.2 Lift-off Signal
The discrete input pin of the GPS Orion-HD main board can be employed to automatically
sense the lift-off time of a sounding rocket and set the reference epoch for the trajectory aid-
ing. The lift-off signal is defined to remain low while the rocket is grounded and switch to high
level at lift-off. While set to low, the receiver continuously overwrites the reference time for
the trajectory polynomials by the current time. This update is performed at each TIC and is
thus accurate to about 0.1 s. For proper function, the lift-off signal must remain high through-
out the entire flight.
3.2.3 IIP Prediction
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 cen-
ter following e.g. a guidance error during the boost phase. As part of the range safety opera-
tions during a sounding rocket launch, a real-time prediction of the IIP is performed to moni-
tor the expected touch down point in case of a boost termination. The computation and dis-