Within a joint project of DLR and Kayser-Threde, an Orion GPS receiver (Picture on the right) was flown onboard the Maxus-4 and Texus-39 sounding rocket missions at ESRANGE, Kiruna, to assess its reliability and performance in comparison to a commercial Ashtech G12 HDMA receiver.
The Maxus-4 rocket (Picture on the left) was launched on April 29, 2001 (11:28 UTC). The payload segment weighed a total of 803 kg and comprised seven material sciences experiments in five different experiment units:
A single stage Castor-4B motor (Fig. 2) carried the payload to an altitude of 703.4 km and allowed for a zero-g time of more than twelve minutes. At the end of the boost phase, which took place some 64 s after lift off, the rocket achieved a peak climb rate of 3.3 km/s.
Due to problems in the guidance system, the flight path started to deviate from the planned trajectory from about 10 s after lift-off. As a result, the ground track was shifted in a westerly direction and the payload ultimately crossed the border before landing in Norway. Furthermore, the main parachute was destroyed after being deployed too early due to a malfunction, which resulted in a final sink rate of about 90 m/s. While the payload segment was badly damaged due to the abrupt deceleration, valuable data and samples required for the scientific post-mission analysis could still be recovered. Likewise, the Orion GPS receiver experienced no evident damage during touch down and was found to be electrically functioning after disassembly of the payload module.
The Orion receiver tracked the position and velocity of the Maxus-4 payload from lift-off to landing with the exception of two outages related to antenna switching. 3D navigation was first lost for 5.5 seconds after the boost end, even though one to two satellites were continuously tracked throughout this interval. Prior to the reentry, no tracking was available for 92 s after separation of the can. Position and velocity information was regained as soon as the parachute was deployed, but telemetry transmission was terminated briefly thereafter.
During the pre-launch free flight phase the Orion and G12 navigation solutions agree to roughly 10 m and 0.5 m/s. Only moderately larger velocity errors are encountered during the boost phase, and major tracking errors may only be observed right after the end of boost. Here notable variations in the frequency of the reference oscillator may be observed in response to the peak yerk of -7g/s, which cause associated errors in the Doppler based velocity measurements and - to a less extent - the position measurements.
During the re-entry no GPS tracking of the Maxus-4 payload was available due to the lack of an antenna after the canister on top of the recovery system had been removed. At an altitude of roughly 5 km the heatshield was finally released and the drogue parachute was deployed. The main parachute was planned to be released 16 s later, after which GPS tracking would again be possible via a patch antenna located in between the four leg bridle of the main chute. However, due to a malfunction of the stage line the main parachute was deployed to early and destroyed consequently. This is evident from the fact that a first GPS satellite was acquired by the Orion within 1.5 s after the heatshield release and 3D navigation was achieved about another second later. Since no tracking would have been possible from an antenna wrapped inside the packed main parachute, the main chute must have been prematurely released.
Nine days after Maxus-4, a second microgravity misison was launch from Esrange, Kiruna. Following various launch delays, Texus-39 left the launch tower at 11:28 UTC on May 8, 2001. The payload segment weighed a total of 362 kg and comprised five biological and material sciences experiments in four distinct experiment units:
A dual stage Skylark 7 motor (Goldfinch II & Raven XI) carried the payload to an altitude of almost 250 km and allowed for a zero-g time of six minutes. The first stage accelerated the rocket with up to 11 g during the first six seconds. At the burn-out of the second stage, which took place within 43 s after lift off, the rocket achieved a peak climb rate of about 1940 m/s.
The Orion receiver tracked the position and velocity of the Texus-39 payload from launch to reentry with the exception of a brief gap following the lift-off. 3D navigation was lost for a total of four seconds (t=2.7 s to 6.6 s), even though one to three satellites were continuously tracked throughout this interval. No signal interrupts were recognized when the tip was separated and tracking was performed via the can antenna. Prior to the reentry, tracking was terminated at t=468.6 s, when the can and antenna were removed to uncover the parachute system. Other than expected, tracking could not, however, be regained after full deployment of the main parachute, when a third antenna mounted in the four-leg bridle achieved full sky visibility. Apparently, the antenna cable had been injured during assembly of the parachute system and did not provide a sufficient signal level for GPS tracking.
The Orion and G12 navigation solutions agree to roughly 20 m and 1 m/s during the free flight phase, prior to which no reference data were available due to a loss of tracking of the G12 receiver. similar to the Maxus mission, the Orion receiver was notably affected by high yerk, which resulted in a pronounced degradation of the navigation solution during the boost phase and the early parabolic flight.
Montenbruck O., Markgraf M.;Maxus-4 Orion GPS Tracking System Flight Report;MAX4-DLR-RP-0001; DLR/GSOC, Oberpfaffenhofen (2001).
Montenbruck O., Markgraf M.;Texus-39 Orion GPS Tracking System Flight Report ;TEX39-DLR-RP-0001; DLR/GSOC, Oberpfaffenhofen (2001).