The Mobile Rocket Base (MORABA) of DLR's German Space Operations Center plans, prepares and implements scientific sounding rocket and balloon campaigns in the fields of aeronomy, magnetospheric research, astronomy and microgravity. Tracking services are currently performed using C-band radars (AN/MPS-36, RIR-774C), which are relocatable but comprise bulky equipment and result in costly ground operations. As an alternative, the development of a low cost, GPS based tracking system for sounding rockets has therefore been initiated at DLR.
Orion Receiver Development and Testing
The Orion receiver itself has been built by DLR based on Mitel design information. It makes use of the GP2000 chipset, which comprises a GP2015 RF down-converter, a DW9255 SAW filter, a GP2021 correlator and a 32-bit ARM-60B microprocessor. Using a single active antenna and RF frontend, the receiver supports C/A code tracking of up to 12 channels on the L1 frequency. The small size (10cm x 5cm) and the open-source policy makes the Orion receiver particularly interesting for the desired application on a sounding rocket.
To cope with the highly dynamical environment, various modifications of the standard receiver software have been made and tested in a hardware-in-the-loop simulation using a GPS signal simulator. A position-velocity aiding concept has been developed, which makes use of a piece-wise polynomial approximation of the nominal flight path in Cartesian WGS84 oordinates. Based on a polynomial approximation of the nominal trajectory, the reference position and velocity of the sounding rocket in the WGS84 reference frame are computed once per second. The result is then used to obtain the line-of-sight velocity and Doppler frequency shift for each visible satellite, which in turn serve as initial values for the steering of the delay and frequency locked loops. The position-velocity aiding thus assists the receiver in a fast acquisition or re-acquisition of the GPS signals and ensures near-continuous tracking throughout the boost and free-flight phase of the sounding rocket trajectory. The modified receiver has been extensively tested in a signal simulator environment for both a VS40 and a Texus-37 scenario. During the tests the signal was intentionally interrupted for several seconds at well defined points of the trajectory. In all cases, the aided receiver was back on track much faster than the unaided receiver (usually within a few seconds).
A helical antenna mounted in the tip of the rocket cone provided near hemispherical coverage during the ascent trajectory (Picture on the left). After separation of the cone, an R/F switch connected the GPS receiver to a pair of blade antennas mounted opposite to each other at the walls of the service module (Picture on the right) and combined via a power divider. This provided a near omni-directional coverage and could thus be applied even in case of a tumbling motion of the module. Finally, a separate antenna was mounted on the arm of the launch pad and connected to the receiver through a supplementary R/F switch up to lift-off. Thus the receiver could be properly initialized and acquire all visible GPS satellites prior to launch.
Test Maxus-4 Campaign
As part of the Test Maxus-4 campaign, which was performed at ESRANGE, Kiruna, on 19 Feb. 2001, the Orion receiver receiver was first flown on a sounding rocket. The experiment aimed at the validation of dedicated low-cost antenna concepts as well as GPS receiver technology.
The ORION rocket (yes, it's named like our GPS receiver) is a single stage rocket reaching an altitude of about 100 km in the improved version. During the 24s boost phase, the rocket built up a spin rate of 3.8 Hz along the longitudinal axis. Accordingly, the rocket maintained a constant and stable attitude with a near zenith-facing tip. In the first 6s boost phase a maximum acceleration of 18g was reached, followed by a sustenance phase of 1g and 5g. After burnout a maximum rate of climb of 1100 m/s and a speed over ground of 280 m/s were measured. The rocket reached the apogee 2 minutes and 17 seconds after lift-off at an altitude of 81 km. Briefly thereafter the spin was removed by a yo-yo system and the top cone as well as the motor have been separated. The service and recovery module started a tumbling motion from about h=40 km downwards. Between 25 and 15 km altitude the module decelerated to sub-sonic speed before parachute deployment at h=5 km. The payload and nose cone landed at a distance of 60 km from the range and were finally recovered by helicopter.
Throughout the flight, the Orion receiver has continuously obtained a 3D-navigation solution up to the time when telemetry lost contact to the payload near landing. Typically, the receiver had between 10 and 11 GPS satellites in lock. Only during the first few seconds of the boost phase and during atmospheric reentry a loss of some satellites can be observed. Within a post-processing of the raw pseudorange measurements, r.m.s. position errors of 1.5 m were achieved in comparison to an Ashtech G12 HDMA receiver flown by NASA on the same rocket. Aside from these positive results, it is evident, however, that the measurements collected during the initial boost phase are notably deteriorated by frequency variations of the reference oscillator for about ten seconds. A similar but less pronounced degradation is again observed during the atmospheric reentry. Due to the use of off-the-shelf components with mechanical tun-ing elements the observed behavior is not, however, entirely unexpected. As a corrective measure, qualified oscillators for highly dynamical loads will be employed in future receiver models.
Markgraf M., Montenbruck O.;Orion GPS Post-flight Data Analysis Report - TestMaxus-4 Campaign;TMX4-DLR-RP-0001; DLR/GSOC, Oberpfaffenhofen (2001).
Markgraf M., Montenbruck O., Hassenpflug F., Turner P., Bull B.;A Low cost GPS System for Real-time Tracking of Sounding Rockets;15thEuropean Symposium on European Rocket and Balloon Programmes and Related Research, Biarritz, 28 May - 1 June (2001).
Montenbruck O., Enderle W., Schesny M., Gabosch V., Ricken S., Turner P.;Position-Velocity Aiding of a Mitel ORION Receiver for Sounding-Rocket Tracking;C5-5; ION GPS 2000 Conference, Salt Lake City, 19-22 Sept. 2000 (2000).
Markgraf M., Montenbruck O., Hassenpflug F.;A Flexible GPS Antenna Concept for Sounding Rockets;DLR-GSOC TN 01-04; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2001).
Montenbruck O., Markgraf M., Hassenpflug F.;Pre-flight Assessment of a Dual Blade Antenna System for GPS Tracking of Sounding Rockets;DLR-GSOC TN 01-03; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2001).