The VSB30 Mission
In the framework of a Brazilian-German cooperation a new dual-stage sounding rocket, named VSB30, has been developed. In October 2004 the system performed its first flight from the Centro de Lancamento de Alcantara (CLA) in the north of Brazil. The main goal of this mission was to qualify the vehicle for a future use in European and Brazilian sounding rocket programs. Furthermore, the mission offered the opportunity to flight qualify the Phoenix-HD GPS tracking system under representative dynamical conditions of suborbital vehicles. The VSB30 rocket is composed of a S31 boost motor and a S30 motor as a second stage. Both of the above are solid-propellant motors, developed and manufactured in Brazil. The vehicle was flown in an unguided configuration and thus the trajectory could not be altered after liftoff. While the S30 motor has already successfully completed numerous flights in a single stage as well as dual stage configuration, the present flight represents the maiden flight of the S31 motor. In addition to the motors, various other key components of the rocket have been verified during the mission which were in part newly developed for the VSB30 vehicle (fin assembly second stage, boost adapter, etc.) and partly adopted from existing systems (fin assembly first stage, payload adapter, de-spin system, etc.).
The rocket left the launch pad on 23 Oct. 2004 at 16:51:00 UTC. During the 45 s total burn time of the both motors the vehicle was accelerated to a final velocity of 1676 m/s. The payload segment with an approximate weight of 400 kg was carried to an apogee height of almost 240 km, which was reached 252 s after lift-off. About 650 seconds subsequent to ignition of the first stage the payload landed in a distance of roughly 190 km form the launch site in northeastern direction in the ocean, only 700 meters away form the nominal impact point.
The Phoenix-HD GPS Receiver
With the exception of two brief telemetry outages, navigation and status information were continuously sent from the GPS receiver to the ground station from lift-off to landing. Data transmission was first lost at t+155.0s for about 14.5 seconds and again shortly thereafter at t+186.5s for another nine seconds. Furthermore, the receiver temporarily lost 3D-navigation for approximately 60 second near apogee.
The latter problem could be traced down to a frame sychronization problem on one receiver channel after reacquiring signals under poor conditions and as henceforth been fixed in the receiver software. Since all other channels tracked properly, navigation solutions for the affected time interval could be reconstructed on ground from the transmitted raw pseudorange and Doppler measurements. Overall, between 9 and 12 GPS satellites were tracked in all flight phases.
While near-constant carrier-to-noise ratio (C/N0) readings with typical values of 42-48 dB/Hz were obtained for all tracked satellites during the first 90s of the flight, a slow but notable drop in the signal levels was encountered thereafter (Fig. 3). This gradual decrease of the signal strength, observed across all used channels, persisted for roughly 100s and is best attributed to a gradual change in the orientation of the rocket and the antenna boresight. After passing a local minimum with C/N0 values of 32-39 dB/Hz around t+200s, the signal levels started to increase again and reached normal values about 300 seconds after ignition. A second minimum was recognized about 420s after launch, again followed by a period with gradually rising C/N0 values. The start of the atmospheric reentry at around t+450s was finally marked by eratic C/N0 variations across all channels, that are evidently caused by the tumbling motion of the payload module during re-entry into the atmosphere.
Markgraf M.;VSB30 - Phoenix GPS Tracking System - Flight Report;VSB30-DLR-RP-0001; Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen (2005).
Markgraf M., Montenbruck O.,Phoenix-HD - A Miniature GPS Tracking System for Commercial and Scientific Rocket Launches;6th International Symposium on Launcher Technologies, 8-11 Nov. 2005, Munich Germany (2005).
Montenbruck O., Markgraf M.;A GPS Tracking System with Onboard IIP Prediction for Sounding Rockets;Journal of Spacecraft and Rockets 41, no.4, 644-650 (2004).