No. 239 - Nominal operations and Delta Differential One-way Ranging measurements
08 Nov 2011 10:00
Report for the period 25 September to 22 October 2011 (week 39 - week 42)
The reporting period covers four weeks of Venus Express mission operations. During this period routine planned operations were conducted at very low telemetry downlink data rates.
Cebreros ground station
During this period the X-Band Low Power Amplifier (X-LPA) was repaired to fix the problems noted in the previous status report (No. 238). The X-Band High Power Amplifier (X-HPA) was used as the prime uplink amplifier until 25 September, when the X-LPA was set back as prime.
In the week of 16 October, new software was installed in two of the Cebreros (CEB) ground systems. The software contains new functionalities required in order to provide support to the Phobos-Soil (Phobos-Grunt) mission. None of the new functionalities are used for Venus Express.
One outcome of this upgrade was a greater ground-station delay; this was updated at the station itself. For some operations it is important to know the timing of a signal between transmission and subsequent reception at the ground. One example is spacecraft ranging, i.e., determining the distance to a spacecraft. This distance is measured by transmitting a signal, measuring the time it takes to be sent to the spacecraft and returned, and then dividing that time in two. Using the speed of light as the signal speed, and knowing the signal time, gives the spacecraft distance. It is crucial that the time it takes for the signal to pass through each step of the process is known. The end-to-end system must be calibrated and the time for a signal to pass through the mission control computer console, through the ground station electronics, through the spacecraft receivers and transmitters, and back through the ground system to the computer console, must be measured. When the software for Phobos-Soil support was installed, it changed the amount of time it took a signal to pass through the ground system electronics. This value had to be measured, calibrated and recorded in the ground system databases.
High accuracy spacecraft ranging
Delta Differential One-way Ranging (DDOR) was performed on 27 September and 8 October. These measurements are carried out with the Venus Express spacecraft on a regular basis to support the accurate determination of the ephemeris for the planet Venus that is maintained by NASA's Solar System Dynamics Group. The DDOR measurements pinpoint the location of the Venus Express spacecraft; the location of the spacecraft with respect to the planet centre is known to high accuracy. Combining the two allows the orbit of Venus to be determined with very high accuracy. Modern-day DDOR campaigns are carrying on a tradition started by ancient civilizations - the orbit of Venus has been measured throughout the ages, with increasing accuracy; it is increasingly important in the space age.
The DDOR technique uses two widely separated antennas to simultaneously track the location of a transmitter in space in order to measure the time delay between signals arriving at the two stations. Theoretically, the delay depends only on the positions of the two antennas and the spacecraft. In reality, the delay is affected by several sources of error: for example, the radio waves travelling through the solar plasma between Venus and Earth; the ionosphere and troposphere of the Earth; and clock instabilities at the ground station. DDOR corrects these errors by 'tracking' a quasar in a direction close to the spacecraft for calibration. The quasar's direction is already known to very high accuracy by astronomical measurements, typically to better than 50 billionths of a degree (a nanoradian). The quasar is usually within 10 degrees of the spacecraft so that their signal paths through space and the Earth's atmosphere are similar. In principle, the delay time of the quasar is subtracted from that of the spacecraft's to provide the DDOR measurement.
(Further details about DDOR can be found in the ESA Bulletin article "Delta-DOR – A New Technique for ESA's Deep Space Navigation" - see link in right-hand column.)
Orbital correction manoeuvres
Atmospheric Drag Experiment Campaign #6 ended in the period covered by the previous report. Two orbital correction manoeuvres (OCMs) were performed to finalize the experiment. On 25 September, a pericentre burn was executed to lower the apocenter. On 26 September, a burn was performed at apocentre to raise the pericentre. Both burns executed successfully, ending ADE #6.
The OCM executed on 25 September had a nominal magnitude of 0.7134 metres per second. It was a retrograde manoeuvre (the burn was opposite to the direction of the spacecraft travel) at pericentre, with the intention of lowering the apocentre altitude by 134 kilometres and decreasing the orbital period by 218 seconds.
The OCM executed on 26 September had a nominal magnitude of 9.5995 metres per second. It was a prograde manoeuvre (burn was in the direction of travel) at apocentre, with the intention of raising the pericentre altitude by 155 kilometres and increasing the orbital period by 256 seconds.
The combined performance of the two manoeuvres was excellent, and the overall increase in the orbital period was only 0.10 seconds more than planned.
Summary of main activities
During the reporting period, routine mission operations were conducted using the ESA Cebreros (CEB) ground station. Two DDOR measurements were carried out using the ESA New Norcia (NNO) ground station and CEB. Two orbital correction manoeuvres were performed to complete ADE #6. Two communication passes at CEB were shortened because of maintenance at the station.
The table below shows a chronology of the main spacecraft bus activities in the reporting period:
At the end of the reporting period on 22 October 2011, Venus Express was 240 million kilometers from Earth. The one-way signal travel time was 801 seconds.
At the end of the reporting period, the final oxidizer mass was estimated to be 32.108 kilograms, and the final fuel mass estimate was 19.995 kilograms.
The instruments were operated nominally according to the plans of each instrument team.