The Prisma operation team is now back from a well deserved vacation and ready to take on the tasks of this autumn.

First we took back the controls of Prisma from Esrange, who operated the satellite during July. We are very grateful for their well performed operations.

We turned Prisma back into safe celestial mode, which also meant that we turned on a lot of systems again, including the startrackers and GPS. This was uneventful and Prisma performed just as it should. We are very happy with how well behaved Prisma is.

Next we turned on and power cycled the GPS on Target to ensure that it worked. This too resulted in happy faces as everything worked nominally. 

We then initiated the DVS at a high frame rate (2Hz) in preparation for the Target separation sequence. The resulting clip can be seen here.

This was taken during one passage. During the first part of the sequence we are space pointing. Then we start to point nadir towards Earth. Since the autoexposite is on, it takes a little while for it to adjust from the black space to a "shining" Earth.

We are now uploading new software to the the Main satellite.

On the 11^th of July, a Sun Eclipse occurred and was observed on-board PRISMA. Indeed the Sun has been masked for a few hours by the Moon, and on Earth this could be followed mainly on islands in the South Pacific Ocean (see also Total solar eclipse).

On orbit, PRISMA observed the phenomenon 3 times, with a minimum of sun intensity the 3^rd time (the sun was then more than 90% shadowed by the Moon).

Onboard, the events had impacts on power parameters (it had to switch to battery power supply), on the Sun Sensors measurements (the sun direction could not be estimated anymore), as well as on equipment temperatures (some of the equipments got slightly colder).

The spacecraft handled the situation completely autonomously, without any ground intervention, and its behaviour was perfectly nominal.

Solar eclipse as seen by Prisma

Effect of eclipse on solar array power

After a successful initial commissioning phase PRISMA is put into a safe configuration for a period of four weeks. In the meantime we are preparing for the separation of the two satellites which is planned for the 10th of August. Routine operations are performed by Esrange and Solna staff is providing support and performs data analysis.

This video was recorded by PRISMA Digital Video System (DVS) while the Spacecraft was rotating to get the DVS pointing towards Nadir.

We had a little taste of it last week when we fired the HPGP thrusters, but last night at 04:04 UTC we had the first complete controlled firing of a thruster onboard Mango, this time of a hydrazine thruster. After commissioning of the hydrazine system, we need to fire each of the six thrusters a number of times to evaluate the thrust level. Only with this knowledge we can effectively start using them when we separate the Tango spacecraft. This is not only a firing of a thruster but it comprises an orchestrated operation of a lot of onboard functions. When a thruster is fired, it creates momentum on the spacecraft and the attitude control is designed to anticipate this momentum change, and therefore it starts spinning up the appropriate reaction wheels when the time to fire the thruster is there. The accelerometers are treated specially since some of the onboard estimators need to take into account that we are going to fire thrusters. The onboard orbit estimator takes the accelerometer data into account to autonomously estimate the orbit change. In addition we fired the thruster by means of a high level command, where we tell the spacecraft where we want to have the change in velocity and the spacecraft does the rest autonomously. This is a major step forward from last weeks thruster firings as we now can see how the spacecraft will respond during the formation flying experiments. In fact the only thing that is not in this, is that the position with respect to Tango changes, because that spacecraft is still clamped to its mother ship.

We are going to do a full day of thruster firings (going on while I write this). Below you find the first thruster firing: the temperatures of the thrusters are shown and you see one of the six thrusters heating up rapidly when it fires. The other graph shows the change in the half long axis of the orbit. The thruster is fired at the red mark. These are outcomes from our onboard navigation filter based on GPS and accelerometers.

The commissioning of the PRISMA Hydrazine propulsion system was completed tonight at 00:45 UTC (02:45 local time) on June 28, 2010.The commissioning was nominal. Thruster checkout firings will follow.

After a few attempts we manage to get a real nice picture with the Digital Video System delivered by Technosystems.

The picture was taken over Russia (middle of image at Lat 62.07, Long 55.40) at 01:06:50 UTC.

Since the Satellites are still mated we can see parts of Tango in the picture above.

The first firing was performed at 03:47:08 (local time) on June 24, 2010. Between 05:26:42 and 05:37:09(local time) the first in-space firing sequence was performed by firing a pulse train of 40 pulses with 100 ms firing duration at a duty cycle of 1%. The thrust level was nominal. Tonight’s HPGP operations were concluded at 07:05 (local time) after verification of stable propellant tank pressure and temperature.

The HPGP propulsion system is declared "Ready for Action".

This video includes the same pointing maneuver as in the previous post (in this video from 20s to 35s), as well as the maneuver before and after, but displayed in a format that needs a bit less explanations.
It should be noted that the first format gives a more precise view of MANGOs pointing, when you are used to the format.

MANGO FFRF sensor has been powered on

The FFRF sensor is the Formation Flying Radio Frequency sensor provided by CNES on PRISMA. This radio frequency based sensor is designed to handle the first stage omni-directional metrology for future formation flying missions. It is composed of two terminals, one on MANGO and one on TANGO, and a set of antennas. More about this sensor and the CNES experiment on PRISMA (named FFIORD) can be found on at:

http://www.ffiord.fr/

Last night the FFRF terminal on MANGO was checked out successfully. An 8 min test session was dedicated to the checkout test and enabled to validate the electrical behavior of the sensor, the command and control interface and the time synchronization mechanism. The following figure shows the power consumption of the sensor and the local time which starts increasing after an initialization phase of a bit more than 3 min.

In a few days a similar test will be performed for the TANGO FFRF terminal. Then we will have to wait until MANGO and TANGO separation and the 23^rd of August to fully activate the FFRF sensor.

The transition went perfect. It had us clamped to our screens, watching the wheels spin up, and the sun moving away from the solar panel for the first time. We now have an angle with the sun of 14 degrees, one star tracker is looking down to the Earth and the other one sees the starry sky.

 This plot shows where everything is seen from the spacecraft. The Z axis is on the top, the X axis in the middle. One can see the sun, slightly away from the -Y axis, and the blue circle is the earth. Nadir is the vector from the spacecraft to the middle of the Earth. The two eye like figures with the crosses are the star trackers. If the Earth or the Sun gets into these zones, then the star trackers can't see anything. So here there is one star tracker blinded by the Earth and the other is seeing the sky. Just as it should be.

Third day in orbit, commissioning activities are proceeding with increasing complexity. On the 33 orbit around Earth we achieved for the first time a working relative GPS navigation. The estimated relative position of MANGO with respect to TANGO is shown in the figure below.

Note that between 22:00 and 22:20(UTC time, 00:00-00:20 Swedish time) both GPS receivers were switched off and the navigation is propagating the satellites locations without measurement updates. The GPS navigation is also unaware of the fact that the two satellites are still mated. This leads to a situation where it looks like we have just separated MANGO and TANGO! But as can be seen when we enable the GPS receivers again they are still very close together.

Separation is a major milestone in our timeline, but this is scheduled first in beginning of August. First we have a few intensive weeks ahead of us where all systems will be properly tested. The data shown here is just the first example of relative navigation. It does not at all have the performance that it should have, as we still have to do a bit more on Commissioning of the navigation system.