DESCRIPTION

 

The mechanics and the thermic

IDEFIX
Status Report 18 May 2002

Launch by ARIANESPACE, May 3, 2002 was flawless. IDEFIX is now on orbit and presents excellent parameters. Both payloads transmit messages and telemetry.

Internal temperature fluctuates between -20 and +32 degrees centigrade as a consequence of the rotating Ariane 4 third stage. The lithium batteries by SAFT prove excellent condition and should power IDEFIX 1 and 2 as expected. The same batteries will power the Rosetta comet probe to be launched January 2003.

The antennas built by EGC-ESPACE according to plans produced by AMSAT-France are doing a good job. Hutchinson Vibrachoc calibrated and tested the shock absorbers which provided perfect protection during launch. ENSMA, Poitiers delivered several mechanical parts.

Project's timeline :

Based on a prototype built in 2001, the IDEFIX payloads largely use cellular telephone techniques, components as well as energy saving methods. The thermal approach is based on a total isolation concept.

Payload IDEFIX 1 comprises a voice recorder featuring 18 messages in French, German, English and Japanese. It also provides an 8 channels telemetry processor measuring light and battery voltages and temperatures. The 120 mW transmitter alternates messages and telemetry every 12 seconds on 145.840 MHz. Its prime goal is educative. Five successful live presentations had already been done in highschools Saturday, May 11.
Payload IDEFIX 2 comprises the same items but, the prime goal being instrumentation, it transmits telemetry every 20 seconds as well as a message every 110 seconds. Payload 2 telemetry transmits also mean light and temperature values, averaged over 10 and 90 minutes. Its higher power amounts to 1.2W on 435.270MHz.

IDEFIX has already performed its primary task to demonstrate and validate the 'Satellite's Last Stage' concept, showing the feasibility to use orbiting discarded last stages as low cost carriers for experimental payloads, despite totally uncontrolled attitude and solar illumination conditions.
At the end of the experiment, as a secondary benefit, the temperature and current conditions imposed on the lithium batteries will contribute to validate them for the Rosetta mission.

Without the support and the outstanding contribution by ARIANESPACE, IDEFIX would not be there.
More information, including kepler data, is available at <http://www.amsat-france.org./idefix-france/.net>. On request, a complete description in .pdf format (4Mb) of IDEFIX and the launch campaign will be forwarded (French).

(Translation by Gaston Bertels, ON4WF )

Last news:
The internal temperature is around 32 deg celsius, decreasing now. Last monday, it was around -20 deg celsius after having been in total obscurity for more than 24 hours not even seeing the earth reflection as indicated by the telemetry. The third stage slowly revolves around itself and spins too. Both effect combine to affect illumination. In fact, we expected even worse conditions and the temperature range is within the limits.

The batteries are in good health.

Starting from wednesday-thursday night, the telemetry channels of CU1 are all zero while the processor still keeps on transmitting frames and timing the payload. There are three possible causes in the order of highest to lowest probability :

- thermal crackdown in the pcb affecting the ADC SPI bus or power supply.
- thermal crackdown in the ADC chip
- chip destruction by high energy particule.
- uP port destruction.

When the payload cools down and the pcb shrinks, we'll have a chance to recover the telemetry if the first hypothesis is the right one.

As for the translation of the terms:

OPTRO :each payload is fitted with two light sensors which are basically a BPW21 photodiode behind a 2mm thick PTFE windows giving a cosine response of incident light flux. The photodiode is wired as a photocell loaded by a 5k resistor. Under full sun at AM0, the max voltage is around 150 mV depending on temperature. One the two light sensor is fitted with a LM335Z temperature sensor to allow for correction (TEMP OPTRO channel).

On the pictures, the light sensors are the two brass bolt like extrusions ontop the payload housing (Z+ side). One is besides the X+ side (coax connectors), the other at X- side (arming plug connector).

The description of the light sensors is given at http://www.amsat-france.org/satedu/

TEMP BOITIER : case temperature taken at the bottom of the X+ side of the payload. You can see the exact place in the pictures at http://www.amsat-france.org/idefix-france/ LM335Z sensor.

T ISD : a temp sensor glued on the ISD25120 voice recorder chip. One of the voice message is a 1800 Hz tone recorded at 20 deg celsius. As the internal free running clock of the ISD is temperature dependent, the measure of the received tone frequency gives a very rough estimate of the internal temperature of the payload. < 1800 Hz => < 20 deg, > 1800 Hz => >20 deg .

MOY ORBITE : on CU2, three channels are monitored for mean, max and min values.
OPTRO1
OPTRO2
TEMP OPTRO

There are two mean values : a 10 minutes one, which integrates the values of the last ten minutes and an 90 minutes one. The last one is reset every 90 minutes while the first one is a sliding mean.

That was devised to get some tlm values around the orbit as it was impossible to have amateur stations continuously monitoring the sat.

In the decoded tlm channels, you read :
OPTRO1 mean value, 10 and 90 minutes (orbit)
OPTRO1 min value since mean reset and time stamp
OPTRO1 max value since mean reset and time stamp and the same applies to OPTRO2 and OPTRO TEMP.

So we need only a full super frame ( 5 frames, each with subframes) every 90 minutes.

The instantaneous light sensor value (E subframe on CU2, A and B subframe on CU1) is transmitted during each frame i.e. every 12 seconds on CU1 and every 22 seconds on CU2 to adequately sample the spin rotation.

The place of the other temp sensors is visible in the pictures on the web site.

The voltage of the Li Thyonile batteries is measured after the schottky coupling diodes (400 mV drop).

On CU1, it is measured during standby, on CU2, it is measured during transmit.

On CU2, the RF power indication is taken from a diode rectifier located near the PA output coil and the spacing has been adjusted to get roughly 600 mV at 1.25W ouptut. The indication is temperature dependent and the TX temp sensor is located just besides the PA which is a BLT50.

The processor is a 80c32X2 on the two payloads.

On CU1, it comes with a 32k ROM and its internal RAM.

On CU2, 32kRAM are added to store the values for the mean, max, min calculations.

On both CU, the processor generates directly the 1200 Hz bpsk Manchester modulated subcarrier which feeds the FM TX BF input. It also drives the SPI bus (1 MAX186 on CU1, 2 MAX186 on CU2), the ISD25120 voice recorder and TX on/off control. The clock frequency is 6.144 MHz.

The processor also generates an internal time stamp which slightly drifts with time, that's the reason why the telemetry decoding software adds the PC date to appreciate the time stamp drift quite exactly. The drift is regular and has almost a fixed value slightly depending on temperature. The time stamp is generated with the internal timer.

The processor is placed in sleep mode when not in use to reduce the current drain.

The processor program architecture is a basically a state machine whose all states are defined and looped allowing for the processor to reset and recover even in the case the stack is corrupted by a SEU.

Two switching mode power supplies are used, one for 5V processor and sensors, one for 7.5V Tx generation. The 5V TX for vco/buffer and synth (a few mA) is derived from the 7.5V via a linear regulator. The on/off control of the switcher is used for TX control.

Mechanically, the PCB are located on an H like structure 'floating' in the case along with two batteries.

The 10 other ones are fitted on the bottom of the case with thermal insulation.

The cases have been mirror polished while the internal faces have been left rough. That was the compromise we choosed to cope with the totally unknown illumination or dark periods along with internal dissipation. It seems to be right altough we have to carefully study it with a post analysis of the telemetry.

There are 12 Li Thyonile, 13 A.h, 3.6 V batteries (LSH20 SAFT, space qualified model) arranged in 4 series 3 parallel giving 14.4 V no load. A better arrangement would have been to get higher voltage and lower current as the capacity is inversely related to current draw. The switchers allowed for that but there was a technical issue that we hadn't the time to care for leading to that suboptimal stacking.

Hope it gives you some insight of IDEFIX and answers your questions.

Best 73.
Ghislain F1HDD/ON1RG