Answers to some MIDES Frequently Asked Questions (FAQS)

1. What processor do we use?

We use an 8OC552, 8 bit microprocessor made by Phillips.

2. Is the processor rad hard?

No, however it could be replaced with a rad hard device with minimal hardware changes and no software changes.

3. How do we compensate for temperature variations, ie., one side in sunlight, the other in shadow?

The MIDES is an AC (pyroelectric) device, it measures the difference in temperature between the chopper and the scene temperature at 15 microns. Therefore the ratio between the earth & space temperature as measured by the voltages produced by the detector remain the same. The pyroelectric detector is a linear device and therefore doesn't change it's output as a function of IR level. This means that if there were a difference in the chopper temperature caused by the sunlight, our signal output would remain constant.

It is also noted that the detectivity factor of the detector element is also constant over the system temperature range, therefore requiring no heating or cooling to stabilize the detector. The pitch and roll detector hybrids and the chopper are located within a shielded common housing and share the same mounting plate which mounts to the satellite frame.

4. What correction factors do we apply to individual pixels?

We calibrate all the pixels by measuring the output from each pixel when the unit is looking at a heat source which is equivalent to space. The temperature difference between the space and chopper is then recorded. This technique can be used because the detector is an AC device and responds to temperature differences. The polarity of the output voltage is opposite when heat is used to simulate cold. All output voltages are recorded from the digitized output and a lookup table is generated , with the differences normalized to the highest pixel output. These factors are then used during signal processing in our algorithm. This correction factor is stored in the internal fuselink prom.

It should be noted that the option to obtain raw pixel data also exists under software control as do various other data alternatives.

5. Why do we NOT have to calibrate in a vacuum?

The device is not affected by the difference in temperature of the detector head as explained above, and the output is a normalized digital word and not a DC level that changes with detector temperature. Therefore the thermal gradients, that are different when in a vacuum, change the output of separated thermally sensitive detectors. These gradients do not affect MIDES as the (Space to Chopper)- (Earth to Chopper) voltage ratio remains constant, and independent of the chopper temperature.

6. How is the radiance model applied?

We use the LOTRAN generated radiance model to generate a series of curves as a function of Earth temperature. As the Earth's temperature varies the difference between the Space and Earth temperature, as measured by the pixel voltage, varies. We generate a series of normalized lookup tables which follow the LOTRAN model and correct for this difference. We determine the Earth temperature by looking at the output of our array of 16 pixels to select which curve to use in the algorithm. There is always at least one Earth looking pixel in the array and typically two pixels viewing the horizon. The 1 0 bit word representing the voltage from these pixels is then used to look up the actual angle.

7. Are we subject to SEU?

The processing circuit contains a watchdog timer which resets the program stored in the fuse-linked prom if a SEU occurs.

8. How does oblatness affect the output?

The MIDES does not correct for oblatness. It produces an output angle in X & Y with respect to the mounting surface of the MIDES on the Satellite. Further corrections which are needed to determine actual pitch & roll such as oblatness and altitude must be external inputs from other sources and do not come from MIDES.

9. Does changes in temperature affect the actual mechanical alignment?

The finite element analysis performed on the thermal model shows no significant shift

due to temperature.

10. Will the sensor be affected by other 1OHz sources on the spacecraft?

The sensor would be slightly sensitive to 10 Hz vibration which could couple into the chopper or detector circuit. There is a significant amount of immunity to this due to the high mechanical Q of the chopper (>1000) and the pyroelectric detector's mounting scheme.

11. How does age and radiation exposure affect the detector calibration?

To date our tests on the front-end circuitry show that the detector element is insensitive to radiation. The input preamp does change its leakage current and noise, however we have designed-in significant headroom to the circuit and shielding such that the detector should continue to perform for extended periods satisfactorily. The calibration should not be affected by the addition of the noise, and the bandpass filters in the detector preamp circuit remove the DC offsets due to increased leakage current. It should be noted that periodic recalibration can be done in-orbit by pointing MIDES to Space and regenerating the calibration table.

12. What is the memory required for the host to run the algorithm?

Approximately 64k

13. What is provided to the customer in the way of an algorithm?

Source code in C plus a flow chart of algorithm

Hit your back arrow to return to MiDES