OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

One four five dot nine seven two

Jan W3GEY attends to AO7 on the shake table.

The Oscar7 satellite has a telemetry beacon on 145.972Mhz give or take a bit of doppler shift. 

Oscar7 sends its telemetry as a series of coded blocks of numbers. It's sent as CW but the satellite downlink FM's frequently and it's sometimes not so easy to copy.  A good data block starts with hi hi and ends in either 649, 650 or 651. All blocks ending in other values should be considered bad.  That that last all-important value is the "Midrange TLM Calibration" , in short, the voltage present on the Analogue to Digital converter's voltage reference.  Oscar7 uses a hard-wired telemetry system built mainly in A series (non-buffered) CMOS IC's. The telemetry channels are sequentially sampled by a clock circuit and presented to the A/D converter with the resultant values being transmitted as CW.

 Here is an example telemetry block.

hi hi
 100 177 179 147
 208 269 202 254
 375 364 331 354
 444 455 449 453
 541 501 552 529
 600 601 601 651

The data can be copied down and decoded using the following table of equations.

The first row of data consists of the solar panel currents (1A - 1D).

 Rows 2 to 6 take readings from various points within the bird.  Some values were 'stuck' ie faulty, at launch time and should be discarded.

You should note that good data frames always end in 649,650 or 651.  Other values should be regarded as not trustworthy.

Oscar-7 used a single A/D converter with a voltage reference source. The reference voltage of the source is the last value of the last item in the frame (channel 6dd Midrange Telemetry Calibration) and the raw uncalculated value as CW from the bird should be 649, 650 or 651. 

Oscar7 was the first AMSAT satellite to carry a BCR and the first to generate more power than it needed. Now, over 30 years on, the batteries are defunct (open-circuit) and due to blackening of the solar panel cover-slides by severe radiation dose, the panels can barely produce enough power to operate a transponder. AO7 should be considered a QRP satellite. Excessive uplink power should be avoided. 

Al, GM1SXX and John, LA2QAA have a keen interest in this bird. It still is holding on to a few secrets. Many of the onboard systems still appear to work when the power budget permits. When in constant illumination, John discovered that the mode switching timer still worked properly although with some drift in the actual timing of the mode changes. We later discovered (from AO7's chief designer Jan W3GEY) that the timer is NOT crystal controlled, but instead uses a simple R/C oscillator in front of the divider chain used to control the mode switching. The 'official' drawings show a crystal clock, but in fact the timer is R/C controlled and therefore prone to drift dependant on temperature (and possibly also supply voltage).

AO7 Final Assembly	AO7 Solar generator

AO7 switching relays

In the past, since AO7 came back to life, we have also copied telemetry frames that seemed acceptably good.  Those that didn't end in 649/650 or 651 were discarded. 

A photo album detailing the construction, test and launch of AO7 can be found at Oscar7

Readers are urged to listen for themselves to the beacon downlink frequency and send any telemetry they collect to  to both ourselves and AMSAT .  Al can be contacted at the address below.

73 Al.
GM1SXX