Zarya - Soviet, Russian and International Spaceflight
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Tyneside, UK
2017 Sep 22
Friday, Day 265

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This page brings together the tracking information and adds further facts about Omid. It also draws some conclusions from putting them together.

Choice of Launch Time

Omid was launched 2008 February 2 at about 18:35 UTC. Local time was 22:30 so it was dark. A move of lauch time by six hours to optimise solar illumination would have brought daylight into play over Iran during the launch phase.

There is a potentially simple explanation of a launch in darkness - ground-based tracking of the ascending launch vehicle. Given a clear sky, optical tracking would have been possible from lift off through staging, and possibly as far as orbital injection - depending on the second stage propellant mix and visibility of its exhaust plume. The launch site is in northern Iran so the first 1,500 km of ground track was across Iranian territory, with the Safir first stage coming down on land a few hundred kilometres downrange .

Omid and SunThere is another fact that may be relevant to the precise choice of time within the hours of darkness. This map shows a section of ground track from Omid's first circuit of the Earth as it headed northward over Central America.

The map also shows the location of Omid and the sub-Solar point at the precise time Omid went through its orbital Ascending Node for the first time.

Using the Earth's surface as a reference, Omid crossed the equator at longitude 111 degrees west. The longitude of the Sun at that moment was 110 degrees west. This could be chance but with a probability of 1/180, it is unlikely to be so.

A simple expression of this arrangement is that on it's first few revolutions, Omid's nothbound equator crossing occurred when it was noon local time at the sub-satellite point. Southbound equator crossings accordingly were at midnight local time. Why this should be a mission constraint is unclear.

There is a down side to the choice of this particular orbit arrangement - no possibility of optical tracking from Iranian soil for the first week of orbital operations. February 9 marks the start of the first optical observation window. This may indicate that Iran currently has no significant optical tracking ability.

The previous Safir launch in 2008 August lifted off at about the same time of day. Similar lighting conditions at the launch site and on-orbit would have prevailed, indicating that we are seeing a launch constraint rather than a random effect. It may also add weight to the suggestion that the 2008 mission was an orbital attempt.

Initial Transmission Modes and Control

Three Transmission Modes have been observed. At the start of the mission, a pass transmitting as Mode 1 was followed by a Mode 2 pass. The pattern was observed from both Europe and South Africa. There are screen dumps from a radio receiver showing the two signal Modes on the 'Inital Observations' web page accessible from the menu on the left.

On the first pass observed from the UK, the transmitter was seen to switch on after the satellite had risen above the horizon. It was about six minutes between switch on and Omid becoming visible from Iran.

Given that the satellite was not within range of Iran at the time, the transmitter was probably operated by a timer that was set by ground controllers on its previous pass over Iran. This conclusion is further supported in there being no particular pattern of precisely where aound the orbit switching occurs.

The existence and operation of the timer were a good indication that Iran was in control of its satellite.

Monitoring of Omid's frequency from South Africa and Hawaii proved that it it transmitted only when over the Middle East in the region of Iran.

Transmissions Later in the Mission

February 8, Omid switched to Mode 3 transmission consisting of a 20 second burst of 1 kHz tone followed by 100 seconds of silence, wih the transmitter operating at all times.

The clock controlling the Mode 3 switching appeared to be running fast and gaining about 2s per day (an accuracy of one part in about 45,000). It would have been reasonable to expect the clock to be controlled by the GPS receiver. This time drift - comparable with a simple electronic clock - may indicate that the GPS was not controlling the onboard clock.

Omid transmitted for about 48 hours in Mode 3 before apparently falling silent.

Observations from Chile, Brazil and the USA indicate that Omid spent a second 48 hour period transmitting Mode 3 between February 16 and February 18. After that, it fell into a low power transmission mode before finally falling silent on February 20.

Purpose of the Mode 3 Transmissions

Mode 3 seems to have been a compromise between the limited-period transmissions while over Iran and true continuous transmission.

About 49 hours elapsed between Mode 3 being switched on and the transmitter being switched permanently off. With the it being on for just over 20 seconds in every 120 seconds, there was about eight hours of live transmitter time.

Observations of the earlier transmitting regime suggest that Omid was transmitting for about 30 minutes on each of four passes per day over Iran, making two hours per day of transmitter usage. The transmitter is probably the biggest single drain on the batteries.

As to the question of why Omid switched to Mode 3, there are numerous possibilities ranging from onboard equipment failure making periodic data transmission pointless, to wanting to have Omid heard all round the world. The transmission might be a trigger for ground-based store/dump data transmitters.

Solar Panels or Simple Batteries?

Photographs of Omid show small square patches on four sides of the cube, including one that carries a set of aerial elements. Consideration was given to a suggestion that they are small panels of solar cells but it is not the case. Sven Grahn suggested that they are a set of patch aerials for the GPS receiver. They are set around the periphery of Omid to ensure continuous reception of GPS signals as Omid rotates. This interpretation is borne out by comments from Iran about a special GPS receiving aerial arrangment to deal with a rotating satellite.

Above are components of the Omid GPS, consisting of standard items sourced from commercial companies. The four-way splitter allows output from the four patch aerials to be combined into a single connection to the receiver.

Another clue that the patches are not solar cells is the position of Omid's orbit plane with respect to the Sun at launch. It was not set up for optimal solar illumination of the solar cells of a spinning satellite. Initially, Omid was in sunlight for 55 out of the 91 minutes of each circuit of the Earth. Modelling of the first few weeks in orbit shows it increasing to 69 minutes over the next week or so and then falling back to 55 minutes before rising again in a repeating cycle.

Had the launch taken place six hours earlier or later then solar illumination could have been improved significantly, tending to support the fact that the solar lighting was not crucial to battery life.

Omid has four battery blocks, each consisting of fifteen model LSH-20 Lithium-thionyl Chloride cells - a standard product of by the French company SAFT. Each block seems to be wired in series that would give give 50 volts. More likely, the cells are wired in three sets to give about 17 volts, being a more-standard bus voltage in electronic systems. The capacity of a single block is about 650 Watt-hours, giving 2600 Watt-hours in total.


Sven Grahn has further information and analysis at his web site. The link should open in a new tab or window.

Copyright © Robert Christy, all rights reserved
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