Changes in the ESTCube-1 Orbital Elements to Date

  ESTCube-1 has has been in orbit for a little over three weeks now. It has already downloaded a few images using the onboard camera. The orbit has decayed slightly since the cubesat was placed in orbit as can be seen from the plots of the orbital elements obtained from the Space-Track TLEs. The horizontal axis is the day of the year. The inclination and ascending node specify the plane of the satellites orbit. The eccentricity, e, and the argument of perigee give the shape and orientation of the orbit. And the period, T, and the semimajor axis, a, which are related specify the size of the orbital ellipse. There is still a trace amount of atmosphere left at the satellite's altitude which causes it to slowy decay. Torques caused by gravitation due to the shape of the Earth, the Moon and the Sun cause the satellite to slowly precess changing the ascending node, alters the eccentricity and shifts the argument of perigee.

NASA Says A Meteor Impacted the Moon Last March

src: youtube

  A recent NASA report says that a large meteor impacted the Moon on March 17th in Mare Imbrium creating a momentary bright flash of light.

The IGY and the Progress in Geodesy

  The International Geophysical Year spurred the launch of the first artificial Earth satellites. The year was devoted to a study of the Earth and led to major gains in the field of geodesy. There is a good review on the subject at NOAA entitled Geodesy for the Layman. In the chapter on Satellite Geodesy there is a figure with a close-approach diagram similar to the one I used. I could not find a formula for the frequency vs time curve but that could mean at the time it was available elsewhere. Some of Guier's papers are cited in Kaula, Theory of Satellite Geodesy, 1966.

Guier & Weiffenbach on Doppler Tracking

  Here are a few papers on the history of doppler satellite tracking:

Yionoulis, The Transit Satellite Geodesy Program (1998)

Guier, Weiffenbach & George, Theoretical Analysis of Doppler Radio Signals from Earth Satellites (1958)


Unclassified reports available to the public:

Guier & Weiffenbach, The Doppler Determinations of Orbits (1959)

Guier, W. H., Ionospheric Contribution to the Doppler Shift at VHF from Near-Earth Satellites (1963)

Transit Satellites (NAVSAT)

src: YouTube

  Doppler shifts were used to determine Sputnik's orbit in 1957. Subsequently, the US Navy's Transit satellites which were part of its NAVSAT system used the Doppler effect to determine positions on the Earth.

Using the Doppler Effect to Track Microsatellites

  ESA, the European Space Agency, has made space more accessible to some new users as can be seen from the Vega VV02 launch Tuesday morning. Riding up with the Proba-V was Vietnam's VNREDSat 1A and Estonia's ESTCube-1 student satellite. Tracking satellites visually can be difficult because they have to be in sunlight to be seen and then daytime is too bright and at night this just leaves times just before sunrise and after sunset when they are not in the Earth's shadow. ESTCube-1 sends out coded messages on two amateur radio frequencies. This suggests the possibility that one can use the Doppler Effect to acquire some information about the satellite's location. Consider what happens when the satellite makes a close approach to a known position, x₀, of the Earth's surface.

Over a short period of time the satellite's motion can be approximated by a uniform movement on a straight line. The range of the satellite is r and at some point in time it reached the close approach distance r_m.  As the satellite passes by the frequency received is affected by its relative motion and a plot of the frequency vs time looks something like the plot below which was done for an object emitting a steady tone as it passes by an observer on the ground. As the object approaches the observed frequency is higher than the emitted frequency and as it moves away it is lower.

To use this information we need to relate the shape of the frequency curve to range and velocity of the object. One can derive the following set of formulas where omega is the angular frequency and c the velocity of the transmitted wave.

One gets the formula used to plot the frequency curve which contains two unknown parameters, the minimum distance, x_m here, and the velocity. The velocity can be found from the minimum and maximum ratios of the transmitted frequency and the observed frequency. The close approach distance can then be estimated by fitting it to the frequency observations. So it's not too difficult to estimate the range and velocity at the time of close approach. One may want to keep tabs on ESTCube-1 since it will be testing an e-sail tether in an attempt to make controlled changes to its orbital parameters.

Working with Big Numbers

  As was mentioned in previous blogs the maximum precision available in Mathcad 11 is about 15 decimal places. The limit is somewhat fuzzy and the roundoff errors can accumulate with the sum of a large number of terms. If the roundoff error is δ=0.5x10¹⁵ for a number then the error for the sum of n numbers can be as much as n·δ so one needs to take the necessary care to avoid errors of this kind. I wrote some utilities to work with "big numbers" in Mathcad and did a check on the Mathcad's performance.

  Note that Mathcad made a roundoff error on the sum of u and v above. The correlation matrix for the curve fit procedure involves the sum of squares and other even powers of x_k which can bias the result. The expected value for the error of the sum is zero but the standard deviation of this sum is proportional to the square root of the number of data points.