IEEE Workshop on Intelligent Data Acquisition and Advanced Computing Systems: Technology and Applications5-7 September 2005, Sofia, Bulgaria

Guidelines for Satellite Tracking (NAVSTAR Software)1 ~~~~~,2 3Dusan Vuckovic 1, Petar Rajkovic , Dragan Jankovic 3

Faculty of Electronic Engineering, Aleksandra Medvedeva 14, 18000 Nis, Serbia&Montenegro1) dvuckovicgelfak.ni.ac.yu2) rajkovicpgelfak.ni.ac.yu

3) gagagelfak.ni.ac.yu

Abstract - Since the number of object in Space is continuouslygrowing, the need for their tracking is raising. Differentmethods can be used for that purpose. Satellite trackingsoftware is an irreplaceable tool used to find satellites in thesky if one's using visual observing methods, to see satellitesignal coverage for radio amateurs or even to predict spaceobjects' route in thefuture.NAVSTAR is solution developed to cover all major aspects ofsatellite tracking. Methods and mathematical models used forthis program are explained in this paper.

Keywords - satellite, tracking, glscene, orbit, sgp

I. INTRODUCTION

There are over 8,000 objects in Space for which orbitalinformation is kept by military and civilian spaceorganizations. Over 2,500 are satellites, operative andinoperative. The remaining objects are orbital debris:parts such as nosecone shrouds, lens, hatch covers, rocketbodies, payloads that have disintegrated or exploded, and

Nanie of Satellite(11 characters)

even objects that "escape" from manned spacecraftduring operations. Most of these objects are small butquite a few are large enough to be seen with the unaidedeye. And some, like the Space Shuttle and theInternational Space Station (ISS), are considerably largeand appear very bright.

A. Satellite Observing DifficultiesSince satellites do not give off light of their own, they

can only be seen when there is sunlight reflecting off oftheir surface. But the daytime sky is too bright to seetheir reflected light. So only during a comparatively briefperiod after sunset and before dawn, when the sun isbelow the horizon for Earth based observers but is stillilluminating space overhead, are they reflecting light in adark sky. For that hour to hour-and-a-half it is easy, ifyou are patient, to see artificial satellites. The darker thesky, of course, the more (and fainter) you can see.

1st derivative of MeanMotion or Ballistic CoefficieLnt

International Epoch Year &Designator Day Frctiorn

2nd derivative of DMotion, usually bla

Drag term orradiationpressure coefficient

dean Element Number3nk Ephemeris & Check sum

TypeSTS-89

1 25143 98003 9802854150584 l00015530 00000- 0 20178-3 0 911325143 516567 132.2111 0004328 294r0456 T 66.0247 15.6185860 866

Inclination Etcentricity Mean Anomaly

Right Ascension Argument Mean Motionof the endingNode

of Perigee Revolution numnber atepoch & check sum

Fig. 1. Two Line Elements set explained.

Generally, it is going to be difficult to determine what

satellite you are viewing. But you can usually figure itout if you have an orbit tracking program and currentspacecraft orbital elements. Even more exciting, whenthere is a current space shuttle mission underway or ifyou want to see the International Space Station you can

determine when they will actually be visible and plan togo out and observe at the right time. With bright objectslike the shuttle and ISS you can view from even lightpolluted city skies.

There are orbit tracking programs for Macintosh andIBM compatible computers. They are either free or

0-7803-9446-1/05/$20.00 C2005 IEEE

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shareware and can be obtained from public access sitesvia a modem or Internet connection. They requireupdated orbital elements which can also be easilyobtained.

This paper presents the NAVSTAR as a new solutionfor the problem discussed, developed as a tool for bothprofessionals and amateurs involved in space objects'observing.

II. FEATURES OF PROPAGATION MODELS

General perturbation element sets on all resident spaceobjects are maintained by NORAD1. These sets areperiodically refined to maintain prediction capability onall space objects, and in that form provided to users.

It is very important to notice that not just anyprediction model will suffice. The "mean" values (TheNORAD element sets) are obtained by removing periodicvariations in a particular way. These periodic variationsmust be reconstructed in order to obtain good predictionsin exactly the same way they were removed by NORAD.On the other hand, inputting NORAD element sets into adifferent model will result in degraded predictions.

It is hard to expect that the arithmetic and geometricmeans of a set of data have the same value. Similar, itcannot be expected for mean elements from differentelement sets calculated using different orbital modelsto have the same value. The short answer is that datacannot be simply reformatted unless prediction withunpredictable errors is acceptable.

If the space object's period is less than 225 minutesit's classified as near-Earth. Otherwise, it's treated asdeep-space object. The NORAD element sets areautomatically generated with the near-Earth or deep-space model depending on the period, allowing the userto calculate the satellite period and to know whichprediction model to use.

During the time, five mathematical models weredeveloped. First by Hilton & Kuhlman in 1966 for near-Earth satellites.

Second, by Ken Cranford in 1970 for near-Earthsatellites (SGP4 model)[3]. This model was obtained bysimplification of the more extensive analytical theory ofLane and Cranford in 1969 which uses solution ofBrouwer for its gravitational model and a power densityfunction for its atmospheric model.Few years later, a new model has been developed.

Named SDP4 [3], it was extension of SGP4 to be usedfor deep-space satellites, developed by Hujsak (1979)and model the gravitational effects of the moon and sunas well as certain sectoral and tesseral Earth harmonicswhich are of particular importance for half-day and one-day period orbits.The Hoots's 1980 model SGP8 [3] for near-Earth

North American Aerospace Defense Command

satellites is obtained by simplification of an extensiveanalytical theory of his which uses the same gravitationaland atmospheric models as Lane and Cranford didintegrates the differential equations in a much differentmanner.SDP8 [3] model was developed as an extension of

SGP8 to be used for deep-space satellites.The difference between SGP models is elaborated in

[4] with several satellites used as examples.The complete insight in all of these models can be

obtained by examining their mathematical description orFORTRAN IV implementation [3].

III. ORBIT TRACKING PROGRAMS

Orbit tracking programs require information about theshape and orientation of satellite orbits. These data areknown as "Keplerian elements," named for JohannesKepler. There are two forms of Keplerian elements; thelong version which has each value described, and theshort form or "two-line elements" (tle) which are astandard way of formatting the data so computerprograms can read them automatically. Below areexamples of both forms from the Space Shuttle missionflown in January 1998.

TABLE I.SATELLITE DATA FORMATS

'iwo-irne elements:STS-89

1 25143U 98003A 98028.54150584 .00015530 00000-0 20178-3 0 91132 25143 51.6567 32.2111 0004328 294.0456 66.0247 15.61858603 866

Explanation for numbers in TLE is given in detail inFig.1.

Comparing the numbers in the two-line element setwith the long form ,it can be seen that the data in both isthe same.

The elements in the two-line element sets are meanelements calculated to fit a set of observations using aspecific model the SGP4/SDP4 orbital model.The accuracy of two line elements set depends on the

number of elements. These range from particular sensors

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used and amount of data collected to the type of orbit andcondition of the space environment. Unfortunately, sincethese factors vary for each element set, which influencethe accuracy. NORAD did experimented with methods toincorporate prediction quality into the element set, butwithout much success.

IV. Two LINE ELEMENTS AcQuiSITION

Element sets are generated by NORAD on an as-needed basis rather than according to an establishedtimetable. The frequency of this updates depends uponnumber of factors, such as

* The orbit typeLow-earth orbit satellite, like for example, US

space shuttle-would have its element sets updatedseveral times a day because of the somewhatunpredictable results of atmospheric drag as it variesits attitude and the maneuvering being performed;

* Maneuvering capabilityA satellite in a low-drag orbit which doesn't

maneuver such as LAGEOS II might only needupdates once or twice a week;

* Type of objectObjects such as rocket bodies, defunct payloads,

or other space debris, won't be updated as frequently,either unless there is a prediction of a closeapproach with an operational payload;

* Object prioritySpecial-interest objects such as a large object

reentering the earth's atmosphere normally getspecial treatment.

V. SATELLITE TRACKING SOFTWARE - NAVSTAR

NAVSTAR satellite tracking software presented in thispaper, is also based on the mathematical SGP4/SDP4model. Program uses two line elements set as an input tocalculate and visualize satellite's position in Space. It canbe used to navigate telescopes to space objects passingover certain point on Earth. The complete mathematicalmodel is encapsulated in ActiveXR control, so it acts likea black box. The data is provided from TLE and on theother end viewport coordinates are calculated.NAVSTAR has three basic functions:

* Graphical display of satellite positions in real-time, simulation, and manual modes;

* Tabular display of satellite information in thesame modes;

* Generation of tables (ephemerides) of past orfuture satellite information for planning oranalysis of satellite orbits.

The principal feature ofNAVSTAR is a series of MapWindows, which display the current position of satellitesand observers on a simple world map, together withinformation such as bearing (azimuth), distance, andelevation above the observer's horizon. The maps can beupdated in real time, simulated time, or manually set to

show the situation at any time past or future.An additional Table Window displays much more-

detailed information about one or more satellites in atabular form. The tabulated items can be selected andrearranged to fit the screen. This information likewisecan be updated in real-time, manual, or simulation modesas illustrated in Fig. 2.

Fig.2. Satellite selection dialog and Table Window.

Also, satellite 2D footprint (Fig.3) tracking isavailable, as well as a 3D view (Fig.4). Trackingalgorithms SGP4 and SDP4 give considerable accuracyand opportunity of efficient computation of viewingopportunities. It's also possible in 3D view to make aprediction on satellites position in the future, or to see it'sposition in the past. All is based on the informationgathered from TLE's .The preciseness of visualization depends on accuracy

and age of gathered TLE data.

Fig.3. 2D View.

Regarding the 3D View (Fig.4), options for variableview angle, zoom and time increment are implemented.This gives a user the opportunity to view satellite fromall angles and possibility to see its path (orbit), area onthe Earth covered by its signal (in a form of beam) andreal-time movement, as well as possible faster movementcaused by a time speed up.The part of Earth not covered with the Sun light is

dimmed on the globe, so the user can predict when it will

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become possible to see the satellite by a naked eye.

Fig.4. 3D Satellite tracking view.

VI. DEVELOPMENT AND TESTING

Software NAVSTAR has been developed usingBorland Delphi 7, along with freeware GLScenesolution. GLScene is technology mentioned to be anOpenGL solution for Delphi. It allows almost the samewide programming possibilities like in any otherdevelopment tool with implemented OpenGL.The software was tested in parallel with Maghelan

GPS device and it acted very well. The coordinates of thesatellite given by the GPS device matched with dataprovided by NAVSTAR software perfectly. At night itwas possible to observe low-orbit satellites visually sincethey regularly reflect light in various directions due torotation. That was the third way to check software'sprediction capabilities.NAVSTAR was also compared with some of most

popular satellite tracking programs.

VII. COMPARISSON To OTHER SOLUTIONS

At this moment a very large number of trackingsolutions is available. Many of them are commercial.Some like NASA's J-Track [5] offer on-line space

object tracking. Despite the obvious speed problems thisis very accurate solution.On the other hand solutions like "Nova for Windows"

[6] offer very precise tracking in 3D and 2D but it is acommercial one.

"Orbitron" is a satellite tracking for radio amateur andobserving purposes. It's also used by weatherprofessionals, satellite communication users,astronomers, UFO hobbyist and even astrologers.According to opinions of thousands of users from allover the world, this is probably one of the easiest andmost powerful satellite tracker. It's freeware but it hasonly 2D and Radar View. 3D View is not implemented,yet. One of the Orbitron's greatest advantages is verylarge database of world cities, and satellites [7].Comparing NAVSTAR to others, it offers combined

features from all other programs, with some speedimprovements achieved by GL Scene engine for Delphi.Modifications were mostly done in 3D View section.A cross comparison of few satellite tracking packages

can be seen in Table II.

TABLE II.PROGRAMS CROSS COMPARISON

Yes I Yes Yes YesYes YesYes Yes No YesNo No Yes NoYes No Yes NoYes No Yes Yes

VIII. FUTURE DEVELOPMENT

Future versions will include auto update feature,extended database and radar tracking. Also, localizationoptions will be available very soon, so Serbian andJapanese language support is expected.NAVSTAR is freeware software, and the complete

source code is available upon request.

REFERENCES

[1] R.E.Burns, Solution of the angles-only satellite tracking problem,National Aeronautics and Space Administration, Marshall SpaceFlight Center National Technical Information Service, 1997.

[2] Peter Drake Thompson, Jr., A General Technique for SatelliteTracking, QST, November 1975, p. 29.

[3] Felix R. Hoots, Ronald L. Roehrich, Models for Propagation ofNORAD Element Sets, Defense Documentation Center,December 1980, p.6-p.61

[4] R. Vilhena de Moraes, H.K. Kuga, D.Y. Campos, OrbitalPropagation for Brazilian Satellites Using Norad Models, Adv.Space Res. Vol. 30, No.2, p.1 -p.3

[5] http:Hscience.nasa.gov[6] http://www.nlsa.com[7] http://www.stoff.pl

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