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Notes for Users of the !Spotter Application, version 1.06 ========================================================= Introduction ============ To my knowledge, !Spotter is the first satellite orbit prediction utility implemented on the Archimedes, although many similar programs exist on other platforms. !Spotter reads in a set of NORAD two line elements (from the elements file provided), and allows the user to perform real time analysis on these elements. This means that the user can display the location of the satellite in any of three ways, and/or the position of the satellite as viewed from any number of locations on the Earth's surface, all in real time. In addition to this, it is possible to create a printout, (or WIMPY *) for any time period, detailing the visibility coverage for up to five stations within that period. The !Spotter package comprises two Wimp applications. The first of these is the front end, which installs itself on the Icon bar when the application is first run. This program controls the contents of the configuration file (details later), and also launches any number of instances of the second type of application. These are the satellite tasks. Each satellite task controls all the functions (i.e. position displays and WIMPY (*) generation) for a particular satellite, and any number of satellite tasks may be running simultaneously. In addition, when a satellite task is running, the front end task is no longer required and so may be cancelled. The following sections describe the two types of task in more detail. These notes are by no means exhaustive, but I hope that they will eventually evolve into a complete user guide in the future. In the meantime, please do not hesitate to contact me in the event of queries, bug reports, suggestions etc. (*) Within ESOC, a spacecraft visibility printout is known as a WIMPY for historical reasons. (The best explanation that I have heard is that it is colloquial expansion of the initials W.M.P., from World Map Prediction). The reason I have used this accronym in !Spotter is that WIMPY is a lot easier to fit in a menu list than any alternative I could think of !! The !Spotter Frontend ===================== When the !Spotter application is first run (i.e. a double-click on the !Spotter icon from within a filer window has occurred), the !FrontEnd task is started, and the icon installs itself on the icon bar. Clicking SELECT over the iconbar icon will display a welcome window, but the main functions are reached via the MENU button. The items in this menu are presented in order below: Info: Leads to window containing standard application info Load Satellite: This item leads to a menu structure from where any satellite currently known to !Spotter can be loaded. Clicking on a satellite name from a list will start a satellite task for that satellite. Clicking with the 'Alt' key pressed on any menu other than the 'User List' submenu will toggle inclusion of that satellite in the user list menu. The 'Find Sat' option leads to a writable menu. Entering a string in this field will initiate a search for this string in the satellite list, and a menu of matches (if any) will be displayed. Add Satellite: Not currently implemented. Presently, to add to the list of available satellites, a set of NORAD format elements must be preprepared, and then dragged to the icon bar icon. Save Elements: This menu item leads to a "save_as" dialogue for storing the currently loaded element sets, including the information on the satellite user list contents. The default save file is the !Spotter internal file, so to update the elements loaded by !Spotter on start up, simply click on the OK icon. Load Planet: Not currently implemented. I may or may not include the facility for predicting motions of other solar system bodies in a future release of !Spotter. Change Setup: This option enables the user to edit the configuration file. Clicking on this option displays the configuration control panel, and the contents of the file can be edited from here. More information on the configuration file can be found below. Quit: Quits the application. Any satellite tasks started by the task now being cancelled will, however, remain active. Files Read By !Frontend ----------------------- The Elements File ----------------- The names of satellites along with the two line elements are read from the file 'elements', which should reside in the !Spotter directory. Updating The Elements File -------------------------- To use a different elements file, drag the new file to the !Spotter icon on the icon bar. The FrontEnd task will then read in the contents of the new file, and then open a window above the icon bar. This window displays the total number of element sets in the new file, how many of these sets are more recent elements for satellites already in the database, and how many of them are for satellites not presently defined. The user can update the elements currently loaded by clicking on the OK icon, but the nature of the update is determined by the status of the radio icons as follows: "Replace old data": If this option is selected, the data from the new file will completely overwrite the data currently loaded. The currently defined "User List" will be lost. "Merge data": If this option is selected, the new data will be merged with the old data, with the manner of the merging being determined by the following radio icons: "New": All satellites in the new file that do not appear in the currently loaded satellite database will be appended to the database. "Updated": All satellites for which newer elements have been received will be updated in the loaded database. "Selection": Satellites explicitly selected from the list of elements in the new file (list obtained by clicking menu) will be updated. Useful if you need to load old elements for some reason. In order that the updated elements are available to !Spotter the next time the application is started, the elements should be saved. To do this, simply Click OK in the "Save Elements" submenu. The Configuration File ---------------------- This file is intended to hold any user configurable parameters. Currently, this file contains only two types of information. The first of these is the local time offset from Coordinated Universal Time (UTC, or, more often GMT). All orbit work is done in UTC, and the offset is needed to relate this time to the time read from the computer clock. If your computer is running in GMT, then the offset should be set to 0 hours, 0 minutes, 0 seconds. If your computer uses British Summer Time (1 hour ahead of GMT), set the offset to 1 hour, 0 minutes and 0 seconds, and similarly for other locations. It is worth noting that the sign of the time offset applies to all three fields, i.e. - 02 hours 30 minutes means -2.5 hours, and not -1.5 hours. The second type of information currently contained in the configuration file is the list of observation stations set up by the user. The list comprises names and coordinates of any number of stations, where the coordinates are given as longitude, geodetic latitude and altitude relative to the WGS 72 geoid. The stations provided in the file with the distribution are a few of the locations that I have used in the past, with varying levels of precision. It is unlikely that any of these will be of any use to you, so feel free to delete them and add your own. All information in the configuration file may be edited via the 'Change Setup' option in the !FrontEnd main menu. Stations may be added, deleted or modified, and coordinates may be entered as geodetic, geocentric or cartesian. Alternatively, the 'config' file may be edited directly, but make sure that the numer of stations given in the second line matches the number of stations following. Some Additional Points ---------------------- When a satellite task is loaded, the configuration is the same as the configuration in the Front End task, i.e. any changes made in the 'Change Set Up' panel will be transmitted to ant initialised satellite tasks. This does not apply to versions previous to 1.02. The Satellite Subtasks ====================== As described in the previous section, satellite specific subtasks are started from the !FrontEnd menu. These tasks start by displaying their main window in the centre of the screen, and are quit by clicking on the exit icon of this window. While this window exists, the task is still active. The main window displays some basic information on the satellite that has been loaded, i.e. the satellite name, NORAD number, epoch of the loaded two-line elements (which gives an indication of the accuracy of predictions), the orbital period and the orbital inclination. Clicking MENU within this window displays the following list of options: Time Control: This menu item leads to a window controlling the epoch used for the display windows, and also for the frequeny of update of the control windows. To set the epoch to a particular time, simply enter the time and click the 'SET' icon. The time can then be reset to the computer time (i.e. actual time) by clicking on 'RESET EPOCH', and the epoch can be frozen by toggling the 'Freeze Epoch' radio icon. To reduce the update frequency of the display windows, deselect the 'Update Frequency' icon, and change the interval to the desired value. This can be useful to improve the response time for other applications. Display: This menu item controls the real time displays for this satellite task. Hanging off of this menu is a list of the types of display available, and these are dealt with in more detail in the next section. Create Hardcopy: All output product files generated by the satellite task are controlled by this submenu. This currently consists of ephemeris & WIMPY generation, but I intend to add groundtrack plot generation in the near future. A WIMPY is a chronological list of pass events for up to five different locations. Additionally, the WIMPY contains details of when the spacecraft is lit by the Sun, and hence when the satellite is potentially visible to the naked eye. Presently this is the only way of obtaining this information from within !Spotter (although I intend to rectify this). Details on ephemeris & WIMPY generation are given below. Types of Display Windows ------------------------ As mentioned above, the Display option on the satellite task main menu leads to a submenu containing a list of the different types of real time displays available. The items on this submenu are presented in order below: UTC Epoch: Clicking on this item opens a window in which the UTC time for which the other display windows are valid is displayed. Inertial State: Clicking on this item opens a window displaying the current Earth Centred Inertial state vector (as defined by NORAD: mean equinox, true equator of date). Earth-Fixed State: This item displays the current Earth-Fixed state, calculated by rotating the ECI state through an angle corresponding to the current Greenwich Mean Sidereal Time. Lat/Lon/Altitude: This menu item opens a window containing the Earth-Fixed state expressed as latitude, longitude and altitude relative to the WGS-72 geoid. Station View: This menu item leads to another submenu containing a list of the stations in the configuration file. Each of these submenu items opens a window containing the position of the spacecraft relative to the viewing station. This information is presented as range (in km), azimuth (in deg), and elevation (in deg), but it is the last two of these that will enable you to see the satellite. Azimuth is defined as 0 deg due North, increasing to the East, i.e. due East is 90 deg, South is 180 deg, and West is 270, or -90 deg). Elevation is defined as elevation above the horizon, so that if the satellite is visible, the elevation will be in the range 0 to 90 deg. World View: Not currently implemented. It is intended that this option toggles the real time display of the satellite position superimposed on a map of the world. When any of these windows are opened, then the menu item that was used to open the window is greyed out to avoid duplicate windows. When the window is closed (by clicking on the close icon), then the appropriate menu option is reenabled. WIMPY Generation ---------------- If you wish to find out when (or if) a particular satellite will be visible within a specific time interval, then the WIMPY will give you this information. To generate a WIMPY for a particular satellite, select the 'WIMPY' option from the 'Create Hardcopy' submenu of the satellite task main menu, and then a WIMPY options panel will appear. Either by clicking on the raise and lower icons,or entering the times manually, set the start and end times for the new WIMPY to cover the desired interval. The list of available stations is obtained by clicking MENU in the central section of the control panel. Clicking on a station name toggles its inclusion in the WIMPY, up to the maximum of 5 stations. As is standard RISC OS practise, clicking with ADJUST will keep the menu on the screen. Next, you can change the time step used between records in the WIMPY by directly overwriting the default value. However, the default value of 60 seconds should be adequate for most purposes. In a future release, it may be possible to change the elevation cutoff applied to the rise and set times, but this is not presently implemented. Clicking on GO will begin present the user with a "save_as" box, where the user can drag the file icon to a filer window to indicate where the WIMPY should be stored. When this has been done, the WIMPY generation will commence. Whilst the WIMPY generation is in progress, a window will be present in the centre of the screen showing the status of the generation. Although the generation does multitask, it will go much faster if no other display windows are active at the same time. Ephemeris Generation -------------------- For some purposes (for example when making plots), a list of state vectors (called an ephemeris) is required. Three types of ephemeris can be created by !Spotter - Inertial, Earth-Fixed, or the groundtrack of the subsatellite point (as latitude & longitude). Selecting any of these options from the 'Create Hardcopy' submenu will pop up a options menu for the required ephemeris. There, you can select the time range required and the step size, and also (by dragging the filer icon) select the name of the file to hold the ephemeris. When ready, a click on the 'OK' icon will begin ephemeris generation. If sufficient RAM is available, then the ephemeris will be generated into RAM and written to the file in one go when complete. Otherwise, writing to the output file will begin straight away. As for the WIMPY generation, a window displaying the status of the ephemeris generation will be created. Groundtrack Plot Generation --------------------------- To visualise the path of a spacecraft over the Earth, it is possible to create a plot of the path of the sub-satellite point over the Earth's surface. Selecting 'Groundtrack Plot' from the hard copy submenu will open a plot options window. Here you can set a number of options defining the features of the plot. First, the time range for the groundtrack path can be chosen, and by clicking on the node arrows, it is possible to force the path to start or end at an ascending node. It is also possible to select the page size for the plot if it is to be printed, and the longitude to be at the centre of the map is also selectable. The Map can contain a latitude/longitude grid, with the step size configurable, and may also be surrounded by tick marks (to enable labelling). Large tick marks are put at specified interval, with 10 smaller tick marks in between. Clicking MENU over colour icons opens up a !Draw-like colour selection menu, where the colours for the land and sea in the plot can be selected. Clicking menu over the 'Stations' selection of the window will open a list of available ground stations, and selecting any of these will put the visibility circle for this station in the ground track plot. Unlike WIMPY generation, there is no limit to the number of stations used for the groundtrack plot. Finally, by dragging the file icon, it is possible to choose where the file will be stored. Clicking on the OK icon will start the generation of the plot. The path and map data are read first into RAM, and then all data is written to the output file in one go. As for the other hardcopy generation, a window displaying the status of the groundtrack generation will be created. Final Remarks ============= As I mentioned in the introduction, these notes are just that, notes, and do not describe every detail of the !Spotter application. However, I hope that they contain enough information to enable you to make some use of the program, and also to inform you of some of the drawbacks of the current version. I have tried to point out where I think improvements are necessary, but a more specific list of my intended updates is given in the file 'To_Do' in this directory. Of course, if you have any comments yourself, I would be delighted to hear them. Finally, to those new to satellite spotting, I have include a file 'Hints' which contains a few pointers that should enable you to find you first satellite. Happy Spotting, Stuart Martin, 18th July 1993.