The TIROS-N series satellites were designed to operate in a near-polar, sun-synchronous orbit. The orbital period is about 102 minutes which produces 14.1 orbits per day. Because the number of orbits per day is not an integer, the sub-orbital tracks do not repeat on a daily basis, although the local solar time of the satellite's passage is essentially unchanged for any latitude.

However, the satellite's orbits drift over time (Price 1991). This drift causes a systematic change of illumination conditions and local time of observation which is the major source of non-uniformity in multi-annual satellite time series.

Table 1.2-1 contains the approximate times of the ascending node (northbound Equator crossing) and the descending node (southbound Equator crossing) in Local Solar Time (LST) for the TIROS-N series when the satellites were launched. This table also contains the ascending and descending nodes as of March 1995 for the active satellites.

Table 1.2-2 summarizes the important dates for the satellites which have already been launched from the TIROS-N series. The date range in this table is at best an approximation. There may be scattered data sets available before or after these dates.

SSB has available specific orbital reference information regarding each orbit of the polar orbiters. This information consists of the orbit number, longitude of ascending and descending nodes, height of satellite at each node, and date and local time. SSB routinely receives this nodal information from SOCC two or three weeks in advance of the actual orbit.

A user may want to know the sub-orbital track and areal coverage available for a polar orbiter. The following paragraph describes how to make a "spinner" which would show the user this information. A spinner consists of a base map which is overlaid with a piece of clear acetate containing the sub-orbital track of the satellite. The acetate track is rotated over the base map as desired.

To make a spinner, the Polar-Stereographic map of the Northern Hemisphere in Figure 1.1-1 should be mounted on stiff cardboard or similar material. The sub-orbital track and width of the orbital swath for the TIROS-N series which is shown in Figure 1.2-1 should be traced onto a piece of clear acetate and overlaid on the base map. Note for Figures 1.2-1 and 1.2-2, the outer solid lines indicate a 15 degree swath (the actual width of an orbital swath is approximately 25 degrees). The area under the 15 degree swath contains good, usable data with little or no distortion at the edges. A small map pin should be inserted through the "x" on the acetate and into the center (North Pole) of the base map. The numbers indicated on the sub-orbital track are the minutes after the ascending node. The user need only rotate the acetate around the map base until the orbital track is over the desired area and read off the ascending node longitude. Or, conversely, if the orbit number and ascending node longitude are known, then the spinner can be rotated to the proper longitude and the orbital coverage will be shown as that area covered by the spinner. Similarly, a Southern Hemisphere spinner can be made using the base map in Figure 1.1-2 and the sub-orbital track in Figure 1.2-2.

Users now have the option of downloading a self-extracting file XTRCTORB.EXE to their PC's hard drive. This file generates a program, GNRLORB.EXE and associated files which is the equivalent of making the spinner described in this section. By inputting the longitude of the ascending node (which is also available on the same WWW site), GNRLORB will display the subtrack of a nominal TIROS-N series satellite with marks at five minute intervals from the ascending node and the limits of an AVHRR scan on a choice of map bases: 1) rectangular equal spaced projection from 70S to 70N latitude; 2) Northern Hemisphere Polar Stereographic projection; and 3) Southern Hemisphere Polar Stereographic projection. Users may access this software from NOAA/NESDIS' Product Systems Branch (PSB) Home Page which has a URL of: http://www.osdpd.noaa.gov/PSB/PPP/NAVIGATION/navpage.html. Users should click on the "Graphical Orbit Locator" to initiate the ftp download process. This same site also contains an overview of the NESDIS polar earth location process, polar satellite equator crossing information and clock drift files for polar satellites, as well as links to TBUS information and the Brouwer/Lyddane Software package.

Another excellent source of satellite navigation information is located at the NOAA Satellite Information System (NOAASIS) Internet site which has the following URL: http://www.osdpd.noaa.gov/EBB/noaasis.html . Users should click on the "Navigation" button to access TBUS bulletins, equator crossings, orbital elements and two line elements for both GOES and POES satellites. Also included is the navigation summary for the GOES satellites and the Monthly Predict elements for the POES. Further information on NOAASIS is included in Appendix G.

The United States Space Command (USSC, formerly NORAD) tracks TIROS-N series satellites for NESDIS. USSC sends orbital information to NESDIS each day. NESDIS regularly produces Brouwer mean orbital elements for its polar orbiting satellites and SSB archives them weekly (in hardcopy form). An example of the Brouwer mean orbital elements for TIROS-N (as of October 24, 1978 at 0000 UTC) is contained in Table 1.2-3.

SOCC began recording TBUS (APT predict bulletin) messages on magnetic tape beginning on June 1, 1990. These TBUS messages contain information about each operational polar orbiter satellite. Specifically, TBUS Part IV contains the Brouwer mean elements which can be used in a stand alone Brouwer-Lyddane orbit prediction package to determine orbit position information at any time t-t0 where t0 represents the time of the Brouwer mean elements in Part IV and t represents the user request time. The Brouwer-Lyddane algorithm is an analytical solution of satellite motion for a simplified disturbing potential field limited to zonal harmonic coefficients for J2 through J5. Lyddane modified Brouwer's formulation to obtain algorithms applicable for zero eccentricity and zero inclination. This stand alone software package is included in Appendix F. Users may also request TBUS messages in hardcopy form from SSB.

Based on the information in Table 1.2-3, orbital reference information (ephemeris data) for the first operational TIROS-N orbit on November 1, 1978 is given in Table 1.2-4.

The ephemeris data is updated regularly and maintained on file (hardcopy) at SSB. Since June 1, 1992, SSB has also maintained an archive of ephemeris data containing 4-line elements and the PSCEAR initialization reports that IPD and Satellite Operations Control Center (SOCC) use to precisely locate each NOAA operational polar orbiting spacecraft for navigational purposes. These data are available from SSB.

The 4-line elements are generated by the NAVY's U.S. Space Command (USSC, formerly NORAD) and are transmitted to SOCC around 0000Z each day. IPD uses them to reinitialize or update the user ephemeris files each day. The 4-line elements contain the following parameters: the satellite ID, epoch time, epoch revolution number, the x,y,z components of the position and velocity vectors (Cartesian coordinates), ballistic coefficients, daily solar flux, average solar flux for 90 days, planetary magnetic index, drag modulation coefficient, radiation pressure coefficient, time of ascending node, revolution number at time of ascending node, the universal time correction and the universal time correction rate.

The PSCEAR initialization reports contain information from the User Ephemeris Files (UEF) header. The PSCEAR reports contain osculating Keplerian elements at epoch, the inertial Cartesian elements, the Brouwer mean elements at epoch, the anomalistic and nodal periods, orbit number at epoch, plus the first time derivatives of: the right ascension of the ascending node, argument of perigee and the mean anomaly. Contact SSB (see Section 6) to order these ephemeris data.

Source: http://www.ncdc.noaa.gov