ERS-1 and ERS-2 Cycle and Orbit Start Times


The organisation of orbits, passes and cycles for ERS-1 and ERS-2 may be quite confusing for the non-specialist (well, even for the specialist). So I have made a little effort to give some more insight into the issue. For reference, you can read the ESA definition of ERS-1 and ERS-2 Mission Phases.

First we need to define the terms mission phase, cycle, orbit, and pass.

Mission Phase
Part of a satellite mission with a ground track pattern distinctive from the previous and following. So for any change of repeat cycle period we have a new mission phase. ERS-1 has mission phases A through G; ERS-2 only has mission phase A.
A cycle is one full completion of the repeat period. A cycle starts by definition on an ascending pass closests to the Greenwich Meridian. In the ESA definition it starts at the equator on that pass, but it is more practical (as further herein) to start the cycle at the start of the ascending pass, i.e. at the crossing of the south pole.
An orbit is one full revolution of the satellite starting and ending at an ascending node (ascending equator passage). This definition however is impractical and not used any more.
A pass spans half an orbital revolution and is either ascending (South-North) or descending (North-South). This means that a pass always starts at the turnover point, i.e. the crossing of the South or North Pole. Odd pass numbers are ascending, even are descending.

For ERS-2 the story is relatively simple. The first full cycle (Cycle 1) started on 15 May 1995, 22:29:38 UTC with the equator crossing on an ascending arc closest to the Greenwich Meridian. Since then every 35.000 days a new cycle starts.

Since ERS-1 had various mission phases with different repeat cycle lengths (3, 35, and 168 days) there is not such a nice pattern of epochs that mark the start of a cycle. Don't forget also the transitions between one mission phase and the next where the cycle number is also increased and many relative orbit numbers are skipped.

On top of this it is much more practical to count passes in stead of orbits. This scheme gives a more natural place to cut orbits: in places where people generally are not interested in. Also the pass number distinguishes immediately ascending from descending.

Pass information

We have build two tables with the start times and equator crossings of all ERS-1 and ERS-2 passes. Note: these tables are quite big (about 3MBytes). To give you a hint at how they look, a small sample is given below. The time coordinate is seconds from 1.0 January 1985, like Geosat.
# This table lists information about ERS-1 passes
# Per record is listed:
# - Mission phase
# - Cycle number
# - Pass number
# - Start time in UTC seconds from 1.0 Jan 1985
# - Stop time in UTC seconds from 1.0 Jan 1985
# - Equator passage (node) time in UTC seconds from 1.0 Jan 1985
# - Longitude of node in degrees
# - Number of records for this pass
# ph cyc pass      starttime          endtime           nodetime     nodelong  #recs
  a  001 0061  207620704.644073  207621448.131434  207620013.326420 334.131767  484
  a  001 0062  207621605.588618  207624062.214146  207623022.802020 141.591060 1540
  a  001 0063  207626829.654924  207627415.036909  207626041.207595 309.014843  127
  g  156 1000  360358463.250731  360361188.901920  360359921.395570 217.880637 2170
  g  156 1001  360361793.336316  360364441.080044  360362943.804779  25.287198 1112
  g  156 1002  360364466.396941  360367166.618659  360365957.258287 192.731278 2280

Cycle information

From the aforementioned pass tables it is relatively easy to construct cycle tables, with the start times of each cycle. They can be retrieved by clicking the references below. A sample is given at the bottom of this page.
# This table lists information about ERS-1 cycles
# Each record lists the information for one repeat cycle:
# - Mission phase
# - Cycle number
# - First internal pass number
# - Last internal pass number
# - Start time in UTC YYMMDDHHMMSS.sss (with sss fraction of seconds)
# - Stop time in UTC YYMMDDHHMMSS.sss (with sss fraction of seconds)
# - Number of tracks available
# - Number of records for this cycle
# ph cyc pass0 pass1 starttime        endtime          #trx  #recs
  a  001 0061  0086 910801002504.644 910801212941.199   26   41083
  a  002 0001  0086 910801213941.825 910804213052.169   84  144756
  a  003 0001  0086 910804213942.262 910807213051.658   85  141021
  g  154 0001  1002 960218220942.934 960324215920.079  991 1794014
  g  155 0001  1002 960324220945.755 960428215925.087  973 1743525
  g  156 0001  1002 960428221003.290 960602215926.619  989 1749755

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Remko Scharroo,