3.4.1 ERS Waveform Product
T he W aveform P roduct (W A P), produced by the U K -PA F, is the m ain source o f altim eter data used for this work. It contains a record o f the altim eter return echoes w hich are each averaged over 50 w aveform s by the on-board processing system , and the relevant scientific data associated w ith each waveform . D ata is recorded at a rate
o f 20 fram es p er second. C udlip and M ilnes [1994] provide an overview o f the
processing used to generate the WAP.
W A P data is then converted to a more m anageable format, developed at M SSL, called the ERS A ltim eter (EA) form at, w hich retains the original 20 H z d ata rate. A sum m ary o f the relevant data appearing in each EA form at data record is as follows:
(i) Tim e/location information: Tim e tag, location, satellite altitude
(ii) Estim ate o f range
(iii) 64 samples o f return power, representing the averaged w aveform
(iv) Tracking inform ation: HTL and A GC discrim inator outputs, range rate,
tracking mode details
(v) A ltim eter-derived parameters: elevation, SW H, backscatter coefficient
(c^), w aveform peakiness
(vi) Corrections for: Doppler effect, dry and w et troposphere, ionosphere
(vii) A uxiliary inform ation: geoid elevation, ocean tide, ocean loading tide,
(viii) Spacecraft health param eters
(ix) Param eter quality information.
The estim ate o f range is calculated from the observed radar echo tim e delay. V arious corrections are taken into account, including the ground calibration correction and the
internal and external range corrections [Cudlip et a l , 1994].
3.4.2 ERS Ocean Product
The O cean Product (OPR), produced by the French PAP, contains a 1 Hz record o f altim eter data m ostly acquired over regions o f open ocean (the m ajority o f the sea ice data is edited out prior to distribution), and is similar to the G eophysical D ata Records (G D Rs) available from other altim eter m issions. It is a higher level product than the W A P, and contains no record o f the averaged w aveform s and very little inform ation on the m ore technical aspects o f instrum ent operation. As a result, the O PR data volum e is about 350 M bytes per m onth, com pared to about 15 G bytes per m onth for
the W A P [ESA/Earthnet, 1993]. The product user m anual describes the O PR in depth
[CERSAT, 1993].
The values o f range in this product are derived by applying a retracking algorithm to the averaged w aveform s, w hich generates m ore accurate estim ates o f range than those generated on-board. V arious calibration corrections are also applied to the estim ates, including drift o f the U ltra-S table O scillator (U SO ). The 20 H z data set is then red uced to 1 H z by perform ing a linear regressio n at the cen tre o f each 1 Hz m easurem ent.
This data w ill be used to com pare elevation m easurem ents acquired over open ocean w ith those acquired over sea ice. Bearing in m ind that the tracking system in O cean M ode w as designed to deal w ith Brow n m odel w aveform s, we w ill be able to use these com parisons to gauge the perform ance o f the system w hen specular echoes are present. The estim ates provided in the O PR have undergone num erous validation e x e rc ise s, and h ave alread y b een used effe c tiv e ly fo r m any o cea n o g rap h ic applications. A fter updating the auxiliary inform ation in the O PR, in particular the
orbits and tide m odel, Scharroo and Visser [1998] report an overall accuracy for the
O PR sea surface height estim ates o f 6.4 cm.
3.5 Summary
The ERS satellites operate in Sun synchronous orbits, w ith a latitudinal coverage o f ±81.5°. ERS-1 operated in a num ber o f different phases w ith various repeat periods, w hereas ERS-2 w ill operate in one 35 day repeat phase for the entire mission.
The exact form o f the return w aveform depends on the transm itted pulse p ro file, the flat su rface im pu lse resp o n se and th e su rface h eig h t probability density function.
In the ERS system s, 50 w aveform s are averaged together to counter the large pow er fluctuations present on individual w aveform s. The w ay in w hich this is perform ed leads to a quantisation effect in the w aveform s, resulting in the suppression o f low -level signals.
The ERS altim eters can operate in either O cean M ode, w hich assum es the presence o f Brow nian returns at the receiver, or Ice M ode for operation over non-ocean surfaces. The w ork presented in this thesis solely uses Ocean M ode data.
The tracking system consists o f 3 tracking loops - the height tracking loop (H TL), the slope tracking loop (STL) and the autom atic gain co ntrol (A G C ) tracking loop. Each loop con sists o f a d iscrim in ator, w hich
generates an error signal, and a second order low -pass filter.
T he three erro r signals are gen erated by applyin g the S u b -o p tim al M axim um Likelihood Estim ator (SM LE) algorithm to the last available averaged waveform.
U pper and low er lim its are applied to the H TL error signal in order to im prove tracking over sea ice. This softw are patch update w as perform ed on 17 June 1993, before w hich the perform ance o f the tracking system over sea ice was very poor.
The datation is w ith respect to the 37th pulse in the 50 pulse averaging sequence, and the telem etered range is coincident w ith the centre o f the range window.