This webpage is about Dr Elizabeth Essex-Cohen (1940-2004)
who in 1974 was the only Australian involved in the design of
the satellite navigation system NAVSAT later renamed GPS.
After her initial contribution in the GPS design stage,
she returned to the US Air Force Geophysical Research Station in 1978
as a NASA Senior Post-doctoral Fellow, engaged then in further ionospheric research.
Returning to Australia in 1979, she actively continued her research on the ionosphere,
and was active in lobbying for the second Australian satellite, FedSat.
She was in charge of GPS science program for FedSat.
To appreciate Elizabeth's pioneering work one needs an overview of GPS, while on this page an account is given of the GPS satellite constellation system, and brief explanation how and why the ionosphere interfere's with the use of GPS
Elizabeth Annette Essex was born 1940 in Grafton, NSW,
where the world's first female radio-astronomer -
Ruby Payne-Scott had been born in 1912.
She attended the co-educational Grafton High School, before completing a physics degree at the University of New England (UNE),
in Armidale NSW. She stayed at UNE to complete a PhD, to become
the fourth woman to gain a PhD in Physics in Australia.
In her PhD and throughout her career she studied the physics of the ionoshere -
the always moving band of ionised gas (plasma) that surrounds the earth. The ionosphere
the transmission of radio waves, and in fact makes possible short-wave radio.
Her PhD research (see her PhD
thesis abstract) had involved bouncing radio-waves off the underside of the ionosphere,
effectively determining the (variable) change in height.
From 1969 at La Trobe University she began to
study on the earth's surface the features of radio waves emitted by satellites -
termed beacon satellites.
She used a rotating aerial to determine the polarization plane from the geostationary satellite
ATS-1 and hence could determine the Total Electron Content (TEC) along the radio path to the
beacon satelite. This lead to her participation in the first international beacon satellite
conference in Graz, Austria in 1972
In 1973 she attended a conference on beacon satellites held in Kyoto, Japan, where,
notably she met US Air Force scientific staff interested in the reception of radio signals from satellites.
Subsequently she received an invitation to work during 1974 with Jack Klobuchar,
of the US Air Force at a research lab located on a US Air Force Base, Hanscom Fields, located just outside Boston.
This research lab -- late in 1974 - was designated the US Air Force Geophysical Research Laboratory (AFGRL)
At AFGRL in 1974, she worked with future Institute of Navigation Fellow, Jack Klobuchar, USAF physical research scientist, who was developing the gamut of ionospheric corrections for GPS that enable centimeter precision. During her 1974 period at AFCRL she was given the task of plotting the path of radio waves emitted by low earth orbitting satellites. The USAF supplied a civilian contractor to perform the programming for the plotting while Elizabeth set up the actual calculation framework. Using then extant knowlege of the (time-variable) plasma density -- the curved path of rays could be determined as they progressed to the Earth's surface. This work - known to this writer as the civilian programmer would regularly visit Elizabeth off-base to show latest plots -- was never published in the scientific literature; but was used to validate satellite constellation design for which at least 5 satellites had to be visible at (almost all) points of the Earth. However related work was presented (on her behalf) at the Conference IES75 (proceeding front page to the left) dealing with travelling ionospheric disturbances (TIDs), and compared her computed results obtained based on ray tracing method with a diverse number of experimental studies, of which just one was her TEC studies using the geostationary satellite ATS-1 The full IES75 paper is here.
In 1978/9 Essex-Cohen returned to the US AFGRL at Hanscom Field as a NRC Senior Post-doctoral Research Fellow (while her husband was a Visiting Professor in the MIT Artificial Intelligence Laboratory). As in 1974 she worked with leading researcher Jack Klobuchar.
Essex-Cohen's position as a prime researcher on microsatellites at the start of the 21st century was recognised by the internation community: in fact in one nineteen months period before the launch of FedSat she was invited, and attended at JPL's expense, five separate workshops on microsatellites at the Jet Propulsion Laboratory, Pasadena. At the actual date of the launch of FedSat, Saturday December 14, 2002, she fell ill, was in hospital by Xmas. In January 2004, in a short period of remission of her cancer, she attended a Beacon Satellite Workshop in Hobart, attended by international scientists, including notably her first research student, Prof Brenton Watkins of Alaska. She died March 2004.
A sampling of Essex-Cohen's publications - with bibliographic details and abstracts, is here
Plasmasphere effects on GPS
To right is a map of the ionosphere and plasmasphere at the indicated date, from a 1999 paper by Essex-Cohen and PhD student Phillip Webb (Abstract is here .) Note that the colour coding indicates the electron density. Although electron density in the plasmasphere is much less than in the ionosphere, radio waves from GPS satellites to earth have to traverse the far greater depth of the plasmasphere to reach earth.
Artist's representation -- not to scale -- of the GPS satellite constellation.
for proposed NAVSTAR Satellite Navigation System
Satellite Navigation - the idea
Once the Cold War started the US became seriously involved in the development of Inter-continental ballistic missiles -- missiles that in the event of War with the USSR would transport nuclear bombs to selected targets in the USSR and elsewhere. Initially such missiles were only able to navigate by dead reckoning --utilising gyrocompasses to provide reference axes. Dead reckoning navigation introduces huge errors -- limiting applicability to missiles carrying larger bombs against large targets such as cities - but incapable of pin-point navigation against Soviet missile launching sites. Of course -- just as the Japanese bombing fleet that bombed Pearl Harbour used Honolulu radio stations to fix its bearings -- so static sources in the attacked region would be capable of exploitation -- but equally vulnerable to decoy emissions and jamming. There is only a limited capability for the use of celestrial navigation. The US needed a reliable means of navigation. and satellite navigation was conceived. Using "fixes" on at least 5 satellies -- such fixes to include data on distance from each satellite (and thus the distance to each) it is possible to accurately determine one's location. see our discussion of the computational basis of of NAVSTAR (and later GPS).
But there's the Ionosphere Up There
Artists conception of the NAVSTAR-GPS satellite constellation system, comprising 23 satellites (+ 1 spare). The constellation parameters were governed by the requirement that at (almost) every point of earth mostly 6 and at least 5 satellites could be observed. But around the earth is the ionospere a belt of radiation which is in direct intereaction with the solar wind (gusts of ionised particles from the sun). Could the ionosphere have been avoided? No - although signals from Low Earth (LEO) satellites (such as FEDSAT) would [much of the time!] avoid direct ionospheric effects -- the high speed of LEO satellites in the sky would seriously limit accuracy. Thus a satellite constellation with orbits ABOVE the ionosphere -- whose effective height in fact fluctuates -- but their high speed in the sky Orbits below the ionosphere would be Low Earth Orbit satellites of short period - and so of excessive speed for high accuracy fixes But there was a catch to the simple picture. in orbit above the ionosphere, . But setting the constellation above the ionosphere introduces another issue. The ionised plasma that comprises the ionosphere bends the path of radio waves
Operational Details re GPS
The GPS Operational Constellation consists of 24 satellites: 21 navigational SVs and 3 active spares orbit the earth in 12 hour orbits. These orbits repeat the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth. The Master Control facility is located at Falcon Air Force Base in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite. The Master Control station uploads ephemeris and clock data to the SVs. The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals.
For an account of how GPS works and its basic features see the web page
About GPS or click