DOPPLER SATELLITE POSITIONING

Doppler satellite positioning has its origin in early satellite orbit determinations that used
measurements of the Doppler shifts on radio signals received from the first artificial Earth
satellites. It was soon realized that Doppler shift measurements could be used to determine
ground positions if the satellite orbits were known (McClure, 1958). This discovery gave the
impetus for the development of the Navy Navigation Satellite System (NNSS), also known as
TRANSIT. Geodetic Doppler positioning methods almost exclusively utilize NNSS satellites, though
Doppler shift measurements have been used for tracking and orbit determination of geophysical
and oceanographic satellites (see Satellite Altimetry).
Doppler shifts on radio signals/ Doppler Effect
The changes in frequency of any kind of sound or light wave produced by a moving source
with respect to an observer. Waves emitted by an object traveling toward an observer get
compressed- prompting a higher frequency- as the approaches the observer.
When wave energy like sound or radio waves travels from two objects, the wavelength
can seem to be changed if one or both of them are moving. This is called the Doppler effect.
The Doppler effect causes the received frequency of a source (how it is perceived when it
gets to its destination) to differ from the sent frequency if there is motion that is increasing or
decreasing the distance between the source and the receiver. This effect is readily observable as
variation in the pitch of sound between a moving source and a stationary observer.

When the distance between the source and receiver of electromagnetic waves remains
constant, the frequency wave is the same in the both places. When the distance between the
source and receiver of electromagnetic waves is increasing , the frequency of the received wave
form is lower than the frequency of the source wave form. When the distance is decreasing, the
frequency of the received wave form will be higher than the source wave form.
Doppler shifts
As the satellite passes overhead, the range between the receiver and the satellite
changes; that steady change is reflected in a smooth and continuous movement of the phase of
the signal coming into the receiver. The rate of that change is reflected in the constant variation
of the signal’s Doppler shift. But if the receiver’s oscillator frequency is matching these variations
exactly, as they are happening, it will duplicate the incoming signal’s Doppler shift and phase.
This strategy of making measurements using the carrier beat phase observable is a matter of
counting the elapsed cycles and adding the fractional phase of the receiver’s own oscillator.
But perhaps the most important application of Doppler data is the determination of
the range rate between a receiver and a satellite. Range rate is a term used to mean the rate at
which the range between a satellite and a receiver changes over a particular period of time.
Navy Navigation Satellite System (NNSS), also known as TRANSIT
The Transit system, also known as NAVSAT or NNSS (for Navy Navigation Satellite System),
was the (launched 1959) first satellite navigation system to be used operationally. The system
was primarily used by the U.S. Navy to provide accurate location information to its Polaris ballistic
missile submarines, and it was also used as a navigation system by the Navy's surface ships, as
well as for hydrographic survey and geodetic surveying. Transit provided continuous navigation
satellite service from 1964.
NNSS system that operated on the Doppler shift. And the GPS system uses the Doppler
shift as an observable. It is useful to have a concept of how much shift is typical with a GPS
satellite. This graphic is intended to indicate that. As you see on the left, with the satellite rising
or moving toward the receiver, the Doppler shift is approximately 4 1/2 to 5 cycles per
millisecond. At zenith or at its closest approach, the shift is nominally zero. It then goes from the
positive to negative, returning again to approximately 4 1/2 to 5 cycles per millisecond as it's
moving away and about to set relative to the receiver. This steady shift is caused by the
continuous movement of the satellite relative to the receiver. It is very predictable. That
predictability, the constant variation of the signal's Doppler shift, makes it a good observable. If
the receiver's oscillator frequency is adjusted to match these variations exactly, as they're
happening, it will duplicate the incoming signal's shift and phase. This strategy of making
measurements using the carrier beat phase observable is a matter of counting the elapsed cycles
and adding the fractional phase of the receiver's own oscillator. This is one way that the phase
lock loop maintains its lock on the signal as the Doppler shift occurs with each of the satellites
that it is tracking.
Altimetry
Altimetry is a technique for measuring height. Satellite altimetry measures the time taken by a
radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver.
Combined with precise satellite location data, altimetry measurements yield sea-surface heights.
HOW ALTIMETRY WORKS
Altimetry satellites basically determine the distance from the satellite to a target surface by
measuring the satellite-to-surface round-trip time of a radar pulse. However, this is not the only
measurement made in the process, and a lot of other information can be extracted from
altimetry.
The magnitude and shape of the echoes (or waveforms) also contain information about the
characteristics of the surface which caused the reflection. The best results are obtained over the
ocean, which is spatially homogeneous, and has a surface which conforms with known statistics.
Surfaces which are not homogeneous, which contain discontinuities or significant slopes, such as
some ice, rivers or land surfaces, make accurate interpretation more difficult.