Users manual
NR3700-O/G
Revision #:
A
Date:
9-13-16
Page #:
15 of 19
The receiver needs to be able to see at least four satellite vehicles (SVs) to obtain an
accurate 3-D position fix. When travelling in a valley, or built-up area, or under heavy
tree cover, you will experience difficulty acquiring and maintaining a coherent satellite
lock. Complete satellite lock may be lost, or only enough satellites (3) tracked to be able
to compute a 2-D position fix, or a poor 3D fix due to insufficient satellite geometry (i.e.
poor DOP). Inside a building or beneath a bridge, it may not be possible to update a
position fix. The receiver can operate in 2-D mode if it goes down to seeing only three
satellites by assuming its height remains constant. But this assumption can lead to very
large errors, especially when a change in height does occur. A 2-D position fix is not
considered a good or accurate fix; it i
s simply “better than nothing”.
The receiver’s antenna must have a clear view of the sky to acquire satellite lock.
Remember, it is the location of the antenna that will be given as the position fix. If the
antenna is mounted on a vehicle, survey pole, or backpack, allowance for this must be
made when using the solution.
To measure the range from the satellite to the receiver, two criteria are required: signal
transmission time and signal reception time. All GPS satellites have several atomic
clocks that keep precise time and are used to time-tag the message (i.e. code the
transmission time onto the signal) and to control the transmission sequence of the
coded signal. The receiver has an internal clock to precisely identify the arrival time of
the signal. Transit speed of the signal is a known constant (the speed of light),
therefore: time x speed of light = distance.
Once the receiver calculates the range to a satellite, it knows that it lies somewhere on
an imaginary sphere whose radius is equal to this range. If a second satellite is then
found, a second sphere can again be calculated from this range information. The
receiver will now know that it lies somewhere on the circle of points produced where
these two spheres intersect.
When a third satellite is detected and a range determined, a third sphere intersects the
area formed by the other two. This intersection occurs at just two points. A fourth
satellite is then used to synchronize the receiver clock to the satellite clocks.
In practice, just four satellite measurements are sufficient for the receiver to determine a
position, as one of the two points will be totally unreasonable (possibly many kilometers
out into space). This assumes the satellite and receiver timing to be identical. In reality,
when the receiver compares the incoming signal with its own internal copy of the code
and clock, the two will no longer be synchronized. Timing error in the satellite clocks,
the receiver and other anomalies mean that the measurement of the signal transit time
is in error. This, effectively, is a constant for all satellites since each measurement is
made simultaneously on parallel tracking channels. Because of this, the resulting
ranges calculated are known as “pseudo-ranges”.
