Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 320

7.2.7 Hardware Bias Errors
7.2.7.1 Satellite Biases
Upon signal transmission, the GPS signals on each carrier frequency and among fre-
quencies are imperfectly synchronized due to the different digital and analog signal
paths corresponding to each signal. The timing bias between the L1 and L2 P(Y)
code signals is inconsequential for most dual-frequency users since the broadcast
clock corrections compensate for this bias under the presumption that the user is
combining L1 and L2 pseudorange measurements via the ionospheric-free

pseudorange equation.
Single-frequency users (L1 or L2) employing the broadcast clock corrections,
however, must correct for the L1-L2 timing bias by using a broadcast correction,
T GD , contained in word 7 of subframe 1 of the GPS navigation message. The absolute
value of the uncorrected L1-L2 group delay bias is specified to be less than 15 ns
with random variations about the mean less than 3 ns (2 sigma) [4]. Observed values
are generally less than 8 ns in magnitude. Until 1999, broadcast T GD values were
derived from factory measurements. Since April 1999, the broadcast T GD values have
been provided to the Air Force by JPL [27, 28]. At present, the accuracy of the
broadcast values is limited by a nearly 0.5-ns message quantization error.
C/A code users have an additional timing bias of the transmitted signals to
account for, which is the bias between the L1 C/A code and P(Y) code signals. This
bias is specified to be less than 10 ns (2 sigma) [4]. Typical observed magnitudes are
less than 3 ns. Although various organizations, including JPL, routinely estimate this
bias, the present GPS navigation message does not include a field for this data.
Future GPS navigation messages, however, will disseminate corrections for the L1
C/A code to P(Y) code bias, as well as a number of additional group delay correc-
tions, referred to as intersignal corrections (ISCs) that will be introduced on future
satellites (i.e., Blocks IIR-M and beyond) that will broadcast the new L2C, M code,
and L5 signals [4, 29].

7.2.7.2 User Equipment Biases
User equipment bias errors introduced by the receiver hardware are often ignored
because they are relatively small in comparison to other error sources, especially
when cancellation is considered. GPS signals are delayed as they travel through the
antenna, analog hardware (e.g., RF and IF filters, low-noise amplifiers, and mix-
ers) and digital processing until the point where pseudorange and carrier-phase
measurements are physically made within the digital receiver channels (see Chap-
ter 5). Although the absolute delay values for propagation from the antenna phase
center until the digital channels may be quite large (over 1

s with long
antenna-receiver cable runs or when SAW filters are employed), for similar signals
on the same carrier frequency the delays experienced for the set of visible signals
are nearly exactly equal. The absolute delay is important for timing applications
and must be calibrated out. For many applications, however, the common delay
does not affect performance, since it does not influence positioning accuracy, but
rather directly appears only in the least-squares estimate of receiver clock bias. The
C/A code signals have measurably different power spectra due to their short PRN

µ